ACI 355.1R.pdf

ACI 355.1R-91 (Reapproved 1997)

STATE-OF-THE-ART REPORT ON ANCHORAGE TO CONCRETE Reported by ACI Committee 355 Harry A. Chambers Secretary

Patrick J. Creegan Chairman Edwin A. Burdette Robert W. Cannon Peter J. Carrato Peter D. Courtois Rolf Eligehausen

Raymond R. Funk C. Raymond Hays Paul R. Hollenbach Gerard B. Hassehvander Harry B. Lancelot III*

Douglas D. Lee Alexander Makitka, Jr. Donald F. Meinheit Richard S. Orr Moorman L Scott

George A. Senkiw Harry Wiewel Jim L Williams Richard E. Wollmershauser

*Committee Chairman during the formative years of this report.

For the first time concrete anchoring knowledge based on worldwide test programs is presented in a state-of-the-art document. Performance of different anchor types, including cast-in-place, grouted, expansion, torque-controlled, chemical (adhesive), and undercut anchors is presented in both uncracked and cracked concrete. Failure modes in tension and shear, spacing and edge distance, group performance, and load displacements are offered. The effect of loading conditions for structural supports, column bases, and pipe supports as well as base plate flexibility, how load is transferred to anchors, and ductility are discussed. Design criteria and existing code requirements, both domestic and foreign, are presented.

KEYWORDS: Adhesive anchors; anchorages; anchors; anchor groups; base plates; bolts; cast-in-place anchors; chemical anchors; code requirements; combined loads; compression zone; concrete; cracked concrete; creep; deformation; design criteria; drilling; ductility; dynamic loads; edge distance; embedment; expansion anchors; failure modes; fatigue loads; fasteners; flexible base plates; grouting; loads; load transfer; load-displacement; post-installed anchors; preload; pullout; seismic loads; shear loads; slip; spacing; spalling; static loads; stiffness; studs; structural design; tensile strength; tension loads; tension zone; temperature; torque; torque-controlled anchors; ultimate strength; undercut anchor, yield strength.

FORWARD This state-of-the-art report on anchorage to concrete is the first of a two-volume project being undertaken by ACI Committee 355. The second volume, currently being developed, is a design manual. This first volume includes no design aids or procedures, per se, but with emphasis on behavior will serve as the guide for preparation of the second volume. Committee 355 is working with Committees 349 and 318 toward the objective of including the subject of anchorage to concrete in ACI 318-95.

ACI Committee Reports, Guides, Standard Practices, and Commentaries are intended for guidance in designing, planning, executing, or inspecting construction, and in preparing specifications. Reference to these documents shall not be made in the Project Documents. If items found in these documents are desired to be a part of the Project Documents, they should be phrased in mandatory language and incorporated into the Project Documents.

ACI 355.1R-91 became effective JuIy 1, 1991. Copyright 0 1991, American Concrete Institute.

All rights reserved including rights of reproduction and use in any form or by any means, including the making of copies by any photo process, or by any electronic or mechanical device, printed or written or

oral, or recording for sound or visual reproduction or for use in any knowledge or retrieval system or device, unless permission in writing is obtained from the copyright proprietors.

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Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

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355.1R-2

MANUAL OF CONCRETE PRACTICE

TABLE OF CONTENTS Chapter 1-Introduction, p 355.1R-2

1.1 Purpose 1.2 Significance of the subject 1.3 Scope --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

Chapter 2-Types of anchoring devices, p 355.1R-2

2.1 2.2 2.3 2.4 2.5

Introduction Scope Anchor systems Cast-in-place systems Post-installed systems

Chapter 3-Behavior of anchors, p 355.1R-9

3.1 3.2 3.3 3.4

Introduction Behavior of anchors in uncracked concrete Behavior of anchors in cracked concrete Behavior of cast-in-place anchor bolts in uncracked concrete piers 3.5 References Chapter 4-Design considerations, p 355.1R-53

4.1 Introduction 4.2 Functional requirements 4.3 Materials 4.4 Design basis 4.5 Construction practices 4.6 References Chapter 5-Construction considerations, p 355.1R-60

5.1 Introduction 5.2 Shop drawings/submittals 5.3 Tolerances 5.4 Installation of anchors 5.5 Inspection 5.6 Grouting 5.7 Field problems Chapter 6-Requirements in existing codes and specifications, p 355.1R-66

6.1 Introduction 6.2 Existing codes and specifications 6.3 Application and development of codes 6.4 References Appendix A-Conversion factors, p 355.1R-71 Appendix B-Notations, p 355.1R-71 Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

CHAPTER 1 -INTRODUCTION

1.1-Purpose The purpose of this document is to summarize the current state of the art in anchorage to concrete. 1.2-Significance of the subject To date, anchorage to concrete has received little attention in structural codes. Emphasis has been primarily on the tensile and shear capacities of anchorage devices. As designs became more sophisticated and analyses more exacting, more emphasis was placed on the transfer of loads through single anchors and anchor systems. It was recognized that performance of anchors controlled these load transfers, and that generally, failure modes at ultimate anchor capacities were important. There were no definitive design codes or anchorage performance criteria on which designers and installers could rely. Subsequently, a myriad of approaches were developed. 1.3-Scope This state-of-the-art report summarizes anchor types and provides an overview of anchor performance and failure modes under various loading conditions in both uncracked and cracked concrete. It covers design and construction considerations and summarizes existing requirements in codes and specifications. References are given for further review. CHAPTER 2 -TYPES OF ANCHORING DEVICES

2.1-Introduction There are many types of devices used for anchoring structures or structural members to concrete. The design of anchorages, involving the selection and positioning of these devices has been based on the Engineer’s experience and judgment, private test data, manufacturers’ data, and existing (sometimes obsolete) code requirements. It is proposed to promote a design of anchorages that more consistently reflects the performance potential of each type of anchor. 2.2-Scope This report relates to the most widely used types of anchor, in sizes ranging from 1/4 in. (6.35 mm) to 2 l/2 in. (63.5 mm) in diameter. Included for consideration are only those devices which can generally be considered bolt and insert-type

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ANCHORAGE TO CONCRETE

anchors. Excluded from consideration are shear lugs, structural shapes, powder actuated fasteners, light plastic or lead inserts, hammer driven concrete nails, screw driven systems, and cables. These are excluded because there is a paucity of test data regarding their performance. The anchors included in this report are either commercially available or may be fabricated. 2.3-Anchor Systems According to present practice, there are two broad groups of anchoring systems: cast-in-place systems (anchors installed before the concrete is cast) and post-installed systems (anchors installed in holes drilled after the concrete has been cast and cured). Table 2.1 identifies these two groups of anchors. Table 2.1 -Types of anchors in concrete Cast-in-place systems Embedded, nonadjustable Common bolts Hooked "J" & "L" bolts Threaded rod Reinforcing steel Threaded inserts Stud-welded plates Bolted connections Adjustable anchors

Fig. 2.1 Fig. 2.2 Fig. 2.3 Fig. 2.4 Fig. 2.5 Fig. 2.6 Fig. 2.7 Fig. 2.8

Post-installed systems Bonded anchors Grouted anchors Headed bolts or anchor

Fig. 2.9

Chemical anchors With threaded rod With reinforcing steel

Fig. 2.10 Fig. 2.11

Expansion anchors Torque-controlled Heavy-duty sleeve anchor Sleeve anchor Shell expansion anchor Wedge anchor Rock/concrete expansion anchor

Fig. 2.12 Fig. 2.13 Fig. 2.14 Fig. 2.15

Deformation controlled Drop-in anchor Self-drilling anchor Stud anchor

Fig. 2.17 Fig. 2.18 Fig. 2.19

Undercut With predrilled under-cut hole Self undercutting

2.4-Cast-in-place systems 2 . 4 . 1 - Embedded Anchors, Non Adjustable - These anchors may have an end attachment, such as a coil loop, head, nut, or plate, which will enhance anchorage properties and develop full potential strength by means of bond, and/or bearing, or both. Typical examples of these anchors are: Common bolts

- structural steel bolts placed with the head into the concrete. (Fig. 2.1)

Hooked "J" or "L" bolts

-bent, smooth or deformed threaded bars. Have been known to straighten out in pull-out tests. (Fig. 2.2) Threaded rod - straight threaded rod, usually with coarse threads. (Fig. 2.3) Reinforcing steel - Stock or trade-name reinforcing bar (Fig. 2.4) wire form or internally Threaded inserts threaded ferrule inserts, or coils, usually manufactured with internal or external threads, with wire loop struts. Headed anchors made from smooth or reinforcing steel bar also fall into this category. (Fig. 2.5) Stud welded plates - steel plates which have smooth bent hooked bars, deformed bars, or headed stud anchors. (Fig. 2.6) 2.4.2 Bolted connections-These anchors consist of headed bolts, as embedded or throughconnectors. (Fig. 2.7). Plastic

Fig. 2.16

L Steel plate

Fig. 2.20 Fig. 2.20

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Fig. 2.7-Bolted connections Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


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D P

Washer tack welded . Note

Fig. 2.1- Common bolts

-

: E i t h e r ' J ' o r ' L ' ’ b o I ts c a n

be

made

from plain or threaded rod

Fig. 2.2-J- and L-bolts (not recommended)

. b

v

*

Fig. 2.4 -Reinforcing steel

Fig. 2.3 - Threaded rod




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a

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Fig. 2.5 - Threaded inserts

We I d

Fig. 2.6 - Stud- welded plates



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2.4.3 Adjustable anchors-Adjustable anchors can be adjusted for lateral position or depth (Fig. 2.8). They are normally used for attaching large machines or equipment bases. On thin floor slabs, the anchor bolt often goes through the concrete to develop the required anchor capacity. When the floor slab or foundation is very thick, the anchor can develop full capacity and still be embedded in the concrete. After the equipment or machine base is installed and leveled, grout is used to fill the void around the anchor. The anchor then acts similar to a cast-in-place anchor. --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

. P

-

4

Fig. 2.8-Adjustable anchors

2.5-Post-installed s y s t e m s These anchors are installed in a hole drilled in the cured concrete. There are two basic groups of post-installed systems: bonded and expansion. 2.5.1-Bonded anchors 2.5.1.1 Grouted anchors-Grouted anchors are headed or headless bolts or threaded rods. They are set in predrilled holes with portland cement and sand grout or other commercially available premixed grout. (Fig. 2.9)

2.5.1.2 Chemical anchors-Chemical anchors are usually threaded rods (Fig. 2.10) or deformed bars (Fig. 2.11) which are bonded in place with two-part chemical compounds of polyesters, vinylesters, or epoxies. The chemicals are available in four forms: glass capsules, plastic cartridges, tubes, or bulk. Glass capsules are inserted into the drilled hole, and then broken by the anchor rod when it is rotated and hammered into place, thereby mixing two components to cause a chemical reaction. The plastic cartridges are used with a dispenser and a mixing nozzle which mixes the two parts, initiating a chemical reaction while installing the compound into the drilled hole. The anchor rod is then inserted into the hole completing the installation. The setting time is dependent on temperature, varying from a few minutes at 90o F up to several hours at 30o F. The tube or “sausage” type contains two components which are mixed by kneading the tube, placing the mixture into the hole, and finally, inserting the anchor rod into the hole. The bulk systems predominantly use epoxies, which are either premixed in a pot and used immediately, or pumped through a mixer and injected into the hole. The anchor is installed immediately afterward. Epoxies can be formulated to set up quickly or slowly (up to 36 hr curing time).

*

in .v

chemical

or

from capsule

Fig. 2.10-Chemical anchor with threaded rod

n




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2.5.2 Expansion anchors-Expansion anchors are designed to be inserted into predrilled holes and then expanded by either tightening the nut (torque controlled expansion anchor, Sections 2.5.2.1 to 2.5.2.5), hammering the anchor (deformation controlled expansion anchor, Sections 2.5.2.6 to 2.5.2.8), or expanding into an undercut in the concrete (undercut anchors, Section 2.5.2.9). These anchors transfer the tension load from the bolt to the concrete by expansion pressures or forces through friction and/or keying against the side of the drilled hole. They often are supplied with a bolt, nut, and The following sections describe the washer. various types of expansion anchors. 2.5.2.1 Heavy duty, torque controlled sleeve anchor-This type of anchor consists of a bolt or threaded rod with nut and washer on one end and a cone on the embedded end, (Fig. 2.12). Around the cone is a heavy expansion sleeve. Above the sleeve is a collapsible mechanism, sometimes made of plastic. A spacer sleeve extends to the surface of the drilled hole. The anchor is set by tightening the bolt head or nut which draws the cone up through the expansion sleeve, expanding it against the side of the drilled hole. The anchor develops its tensile capacity by means of a combination of keying into the concrete and high friction between the sleeve and concrete. The spacer sleeve aids in increasing the shear capacity. Tensile capacity depends on the strength of the bolt and its depth of embedment. BEFORE

TORQUING

AFTER

TORQUING

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2.5.2.2 Sleeve anchors- The sleeve anchor consists of a steel stud, an expansion sleeve usually made of sheet metal, and a nut and washer (Fig. 2.13). The bottom of the steel stud has a uniformly tapered mandrel which has the same diameter at the end as the expansion sleeve. The entire length of the bolt below the washer is enclosed in a section or sections of the steel tubing. The bottom of the expansion sleeve is slit longitudinally to provide for expansion. When the nut is tightened, the tapered mandrel moves into and expands the sleeve which in turn bears against the wall of the hole. This anchor is used for medium and light holding requirements.

.

.

Fig. 2.13 - Sleeve anchor

2.5.2.3 Shell expansion anchors - The shell expansion anchor, (Fig. 2.14) is available in two types. One type consists of a two-piece shell held together by steel tabs with a tapered, internally threaded end plug. The second type consists of a two-piece shell section with two tapered steel cones, one at the top end and one at the bottom, which are held together by a steel spring at the center. The bottom cone is internally threaded to accept a bolt or stud. By torquing the fastener into the anchor, the steel cones expand the shell to bear against the wall of the hole.

by

Single-acting (shell expanded single wedge nut)

Double acting expanded (shel by opposing wedge)

Fig. 2.14 - Shell expansion anchor

Fig. 2.12 - Heavy-duty, torque-controlled sleeve anchor


2.5.2.4 Wedge anchors-The wedge anchor, (Fig. 2.15) consists of a steel stud bolt with a nut Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1

355.1R-8


and washer. The bottom of the steel stud has a uniform tapered mandrel around which is positioned an expandable steel clip or separate steel When the nut is wedges with protrusions. tightened, the clip or steel wedges ride up on the tapered mandrel, wedging between the mandrel and the wall of the hole. AFTER TORQUING

BEFORE TORQUING

2.5.2.6 Drop-in anchors-The drop-in anchor consists of a steel shell and an internal steel expander plug (Fig. 2.17). The anchor is internally threaded at the top end while the internal end is machined to a uniform taper, matching the shape of the steel plug inside the anchor. The lower portion of the shell is slit longitudinally into equal segments to allow the anchor to expand when the internal plug is hammered with a setting tool. By hammering the plug into the shell, the lower portion of the shell expands to bear against the wall of the hole. BEFORE

AFTER

Fig. 2.15- Wedge anchor

2.5.2.5 Rock/concrete expansion anchor-The rock/concrete expansion anchor, (Fig. 2.16) consists of a stud bolt that is threaded on the top end for a hex nut. The bottom end consists of a large mechanical expansion anchor. To set the expansion anchor, the stud bolt is rotated in a clockwise direction. Grouting is optional down the center of the bolt to fill the annular space between the rod and the drilled hole for corrosion protection. Grout hole Threaded rob Nut Air tube Plate

Fig. 2.17-Drop-in anchor

2.5.2.7 Self-drilling anchors-The self-drilling anchor, (Fig. 2.18) consists of a steel shell and a tapered steel end plug. The bottom of the shell has teeth for cutting its own hole in the concrete. The top of the shell is internally threaded to accept a bolt or stud. The bottom of the shell is expanded by hammer drilling the anchor over the steel plug. The plug expands the bottom of the shell which bears against the wall of the drilled hole. BEFORE

AFTER

Hollow bar

Grout hole Thrust rl Mal leable e shell

Fig. 2. I6 - Rock/concrete expansion anchor (grouted)

b 0.

Fig. 2.18 -Self-drilling anchor

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355.1R-9

2.5.2.8 Stud anchors -The stud anchor consists of a steel stud, threaded at the top end, and has a drilled hole with longitudinal slits at the bottom end, which accepts a tapered steel plug (Fig. 2.19). The top of the threaded section is raised to provide a surface for hammering. By hammering the top of the stud, the tapered plug expands the bottom end of the bolt causing it to bear against the wall of the hole. BEFORE

b

.

v

-

’ .

.

t .

.v Q

AFTER

V

*

.

BEFORE

. 4 . .

n .

D

V

v

v-

4

Q

A

AFTER

.

.

bolt and tapered cone are drawn up into the expansion sleeve, keeping the bottom of the expansion sleeve in the undercut.

A .b

I

.

V’

Fig. 2.19 - Stud anchor

2.5.2.9 Undercut anchors-There are two primary designs of undercut anchors available (Fig. 2.20). They all operate by keying and bearing against an undercut in the concrete at the bottom of the drilled hole. They cause little or no expansion force in the concrete, but generate high tensile-loading capacities. The first type requires a second drilling operation to create an undercut at the bottom of the first drilled hole. The anchor is installed with the bottom of the expansion sleeve at the undercut. When the nut is tightened, the tapered expander plug expands the bottom of the steel expansion sleeve into the undercut. The second type cuts its own undercut at the bottom of the drilled hole. A sleeve is hammered by a rotary hammer drill with a special setting tool. The bottom of the expansion sleeve is driven over a cone at the bottom of the hole. The bottom of the expansion sleeve has a sharp edge which, on expansion, cuts its own undercut into the wall of the hole. By tightening the nut, the

Fig. 2.20 - Undercut anchor CHAPTER 3-BEHAVIOR 3.1 - Introduction

Understanding anchor behavior is necessary in specifying the appropriate anchorage for a given application. This includes an understanding of failure modes and strengths as well as loaddisplacement and relaxation characteristics of various anchor types. This chapter covers anchor behavior in uncracked concrete and in cracked concrete. Anchors are primarily loaded through attachments to the embedded anchor. The loading can be in tension and shear or combinations of tension and shear (Fig. 3.1). They may also be subjected to bending depending on the details of shear transfer through the attachment. The behavior of anchors in tension is of primary importance and will be discussed first.

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OF ANCHORS


355.1R-10


combined tension and shear loading

tension loading

[ shear

loading

bending

Fig. 3.1 -Possible loadings of anchors

By far, most anchor testing to date has been performed in uncracked concrete. While cracking occurs in almost all concrete, testing in uncracked concrete provides the basis for understanding anchor behavior.

The various types of anchors have different displacement characteristics depending on preload, load transfer mechanism, and failure mode. Fig. 3.3(a)-3.3(c) present three load-displacement graphs. Fig. 3.3(a) gives the characteristic curves for headed and undercut anchors while Fig. 3.3(b) presents curves for torque-controlled, drop-in, and self-drilling expansion anchors. Fig. 3.3(c) gives load displacement curves for adhesive anchors. The displacements shown represent the displacement (slip) of the embedded anchor and the deformation of the concrete as well as the deformation of the anchor. When a preload is applied to an anchor, typically by tightening the nut to a prescribed moment torque, the displacement caused by an externally applied load is affected. The preloaded

3.2-Behavior of anchors in uncracked concrete

3.2.1 Load-displacement behavior and failure modes under tension loading- The five primary failure modes of anchors in tension are (Fig. 3.2): (a) Steel failure (b) Pull-out failure (c) Concrete splitting failure (d) Concrete cone failure (e) Spacing and edge cone failure

a) steel failure

d) concrete cone failure

b) pull-out failure

c) concrete splitting failure

e) spacing and edge cone failure

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Fig. 3.2 - Typical failure modes of anchors loaded in tension



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load F [kN]

l o a d F [kN]

0 4

6 8 10 Displacement s [mm]

Fig. 3.3(a) - Typical load-displacement relationships of headed and undercut anchors (from Rehm, Eligehausen, and Mallee 1988)

2

I I Iine

anchor type

6 8 4 d i s p l a c e m e n t s [mm] bolt diameter anchorage depth mm mm I

Fig. 3.3(b) - Typical load-displacement relationships of expansion anchors under tension loading (from Eligehausen and Pusill-Wachtsmuth 1982)

d i s p l a c e m e n t [mm]

Fig. 3.3(c)- Typical load-displacement behavior of chemical anchors under tension and shear loading (from Eligehausen and Pusill- Wachtsmuth 1982)



355.1R-12


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anchor shows little displacement with increasing external loading until the preload in the anchor (and resulting clamping force on the concrete) is overcome. The preload has no effect on the ultimate static tensile capacity of the anchorage, but significantly reduces the anchor total displacement. In the case of steel failure (Fig. 3.3(a), Line 3) the ductility depends on the relationship between tensile strength and yield strength of the steel and the anchor length. Inelastic displacements of headed anchors due to concrete deformations under the head may be expected at relatively low loads unless preloaded. Increasing the bearing area under the head may reduce inelastic displacements but will have little influence on the failure load [compare Lines 1 and 2 in Fig. 3.3(a)]. Headed anchors that fail due to fracture of the concrete will exhibit a brittle failure (Fig. 3.3(b), line 2). The behavior of drop-in anchors is dependent on the magnitude of the expansion force created in setting the anchor. When expanded properly during installation, high expansion forces are induced and the load displacement curve may remain almost linear up to failure [Fig. 3.3(b), Line 2). The expansion force, at installation, of torquecontrolled expansion anchors is smaller than that of drop-in anchors and, therefore, the displacements are larger for equal loads. If the external load exceeds the preloading force in the bolt generated by the torquing during installation, the spreading cone is pulled further into the sleeve, leading to increased displacement. At failure the deformations are much larger than for comparable drop-in anchors [Fig. 3.3(b)]. Self-drilling anchors show larger displacements in the total load range than torque-controlled expansion and drop-in anchors [Fig. 3.3(b)]. This happens because load transfer is mainly by mechanical interlock which causes high pressure on the concrete and large concrete deformations. The displacement behavior of undercut anchors depends primarily on the bearing area (undercut area) and the installation torque. Therefore relatively large deformations may be expected with some undercut anchors while others exhibit elastic behavior well above service load [Fig. 3.3(a)]. Adhesive anchors exhibit elastic behavior up to nearly maximum load [Fig. 3.3(c)]. While the load-displacement curves of adhesive anchors exhibit relatively low coefficients of variation in


comparison to torque-controlled expansion and drop-in anchors, the bond strengths vary considerably depending on the adhesive component mix used and the installation procedure. Under working loads all categories of anchors should behave elastically with little additional displacement after installation. However, at ultimate load a plastic behavior and in the case of cyclic loading only a limited strength degradation is desired. Fig. 3.3(a)-3.3(c) show that the actual load-displacement behavior of the currently available expansion, undercut, adhesive, and headed anchors differs somewhat from this plastic behavior. Under sustained loads displacements will increase with time due to creep of concrete in the highly stressed load transfer area (bearing area in the case of headed or undercut anchors, contact area in the case of expansion anchors, bonded area in the case of adhesive or grouted anchors). As an example, in Fig. 3.4 (see Seghezzi and Vollmer, 1982) the displacements of a torquecontrolled expansion anchor loaded with a constant tensile force corresponding to approximately 70 percent of the static ultimate strength, are plotted as a function of load duration on a double logarithmic scale. It can be seen that the displacement velocity (tangent to the displacement-time curve) decreases with increasing time and, therefore, the displacements approach a limiting final value. The increase in displacements is smaller for lower sustained loads. If the load is increased after a sustained load test, the displacement curve is rather steep until it reaches the static envelope which is followed thereafter. Failure load and displacement at maximum load are not negatively influenced by a previous sustained load smaller than about 70 to 80 percent of the static failure load.

10*

10

10 2

Duration [Days]

Fig. 3.4 -Increase of displacement during sustained loading Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


In principle, the same behavior is valid for cyclic loadings with up to 1 x lo6 load repetitions and an upper load (where the cyclic load ranges between an upper and lower value, both of which are tension) smaller than about 50 percent of the static failure load (provided no fatigue failure of the bolt occurs). For higher upper loads the displacements may increase significantly and a fatigue failure of the concrete might occur (Rehm, Eligehausen, and Mallee 1988). Sustained and cyclic loadings in the workingload range have the same influence on displacements and ultimate loads of headed anchors as for expansion and undercut anchors. 3.2.2 Relaxation -If headed anchors are preloaded, the initial force induced in the anchor is reduced with time due to creep of the highly stressed concrete under the anchor head. The final value of the tension force in the anchor depends primarily on the value of bearing stresses under the head, the concrete deformation and the anchorage depth. In typical cases the value of that final force will approach 40 to 80 percent of the initial preload (40 percent for short anchors, 80 percent for long anchors). Torque-controlled expansion anchors are usually preloaded by tightening the nut during installation. This preload is essential for the proper performance of such anchors. In a typical installation, locally high concrete stresses are created around the embedded anchor wedges or expansion devices as the anchor is preloaded. Creep of concrete under these high stresses results in a slight movement of the embedded anchor, and in turn, in a reduction in the load in the bolt. Fig. 3.5 shows a typical load-relaxation test (Burdette, Perry, and Funk 1987). Preload is plotted as a function of time. The shape of the curve is essentially the same for all anchors (including headed anchors). There is an exponential drop-off of load immediately after the applied tension is released, followed by a continued gradual diminishing of the load over an indefinite period. It is estimated that the final preload will be about 40 to 60 percent of the initial value. This is confirmed by other test data (Seghezzi and Vollmer 1982, and Wagner-Grey 1976). After retorquing the anchors, the process of load relaxation starts again, however, the final value of the preload is increased (Fig. 3.6). Retorquing even a short time after anchor installation can be effective (Wagner-Grey 1976).

355.1R-13

I

0

0

20

30

40

50

60 70 Time [Days]

Fig. 3.5 -Reduction of preload as a function of time (after Burdette, Perry, and Funk 1987)

1

i

I

.I Torque Controlled Expansion Anchor M12

0

0

2,5

I

I

5,0

7,5

I

10

12,5 Time [h]

Fig. 3.6 -Influence of retorquing on the final value of preload (from Seghezzi and Vollmer 1982)

Chemical anchors are usually preloaded by applying a predefined torque. Because of the high stresses in the adhesive bond, the preload force in the anchor declines faster and the final value is less than for torque-controlled expansion and headed anchors. Long-term relaxation and creep has been Four Ml6 investigated in several studies. diameter polyester anchors tested at loads of 25, 30, 38, and 40 kN (6, 7, 8.5, and 9 kips), showed displacements still increasing after 5 years, but ranging from 0.090 to 0.140 mm (0.0036 to 0.056 in.)(Elfgren, Anneling, Eriksson, and Granlund 1988). Creep tests were also performed on 26 Ml6 anchors for 3 years at various loads and

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I

10



environmental conditions. At allowable working loads of 15 kN (3.4 kips), anchors tested indoors showed small creep, 0.10 to 0.40 mm (0.004 to 0.016 in.). However, anchors tested outdoors exhibited continually increasing creep. Those tested indoors at 30- and 45- kN (7 and 10 kips), loads exhibited continually increasing creep. A 4 month test on epoxy anchors showed creep less than 0.009 in. (0.2 mm) (Wiewel 1989). The U.S. Army Corps of Engineers performed creep tests on polyester and epoxy anchors, subjecting the anchors to 60 percent of the anchor steel yield strength for 6 months. Cement and epoxy grouted specimens exhibited low slippage, 0.0013 to 0.0008 in. (0.03 to 0.02 mm), while polyester anchors exhibited approximately 30 times as much movement, 0.008 to 0.024 in. (0.2 to 0.6 mm) (Best, Floyd, and McDonald 1989). 3.2.3 Ultimate strength in tension 3.2.3.1 Steel failure -The strength of anchor steel controls failure when the embedment of the anchor is sufficient to preclude concrete failure and when the spreading forces are sufficiently high (expansion anchors) or the bearing area is sufficiently large (headed and undercut anchors) to preclude an anchor slip failure. The failure mode [Fig. 3.2(a)] is rupture of the anchor steel with ductility dependent on the type of anchor steel and embedment length. The ultimate strength can be determined from Eq. 3.1. F u = 4 x f,,, lb

(3 .1)

where As = tensile stress area, in.* f ut = ultimate tensile strength of steel, psi For given material properties and anchor dimensions this case defines the upper limit for the tensile-load-carrying capacity. Fig. 3.7 shows a comparison of the failure loads of headed anchors measured in tests to the values predicted by Eq. 3.1. Because the theoretical failure load was calculated with the nominal steel strength, the ratios of actual to predicted tensile capacity are larger than one.

number of

specimens 10 STEEL FAILURE 5-

Fig. 3.7-Ratio of actual to predicted tensile capacity according to Eq. (3.1) for steel failure (after Klingner and Mendonca 1982)

3.2.3.2 Concrete cone failure -When the embedment of an anchor or group of anchors is insufficient to develop the tensile strength of the anchor steel, a pullout cone failure of the concrete [see Fig. 3.2(d)] is the principal failure mode. When the spacing of anchors or location of an edge [Fig. 3.2(e)] interferes with the development of the full cone strength of an anchor, its capacity will be reduced. The angle of the failure cone, measured from the axis of the anchor, varies along the failure surface and shows considerable scatter. In ACI 349, Appendix B, ACI Committee 349,1985) the angle of the failure cone of headed and expansion anchors is assumed as 45’. According to Cannon*, in the case of expansion anchors the angle varies from about 60’ for short embedments (Id ( 2 in) to 45O for 1, 2 6 in. According to Rehm, Eligehausen, and Mallee 1988, the angle varies between approximately 50° and 60”, (mean value 5S”) and tends to decrease with increasing anchorage depth. The following formulas have been developed to describe behavior of headed studs, expansion, and undercut anchors.

*Cannon, Robert W., correspondence to ACI

Committee 355, Nov.

1986. Cannon, Robert W., correspondence to ACI Committee 355, Sept. 1988. This correspondence is filed at ACI ACI headquarters and is available at cost of reproduction and handling at time of request.



--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

355.1R-14


ACI 349, Appendix B, limits the tensile capacity of of the cone failure of an anchor, or wup . anchors, to a uniform stress of 4 +d$ (psi) on the stress cone surface of the anchors.

(3.2)

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

strength reduction factor 0.85 for uncracked concrete = 0.65 in zone of potential cracking A = the summation of the projected areas (in.2) of individual stress cones minus the areas of overlap and of any area, or areas, cut off by intersecting edges. Note: Other reductions are made based on member thickness relative to embedment and the area of fabricated anchor heads (see Fig. 3.8). ACI 349 has no requirements for minimum center-to-center spacing of single anchors or anchors belonging to a group. Fig. 3.9 shows the frequency diagram of the ratio of actual to Predicted tensile capacity of headed anchors. Theoretical capacity was calculated according to Eq. (3.2). The tests were described by Klingner and Mendonca (1982a), and were evaluated by Cannon*. Tested were individual anchors with large and small edge distances and anchor groups. In all tests a concrete cone failure occurred.

If an anchor is installed too close to an edge, the anchor will fail before developing the concrete cone strength. Therefore, for headed anchors, ACI 349 requires that the minimum edge distance m to the center of the anchor be sufficient to prevent a side cone failure. The following equation is suggested in the ACI 349 Commentary for determining this minimum value. m

, in.

(3.3)

where D = anchor diameter, in. F = ultimate tensile strength of anchor, psi f 'c = compressive strength of concrete, psi If this requirement cannot be satisfied, stirrup or tie reinforcement should be provided. Cannon+ found that for embedments less than 6 in., ACI 349 becomes increasingly conservative with decreasing embedment. He has proposed a modification to Eq. (3.3) to provide a better fit to test data. For embedments less than 6 in., this modification would increase the angle of the failure cone, measured from the axis of the anchor. (3 3 For 1, c 3 in.: cy = 62 - 1.1 (l#, deg For ld 2 3 in. but < 6 in.: (Y = 45 + 0.79 (6-ld) , (3 . 5) deg With respect to the minimum edge distance he reported the results of tests which indicated a direct relationship between anchor load and side cone failure.** He suggested Eq. (3.6) instead of Eq. (3.3) as a more correct lower bound for the edge distance for headed anchors:

-A Frequency [%] n = 45 tests 5i = 1,14 v = 26 O/o

20

355.1 R-1 5

m 10

F ut = ASTM-specified tensile strength of the anchor bolt, kips 1,5

2,0

2,5 Fu,test /Fu,pred

Fig. 3.9 -Ratio of actual to predicted tensile capacity of headed anchors according to Eq. (3.2) (from Cannon, 1984 **)


*Cannon, private correspondence, 1988, previously cited (see footnote p 14). +Cannon, private correspondence, 1986, previously cited (see footnote p 14). **Cannon, Robert W., Letter to ACI 355, “Comparison of Testing Edge Conditions and Anchor Spacing with Predictions”, Dec. 1984.


*EFFECTIVE STRESS AREA,

Ld I41

B

*EFFECTIVE STRESS AREA

A t

L

\L DEDUCT AREA OF ANCHOR HEADS

EFFECTIVE STRESS AREA

P L A N --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

*REDUCE BY THE TOTAL BEARING AREA OF THE ANCHOR STEEL. Pd

Pd

t

t

L

J

(a+2Ld-2h)

. A) Effective stress area for anchorage pullout

EFFECTIVE STRESS AREA .

STRESS AREA REDUCTION FOR LIMITED DEPTH (Ar) Ar= (a+2Ld-2h)(b+2Ld-2h) *REDUCE BY THE TOTAL BEARING AREA OF THE ANCHOR STEEL

B) Stress area reduction for I imited depth A


Fig. 3.8-ACI 349 method for determining effective stress areas Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


The average failure load for a side cone (bursting) failure is given as:

where

F,

= 15m

f - kips

(3.7)

35cCo’

m = actual edge distance, in. For expansion and undercut anchors, Eligehausen, Fuchs, and Mayer (1987 and 1988), derived Eq. (3.8a) from 287 test series with single anchors with large edge distances showing concrete cone failure. (3.8a)

mm (1 9/16 to 20 l/2 in.) and concrete strengths f’, = 20 to 50 N/mm2 (2900 psi to 7150 psi). Fig. 3.11 shows a histogram of the ratio of measured to predicted failure load. The average failure loads given in Eq. (3.8) can only be obtained if the distances between anchors are large enough so that concrete cones do not overlap each other. Assuming an angle of the failure cone cy = 55o the critical distance is approximately three times the embedment depth. The failure load of a two-point fastening results in:

F ul

‘d

f’,

= average ultimate load, N depth (see Fig. 3.10), mm = average compressive strength of concrete cylinders (6 by 12 in.) at time of testing, N/mm2

(3.9)

G = xcr x F,, where

where

Fu

355.1R-17

= embedment

ultimate failure load, single anchor, from Eq. (3.8) =

& = 1 +a/a,,it I 2

(3.10)

where a

= distance between center of anchors a crit = critical distance between center of anchors = 31,, where 1d is the depth of embedment.

Results of an additional 196 tests on headed studs showed a similar relationship (from Rehm, Eligehausen, and Mallee 1988).

FIA4

xai

(3.8b)

= %a1

x Xd

=

ai(acd 5

1 +

x Fur

2

where In the original equation the concrete strength was measured on cubes with a side length of 200 mm (8 in.). Eq. (3.8a) and (3.8b) assume f 'c (cylinder) = 0.82 f 'cc (cube). The tests with expansion, undercut and headed studs included anchorage depths from 40 to 525 Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

a.I = spacing in direction i


(3.11) (3.12)

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

Fig. 3.10 -Illustration of embedment depth as used in Eq. (3.8a) and (3.86)

Eq. (3.9) leads to the x-method for calculating the ultimate capacity of multiple anchor fastenings. For the calculation of the ultimate load of quadruple fastenings the xa factors can be derived separately for both directions and combined in product form as follows.

355.1R-18


40

Frequency [%] n = 196 individual tests si= 1 0 0 v= 1 4 %

30

I 20

10

0.5

1.0

$,

1.5

2.0

0.5

1.0

Fu, test

test ’ %,pred

1.5 /Fu, pred

Fig. 3.11 (a) -Ratio of acutual to predicted tensile capacity for concrete cone failure of individual expansion and undercut anchors away from edges according to Eq. (3.8a). (from Rehm, Eligehausen, and Mallee 1988, and Eligehausen, Fuchs, and Mayer 1987 and 1988)

Fig. 3.11(b) -Ratio of actual to predicted tensile capacity for concrete cone failure of individual headed anchors away from edges according to Eq. (3.86). (from Rehm, Eligehausen, and Mallee 1988)

Fig. 3.12 shows the capacity of quadruple fastenings for headed studs, expansion and undercut anchors as a function of the ratio of anchor spacing to embedment depth as measured in tests and calculated according to Eq. (3.11). Eq. (3.9) and (3.11) can also be extended for multiple anchorages with any number of anchors in any spacing by setting the value of ai as the distance atot between the outer anchors, and the x0- value is limited to xa I n with n = number of anchors in one direction. This is provided that the spacings between the individual anchors are smaller than acrit = 31, and the anchor plate is sufficiently stiff to assure an even distribution of tension forces to all anchors (see Rehm, Eligehausen, and Mallee 1988). The X-method can also be extended to take account of load eccentricities (Riemann 1985). Fig. 3.13 shows the ratio of actual to predicted tensile capacity of groups of headed studs. In the tests the number of anchors was varied between 4

and 36, the spacing of the outer anchors between 100 and 875 mm and the spacing of the individual anchors between 0.541, and 2.2&. The groups were loaded by a concentric tension load which was equally distributed to all anchors. Eq. (3.13) covers the influence of edge distances, a,, smaller than critical:


Fu* = a& * Fy

(3.13)

Xa?n = 0.3 + 0.7 am/a,crit S 1

(3.14)

a m,crit

Fu

= =

critical distance from free edge 1.5 1d

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

where

= ‘ actual embedment length = ultimate failure load, single anchor to be taken from Eq. (3.8)


355.1R-19

5.0

0

I

I

FE according to eqn. ( 3.8 )

4.0

O

I

8 0

3.0

2.0

1.0

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

Fig. 3.12-Ratio of actual failure load of a group of anchors to the predicted value for an individual anchor as a function of the ratio of anchor spacing to embedment depth (from Rehm, Eligehausen, and Mallee 1988)




I

_. l-

,

L

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---




Fig. 3.14 shows a comparison of test results with the theoretical values according to Eq. (3.13). It should be noted, however, that minimum distances from the free edge are necessary for headed studs in order to allow proper concreting and avoid local spalling of concrete. Minimum edge distances for expansion and undercut anchors are necessary to avoid splitting of concrete during installation and expansion of the anchors. If anchors are located in a corner [see Fig. 3.15(b,)], the factors xarn are calculated separately for each direction and then the two x-factors are multiplied.

355.1R-21

Bode and Roik (1987), evaluated data of 106 tests with headed studs to arrive at Eq. (3.15). F” = 12r,3/2(1 + d&,) 8, N

(3.15)

where F, 1d d, f’,

= = = =

average failure load, N embedment length, mm head diameter, mm concrete cylinder strength at time of testing, N/mm2

Fig. 3.16 compares the measured failure loads of headed studs with the values according to Eq. Roik (1987), assume the critical spacing of neighboring headed a cl+ = 41,

(3.16)

Fig. 3.15 - Typical failure modes of anchors Loaded in shear (from Rehm, Eligehausen, and Mallee 1988)

kN

TU’k lN/mmz I

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

mean value

50

75 100 125 150 h [mm]

Fig. 3.16 - Measured failure loads compared to Eq. 3.15 (where p, = concrete splitting strength) (from Bode and Roik 1987) Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott


Not for Resale, 02/05/2007 07:29:17 MSTs/org1

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

.50 o Anchor

0 .00

/

. 00

studs, concrete break-out l Headed studs, local concrete failure ( blow -out) I

/ L

.50

1.00

1.50

Fig. 3.14-Ratio of actual failure load of an individual anchor close to the edge to the predicted value for an anchor with large edge distance (from Rehm, Eligehausen, and Mallee 1988) Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott


Not for Resale, 02/05/2007 07:29:17 MSTs/org1

1.75

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

355.1R-23

With respect to the influence of free edges (see Fig, 3.15) they consider the critical distance beyond which there is no significant influence on load as being in the case of one free ati1 IJ 1.21, and in the case of two or more free edges: acit.2

5 21,

For distances from center of headed stud to the free edge(s) which are smaller than the critical distance according to Eq. (3.17) and (3.18), they fou d that the assumption of a linear decrease of ulti ate failure load in proportion to the ratio of act”al distance/critical distance gives a lower bound of their test results, in much the same ner as shown in Fig. 3.14. raestrup, Nielson, Jense, and Bach (1976), give the predicted failure load as:

FM

= 0.21 x 2; (1 + d,ll&f$ N (3.19)

Eq. (3.19) was deduced by applying the theory of plasticity to headed studs embedded in co rrete. The failure load is assumed to be pronI ortional to the concrete compressive strength. 3.2.3.3 Pullout (slip) of the anchor- Slip failure occurs [Fig. 3.2(b)] with expansion anchors when the expansion force is too small to develop either the strength of the anchor steel or a shear cone failure of the concrete. This is a typical failure mode for wedge anchors at moderate to deep embedments in lower strength concrete where the crushing of the concrete at the wedges allows the bolt to “pull through”. The cause may also be due to an oversize hole. Slip failure may also occur in low strength concrete due to deformation of the wall of the hole. The testing of wedge bolt expansion anchors by Hanks (1973), clearly demonstrated that the primary failure mode for individual anchor tests (uninhibited by edge conditions) was either cone failure of the concrete or anchor slip depending on the depth of anchor for a given size. Only 10 of 464 tension tests indicated any cracking associated with a cone failure. The line of demarcation between shear cone failure and slip Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

failure was approximately six bolt diameters. Under conditions of poor workmanship in the field (e.g., oversize holes) slip failure may occur at a much smaller embedment depth than ld = 6D. Slip failure may also occur with bonded and adhesive anchors of insufficient embedment to develop the strength of the anchor steel or to cause a concrete cone failure. Torque-controlled wedge anchors, which fail by slip, generally fail by slipping the expansion cone past the wedges. This failure mode may also occur with sleeve anchors. However, in some case anchors may fail by pulling the whole anchor (including expansion sleeve) out of the hole. Torque-controlled expansion anchors may also slip to a critical depth and fail the concrete. Deformation-controlled expansion anchors (e.g., drop-in anchors) have a fixed expansion and may slip to a critical depth and then fail the concrete. The slip failure load is dependent on the coefficient of friction between the sliding surfaces and on the spreading force at failure which is a function of the critical expansion force producing failure and the deformability of the concrete which varies with hole depth and concrete properties. All of these factors may vary with anchor type, manufacturer, and installation. The spreading force and thus the slip load of drop-in anchors decreases significantly with increasing diameter of the drilled hole with respect to the diameter of the anchor. Theoretically the slip failure load F, could be calculated from Eq. (3.20). Fit = ps

(3.20)

where I, = coefficient of friction S = spreading force The coefficient of friction depends mainly on the roughness and cleanliness of the drilled hole and of the surface of the expansion sleeve or wedge as well as on the spreading pressure. From Wagner-Grey (1976), the factor p for torque controlled expansion anchors is in the range of 0.2 to 0.3 and for drop-in anchors is approximately 0.35. The difficulty in using Eq. (3.20) lies in properly estimating the spreading force, since complex mechanics are involved. For this reason



the profession relies on test data. However, equations for estimating of the spreading force are given by Wagner-Grey (1976). Because of the large variability of the spreading forces and the coefficient of friction, Eq. (3.20) gives only an approximate estimate of the pullout load (see Eligehausen, and Pusill-Wachtsmuth 1982). Furthermore, in important applications it is advisable to test expansion anchors, which typically fail by slip at specified embedments, in design strength job concrete to confirm slip characteristics. For pullout failures of a chemical anchor, the bond between the wall of the drilled hole and the mortar is critical (see Sell 1973). Assuming a uniform bond stress distribution along the anchorage length, the bond strength is in the order of 1300 psi (9 MPa) with a coefficient of variation of 10 to 15 percent for polyester and vinylester chemical anchors. This value is for a concrete compressive strength of 3000 psi (21 MPa) and an embedment of about nine anchor diameters. The bond strength increases approximately with the square root of the concrete strength. The pullout capacity of chemical anchors increases with increasing embedment depth: however, after about nine anchor diameters the increase is not proportional to embedment. This is due to the high bonding effect resulting in high load transfer to the concrete at the top of the anchorage. The bond stress is no longer uniform, and if the tensile load is sufficiently high, the failure initiates with a concrete failure in the upper portion of the concrete and then the bond fails in the remainder of the embedment. For headed anchors local failure in front of the head will occur when the pressure on the concrete is larger than about 12f’, to 15f’, (Rehm, Eligehausen, and Mallee, 1988). This type of failure is somewhat similar to a pullout failure. 3.2.3.4 Splitting failure of concrete -This failure mode will occur only if the dimensions of the concrete are too small, the anchors are placed too close to an edge or too close to each other [Fig. 3.2(c)], or the expansion forces are too high. The failure load is usually smaller than for a concrete cone failure. Torque-controlled expansion and deformationcontrolled anchors (e.g., drop-in and self-drill anchors are the type anchor most likely to experience splitting failure due to the high lateral thrust required to resist sliding by friction on the Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

steel wedges. Deformation-controlled expansion anchors generate higher spreading forces and require larger edge distances than torquecontrolled expansion and undercut anchors. The capacity of expansion anchors which fail by splitting of the concrete has been evaluated by Pusill-Wachtsmuth (1982), using theoretical considerations. It was assumed that splitting occurs when the tensile stresses averaged over a critical area reach the concrete tensile strength. The size of this area was found by evaluating the results of tests with concentrated loads and of tests with thick concrete rings subjected to a constant inner pressure. According to this theory, the necessary side cover or spacing to preclude a splitting failure before reaching the concrete cone failure load must be about 1.751d or 3.51,, respectively. For drop-in anchors a side cover m I 31d was recommended. The validity of this evaluation was checked by relatively few test results. With respect to the minimum edge distance Cannon* has proposed the following criteria to preclude a splitting failure occuring at a load lower than the capacity for concrete cone failure or pullout failure:

m = D(11.4 - 0.92& in. where

(3.21)

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

355.1R-24

= minimum edge distance = anchor bolt diameter, in. = embedment depth to the bottom of the ld anchor, in Eq. (3.21) is valid for anchor spacings s L 2 in. :

If side cover or spacings of anchors are too small, splitting cracks may occur during installation of anchors. This possibility is greater for drop-in anchors and for self-drilling anchors than for torque-controlled expansion anchors because of the higher initial spreading forces. The minimum edge distance and the minimum spacing to avoid splitting during installation, as recommended by Rehm, Eligehausen, and Mallee (1988), are based on many tests and are given in Table 3.1 for the different types of anchors.

*Cannon, Private correspondence previously cited Dec. 1984 (see footnote p 14).



355.1R-25

Table 3.1 -Minimum edge distance and minimum spacing to avoid splitting failure Torque-controlled expansion anchors with one cone (recent design)

Mihimum edge distance m / 1d to avoid splitting during installation

1.0

2.0

Minimum center-to-center spacing a / 1d to avoid splitting during installation

1.0

1.0

3.2.4 Load-displacement behavior and failure modes in shear-For anchors with an applied preload, the initial friction forces between the baseplate and the concrete have to be overcome by the shear load before there is initial anchor movement (Fig. 3.17). The baseplate slides and the anchor moves to the side of the hole in the second stage of behavior. The third stage of loaddisplacement behavior is a pressure loading against the top surface of the concrete and a surface spa1l of the concrete at the edge of the hole. Depending on edge distance and anchor embedment, the failure may be by shearing of the anchor (for deep embedments) with or without a concrete spa11 preceding the steel failure [Fig. 3.15(a)] or by shearing of the concrete (concrete failure) in the case of anchors loaded near an edge [Fig. 3.15(b1), (b2), (b3)]. Shear loading generally produces larger displacements than tension loading [see Fig. 3.3(c)]. This can be attributed to the bending of the anchor rod and the deformation of the concrete in the direction of loading. This is especially true if the anchor is not flush with the concrete at the hole opening (e.g., when the concrete is spalled during drilling). For cast-inplace anchors, the behavior will depend on the type of anchorage used, the embedment and the steel strength. The distribution of shear from the attachment to anchors of a group depends on the details of the anchors to the attachment connection and on overcoming the frictional resistance of the attachment. The frictional resistance depends on surface conditions, the existing preload (if any) in the anchors and the compressive forces applied


Drop-in anchors

I 3.0

I

through the attachment as a result of direct loads or applied moments. The connection details concern the treatment of connecting surfaces and the fit and manner of connecting the anchors to the attachment.

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

Undercut anchors

Onset of bearing crushing in the concrete

lip of loading plate into bearing on anchor stud

Load transfered by friction to embedment

. ~~~ 0

.50

r

10

r

1.5 0

Deformatlon

Fig. 3.17- Typical load-displacement curve for wedge anchor in shear from Meinheit and Heidbrink 1985)


-7

20

355.1R-26


3.2.5 Ultimate strength in shear 3.2.5.1 Steel failure - Steel failure usually occurs after relatively large displacements and is most common for deep embedments, lower strength steels and large edge distances. The failure load depends on the steel area and the steel strength and is given by Eq. (3.22).

number of specimens

x

I ”

0,: 0.65-4 I

= N A,f,, lb

(3.22)

where the factor N takes account of the steel “shear” strength and has the range 0.6 to 0.7 [Klingner and Mendonca, (1982b)], A, is the tensile stress area (as defined in Eq. (3.1)) and f,t is the ultimate tensile strength. Eligehausen and Fuchs (1988), propose the value N = 0.6 on the basis of an evaluation of 230 tests. 3.2.5.2 Concrete failure -Concrete failures will exhibit two modes; (1) blow out cones due to edge proximity (Fig. 3.15) and (2) concrete spa11 followed by a possible anchor pullout or steel failure away from an edge. 3.2.5.2.1 Edge failure- For all types of anchors loaded in shear toward an adjacent, free edge and exhibiting a concrete failure (Fig. 3.15), the failure load is influenced by the concrete tensile strength, the edge distance m and the stiffness of the anchor. Another influencing factor is the embedment depth. The failure surface has a conical shape that may radiate from the embedded end of the anchor for shallow embedments or from the upper part of the anchorage for deep embedments. In the following paragraphs, several formulas for calculating the failure load for an edge failure are reviewed. ACI 349, Appendix B, Commentary gives a design shear strength of vu = 24$$n2, lb

(3.23)

RATIO OF ACTUAL TO F’FQICTED CAPACITY

Fig. 3.18 -Histogram of actual to predicted capacity

ACI 349, Appendix B further recommends a minimum side cover or edge distance m required to preclude edge failures: be calculated by Eq. (3.24). m=

D = anchor diameter, in. F t = anchor ultimate tensile load, lb fr”, = concrete compressive strength, psi Eligehausen and Fuchs (1988), have suggested, based on the evaluation of some 80 test results with headed and expansion anchors (anchorage depth ld > 4D), the average ultimate failure load of the concrete of a single fastener in shear be calculated by: F,, = 1.4@$n1.s~~, N

where D

f’, m

(3.25)

shank diameter (mm) of headed studs or drill-hole diameter for anchors, D < 25mm average concrete compressive strength (cylinders) at time of testing, N/mm2 distance from anchor to free edge, mm

Fig. 3.18, taken from Klingner and Mendonca (1982b) gives the ratio of actual to predicted shear capacities for this approach. Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

(3.24)

where

where = 0.85 cb f’, = compressive strength of concrete m = distance from anchor to free edge (see Fig. 3.15)

, in.


--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

F8l


355.1R-27

h Xh =-sl 1.4m

where h = member thickness, mm Eq. (3.25) is valid for 1,/D = 4 to 6. Fig. 3.19 shows a comparison between failure loads according to Eq. (3.25) and test results. The thickness of the test specimens was h 1 1.4m. The tests were performed in concretes with different strengths and anchors ranging in diameter between 12 and 22 mm. The test results were normalized to a concrete strength f 'c = 20N/mm2 and D = 18 mm. If an anchor group is loaded in shear toward an edge, a common failure cone may occur [see Fig. 3.15(b2)]. T he corresponding failure load may also be calculated as described in Section 3.2.3.2 for tension loading [Eq. (3.9), (3.11), and (3.12)] according to the x-method. The x-values for shear loads, however, depend on the distance from the free edge measured in load direction. The critical (minimum) distance between two or more anchors beyond which no intersection of failure cone will happen is given by Eligehausen and Fuchs (1988), as: a Wit = 3.5m

Similar expressions are proposed for calculating the failure load of single fastenings or anchor groups situated in a corner or in narrow members. The influence of load eccentricity on the failure load of an anchor group can also be taken into account by the x-method (Rehm, Eligehausen, and Mallee 1988). The method has been extended to anchor groups with an arbitrary number of anchors. Klingner, Mendonca, and Malik (1982), recommend a critical (minimum) edge spacing of:

(3.27)

mkD

For a I a,,i, Eligehausen and Fuchs (1988), have proposed the calculation of the average failure load of a group of anchors (see Fig. 3.20) subjected to shear load by:

Fut in. -,

(3.29)

%@

where #C = 0.90 and the other terms are as given the ACI 349 [Eq. (3.24)]. for

(3.28)

For anchors with small embedment depth situated away from an edge and loaded in shear, the failure mode may be a tensile cone failure as the anchor bends under load and induces a tensile loading into the concrete. Because of ductility requirements and reversible load conditions associated with seismic design, ACI 349 does not distinguish between embedment requirements for shear and tension. This is very conservative if only shear is considered (see Shaikh and Yi, 1985).

where = 1 + a/a,,i, & F, is from Eq. (3.25) Fig. 3.20 (Eligehausen and Fuchs, 1988) shows the ratio of the failure load of a group loaded in shear towards the edge to the failure load of an individual anchor calculated according Eq. (3.25). The failure load ratio is plotted against the ratio of spacing to edge distance. --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---


' 3.5 4.0 a/a, [-]

Fig. 3.20-Ratio of actual shear failure load of anchor group to shear failure load of an individual anchor as a function of spacing between anchors

where m = distance to free edge.

FI(, Group = x,F,

3.0

0


--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

8 cv Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS


8 Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


3.2.5.2.2 Concrete spall-Anchors away from an edge will locally spall the concrete in front of the anchor. The primary factors influencing concrete spall due to shear are tensile strength of the concrete, stiffness of the anchorage, anchor diameter, embedment depth, and deformability of the concrete. The corresponding shear capacity is given by Klingner and Mendonca (1982), and American Institute of Steel Construction (1978), as: (3.30) F, = 0.5 A, fit, lb where Ab = nominal gross cross-sectional area of anchor shank, in.2 f’, = specified compressive strength of concrete, psi E, = elastic modulus of concrete, psi However, according to Eligehausen and Fuchs (1988), the above described local concrete failure does not negatively influence the anchor steel capacity (normal strength steel) and will not cause subsequent pullout of the anchor, provided the embedment depth is 1, L 4D. 3.2.6 Combined tension and shear Loading- The behavior of anchors under combined tension and shear loading lies in between the behavior under tension or shear loading, and for a given depth of embedment, is dependent on the angle of the loading (Fig. 3.21). To calculate the failure load under combined tension and shear loadings three approaches are in use; a straight-line function, a trilinear function and an elliptical function. There are two types of straight-line functions. The first is a shear friction approach used by ACI 349, Appendix B, and given by Eq. (3.31).

355.1R-29

A second straight-line equation is given by Eq. (3.32). T,/T, + VJVu s 1.0

(3.32)

where applied tensile and shear load, respectively V = ultimate tensile and shear load, T”, u respectively These straight-line methods give a conservative approach to combined loading analyses. Bode and Roik (1987), propose for headed studs a trilinear function: Ta*

=

va

vp,

s 1

TJT,, + VJV,, s 1.2

(3.33c)

where T,, Va, TU and Vu as defined for Eq. (3.32). According to Meinheit and Heidbrink (1985), Eq. (3.33) is valid also for expansion anchors (see Fig. 3.22). Load FQ[ kN ] 125

100

75

(3.31) where = applied tension load = 4 F, r” = 0.85 F, according to Eq. (3.22) cr = coefficient of friction = 0.55 to 0.9, depending on the location of the anchor plate in relation to the concrete surface Tall = allowable anchor tensile load TL?

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---


50

25

0

I

I

5

10

I

I

25 15 20 Displacement A,[mml

Fig. 3.21- Shear load-displacement behavior of headed studs for different tension loads (from Bode and Hanenkamp 1985) Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


Many investigators have concluded that shear and tension combine in an elliptical function as given by Eq. (3.34). (3.34) (T,/T,,Y + CV,lVJ s 1.0 where exponents x and y are determined from tests and the other terms are as previously defined for the straight-line equations. The PCI Design Handbook (1978) uses x = y = 4/3 for precast anchors, while the Teledyne Engineering Services report (1979) gives x = y = 5/3 as a good fit for expansion anchors. Fig. 3.22 shows a comparison between test results with expansion anchors and the different approaches as described above. 3.3-Behavior of anchors in cracked concrete 3.3.1 Introduction -When anchors are installed

in the tension zone of reinforced concrete members, it must be assumed that cracks will occur in the concrete because of the rather low concrete tensile strength. The concrete tensile strength may be totally or partially consumed by the restraint of induced deformations due to shrinkage, temperature, or flexure, or from the anchorage itself. Cracks run either in one direction (single cracks) or in two directions (intersecting cracks, in the case of slabs spanning two directions). If concrete cracks, experience has shown that there is a high probability that the crack will propagate through the anchor location (see Cannon 1981 and Eligehausen, Fuchs, Lotze, and Reuter 1989). Theoretical considerations also indicate that cracks should propagate through the anchor location. When the anchor is loaded, the anchor creates splitting (tensile) forces at the anchor embedded end. These tensile stresses in the concrete would add to other tensile stresses from locally high bending moments. (i.e., flexural stresses and restrained shrinkage stresses). For the case when expansion or undercut anchors are used, the drilled hole can also act as a notch or produce a cross section in the concrete member with reduced concrete area. The theoretical considerations discussed above, were confirmed by testing Ml2 (12 mm) torquecontrolled expansion anchors and undercut anchors in a slab reinforced with welded wire mesh (AJbd = 0.004) (see Eligehausen, Fuchs, Lotze, and Reuter 1989). The test anchors were installed with 1d = 80mm (3.2 in.) and in uncracked concrete. The anchorage holes were Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

drilled either 40 mm (1.6 in.) or 80 mm (3.2 in.) away from the transverse acting wires, [spacing of 250 mm (10 in.), in the fabric]. Bending of the slab was in one direction only. All test anchors were pretensioned or pretensioned and loaded with their allowable load before the slab was subjected to flexural loadings. After preloading the anchors, the concrete slab was loaded to its service load. Observations during this part of the testing often showed that cracking started at the section with transverse reinforcement but then deviated from that section to the section that contained the anchor hole. The cracks propagating through the anchor hole also were to the depth of the hole (Fig. 3.23 and 3.24). Testing showed that the displacement characteristics of these anchors remained essentially unchanged until the slab load was about 40 percent of the slab service load. Beyond that point, significant increased displacement occurred (Fig. 3.25). The increased displacement characteristics of the anchor in cracked concrete are caused by the crack propagating through the load transfer zone of the anchor (see Cannon 1981). The crack width can vary over the depth of the member (bending cracks) or can be of constant width (parallel cracks, e.g. due to tension loading). In the worst case the anchor can lie in the intersection of two cracks with constant width over the member depth. If anchors are situated in or beside these cracks, their load displacement behavior and strength may be significantly influenced. 3.3.2 Load-displacement-behavior and failure modes in tension -Fig. 3.26 presents typical loaddisplacement curves of torque-controlled expansion anchors which were set in uncracked concrete and in cracks, and loaded statically to failure. The displacements of anchors located in cracks behave similarly to anchors in uncracked concrete up to a critical load. This critical load depends on the type of crack and the crack width. For higher loads the displacements of anchors in cracks are much higher than the values expected in uncracked concrete and anchor capacity is significantly reduced. The load-displacement behavior of headed or undercut anchors may be affected by cracks in concrete but the displacements at maximum load are less influenced by cracks than are expansion anchors (see Fischer 1984). --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

355.1R-30



0 l l

0

0

I.0

Fig. 3.22- Tension-shear interaction diagram for expansion anchors (from Meinheit and Heidbrink 1985)

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---




I

I______ _;

!

F

I

1. F

K 884 -(8,84cm2/mI

_,,,:‘,

I

_

_

_

__

_

2 kc -I

a

I /

15 , l

100

Id

150

1

150

1

,I

100

I

15

L

torque-controlled undercut expansion anchors -7 anchors --

z --

z ic -i anchor loaded a n c h o r p r e s t r e s s e d but n o t l o a d e d o drill hole

l l

Fig. 3.23 - Torque-controlled expansion anchors and undercut anchors in the cracked tensile zone of a concrete slab (from Eligehausen, Fuchs, Lotze, and Reuter 1989)

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---



355.1R-33


tension t

jr

I-

expansion area

A

A

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

Section A - A

Fig. 3.24- Crack pattern in a drilled hole with expansion anchor (from Eligehausen, Fuchs, Lotze, and Reuter 1989) Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS


355.1R-34


-4-l F . 8 adm -

1.0

-

0.8

0.6 1 r(I 0.4 ,

1

(H

0

,, ,

4

0.2

0.3

0.2

torquee controlled expansion anchors

1 0.1

0.1

0.2

displacement [mm]

crack width [mm] --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

Fig. 3.25-Crack width and anchor displacement as a function of the ratio of applied load to allowable load of the slab (from Eligehausen, Fuchs, Lotze, and Reuter 1989)

Force

-v

r

Torque Controlled Expansion Anchor Tension Loading

Uncracked Cracked Concrete r---

Displacement Fig. 3.26-Influence of cracks on the load-displacement relationship of expansion anchors - schematically (from Rehm and Lehmann 1982) Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS


--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

Fig. 3.27 shows the typical load-displacement relationship of torque-controlled expansion anchors set in intersecting cracks and cycled up to 10’ times between different load levels before loading to failure. For comparison the loaddisplacement relationship for statically loaded anchors is also plotted. Provided the upper load during cycling is smaller than about 50 percent of the static failure load, cyclic loading results in an almost linear increase of the anchor displacement as a function of the logarithm of the number of cycles. The load-displacement curve for higher loads than the upper load during cycling is rather steep up to the static envelope which is followed thereafter. Anchor capacity and displacement at failure are not influenced significantly by cyclic loading with an upper load as given above. Opening and closing of cracks by cycling the reinforced concrete while subjecting the anchor to a constant load has more influence on the anchor behavior than cycling the anchor with the cracks kept open (Rehm and Lehmann 1982). In principle the failure modes described in Sections 3.2.1 and 3.2.3.1 are also valid for anchorages in cracked concrete. However, expansion anchors which produce a concrete cone failure in uncracked concrete may slip and pull out when located in a crack. This possible change of the failure mode is due to the reduction of the spreading force as a result of the cracks (see below). 3.3.3 Relaxation-Expansion and undercut anchors installed in cracks will show an initial displacement during widening of the crack. The amount of this displacement is dependent on the design of the anchor and on the crack width. Usually this initial displacement is large enough to reduce the preload to zero. This is also valid for bonded anchors. The relaxation behavior of headed anchors installed in cracks has not yet been studied. However, one may assume that the residual preload is not significantly smaller than for headed anchors in uncracked concrete.


[kN] Torque ConIt rolled Expa c, sion Anchor

FTER CYCLIC LOADING I

D CYC LES

5

15

Displacement [mm]

Fig. 3.27-Influence of cyclic loading on the loaddisplacement relationship oftorque-controlled expansion anchors (after Rehm and Lehmann 1982)

3.3.4 Ultimate strength in tension-Fig. 3.28 shows the influence of cracks in the concrete on the strength of headed and undercut anchors placed in or close to cracks. The ratios of the failure loads of single anchors measured in cracked concrete to the value in uncracked concrete are plotted as a function of the crack width. The anchors were tested in tension specimens with almost constant crack width over the member depth. After installing the anchors in uncracked concrete or concrete with hairline cracks, the cracks were opened by loading the specimen and then the anchors were statically loaded in tension with the cracks open. Failure occurred by pulling out a concrete cone.


m m


355.1R-36

Fu (crack) / Fu (uncracked c o n c r e t e )

I,OA

I

fi- 20-55N/mm2

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

id = 8O mm

The failure load decreases rapidly up to a crack width of about 0.4 mm (l/64 in.) and is almost constant for larger cracks. The scatter of the data is relatively large. On an average, the ultimate load of anchors installed in or beside cracks with a width > 0.4 mm (l/64 in.) is about 60 percent of the ultimate value in uncracked concrete. It should be noted that, under service load, cracks with a width no greater than 0.4 mm (l/64 in.) are tolerated in reinforced concrete structures. The influence of the type of anchor (headed or undercut) on the failure load reduction is negligible. An almost similar strength reduction was also observed with anchors installed deeper in the tension zone of beams for various anchor-depthto-beam-height ratios (Rehm, Eligehausen, and Mallee 1988). The reduction of the anchor strength is due to the change of the stress distribution in the concrete caused by cracks (Eligehausen 1984 and Eligehausen, Fuchs, and Mayer 1987 and 1988). In the case of uncracked concrete, the stresses in the concrete are radially symmetric to the anchor and tensile hoop stresses are caused by the load transfer into the concrete [Fig. 3.29(a)]. If the anchor is installed in a crack, tensile stresses cannot be transferred across the crack. Therefore, the area which can be used for transmitting the load into the concrete is smaller than in uncracked concrete [Fig. 3.29(b)].

0,4

0,8

1,2 1,6 crack width A w [mm]

Fig. 3.28 -Influence of cracks on the ultimate load of undercut and headed anchors (from Eligehausen 1984)

uncracked concrete

b) cracked concrete

Fig. 3.29 - Load transfer into concrete schematically for a) uncracked concrete and b) cracked concrete (from Eligehausen, Fuchs, and Mayer 1987, 1988) Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS



Furthermore, a part of the concrete cone may be cut off by neighboring cracks. These combined effects cause a strength reduction of approximately 40 percent compared to uncracked concrete. Some tensile stresses can be transmitted over small cracks due to aggregate interlock (Eligehausen and Sawade 1985). This explains the increasing anchor strength for crack widths less than 0.4 mm (l/64 in.). In addition to the above effect, the reduction of the spreading forces by the crack opening must be taken into account for expansion anchors (Fig. 3.30). If the anchor lies in an intersecting crack, the widening of the crack by the width w leads to a reduction of the effective expansion displacement around the circumference of the anchor by w/2 [Fig. 3.30(a)]. Assuming elastic behavior of the concrete, this reduction of the expansion displacement causes a slight reduction of the

355.1R-37

spreading force from F,, to F, [Fig. 3.30(b)]. If, on the other hand, it is assumed that the concrete is subjected to purely plastic deformations during expansion, then theoretically the expansion sleeve will free itself around its circumference from the hole wall and the spreading force will decline to zero [Fig. 3.30(c)]. In reality the concrete is deformed elastically and plastically. Therefore, the actual situation lies between these two extremes. However, due to the steep gradient of the unloading curve, it has to be expected that even a relatively slight increase in crack width will lead to a substantial reduction of the spreading force [Fig. 3.30(d)]. For anchors situated in cracks running in one direction, the spreading force will also be reduced by the opening of the crack, but the reduction will be less pronounced than in the case shown in Fig. 3.30.

anchor unspread crack opening spread anchor

a) Spreading Force

FO 6

I -7 . -

b) Concrete elastic

c) Concrete plastic

Fig. 3.30-Influence of cracks on spreading force (from Eligehausen and Pusill- Wachtsmuth 1982) Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS


--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

1 Spread. Displ. c


355.1R-38

Properly designed torque-controlled anchors will expand to an upper bound when they are loaded. This causes an increase of the spreading force until the holding capacity is reached. If the crack width is smaller than about 0.4 mm, the holding capacity of heavy-duty, torque-controlled sleeve anchors is often large enough to cause failure by pulling out a concrete cone. Therefore, the reduction of the failure load is ahnost the same as for headed anchors (compare Fig. 3.31 with Fig. 3.28). For larger cracks the expansion cones are often pulled through the expansion sleeves, because the maximum spreading displacement reaches the upper bound and the holding capacity is less than the concrete cone failure load. This results in an additional decrease of the failure load in comparison to headed or undercut anchors. If torque-controlled expansion anchors do not properly expand further or when the spreading displacement is too small, the influence of cracks on the failure load will be much more pronounced than shown in Fig. 3.31. Ku / c r o c k ) / Fu ( u n c r a c k e d

concrete)

Drop-in anchors cannot expand further after they have been properly installed. Due to the reduction of the spreading force caused by cracks (Fig. 3.30), these anchors often fail by pulling out without significant damage of the concrete while in uncracked concrete they produce a concrete cone type failure. Therefore, the reduction of the failure load caused by cracks is much larger than for well-designed torque-controlled expansion anchors (compare Fig. 3.32 with Fig. 3.31).

1,0

bcrack) / F ubncracked concrete)

0,8

0,6

0,4

0,2

0 0,4

0,8

1,2 crack width _w [mm]

^

Fig. 3.32-Influence of cracks on the ultimate load of drop-in anchors (from Eligehausen, Fuchs, and Mayer 1987 and 1988)

0,8

1,2 1,6 crack width _ w [mm]

^

For self-drilling anchors the ratio of failure load in cracked concrete to failure load in uncracked concrete seems to be independent of the anchor diameter for constant crack width to maximum expansion displacement ratio (Fig. 3.33). Because the maximum expansion displacement increases with increasing anchor diameter, the reduction of the failure load for constant crack width is larger for smaller anchors than for bigger anchors.

Fig. 3.31 -Influence of cracks on the ultimate load of torque controlled expansion anchors (from Eligehausen 1984)

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---



FU (Anchor in Crack)

r .rU (Anchor in uncracked Concrete)

7

1 Single Cracks ,

,M12

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

*

0

0,1

0,2

0,3

I

I

I

I

0,4

0,5

0,6

0,7

L-7 0,8

0,9

1

1,0

w/a

Fig 3.33 -Relative strength of self-drilling anchors as a function of the ratio of crack width to expansion displacement (from Eligehausen 1987) F, (crack) / F, (uncracked concrete)

0,4 0,6 crack width w [mm]

Fig. 3.34 -Ratio of the failure load of chemical anchors installed in cracks to the failure load in uncracked concrete as a function of crack width (from Eligehausen, Mallee, and Rehm 1984) Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

In the case of grouted anchors (grouted by cement-based or chemical-based mortar) cracks may disturb the bond between the grout-concrete interface. Therefore, the failure load of grouted anchors in cracks is significantly smaller than the value measured in uncracked concrete (Fig. 3.34). The large scatter of the results is caused by the random distribution of the crack around the anchor hole and along the anchor length. If the crack widths are changing due to fluctuating loads, the anchor failure load is even more reduced or the anchor may even be pulled out (Cannon 1981). Under constant conditions anchors placed in the intersection of two cracks fail at approximately 20 percent lower loads than anchors set in cracks running in one direction only (Eligehausen, Fuchs, and Mayer 1987 and 1988). This can be explained by the fact that the effects described above will occur in both directions and not in one direction as in the case of single cracks. Anchors are often installed in groups where the individual anchors a r e c o n n e c t e d b y a n attachment. In this case some anchors might sit in uncracked concrete while others are located in


--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

355.1R-40


cracks. The average strength of groups situated in cracked concrete was about 30 percent lower than the value applicable for anchor groups set in uncracked concrete (Eligehausen, Fuchs, and Mayer 1987 and 1988). Approximately the same strength reduction was measured for single anchors installed in cracks. Failure of all fastenings was caused by pulling out a concrete cone. The strength of the entire anchor group is constant for one or more of the anchors in a concrete crack. The reduction is almost the same whether one anchor or all are in concrete cracks (Fig. 3.35). In the test, the anchor plate was connected flexibly (by hinges) to the hydraulic cylinder. Fu [kN] 150 -

125

100

75

50

Number of anchors in cracks

Fig. 3.35-Strength of fastenings with four anchors as a function of the number of anchors in cracks (from Eligehausen, Fuchs, and Mayer 1987 and 1988)

Theoretical studies showed that the results described are also valid for larger groups of anchors and for applications when the anchor plate is rigidly attached (Eligehausen, Fuchs, and Mayer 1987 and 1988). Based on these results, it can be stated that the strength of anchor groups placed in cracked concrete can be taken as n-times (n = number of individual anchors of the group) the value expected for one anchor if the influence of cracks Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

and anchor spacing is taken into account simultaneously. This is valid for anchors with a steadily increasing load-displacement relationship in both uncracked and cracked concrete. Fig. 3.36 describes the influence of the loaddisplacement relationship of expansion anchors placed in cracks on the failure load of anchor groups. It is assumed that three anchors of a quadruple fastening (large spacing) are located in cracks and one anchor is sitting between cracks in uncracked concrete. If the anchors show a steadily increasing load displacement relationship in uncracked and cracked concrete (Lines a1 and a2 of Fig. 3.36), the failure load of the group is about four times the failure load of one anchor placed in a crack. (This theoretical result is in accordance with Fig. 3.35.) Expansion anchors located in cracks may slip in the hole before expanding further and take up more load (Line b of Fig. 3.36) or may be pulled out at rather low loads (Line c of Fig. 3.36). If only one of the anchors shows a load-displacement behavior according to Lines b or c, the failure load of the group may be reduced by more than 40 percent. Anchors which are being used in areas where cracks may occur, such as the tension zone of a concrete member, must be suitable for this application. 3.3.4.1 Influence of tensile stresses generated by structural action on anchor strength -In tests summarized to this point, the anchors were placed in the tension zone with constant stress of the reinforcement, and therefore, tensile stresses in the concrete were mainly induced by the anchors. However, if the anchors are placed in the shear region of beams and slabs and in the region of anchorages and lap splices of deformed bars, locally high tensile stresses are already induced in the concrete due to the loading of the structure. If anchors are placed in this region, the tensile stresses that they induce in the concrete combine with the tensile stresses due to loading of the structure. An example is shown in Fig. 3.37. It is assumed that an anchor is placed in the end region of lapped splices of large reinforcing bars. Plotted are stresses in the concrete due to splicing of the bar and loading of the anchor. The tensile stresses along the failure surface of the concrete cone overlap. Therefore, a reduction of the pullout load compared to anchors placed in otherwise unloaded concrete must be expected which, according to tests, is up to 25 percent in Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


355.1R-41

F

anchor in crack

FUC load displacement relationship

0 a, 0b C .o

Fu 4 Fuc

0,94 0,64 0,50

vuc

V

Fig. 3.36- Influence nf load-displacement relationships of expansion anchors on the ultimate load of an anchor group (from Mayer and Eligehausen 1984) \ -

stresses caused by r e i n f o r c e m e n t

~~

expansion anchor

. \

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

ds = 28mm-,

a

-

stresses caused by anchor

1

+c

+1

Fig. 3.37-Anchor in the region of an overlap splice (cross section). Overlapping of stresses caused by the bars and by the anchor (from Eligehausen 1984) Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS


355.1R-42


the assumed case (Eligehausen 1984). Reducing the size of the reinforcing bars, increasing the embedment depth of the anchor, or both reduces the influence of these intersecting stresses. In summary, the influence of these intersecting stresses on the failure load is smaller than the influence of cracks. In the tests summarized in Fig. 3.28 and 3.31 to 3.35, anchors were used which extended beyond the tension reinforcement. If short anchors are used, they are anchored in the concrete cover or between the bars. In this circumstance, high tensile stresses are induced in the concrete cover by the bond action of the reinforcing bars. These stresses intersect the tensile stresses in the concrete induced by the anchor. The strength of the concrete in the cover and in the region of the bars may be lower than in the core of the specimen due to poor compaction, especially in sections with closely spaced reinforcement. Furthermore, this reinforcement reduces the concrete area available for transmitting tensile forces. Because of these conditions a significant reduction of the failure load of all types of anchors must be expected. This was confirmed by tests with expansion and undercut anchors placed in the cover of a beam with rather heavy reinforcement (Eligehausen, Fuchs, and Mayer 1987 and 1988) (Fig. 3.38). After loading the beams to service load (crack width w = 0.3 to 0.4 mm) the anchors were loaded to failure. The anchor failed when the concrete cover between two adjacent cracks was pulled off (Fig. 3.39). On an average the ratio of failure load in cracked concrete to the value for uncracked concrete was about 30 percent smaller than shown in Fig. 3.28 and 3.31. 3.3.4.2 Influence of load transfer into the tension zone on the behavior of the structural element-The overlapping of concrete tensile stresses caused by loading the structure, and stresses induced locally by the loaded anchor affects the strength of the anchor and may reduce the strength of the member where the anchor is placed (Rehm and Eligehausen 1986). Transfer of high tensile forces into the concrete in the region of overlap splices and of anchorages of reinforcing bars may be critical especially if the splice reinforcement is not enclosed by stirrups (Rehm and Eligehausen 1986). Another critical application is the transfer of forces into the tension zone in the shear region of slabs without shear reinforcement.

a 0

135 = 2.15 Id b 0

180=3ld

t

c

dimensions in mm

Fig. 3.38 - Test specimens (from Eligehausen, Fuchs, and Mayer 1987 and 1988)

Investigations of this case are described by Eligehausen and Reuter (1986) and Lieberum, Reinhardt, and Walvaren (1987). Slabs 300 mm thick were tested by Eligehausen and Reuter without shear reinforcement. The shear-span ratio a/h ranged from 3 to 4.5. A fraction of the total load was transmitted by anchors into the tension zone and the rest by loading plates into the compression zone. Types of anchors examined were expansion, undercut, and headed studs. The embedment depth (40 to 130 mm) and the ratio of anchor load to total load (0 to 100 percent) were varied. In all cases the slabs failed by an inclined shear crack. Fig. 3.40 shows the cracking pattern of one specimen at about 95 percent of the failure load of the member. The anchor loads must be transmitted over the tip of the inclined crack to the supports. This causes high tensile stresses at the crack tip. Therefore, the failure crack (shown as a broken line) will occur at a lower total shear force than loading the slab in the compression zone only. In the tests a reduction of the shear carrying capacity of the slabs up to between 15 and 20 percent was found when all the loads were transmitted into the tension zone and not into the compression zone. The strength reduction was smaller when only a fraction of the total load was transferred into the tension zone. A similar strength reduction was found by Lieberum, Reinhardt, and Walvaren (1987), under

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---



355.1R-43


.b..

_ I

b

Fig. 3.39 - Concrete failure of an anchor group (from Eligehausen, Fuchs, and Mayer 1987 and 1988)

1

7

a =3d

L

I

Fig. 3.40- Crack pattern. of a slab without shear reinforcement (from Eligehausen and Reuter 1986)

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---




355.1R-44

these test conditions. If the anchors are placed close to the support the strength reduction will be much more significant. This reduction of the shear capacity may, depending on the design of the slab, significantly change the type of failure from a ductile bending failure to a brittle shear failure (Eligehausen and Reuter 1986). To avoid this problem, it is recommended that the shear forces transmitted directly into the tension zone should be limited to about 40 percent of the total shear force, or alternatively, the shear stress should be limited to about 80 percent of allowable values. Composite structures (precast concrete elements with bonded cast-in-place concrete) without reinforcement connecting the precast and cast-in-place concrete, are especially critical. Failure of this type of structure will often be caused by a crack in the contact area between the precast and the cast-in-place concrete. If the load is transmitted into the precast concrete element, high tensile stresses are generated in the contact area. Therefore, the shear stress at failure is significantly lower than in the case of loading the specimen in the usual way at the top (Fig. 3.41).

3.3.5 Shear loading-Little investigation of the influence of cracks on the behavior of anchors loaded in shear has been conducted. The few available test results can be summarized as follows. Anchors placed in cracked concrete and loaded in shear will fail the concrete (small edge distances), or the bolt (large edge distances), or a combination of both. Under otherwise constant conditions, the failure load of anchors with a small edge distance and loaded towards the edge will be smaller in cracked concrete than in uncracked concrete due to the disturbance of the distribution of stresses in the concrete by cracks. It can be assumed that the strength reduction is almost the same as for tension loading (reduction by about 40 percent). The strength reduction will be smaller if edge reinforcement is present. The ultimate load of anchors with large edge distances (steel failure) is not significantly influenced by cracks. The edge distance required to insure a steel failure of the anchor is about 30 to 40 percent larger in cracked concrete than in uncracked concrete. 3.4-Behavior of cast-in-place anchor bolts in uncracked concrete piers

join f

-7

2

4

a/d

Fig. 3.41 -Shear stress failure of a composite slab without connecting reinforcement between precast and cast in place concrete (after Rehm and Eligehausen 1986) --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---


6

3.4.1 Introduction -Anchor bolts are commonly used in highway and bridge structures to connect light standards, sign supports, and traffic signal poles. They are also used to connect steel columns in industrial structures to structural concrete members. The anchor bolt installation discussed in this section is one of the most widely used cast-in-place anchorage systems. The anchor bolts used typically have long embedment lengths and small edge distances. Such installation should be distinguished from bolts embedded for short lengths in mass concrete with very large edge distances. The supporting concrete members associated with this installation are usually piers, drilled shafts, or other foundation elements with limited plan dimensions; however, the concrete is usually well confined by reinforcement. The structural behavior of cast-in-place anchor bolts with long embedment lengths installed in supporting members with limited dimensions is distinctly different from that described in the preceding sections. This section summarizes some significant results from extensive research conducted for this type of anchor bolt application at the University of Texas at Austin (see Breen 1964; Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


Lee and Breen 1966; Lee and Breen 1970; Hasselwander, Jirsa, and Breen 1974; Hasselwander, Jirsa, Breen, and Lo 1977; and Jirsa, Cichy, Calzadilla, Smart, Pavluvcik, and Breen 1984). The test results and design recommendations are valid for anchors in wellconfined concrete. These studies focused on many significant factors affecting anchor bolt behavior including clear cover, embedment length, bolt diameter, bearing area, type of anchorage device, concrete strength, steel yield strength, shape of piers, and bolt group configuration. In addition, a series of exploratory and supplementary studies were made to determine the influence of cyclic loading, lateral loading, transverse reinforcement, and method of loading on the bolt behavior. Diameters of anchor bolts ranged from 1 to 3 in. Steel yield strengths ranged from 33 ksi (A7) to 105 ksi (A139). Embedment lengths ranged from 10 bolt diameters to 20 bolt diameters. A typical test specimen geometry is shown in Fig. 3.42.

355.1R-45

3.4.2 General behavior under loading-A single anchor bolt transfers tension load to the concrete member in three successive stages: (1) steel-toconcrete bond, (2) bearing against the washer of the anchorage device, and (3) a wedging action by the cone of crushed and compacted concrete in front of the anchorage device. These three stages are not entirely distinct, but the exact nature of the transition from one stage to the next is highly indeterminate and can only be discussed in a general manner. Fig. 3.43 shows tail stress plotted against lead stress for three 1 3/4 in. anchor bolts with clear covers of 3 l/2 in. and three different 10, 15, and 20 bolt diameters. embedments: Adhesion or bond between the bolt and concrete is the predominant load carrying mechanism for early stages of loading; little increase in tail stress is observed with increasing lead stress. The longer the bolt, the more load the bolt can carry by the bond mechanism. Under increasing load, bond strength decreases along the length of the bolt and

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8'- o”_

I'' ANCHOR 8OLlI J/4” ANCHOR B O L T -

SECTION A-A


SECTION B-B Fig. 3.42 - Typical specimen geometry Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


355.1R-46

Bolt failures occurred in several bolts by necking in the threaded portion of the bolts. Little damage to the concrete cover over the bolt was observed at bolt failure. A relatively sudden spalling of the concrete cover over the anchorage device at low loads characterized the failure of bolts with small amounts of clear cover [Fig. 3.44(a)]. For larger amount of clear cover, the failures were characterized by the splitting and spalling of the concrete cover into distinct blocks by the wedging action of a cone of crushed and compacted concrete which formed in front of the anchorage device [Fig. 3.44(b)]. The distinguishing feature of a wedge-splitting failure was the diagonal cracks [marked B in Fig. 3.44(b)] which started just in front of the washer on the bolt centerline and extended toward the front and each side of the specimen. These diagonal cracks were frequently accompanied by a longitudinal crack along the bolt axis [C in Fig. 3.44(b)], a transverse crack parallel to and near the washer of the anchorage device [A in Fig.

tail stress begins to increase. The load that was previously carried by a bond mechanism must be transferred to a bearing mechanism. In Fig. 3.43 the bond-to-bearing transition is most clearly seen for the bolt with 200 embedment. For a given load increment, the tail stress increases more than the lead stress as the load carried by bond is unloaded into bearing on the anchorage device. The bond-to-bearing transition is dependent on the embedment of the bolt; the shorter the bolt, the shorter and less well-defined the transition. After the bond-to-bearing transition, tail stress increases uniformly with increasing lead stress as the load is carried by bearing or by wedging action. 3.4.3 Failure modes-The failures observed during testing can be described as: (1) bolt failure, (2) concrete cover failure by spalling, and (3) concrete cover failure by wedge-splitting. While these three categories represent distinct failure modes, combinations of these modes were observed in several instances.

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8 10

l

20

I

30

I

40

I

50

s

60

I

70

Tail Stress, ksi Fig. 3.43 - Tail stress versus lead stress for different embedment lengths Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS


Tension

Tension

Cover Spalling Failure

Wedge-Splitting Failure

Fig. 3.44 - Concrete cover failures

3.4.4 Lead-slip relationships (effect of clear cover and embedment length)-Bolt tension versus lead slip curves associated with different clear covers and embedments are shown in Fig. 3.45 and 3.46. Slip of the anchor bolts was measured relative to the front face of the specimen (lead slip). Fig. 3.45 illustrates the effect of clear cover. Since the effect f concrete strength varied approximately with P d lead stress in Fig. 3.45, calculated on the basis of the anchor bolt stress area, was d and plotted against normalized with respect to /-lead slip for four 1 3/4 in. bolts each with an embedment of 15 bolt diameters (15D) and an anchorage device consisting of a nut and a 4 in. diameter, l/2 in. thick washer. As seen in Fig. 3.45, the slopes of the curves are essentially the same until each bolt approaches ultimate capacity. A definite trend of increasing ultimate strength with increasing clear cover is indicated. Fig. 3.46 illustrates the effect of embedment length on the stress-slip relationships of three 1 3/4 in. bolts each with a clear cover of 3 l/2 in. and an anchorage device consisting of a nut and a 4 in. diameter, l/2 in. thick washer. The initial portions of the curves are essentially the same and there is no appreciable difference between the ultimate strengths of the 15D bolt and the 20D bolt; the ultimate strength of the 1OD bolt, however, is noticeably reduced. The failure of the 10D bolt developed initially as a typical wedge-splitting mode until the Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

cracking propagated to the sides and front face of the specimen. The result was the complete loss of a rectangular block of concrete cover extending back to the anchorage device over the full width of the specimen, as opposed to the usual group of triangular wedges with a common apex over the anchorage device. Such a failure indicates that the wedge-splitting mechanism did not fully develop and therefore the ultimate strength of the anchor bolt installation was reduced. The major effect of embedment length on the ultimate strength of an anchor bolt installation is related to the ability of the concrete cover to resist the wedge-splitting action of the cone of crushed and compacted concrete in front of the anchorage device. A certain minimum embedment length is required to develop this resistance. As illustrated in Fig. 3.46, increasing the embedment length beyond this minimum length provides no significant improvement but decreasing the embedment length results in a significant reduction in ultimate strength. A 15D embedment length can be considered a satisfactory minimum embedment length. 3.4.5 Ultimate strength-The ultimate strength of a bolt in a group is clearly not the same as that of an isolated bolt with similar geometry. 3.4.5.1 Single bolt strength -Hasselwander, Jirsa, Breen, and Lo (1977), concluded that clear cover and bearing area are the main variables governing the strength of single anchor bolts. The variables were incorporated into an equation for predicting the strength of isolated anchor bolts, subjected to simple tension and failing in a wedgesplitting mode: Tn = 140A, @[O-7 + ln[2C’/(D, -II)]] (3.35)

where T, =

Ab = D = D, = C' =

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

3.44(b)] or both. Cracking generally started near the anchorage device and extended toward the front, toward the sides of the specimen, or both under increasing load.

ultimate wedge-splitting capacity of a single bolt, lb, with an embedment length not less than 12 (D w - D) net bearing area, in.* , (r/4) $$D”), but not greater than 4D bolt diameter, in. diameter of anchorage device (washer), in. with minimum thickness of Dd8 clear cover to the bolt, in.



355.1R-48

:cu

d

.

s

d

s d

.

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’

I500

1250 _,--L~l5 0

Jr-:

L = 20 D

1000 A-.._.._& --z-

.

LL=l0D

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f A!!L

750

C’ = 3.5 In.

1 0.02

I 0.04

. 0.06

. 0.08

I 0.10

I 0.12

. 0.14

. 0.16

Lead Slip, inches

Fig. 3.46-Effect of embedment length Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS


. 0.18

. 0.20

. 0.22

. 0.24

355.1R-50


The design tensile strength T, was determined

3.5-REFERENCES

as: (3.36)

where 4 As fy

= a capacity reduction factor of 0.75 = tensile area of the anchor bolt, as defined in Eq. (3.1), in.2 = yield stress of the bolt material, psi

The design equation was developed from a regression analysis on test results of bolts failing in A minimum the wedge-splitting mode only. embedment length of 12(D, - D) was suggested to allow the wedge-splitting mechanism to occur. A restriction which accounted for a reduced bearing efficiency observed for large washers, limited the net bearing area to 4D2. A minimum washer thickness, D$?, was suggested to prevent flexibility of the washer. Fig. 3.47 shows graphically the suggested ultimate strength equation and the test data plotted to illustrate the accuracy of the equation. The equation provides a reasonable estimate of strength, yet is simple to use and reflects the critical parameters observed in the test program. 3.4.5.2 Bolt group strength - Jirsa, et al. (1984), evaluated the bolt group interaction and strength reduction by comparing the average test capacity with the predicted capacity of an isolated bolt with similar geometry. It was observed that as bolt spacing decreased, the reduction in strength significantly increased. From a least squares analysis of the available data, the following modification to Eq. (3.35) was produced for the nominal tensile capacity of an anchor bolt in a bolt group based on failure of the concrete. T,, = 140Ab@ {0.7 + ln[2C’/(D,,,-D)]} (3.37) (0.02S + 0.4), in. where

S = bolt spacing, in. (0.02S+0.4) 5 1.0 and other factors are the same as in Eq. (3.35). Eq. (3.37) provides an estimate of the strength of closely spaced anchor bolts with edge cover typical of highway- related structures. The design tensile capacity, Tu, can be determined according to Eq. (3.36). Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

ACI Committee 349,1990, “Code Requirements for Nuclear Safety Related Concrete Structures,” (ACI 349-90) Appendix B, American Concrete Institute, Detroit. American Institute of Steel Construction, 1978, Specifications for the Design, Fabrication and Erection of Structural Steel Buildings, with Commentary, New York, 235 pp. Best, J. FIoyd and McDonald, James E., 1989,: “Evaluation of Polyester Resin, Epoxy, and Cement Grouts for Embedding Reinforcing Steel Bars in Hardened Concrete,” Technical Report REMR-CS-23, US Army Engineer Waterways Experiment Station, Vicksburg, MS. Bode, H. and Roik, K., 1987,: “Headed Studs Embedded in Concrete and Loaded in Tension,” in ACI SP 103 Anchorage to Concrete, G. Hasselwander ed. , Detroit. Bode, H. and Hanenkamp, W., 1985, “Zur Tragfshigkeit von Kopfbolzen bei Zugbeanspruchung,” (For Load Bearing Capacity of Headed Bolts Under Pullout Loads), Bauingenieu pp. 361-367. Braestrup, M.W., Nielson, M.P., Jense, B.C. and Bach, F., 1976, “Axissymetric Punching of Plain and Reinforced Concrete, Copenhagen, Technical University of Denmark, Structural Research Laboratory, Report R 75. Breen, J.E., 1964, “Development Length for Anchor Bolts, Research Report 55-1F, Center for Highway Research, the University of Texas at Austin. --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

T,, si 4 Tn but < A, fy , lb

Burdette, E.G., Perry, T.C. and Funk, R.R., 1987, “Load Relaxation Tests”, ACI SP-103 Anchorage to Concrete, G. Hasselwander ed.,Detroit, pp. 297-311.

Cannon, R.W., 1981,: “Expansion Anchor Performance in Cracked Concrete,” ACI-Journal, November-December, pp. 471-479. Elfgren, L., Anneling, R., Eriksson, A., and Granlund, S., 1988, “Adhesive Anchors, Tests with Cyclic and Long-Time Loads,” Swedish National Testing Institute Report 1987:39, Bor&. Eligehausen, R., 1987, “Anchorage to Concrete by Metallic Expansion Anchors, ACI SP 103 Anchorage to Concrete, G. Hasselwander ed., Detroit, pp.181-201. Eligehausen, R., 1984,: “Wechselbeziehungen zwischen Befestigungstechnik und Stahlbetonbauweise”, (Interactions of Fastenings and Reinforced Concrete Constructions), in “Fortschritte im Konstruktiven Ingenieurbau”, Verlag Wilhelm Ernst & Sohn. Berlin.


ANCHORAGE TO CONCRETE --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

Q

4 (I

w

a

I

00000 *. .* .. .. l .

a

ma40

44 4 l 0

0

0



355.1R-52


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Eligehausen, R. and Fuchs, W., 1988, “Tragverhalten von Dfibelbefestigungen bei Querzug-, Schrsgzug- und Biegebeanspruchung,” (bad-bearing Behaviour of Anchor Fastenings Under Shear, Combined Tension and Shear or Flexural Loading), Betonwerk + Fertigteil-Technik, No. 2, in German and English. EIigehausen, R., Fuchs, W., Lotze, D. and Reuter, M., 1989, “Befestigungen in der Betonzugzone,” (Fastening in the Concrete Tensile Zone), Beton-und Stahlbetonbau 84, No. 2 and 3. Eligehausen, R., Fuchs, W. and Mayer, B., 1987, 1988, “Tragverhalten v o n D i i b e l b e f e s t i g u n g e n bei Zugbeanspruchung,” (Loadbearing Behavior of Anchor Fastenings in Tension), Betonwerk + Fertigteil-Technik, No. 12/1987 und No. l/1988, in German and English. Eligehausen, R., Mallee, R. and Rehm, G., 1984, “Befestigungen mit Verbundankern,” (Fastenings Formed with Chemical Anchors), Betonwerk + Fertigteil-Technik, No. 10, pp. 686-692, No. 11, pp. 781-785, No. 12, pp. 825-829. Eligehausen, R. and Pusill-Wachtsmuth, P., 1982,“Stand der Befestigungstechnik im Stahlbetonbau,” (Fastening Technology in Reinforced Concrete Construction), IVBH Survey S-19/82, IVBH- Periodica l/1982, February. Eligehausen, R. and Reuter, M., 1986, “Tragverhalten von Platten ohne Schubbewehrung bei Einleitung von Lasten in die Betonzugzone”, (Load Characteristics of Plates without Shear Reinforcement by Introduction of Loads in the Tensile Zone of Concrete), Report No. l/17-86/3 of the Institut fiir Werkstoffe im Bauwesen, Universitgt Stuttgart. Eligehausen, R. and Sawade, G., 1985, “Verhalten von Beton auf Zug,” (Behavior of Concrete in Tension), Betonwerk + Fertigteil-Technik, No. 5 and 6, May/June. Fischer, A., 1984, “Befestigen mit Hinterschnittankern,” (Fastenings with Undercut Anchors), in "Fortschritte im Konstruktiven Ingenieurbau”, Verlag Wilhelm Ernst & Sohn, Berlin. Hanks, Abbot A., 1973,: Kwik Bolt Testing Program, Abbot Hanks Testing Laboratories of San Francisco, File H2189-S1, Report No. 8783. Hasselwander, G.B., Jirsa, J.O., Breen, J.E., and L.o, K., 1977, “Strength and Behavior of Anchor Bolts Embedded Near Eclges of Concrete Piers, Research Report 29-2F, Center for Highway Research, The University of Texas at Austin, May. Hasselwander, G.B., Jirsa, J.O., and Breen, J.E., 1974, “A Guide to The Selection of High-Strength Anchor Bolt Materials”, Research Report 29-1, Center for Highway Research, The University of Texas at Austin, October.


Jirsa, J.O., Cichy, N.T., CaIzadilla, M.R., Smart, W.H., Pavluvcik, M.P., & Breen, J.E., 1984, “Strength and Behavior of Bolt Installations Anchored in Concrete Piers,” Research Report 305-IF, Center for Highway Research, The University of Texas at Austin, November. Klingner, R.E. and Mendonca, J.A., 1982a, “Tensile Capacity of Short Anchor Bolts and Welded Studs: A Literature Review,” ACI-Journal, July/August, pp. 270-279. Klingner, R.E. and Mendonca, J.A., 1982b, “Shear Capacity of Short Anchor Bolts and Welded Studs,” A literature review, ACI Journal, Sept/Oct. Klingner, R.E., Mendonca, J.A. and Malik, J.B., 1982, “Effect of Reinforcing Details on the Shear Resistance of Anchor Bolts Under Reversed Cyclic Loading,," ACI Journal, Jan/Feb. Lee, D.W. and Breen, J.E., 1966, “Factors Affecting Anchor Development, “Research Report 881F,” Center for Highway Research, The University of Texas at Austin, August. Lee, D.W., and Breen J.E., 1970, “Model Study of Anchor Bolt Development Factors, Models for Concrete Structures, SP29, American Concrete Institute. Lieberum, K.H., Reinhardt, H.W. and Walraven, J.C., 1987, ’ Lasteinleitung fiber Diibel in der Schubzone von BetonPlattenstreifen,” (Fastening of Anchors in the Shear Zone of Concrete Slabs), Betonwerk + Fertigteil-Technik, No. 10, in German and English.

Mayer, B., and Eligehausen, R., 1984, “Ankergruppen mit Dubeln in der Betonzugzone,” (Anchor Groups with Anchors in the Concrete Tension Zone), Werkstoffe und Konstruktion Institut ffir Werkstoffe im Bauwesen der Universitit Stuttgart Badenand Forschungs-und Materialpriifungsanstalt, Wiirttemberg (Eigenverlag) October, pp. 167-180. Meinheit, D. and Heidbrink, F.D., 1985, “Behavior of Drilled-In Expansion Anchors,” Concrete International, April, pp. 62-66. PCI Design Handbook-Precast and Prestressed Concrete, 1978, Prestressed Concrete Institute, Chicago, 380 pp. Pusill-Wachtsmuth, P., 1982, “Tragverhalten von Metallspreizdiibeln unter zentrischer Zugbelastung bei den Versagensarten Betonausbruch und Spalten des Betons,” (Bearing Behavior of Metallic Expansion Anchors, Loaded in Tension, for the Failure Modes of Concrete Breakage and Splitting), Doctoral Thesis, University of Stuttgart. Rehm, G. and Eligehausen, R., 1986, “Auswirkungen der modernen Befestigungstechnik auf die konstruktive Gestaltung im Stahlbetonbau,” (Effects of Modern Fixing Technology on Structural Design in Reinforcing Concrete Construction), Betonwerk + Fertigteil-Technik, No. 6, in German and English.


ANCHORAGE TO CONCRETE Rehm, G., Eligehausen, R. and Mallee, R., 1988, “Befestigungstechnik,” (Fastening Technique), in “Betonkalender 1988”, Verlag Wilhelm Ernst & Sohn, Berlin. Rehm, G. and Lehmann, R., 1982, “Untersuchungen mit Metallspreizdtibeln in der gerissenen Zugzone von Stahlbetonbauteilen,” (Investigations with Metallic Expansion Anchors in the Cracked Tension Zone of Reinforced Concrete Members)“, Research Report of the Otto-Graf- Institut, Stuttgart, July, unpublished. Riemann, H., 1985, “Das erweiterte x-Verfahren fiir Befestigungsmittel: Bemessung an Beispielen von Kopfbolzenverankerungen,” (The Extended X-Method for the Design of Fastening Devices as Exemplified by Headed Stud Anchorages), Betonwerk + Fertigteil-Technik, No. 12, pp. 806-815, in German and English. Seghezzi, H.D. and Vollmer, H., 1982, “Modern Anchoring Systems for Concrete, ACI SP-103, Anchorage to Concrete, Atlanta, January. Sell, R., 1973, “Festigkeit und Verformung von mit Reaktionsharzmiirtel-Patronen versetzten Ankern,” (Strength and Displacement of Anchors Installed with Reaction Resin Mortar Cartridges), Verbindungstechnik 5, Vol. E, August, in German. Shaikh, A.F. and Yi, W., 1985, “In-Place Strength of Welded Headed Studs,” Journal of the Prestressed Concrete Institute, March/April, pp. 56-81. Teledyne Engineering Services, 1979, Technical Report 3501-1, Revision 1, August 30. Wagner-Grey, U., 1976,: “Experimentelle und Theoretische U n t e r s u c h u n g e n zum Tragverhalten von Spreizdtibeln in Beton”, (Experimental and Theoretical Investigations on the Performance of Expansion Anchors in Concrete), Doctoral Thesis, Technical University of Munich. Wiewel, Harry, 1989,: ” Results of Long-Term Tension Tests on ITW Ramset/Red H e a d E P C O N S y s t e m @ A n c h o r s Installed in Hardrock Concrete,” Techmar Inc, long Beach, CA. J une.

CHAPTER 4-DESIGN CONSIDERATIONS 4.1- Introduction

The purpose of this section is to discuss the various factors which affect the ability of concrete anchorages to perform their intended purpose. These factors should be considered in the design of anchorages. The tendency to design anchors based only on their tensile or shear loading is discouraged, when actually bending, prying action, and redistribution of loads are often involved.

4.2 -Functional requirements

4.2.1 Loading Conditions-Major considerations in determining the requirements for concrete anchorages include the type of loading which the anchorage will experience, and the potential for concrete cracking in the vicinity of the anchors. There is a high probability of coincidental cracking when anchors are located in the tensile zone of a concrete member. As described in Chapter 3, the capacity of anchors under sustained loading in the tensile stress zone of uncracked concrete is only 60 to 75 percent of static load capacity of anchors in unstressed concrete. In cracked concrete, anchor capacity is significantly influenced by anchor type and width of the crack in the region of the anchorage. In regions of tensile stress, since the width of flexural cracks is maximum at the concrete surface and decrease with distance away from the surface, the designer should use deepseated anchors (anchored in the compression zone of the member), or anchors which are designed to perform in cracked concrete. Anchors which perform well, at a given load level in uncracked concrete, may fail completely in cracked concrete under loads of the same magnitude. Criteria for the design and selection of concrete anchorages should account for these factors. Economics or related issues may dictate designing for a selected mode of failure. Installations such as bridge railings and highway signs could potentially receive accidental loadings that are not reasonable design loads. In such cases it may be prudent to design for the failure of the most easily replaced segment of the structure, whether it is the anchor bolt or a separate piece of the structure. Care must be exercised in designing for selected failure modes to maintain the integrity of the primary structural system. 4.2.1.1 Column bases - Simply connected column bases are normally loaded in compression of sufficient magnitude that column shear is transferred through friction and the anchorage serves only for erection purposes. It has been common practice for many years to use L- and Jbolts for erection anchors, which do not have sufficient embedment to develop the strength of the anchor steel. Headed anchors of the same size and length as L- and J-bolts have significantly However, the increase in higher capacities. capacity is often not needed for the simple column base plate connection. Column bases which are designed as moment connections should require a

--`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---


355.1R-53


355.1R-54


rigid base connection and anchors should be selected which can maintain a sufficient residual preload to develop applied moments. These conditions are necessary to achieve fixity of the column base. 4.2.1.2 Machine Foundations-Anchor bolts for machinery foundations are generally specified by the machinery manufacturer and have been sized by experience. Their general purpose is to fix the rigid machine housing to concrete in order to withstand machine vibrations. They are generally installed to a relatively low stress level and may not have sufficient embedment to develop the anchor steel capacity. Seismic loading of machine foundation anchorages can be critical and must be considered. 4.2.1.3 Structural Tension and Shear Connections-The anchorage of principal structural connections requires careful consideration of all possible loading combinations. Failure of structural connections may be catastrophic, particularly when there is no redundancy in the system. It is recommended that all structural connections be ductile. Ductility is defined as the ratio of a structure’s plastic displacement to its maximum elastic (yield) displacement. The ability of a structure to exhibit high values of ductility (ten or greater) is an extremely desirable feature because this can allow for an overload condition to exist without producing a catastrophic failure. It can provide for highly redundant structures (i.e., structures that provide alternative stress paths) that redistribute loads internally. When designing the anchorage of a steel structure to concrete, ductility of the structure, including the connection, should be considered. The desired ductile behavior may occur in any one or all of the following components: the structural steel element being connected, the baseplate attached to the steel member, the steel anchors, or the concrete. Steel is more ductile than concrete and it is better to proportion an anchorage so that the majority of the ductile displacement occurs in the steel elements of the anchorage or in the attached structural member. In cases where this is not possible, extra care should be taken in selecting anchor types, geometry, and safety factors. Temperature changes and the shrinkage of structural elements should also be carefully considered in determining connection details

because of the significant effect which tensile loads have on anchor stress and the manner in which shear is transferred to the concrete. Structural connections should also be investigated for cyclic loadings, vibration loads from wind or machinery, and seismic loads. 4.2.1.4 Pipe Supports-In most structures, pipe supports are dead-load hangers or support brackets. Pipe supports are generally detailed to provide free expansion and contraction of the piping system under changing temperatures. Experience has shown these loosely supported systems function very well under seismic conditions without special design considerations. Vibration problems normally occur under operating conditions and are corrected by adding or shifting supports to alter the response frequency of the system. Design loads for these supports are generally low and sizing of anchors, by experience, usually results in large safety factors. In contrast to this, the pipe supports for nuclear applications are often designed to prevent piping system frequencies from coinciding with predicted structural frequencies generated by an earthquake of prescribed magnitude. As a result, specifications often limit support displacements to low values under conservative combinations of loading. Most anchorages cannot comply with the imposed displacement limitations without rigid bases and oversized anchors. When a pipe has multiple supports and is loaded along its length, evaluation of the stiffness of each support with respect to the longitudinal stiffness of the total support system between expansion joints or bends should be made to insure that a particular support is not overloaded to failure, thus setting up a progressive failure mechanism. 4.2.2 Anchorage Environment- Consideration of the service environment is essential for service longevity, particularly in areas where the anchorage may come in contact with saltwater sprays or deicing salts. Unprotected steel is particularly vulnerable to corrosion when exposed to the atmosphere. For expansion anchors, vulnerability to corrosion exists in the region of the expansion mechanism where space is available for moisture collection. Corrosion will reduce the ability of anchors to function correctly, especially torque-controlled expansion anchors. Where steel is under a sustained high stress,

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there is a higher potential for stress corrosion failure. If the yield strength of the anchor steel is less than 120,000 psi, stress corrosion is less likely to be a problem. However, precautions must be taken when chlorides are used in the anchorage zone either externally or as a part of the concrete mix. Protective coating systems, or the use of corrosion resistant materials, should be considered in corrosive environments. The use of thin zinc coatings will not provide permanent protection against corrosion under normal outside exposure conditions. Proper detailing will insure that runoff water cannot reach anchors in areas of snow and ice removal. Alternate periods of wetting and drying have been known to produce corrosion even in the absence of chlorides. Anchor bolts are often set in sleeves to provide for minor adjustment of the bolt to fit the foundation base. If the foundation is exposed to freezing temperatures, the sleeves should be filled with grout or be otherwise protected against the intrusion of water. Gaps between a steel base plate and the concrete surface should be sealed if the foundation is exposed to an aggressive environment. In a similar fashion, plain sandcement dry-pack pads which are exposed to freezing and thawing should be coated with a sealer to prevent water absorption. Chemical adhesives, lead caulking, or other materials which have a high rate of creep at elevated temperatures should not be used in areas of high temperature or possible exposure to fire. Special investigations may also be necessary to determine the possible effects of process chemicals on anchors in industrial plants. Intermittent exposure may be a more severe service condition than continuous exposure. 4.2.3 Behavior-The behavior of cast-in-place and post-installed anchors is described in Chapter 3. Well-designed, cast-in-place anchors perform better than or equally as well as post-installed anchors, if for no other reason than that they are normally set deeper into the concrete and at ultimate load feature failure in the bolt rather than failure in the concrete. Construction logistics that admit alternative and specifications manufacture of the equipment to be anchored (and therefore alternative anchorage size and location) often make the post-installed anchor more practical. Nonetheless, the designer should consider use of a cast-in-place anchor whenever the size and location of that anchor is known prior Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

355.1R-55

to casting of the concrete. Anchor capacity may be limited by the strength of concrete, by the strength of the anchorage steel, or by slip of the anchorage mechanism. The mode of failure is an important design consideration. Concrete failure may occur before or during slip of the anchor. In general, the properties of steel are well defined and steel failure is predictable and controllable. In contrast to the controlled ductility of a steel failure mechanism, concrete is a brittle material with less well-defined properties. Failure by slip may be either brittle or ductile depending on the ability of the anchorage mechanism to maintain load during slip. 4.3 - MATERIALS 4.3.1 Concrete-When the capacity of the anchorage is controlled by the strength of concrete, it is generally the tensile properties of the concrete which control cone failures, and crushing strength that controls slip failures. Tensile properties of concrete vary more than compressive properties. Tensile properties of the concrete also influence bond and affect those anchor types which depend on bond to develop capacity. The tensile-compressive strength relationship can be complicated by the influence of grain size, type, and distribution of aggregate particles. For this reason, construction practices, which permit segregation of the aggregate will increase the variability of tensile strength more than the compressive strength. Segregation of the aggregate is influenced by the slump of the concrete, the height of the drop of the concrete, and the amount of vibration during placement. For this reason, the capacity of anchors may vary depending on their location in walls and in the top or bottom of slabs. 4.3.2 Steel-The type of steel used in anchors is largely dependent on the method of anchorage but can also be influenced by the method of securing the base plate or attachment to the anchors. It is desirable to limit the yield strength of headed anchors to that of ASTM A 325 or lower strength material, because of the brittle nature of higher strength steels. Zinc plating causes additional brittleness and reduced fatigue resistance for higher strength steel bolts. Steel with yield strengths in excess of 120,000 psi have been found to be highly susceptible to stress corrosion in most anchorage environments.


355.1R-56


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4.4 -Design basis

The safety factor for any element in an anchorage system should be consistent with the other elements in the system. Establishing an allowable stress or load factor must consider overall behavior of the anchorage. The design of concrete anchorages is usually controlled by codes governing both structural steel and concrete. 4.4.1 Types of anchors 4.4.1.1Headed Anchors-Headed anchors may consist of welded studs or bolting material with anchor heads manufactured to established standards. Headed anchors may also be made by welding a rigid plate to the embedded end of the anchor or by threading a bar and using a standard nut. Once the load increases sufficiently to overcome n the shank, subsequent loading anchor head. Headed efficiently if the shank of the This will minimize bond and oad on the anchor by bearing Anchors-When anchor load inishes with depth. The quired to fully develop the of deformations). sustained loadi concrete in the Bonded anchor

Under

typically been manufactured deformed reinforcing bars, s. The basic development Building Code are based on d minimum spacing of an rs. The basic development ars with a hook or 90” bend about 50 percent of the of straight bars. The use of r reinforcement was excluded g Code in 1971 (ACI-ASCE considered as twice that

of deformed ba insure that the

gths given in ACI 318 crete capacity is higher than the When evaluating the concrete e failure modes “splitting of e concrete between ribs” The failure mode


“concrete cone break out” was not considered because typically this mode does not occur when developing reinforcing bars. However, the failure mode “concrete cone break out” is quite typical for shallow anchors (see Chapter 3). Excluding edge and spacing conditions, the yield strength of an individual reinforcing bar can be developed in 3000 psi uncracked concrete in about 15 bar diameters (straight bar) or 10 bar diameters (hooked bar). To preclude a concretecone-break-out failure, the development length may increase by a factor of up to four to account for the effects of cover, number, and spacing of bars. A further increase of the development length by a factor of one and one-half to two is necessary if the anchors are located in the cracked tensile zone of a reinforced concrete member. Most anchorage situations do not involve minimum values for spacing and cover. The code provisions will be very conservative if individual bars are anchored in uncracked concrete well away from edges. However, the code provisions may not be conservative, if a group of bars, with or without small edge distance, is anchored in uncracked concrete or in the (cracked) tension zone of reinforced concrete members. 4.4.1.3 Expansion Anchors-Many patented expansion devices are used to mechanically fasten post-installed anchors to the concrete. Most expansion anchors were originally developed for short embedment depths to provide an anchor which failed in the concrete or by slip. Since ductile steel failure had no opportunity to occur in this situation, there were no restricting strengths applied to the steel in these anchors. More recently developed expansion anchors feature expansion mechanisms that can fully develop the strength of the anchor steel, when used as single anchors. Ductile steels should be specified for this type of anchor if a ductile failure mode is desired. 4.4.2 Concrete tensile failure -The determination of concrete pullout strength (cone failure) of individual anchors and anchor groups is discussed in Section 3.2.2. Concrete cone failure will occur when the capacity of the anchor bolt exceeds the concrete pullout strength. All shell type expansion anchors are designed to fail the concrete when the bolt is embedded to shell depth. Concrete failure can also occur with wedge bolts having shallow embedment depths. The concrete may also fail by splitting tension when there is inadequate lateral confinement of


355.1R-57

(1) Providing for deeper embedment to preclude the tensile-cone-failure mode. (2) Using larger number of smaller anchors at closer spacings to avoid spalling when the edge distance is too small. (3) Preloading the anchorage so that shear is transferred by friction at the interface of the base plate and the concrete rather than through shear in the anchor. 4.4.3 Anchor Slip -Anchors which fail by slip, without causing the concrete to fail in tension, have load-displacement characteristics similar to the post-yield behavior of steel. Typically, wedge Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

bolts and sleeve anchors with embedment depths greater than seven bolt diameters will fail by slip. They cannot be considered ductile, however, because the relatively wide variation in the slope of the deflection curves and ultimate loads distribute loads nonuniformly to the anchors. For these types of anchors, most manufacturers of post-installed expansion anchors recommend limiting normal service loads to 25 percent of the average published failure loads. 4.4.4 Tensile strength of steel - When the concrete-failure-cone strength exceeds the tensile strength of the anchor steel, design is controlled by the strength of steel. For structural attachments, other than simple hangers, load distribution to the attachments is dependent on the stiffness of the attachment and its degree of fixi ty For rigid base connections, anchor stress may be determined assuming that plane sections remain plane. However, if the load is transferred from the attachment to the anchors through a flexible plate, the determination of anchor stress is complicated by plate stiffness, prying action, and the load-displacement characteristics (including preload) of the anchor steel. AISC imposes a minimum safety factor of two, against ultimate, for service loads on high yield materials. Considering the increased loss of preload in concrete anchorages (approximately three times that of steel to steel connections), a minimum safety factor of three for anchor bolts would provide residual service load allowables approximating 85 to 90 percent of the residual preload for bolts initially preloaded close to yield. This would appear to be a reasonable limit considering all the other concrete and anchor variables. Proof load for concrete anchorages should be approximately 110 percent of the service load. For factored load design, AC I Committee 349 (1990) limits maximum stress to 0.9 of yield for all types of connections, and with stresses based on the net tensile area for bolted connections. Assuming an average load factor of 1.6, service load stresses would approximate 0.55 yield for anchors other than bolts. For ASTM A 36 steel, this also closely corresponds to a factor of safety of 3 against tensile strength. The capacity of welded stud anchors appears to be affected by the thickness of the attachment, Tennessee Valley Authority (1979). Apparently prying action, due to the flexibility of the plate, --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

the anchor. This occurs with all types of expansion anchors that have small edge distances. Deformation-controlled expansion anchors (dropin, self-drilling, and stud) are especially sensitive to edge distance because of the high expansion forces developed during anchor installation. Splitting may also occur at close edge distances when the anchorage mechanism expands with load application. In the United States, most manufacturers of expansion anchors recommend limiting normal service loads to 25 percent of the manufacturer’s average test failure load. Investigations by the United States Nuclear Regulatory Commission (1979) indicated that installation problems associated with split-shell type expansion anchors warranted increased safety factors over those applied to torque-type anchors. For the split-shell anchor, and others which cause the concrete to fail, it was recommended that a minimum factor of safety of five against average test values be used. Test results for expansion anchors differ from job to job and with anchor size, type, and modifications in anchor design. Assuming a coefficient of variation of 25 percent, a factor of safety of five on average tested anchor strength is appropriate. The capacities of anchors are affected by embedment depth, edge distance, and spacing. Reinforcing steel in the concrete can be used to enhance the strength of cast-in-place anchors. When the edge distance is small, closely spaced spirals of small diameter wire or mesh may be used to resist the bursting forces. However, more research is required in this area. Other solutions may be more effective. They consist of:


355.1R-58


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induces very high stress and cracking at the interior edge of the heat-affected zone of the weld under relatively low load applications. As a result, testing clearly indicates loss of capacity with increasing plate flexibility. 4.4.5 Shear- Shear may be transferred from base plate to concrete either by friction or by bearing. 4.4.5.1 Shear transfer by friction - If shear is to be transferred by friction, no lateral translation (sliding) of the base plate can occur. The normal force necessary to develop frictional resistance may be caused by direct load, by the compressive reaction of the applied moment, by residual preload in the anchors, or by any combination of the three. If the connection is to transfer shear by friction, the loading combination which controls should be that which produces the minimum compressive reaction in conjunction with maximum shear. If the connection is fastened to hardened concrete, the coefficient of friction used to determine shear resistance should not exceed 0.6. If the surface of a base plate is in intimate contact with concrete or grout, shear resistance will be increased by the cohesion between the two surfaces and the coefficient may be taken as 0.7. All forces contributing to frictional resistance should be conservatively determined in designing for either total or partial shear resistance by friction. Note that: (a) Direct loads normal to the shear plane should be the minimum associated with the loading condition. For cyclic loading, this would be the maximum direct pull-off loading including associated impact factors. (b) The compression component of the moment reaction is dependent on the location of the center of gravity of the compressive reaction. Conservative assumptions should therefore be used concerning its location. Without test verification of the analytical procedure, the location should not be assumed to be farther than pne plate thickness from the compressive edge of the attachment. (c) Residual preload, if any, should be based on conservative assumptions of preload loss. Shallow depth anchors having the capability of failing the concrete in tension may be expected to experience a total loss of preload. When the installation procedure requires a positive means of determining installation preload, residual preload Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

should not be assumed greater than 50 percent of the initial preload without prototype testing. When the installation load is determined by calibrated torque wrench or other less positive means, a higher loss should be assumed. Lost preload may be regained by retorquing, or retightening anchors. There appears to be little advantage in retorquing more than twice. Sufficient time should be allowed for the majority of loss to occur before retorquing, but under no condition should the time period be less than about 1 week. Effective preload should not be assumed without verification requirements in the installation procedure. 4.4.5.2 Shear transfer through bearing- If frictional resistance is not sufficient to resist lateral sliding, shear must be transferred by the plate bearing on anchors, shear lugs, or the concrete at the end of a fully embedded plate. In bearing connections, shear is distributed in proportion to the stiffnesses of the shear-resisting elements, with each element contributing its share. Failure of the stiffer elements will increase lateral translation. The stiffer elements then transfer their load to the remaining elements. 4.4.6 Preload-Concern for fatigue failure is a principal consideration in establishing service stresses. This is particularly true for expansion anchors. If the element is subject to frequent fluctuations in stress, the magnitude of the fluctuating stress range must be restricted to prevent eventual fatigue failure (see discussion of behavior under cyclic loads in Chapter 3). This is best controlled by limiting the maximum level of design stress. If the bolting system can be prestressed with sufficient load that the load remaining after losses exceeds the maximum stress load, it is generally accepted that fatigue is not likely to occur. Under these conditions service load stress should be set at a level that reflects the residual prestress. If a sustaining (residual) prestress cannot be assured, the service load stress, under fluctuating loads, must be set at a low enough level to assure that fatigue failure will not occur. Assuring a level of prestress in concrete anchorages is more complicated than steel-to-steel connections. Preload loss occurs due to creep of the concrete in the highly-stressed regions of load transfer from steel to concrete. For most embedments the major preload loss occurs within a few days of preloading. The loss, in percent, Licensee=Jacobs Engineering ( new WAN ) /3219500102, User=Schoolmeyer, Scott Not for Resale, 02/05/2007 07:29:17 MSTs/org1


diminishes each time the anchorage is retorqued such that losses can be minimized by retorquing at about 1 week intervals. The prestress should not exceed the yield stress of the steel. Loss of preload is a function of the strain relaxation (creep) relative to the total anchor strain. Since the major portion of load relaxation occurs at the zone of load transfer into the concrete, the loss of preload, in percent, can be reduced by increasing the total anchor elongation which increases the strain length of the anchor. If the embedment length of the anchor is the minimum required to develop its tensile strength, it will lose from 40 to 50 percent of its applied preload unless retorqued (Burdette, Perry, and Funk 1987). The loss may be more pronounced if the anchor is situated in cracked concrete. Loss of preload may approach 100 percent for anchors of lesser embedment depths which are capable of failing the concrete. This is especially true for anchors located in cracked concrete. To achieve an effective residual preload, care must be taken to exclude any bonding of the anchor to grout or concrete at the embedment surface. When bond occurs at the surface, the confinement of the surface concrete or grout, by compression of the bearing plate on the surface, is often sufficient to locally transfer the entire load for a limited time. When this occurs, stretch of the bolt may be limited to the thickness of the bearing plate or attachment. For effective preload, threads must be excluded from bonding to either concrete or grout. Grout has significantly higher bonding qualities than concrete, therefore the entire length of bolt above the anchor head should be coated to prevent bond in grouted systems. Effective prestress requires intimate contact of the base plate with concrete or grout at all anchor locations. When the base plate is bolted directly to hardened concrete without grout, effective prestress can be accomplished by placing shims or washers between the plate and concrete at the anchor locations. In most moment connections, shear is transferred to the concrete entirely through friction and bolts transmit tension only. If the combined effect of anchor preload and compressive reaction of the applied moment are not sufficient for shear transfer through friction, then shear must be transferred through the anchors. If this occurs, and shims or washers are used, the combined stress in the anchor would be increased by the increased bending stress in the

bolts in transmitting shear through the added space of the washers. If intimate contact is not achieved, the danger of high stress accumulations can be prevented by initially torquing to maximum values and then loosening the bolts to a minimum torque value after the concrete has had sufficient time to consolidate in the region of the anchor head. This will eliminate nonlinear anchor displacement under load and restrict peak stress accumulation to design stress levels. 4.4.7 Base plate flexibility-The flexibility of the base plate connecting the attachment to the anchorage steel is a controlling factor in determining the magnitude of anchor stress and the distribution of stress to the anchors. If the distance between exterior anchors and attachment is more than two plate thicknesses, the plate may be considered flexible, otherwise, the plate may be considered rigid. If the plate is rigid, anchor stress due to moment is proportional to its distance from the neutral axis and a conventional summation of forces and moments can be used to determine stress. If the plate is flexible, anchor stress is dependent on plate stiffness as well as distance to the neutral axis. It can also be influenced by the effect of other stressed anchors in the group that cause bending in the plate, and on any prying forces caused by plate flexure, which may add directly to the anchor load. Anchor loads, determined by conventional analysis, may be significantly in error if the plate is flexible. 4.4.7.1 Prying action-When load is transferred from attachment to anchor through a flexible plate in full contact with the concrete or grout, rotation of the plate at the anchor will induce a prying force beyond the anchor where the plate bears on the concrete. The prying force increases the load in the anchor. Prying increases with plate flexibility which affects the magnitude of potential downward displacement of the plate edge beyond the anchor. Prying decreases with increased anchor displacement. Preload reduces the displacement characteristics of the anchor under applied loading and increases the counter rotation of the plate beyond the anchor. For this reason anchor stress will increase with applied load irrespective of preload. The rate of stress increase, however, decreases with increasing preload. If the plate is not in contact with the concrete beyond the anchor, no prying will occur until the gap between plate and concrete is closed by the

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355.1R-59


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355.1R-60


downward displacement of the plate edge. If the anchor is not preloaded, the displacement of the stressed anchor will add to the gap requiring closure to develop prying. If the anchor is preloaded to close the gap, the preload force will add to the anchor stress resisting applied loads. 4.5-Construction practices

Design of anchor installations must take into account local construction practice and expected field conditions. Details should be designed so that the probability of concrete honeycombing at anchor locations is minimized. Placement tolerances may or may not be critical and should be determined by the application. See Chapter 5 for more information. 4.6-REFERENCES Abbot A. Hanks, “Summary Report - Kwik-Bolt Testing Program”, File No. H2189-S1, Report No. 8783, Abbot A. Hanks Testing Laboratories, San Francisco, CA ACI Committee 349,1990, “Code Requirements for Nuclear Safety Related Concrete Structures (AC1 349-90), “Appendix B”, American Concrete Institute, Detroit. ACI-ASCE Committee 326, 1962, “Shear and Diagonal Tension”, ACI Journal, Proceedings V 59, No. 2, Feb., pp. 277333. Burdette, E.G., Perry, T., Funk, R.R., 1987, “Load Relaxation Tests”, ACI SP-103 Anchorage to Concrete, Detroit, pp. 297-311. Cannon, Robert W., 1981, “Expansion Anchor Performance in Cracked Concrete”, ACI Journal Proceedings V. 78, November-December, pp. 471-479. Eligehausen, Rolf, 1987, “Anchorage to Concrete by Metallic Expansion Anchors”, Anchorage to Concrete, American Concrete Institute Special Publication SP-103, pp. 181-201. Orangun, C.O., Jirsa, J.O., and Breen, J.E., 1977, “A Reevaluation of Test Data on Development Length and Splices,” ACI Journal, Vol. 74, No. 3, pp. 114-122. Raphael, Jerome M., 1984, “Tensile Strength of Concrete”, ACI Journal No. March - April, pp. 158-165. Tennessee Valley Authority, 1979, “Welded Stud Anchors, Effect of Plate Flexibility on Stud Capacity”, CEB Report No. 79-18, TVA, Knoxville, TN. United States Nuclear Regulatory Commission, 1979, “Pipe Support Base Plate Designs Using Concrete Expansion Anchor Bolts”, IE Bulletin No. 79-02, Office of Inspection and Enforcement, Washington, D.C.


CHAPTER 5-CONSTRUCTION SIDERATIONS 5.1 Introduction

CON-

Quality control is the central issue among construction considerations for anchorage to In construction, the engineering concrete. profession tends to be quite meticulous with respect to tolerance in the fabrication of structural steel, but somewhat less so in masonry, timber Meeting framing, and reinforced concrete. tolerances is expensive and, therefore, required tolerances are limited to what is practical and what “can be covered by the other trades” and still yield an acceptable product. Another concept in establishing tolerances is to weigh the consequences of constructing less accurately than specified. Experience has shown that the secondary costs of compensating for the structural skeleton being out of square or out of plumb justify taking great care in the initial fabrication. This is also true for anchorage to concrete. There are few details in a structure where care during installation pays more dividends, or where carelessness can prove more costly. Sometimes corrective measures can be so expensive that they are not taken and the end product falls far short of what the engineer intended. Anchorage details are at the interface and provide the connecting link between separate structural systems. The axial load, moment, and shear required of the connection are typically quite well defined, and must be accommodated because there is usually no alternative path for load transfer. The joint has a minimum of redundancy to compensate for error in design or construction. Accordingly, it is important that the field engineer understands the intent of the design, to assure that the anchorage be constructed as specified. This relates to having the proper device, with the specified size and material, and having it properly installed. 5.2 Shop drawings/submittals

The first step in quality control is that the plans and specifications must indicate clearly what is intended. The next step is the requirement for submittals and shop drawings for all anchorages. 5.2.1 Cast-in-place systems-For cast-in-place systems, the submittal is the shop drawing and any other certifications required by the construction specifications. With respect to each anchorage


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assembly, the shop drawing should indicate the material of the anchoring device, its coating, length, diameter, length of threaded portion, diameter and thickness of washers, number of nuts (single, double, single or double plus leveling, etc.), and torquing requirements, if any. The shop drawing should also indicate the location of the anchorage in the structure, the location of the bolts (or devices) in a group, and their projection and embedment with respect to the finished concrete grade. When the completed anchorage is specified to be either grouted or dry packed, the dimensional details of the grout or drypack should be shown. When the anchorage consists of embedded dowels of reinforcing steel, the shop drawings for the anchorage are included in the shop drawings for the reinforcing steel. They should indicate the type of steel, details of bending, location (bar or groups of bars), embedment, and projection. Often an anchor assembly includes embedded structural shapes, either as the anchor itself or as a lower template. Shop drawings for these embedded shapes should indicate type of steel; coating; cross-sectional shape (standard designation); dimensions and details of the member or group of members in the assembly (location, type, size, and length of welds); size and location of holes; and embedment depth. 5.2.2 Post-Installed Systems-For post-installed systems, the submittal should include the shop drawings with information similar to that required for cast-in-place systems, plus manufacturer’s literature which adequately describes the device and its capabilities and provides instructions for its proper installation. 5.3 Tolerances

The acceptable variation from the specified positioning is the tolerance. The tolerances should be specified by the engineer and be appropriate for the application. Table 5.1 gives suggested tolerances for anchor positioning and can be used as a guide in determining acceptability. Other sources such as the American Institute of Steel Construction (AISC) and Precast/Prestressed Concrete Institute (PCI) are available. These requirements are rigorous, but meeting them is judged to be more economical than the consequences of not meeting them. Mounting or anchoring certain special equipment may require even closer tolerances. Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

5.4 lnstatllatlon of anchors

5.4.1 Cast-in-place systems 5.4.1.1 Anchors Embedded, NonAdjustable-Anchorages that fall into this category (see Table 2.1) can be grouped as follows: - Bolts installed in plastic concrete - Bolts in cans or blockouts - Bolts, with or without sleeves, positioned without template - Bolts, embedments, weld plates, or inserts attached to the formwork - Bolts or groups of bolts, with or without sleeves, positioned by top or bottom templates, or both - Embedded structural shapes 5.4.1.2 Bolts installed in plastic concrete-Often in wood frame construction the bolts connecting the wood sill to the footing or a wood plate to the top of a wall are installed as soon as the concrete placement is completed. This practice is not recommended because a good bond may not be achieved. 5.4.1.3 Bolts in Cans or Blockouts- T h i s system can be used in cast-in-place or postinstalled construction. Often, for machinery foundations or in situations where it is not desirable to have anchor bolts protruding from a slab or penetrating through a wall form, a can or blockout will be set at the approximate future bolt location. These blockouts can be made of wood, metal, or plastic; can be cylindrical or prismatic; can provide a shear key perpendicular to the floor or wall; or be battered to provide a dovetail effect. For flatwork, the cans or blockouts can be positioned by wood battens or templates which have a soffit elevation equal to the grade at topof-concrete and are secured to the edge forms; or they can be wired to the reinforcement or the edge forms. For vertical surfaces, they can be fastened to the wall form in their predetermined positions. In both cases, the blockout should be wire tied to the reinforcement so that it will not be vibrated out of position during the placement of the concrete. After concreting, wood blockouts are stripped. Metal or plastic units are typically left in place. The pocket is blown clean of debris, the anchor bolt positioned and the pocket grouted.


Table 5.1 -Suggested tolerances for installation of anchors in concrete Installed Location L and P are Specified

Correct Location Plan “A”

/3 Per Plans Suggested tolerances

Vertical Alignment, (r, deg.

Type of anchorage Positioning r, in.

Projection P, in. A. Cast-in-place 1. Common bolt, J- or Gbolt, continuously threaded rods

k /14

I

1/16

I

3.0

f 1/4

1/8

3.0

4. Weld plates

Flush with concrete

l/8

N/A

5. Troughs for adjustable anchors

Flush with concrete

l/4

N/A

6. Temporary embedded inserts

Flush with concrete

l/2

3.0

1. Drilled and grouted-all types

+ l/4

1/16

3.0

2. Expansion types

k 1/8

1/16

3.0

3. Embedded structural shapes

B. Post-installed



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Per recommendations of Committee 117

2. Reinforcing steel

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5.4.1.4 Anchors with or without sleeve, positioned without templates-This practice is generally not recommended, but if proper care is taken it can be successful. Tolerance criteria should be met and maintained throughout the concrete placement. The bolt insert or sleeve should be rigidly tied with wire to the reinforcement, top and bottom. Sometimes, the sleeve, the bolt head, bottom washer, or insert is tack welded, according to approved procedures, to cross bars which in turn are wire tied or tack welded to the existing reinforcement. 5.4.1.5 Bolts, embedments, weld plates or inserts attached to the formwork-This work generally relates to soffit and wall forms. The important step is to accurately scribe the inside of the form for proper location of the anchoring unit. The anchor unit should then be nailed or bolted to the form or wire tied to the reinforcing steel, or both so that neither internal nor external vibration can disturb or move the anchorage unit out of position. 5.4.1.6 Bolts or groups of bolts, with or without sleeve positioned by templates -These installations are generally used in flatwork, where the bolts are vertical. The use of templates is the best technique for guaranteeing that the anchorage is correctly positioned. A top template is often wood, although in “loose base plate” construction (where the superstructure is subsequently welded or otherwise connected to a steel base plate), the base plate itself can be used as the template. The top template for a single bolt or a group of bolts generally has a soffit elevation at or above the top of the finished concrete. Sometimes top templates are plywood with the holes either laid out precisely as the holes in the base plate, or actually drilled using the base plate holes as a guide for the drill. Where only a top template is used, there should be nuts above and below the template to hold the anchor bolt in a plumb position. The bottom template is a steel assembly of angles, channels, or flat bars. Low carbon steel bolts can be precisely positioned and welded directly to the steel template, or set in accurately located holes in the template, and tack welded. When used in conjunction with a top template, it is the top template that controls both the bolt projection and lateral position of the group of bolts. When there is no top template, the bottom template must provide those controls and should Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

be wire tied or welded, according to approved procedures, to the reinforcement so that it will be maintained in correct position while the concrete is being placed. Engineering approval should be obtained before welding to high-strength bolts or reinforcing bar, because material property changes may compromise expected steel capacities. Bottom templates are expensive and usually reserved for larger diameter bolt installations. They also affect the capacity of an anchorage and for this reason should only be used where detailed or approved by the Engineer. 5.4.1.7 Embedded structural shapes -This system is used mainly for transmission towers, although it has been used for other applications. The superstructure can be erected plumb, leveled, and set to grade in holes augered in the ground. Then concrete is cast around the structural shape. Alternatively, the anchoring elements, usually angles, are cast in the footings and the tower or superstructure subsequently bolted to them. The anchor installation is straightforward but care is required through the use of templates and guides to maintain proper location in plan, and at the proper grade, batter, and plane of batter. 5.4.1.8 Adjustable anchors -Anchors of this type are patented devices used principally in flatwork. Most often they are used for machinery installation and are designed to compensate for normal field tolerances in the positioning of anchor bolts. They offer an added advantage in that there are no bolts projecting above the floor prior to setting the machinery. The machinery can be moved into place on its base and then the bolts set. One features a trough set flush with the surface of the concrete and stud anchored to the concrete below. Another features deeply embedded pockets, housing a tapped bottom washer plate and having a sleeve extension up to the surface of the concrete. The devices are positioned and held in position during concrete placement in a manner similar to that described for sleeves. The principal concern is that the insert be maintained level. The bolt is normally grouted in place at the same time that the equipment base plate is grouted. 5.4.1.9 Common bolts pretensioned -Bolt installation is as described in further detail in Section 5.5.1. The shank of the bolt should be coated with bond breaker before placing concrete. After concreting, the annular space around sleeved


355.3R-64


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bolts is grouted. When concrete and grout (and dry pack under baseplate) has cured the specified number of days, screw on the nut and apply the pretensioning load with a torque wrench. Torque should initially be about 50 percent of desired torque, then to 90 percent, working from one bolt to the one diagonally opposite and thus progressing through the group. The final 10 percent of torque should be applied to all bolts in sequence. After 1 week verify that pretension has held, or retension to specified torque, if necessary. 5.4.2 Post-installed systems 5.4.2.1 General anchor types -Anchors in this group include: Common bolts, reinforcing bars, and continuously threaded rods - Bonded (grout and chemical) anchors - Rock bolts - Expansion anchors 5.4.2.2 Common bolts, reinforcing bars, threaded rod-Section 5.5.2 applies for positioning and drilling the hole; Section 5.6 for grouting. 5.4.2.3 Chemical anchors-These are similar to grouted anchors, with an adhesive, such as epoxy, polyester, or vinylester taking the place of the grout. Section 5.5.2 applies as far as positioning and drilling the hole for the anchor. The adhesives are proprietary and installation should follow manufacturer’s instructions. Drilled hole diameters may vary from 1.0 to 2.0 mm larger than the nominal steel diameter without affecting loading capacity for polyester and vinylester anchoring systems. Storage should follow manufacturer’s recommendations to prevent heat, ultraviolet light, or both from shortening the shelf life of the unused product. Anchoring systems using epoxies are not sensitive to these same storage requirements. 5.4.2.4 Rock bolts - Rock bolts occasionally are used for anchoring to concrete. There are many types available. Section 5.5.2 applies as far as positioning and drilling the lead hole. In the case of the split end variety, bondbreaker is applied to part of the shank and then the rock bolt is then inserted in the hole with the wedge lightly set in the split tail of the bolt. The nut is in place on the bolt, flush with the end. The bolt is then rammed down over the wedge until the bolt is well set in the hole. It is then adjusted for vertical alignment and grouted per Section 5.6. 5.4.2.5 Expansion anchors-These systems include a myriad of devices. They are self-drilling, Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

or set in predrilled holes. The wedging action between the device and the sides of the hole is actuated by placing tension on the bolt, by turning the bolt, by hammering the bolt onto a spreader (cone or wedge) in the bottom of the hole, or by hammering a spreader into the bottom expanding portion of the anchor. The manufacturer’s instructions for installation of expansion anchors must be followed meticulously. This applies particularly to the diameter and depth of hole. Some systems afford the opportunity of using the base plate or element being connected as a template in drilling the embedment hole. Others require a larger hole to accommodate a sleeve that bears against the bottom of the connected base plate. Expansion anchors can lose preload under a cyclic loading or from concrete creep due to high local expansion forces unless they are so pretensioned that the bolt is always in tension under all loading conditions. Generally, to develop the pretension load the wedge or expansion device must first be “set” against the side of the hole. With certain types of anchors there may be an initial slip which should be anticipated and designed for. In the case of excessive slip, follow the recommendations in Section 5.7.2. 5.5 Inspection 5.5.1 Cast-in-place systems -The inspector has

the responsibility to verify that the size and location of anchors or anchorage assemblies are in accordance with the construction plans and specifications, prior to the placement of concrete. Anchors must be located properly in plan, have the proper projection, and be rigidly held in place so as not to be disturbed during the placement and finishing of the concrete. Methods of securing the anchorage in place include: - Nailing to the forms (conditions applicable) - Nailing the top template to the forms - Wire tying individual bolts, or their bottom template, to the forms or the reinforcement and - Tack welding to the reinforcement, if approved. (High strength bolts should not be welded) Welding should be to the bottom washer or the bottom template of the bolt head, rather than the shank of the bolt. In the case of bolts that are subsequently to be tensioned, the inspector should verify that


unsleeved bolts, or sleeved bolts that are to be grouted prior to the tensioning, have a bond breaker (grease or other) on the shank that will prevent the bolt from bonding to the concrete or grout. 5.5.2 Post-installed systems -Post-installed systems involve setting the anchor in blockouts or drilled holes. The inspector should verify that the blockouts or holes are properly located. With drilled holes he should verify that the drill bit is of the proper diameter, that the hole is plumb to the surface (bit guides should be used for critical work), that the finished hole has the proper diameter and depth, and that the appropriate drilling equipment is used. This calls for rotary drills (carbide tip or diamond studded bits) or hand hammered star drill bits. Jackhammering should not be permitted because of the damage it does to the concrete immediately around the hole. Once the hole is drilled and blown clean, the anchor should be installed, preloaded, and tested (as required) in accordance with Section 5.5; or the hole should be protected by plugging it with a rag or other suitable stuffing until the time of anchor installation. Guidance for inspecting grouted anchors is given in Section 5.6. 5.6 -Grouting

ACI Committee 351, Foundations for Equipment and Machinery, includes in its work development of information on grouting. Accordingly, reference is made to publications of that committee. The statements which follow are intended to be a brief summary of grouting as it relates to construction considerations for concrete anchorages. 5.6.1 Materials - Grouting materials fall into two broad functional categories: nonprecision grouts and precision, “nonshrinking” grouts. 5.6.1.1 Nonprecision grouts - Nonprecision grouts include mixtures of cement and water, with or without the inclusion of sand or admixtures. The use of the “jobsite mixed” or packaged products not designed to perform as a precision The most significant grout has limitations. limitation is the lack of a mechanism for overcoming drying shrinkage which occurs as free moisture leaves the grout. Dry packing with cement, sand, and only enough water to result in a stiff, but cohesive mixture has been used in grouting for many years Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

355.1R-65

and is an excellent method, but it is labor intensive, and in many installations is impractical. Epoxy grouts also have been used successfully for a number of years. These materials offer high, early strength and provide excellent bond and protection of steel in corrosive environments. There are, however, some limitations in the use of these materials. The concrete and steel surfaces to be in contact with epoxy must be cleaned and, for most epoxies, dry. Epoxies also have a coefficient of thermal expansion several times that of the concrete or steel, which should be taken into consideration. Epoxies can creep under sustained loading of the anchor, and some epoxy grouts lose strength when exposed to temperatures over 120 F. 5.6.1.2 Precision, "Nonshrinking" grout-These portland cement based products are proprietary and sophisticated in terms of their cement chemistry and composition. They comply with the requirements of the U.S. Army Corps of Engineers specifications for nonshrink grouts, CRD-C621. Precision grouts are proportioned to lessen the effects of plastic and drying shrinkage in the plastic and hardened states. Accordingly they are excellent materials to use in complex grouting situations, such as the grouting of machinery bases. 5.6.2 Applications - Grouting of anchorages to concrete falls into three application categories: - Grouting of anchor bolt holes and sleeves prior to base plate installation - Grouting or dry-packing of base plates and machinery bases - Grouting bolt holes after pretensioning of the anchor bolt 5.6.3 Construction procedures 5.6.3.1 Preparation - Anchor bolt holes and sleeves should be clean and free of oil, grease, dirt, or other debris. Bolt holes should preferably have a textured surface, thoroughly moistened prior to grouting, but with no free moisture in the hole. 5.6.3.2 Mixing and placing-Grouts may be mixed in mortar mixers or in smaller vessels, as is appropriate to the work. When using proprietary products, follow the manufacturer’s instructions for mixing. The “pot” life is a very important consideration. Proper placement of grout is important. Whether dry packed or poured at a fluid


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consistency, the material should be placed or poured in a manner which will preclude the entrapment of air which produces voids in the hardened grout. 5.6.3.3 Curing-Curing is important in achieving satisfactory results in any grout installation. Normally this is accomplished by placing water-saturated rags over all exposed grout surfaces as soon as possible after grout placement. These rags should be maintained wet and in place for at least 24 hr after which the exposed surface of the grout is coated with a curing compound if secondary grouting will not follow. Where secondary grouting is to follow, continue the water curing for 7 days, or until placement of the second grout. Proprietary grouts should be cured according to the manufacturer’s recommendations. 5.7 -Field problems 5.7.1 Cast-in-Place Systems-The common

problem encountered in the preconcreting stage is interference with existing reinforcement. In this case a decision has to be made whether to move the anchorage or move the reinforcement. In weighing the consequences of each, the Field Engineer, perhaps after consulting the Engineerof-Record, establishes which has priority. Another common problem is to discover, after the concrete has hardened, that the anchorage has shifted during the placement of the concrete, and that the base plate will not fit in place, or that there is insufficient thread projecting to fully engage the nut. These problems can and should be avoided by proper inspection, or by use of sleeved or adjustable anchors. The specifications should cover these possibilities, and state that it is the contractor’s responsibility to take necessary precautions and corrective measures. Actions taken when field errors are discovered should have the approval of the Engineer-of-Record. Bending of protruding bolts is discouraged because the bending stress which results from the eccentricity of the service load, when added to the design axial and shear stresses, can often exceed the yield strength of the bolt. In welding to compensate for insufficient thread being engaged by the nut, care should be taken that the weld acting alone will develop the strength of the bolt, because the capacity of the welds and the engaged threads are not additive. When any embedded anchor is not installed within allowable tolerances, the structural adequacy of the installation should Copyright American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS

be verified by the Engineer-of-Record and, if necessary, the design should be modified. The single most helpful practice for avoiding the problem of cast-in-place anchor bolts not fitting the base plates is to make holes in column and machinery base plates oversize, and then grout the annular space after the base plate is in place, or use specially designed washers. The following schedule of oversize holes is recommended. - Bolts less than 1 in. diameter - 5/16 in. oversize - Bolts 1 to 2 in. diameter - l/2 in. oversize - Bolts over 2 in. diameter - 1 in. oversize 5.7.2 Post-installed systems-A common field problem in post-installed systems is interference with the in-place reinforcement. The location of that reinforcement can be determined magnetically or radiographically. Sometimes, it is simply discovered when the drill bit, drilling the hole, hits steel. When an anchorage interferes with any inplace reinforcement, the Engineer-of-Record should decide on the remedy. Wherever possible, the anchorage itself should be shifted to a new location where there is no interference. Moment reinforcement should never be welded or cut. With due consideration, temperature reinforcement can be cut. A second problem is excessive slip in pretensioning the bolt. This can be indicative of an oversized hole or a faulty anchoring device. When excessive slip occurs, the assembly should be reinstalled in the hole and the pretensioning applied such that the slip does not exceed the allowable limit (i.e., resulting embedment is adequate). Sometimes the entire anchor will have to be replaced, or possibly the hole drilled to a larger size and the next larger sized anchor installed. --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

355.1R-66

CHAPTER 6-REQUIREMENTS IN EXISTING CODES AND SPECIFICATIONS 6.1 -Introduction

Sources of information relating to codes and specifications on anchorage to concrete are presented in this section. Sources are referenced in alphabetical order. American and international documents are included in this state-of-the-art review.



6.2 -Existing codes and specifications 6.2.1 American Association of State Highway Transportation Officials (AASHTO) 6.2.1.1 Standard Specification for Highway Bridges -For composite bridge decks, AASHTO

uses the ultimate capacity of stud shear connectors and a reduction factor t$ of 0.85 for design. Design checks are required for horizontal shear under working loads. Working loads are compared to allowable loads which include a reduction for fatigue. AASHTO Section 1.7.56 bases the number, required embedment, and size of anchor bolt on the span of the bridge, and requires that the anchor bolt be swedged or threaded to insure a satisfactory grip on material such as the grout. AASHTO requires that anchor bolts subject to tension be designed to engage a mass of concrete which will provide a resistance equal to one and one-half times the calculated uplift. 6.2.2 American Concrete Institute (ACI) 6.2.2.1 ACI 318, Building Code Requirements for Reinforced Concrete - ACI 318-63 contained

allowable bond values for plain (smooth) bars. Many engineers have used these values for determining embedment requirements for cast-inplace anchor bolts. The current edition of ACI 318 does not give allowable bond values for plain or deformed bars. Section 12.6.1 states “Any mechanical device capable of developing the strength of reinforcement without damage to concrete may be used as anchorage.” Section 15.8.3.3 of ACI 318 states “Anchor bolts and mechanical connectors shall be designed to reach their design strength prior to anchorage failure or failure of surrounding concrete.” 6.2.2.2 ACI 349, Code Requirements for Nuclear Safety Structures-Appendix

Related

Concrete

B of ACI 349 gives comprehensive procedures for designing anchorages and steel embedments that are used to transmit loads from attachments to reinforced concrete structures governed by ACI 349. The basic philosophy of anchorage requirements in ACI 349 is consistent with the ultimate strength design philosophy of reinforced concrete. The failure mechanism is controlled by requiring yielding of the steel anchor prior to brittle failure of the concrete. This design method considers not only traditional design parameters, i.e., steel strength, concrete strength, and anchor size, but also other

variables such as anchor type or form, spacing, edge distance, nature of the anchor load, thickness of the concrete member, and concrete stress in the anchor zone. Concrete strength is critical to assure that the reinforced concrete structure exhibits ductile failure, which is also an ACI 318 requirement. Note, however, that many of the post-installed systems feature the brittle concretecone failure. The commentary of ACI 349, Appendix B, provides an excellent source of information on types of anchorage devices, design requirements, modes of failure, and testing. 6.2.3 American Institute of Steel Construction (AISC) 6.2.3.1 Manual of Steel Construction -The AISC “Specification for the Design, Fabrication,

and Erection of Structural Steel for Buildings” sets allowable bolt stresses in Sections 1.5.2 and 1.6.3. These values apply to certain cast-in-place and grouted anchor bolts and are valid for allowable anchor steel stresses, but no values are given which relate to the transfer of these stresses to the surrounding concrete. The AISC specification gives allowable values in shear for stud shear connectors used for composite design in Table 1.11-4. The listed values cannot be used for anchor bolts of the same size. The values used in Table 1.11-4 are based on equations derived from a testing program and the ultimate strength of the composite member, using a factor of safety of 2.0. The AISC code commentary contains the following warning: “The values of q in Table 1.11-4 must not be confused with shear connection values suitable for use when the required number is measured by the parameter VQ/I, where V is the total shear at any given cross-section. Such a misuse could result in providing less than half the number required by Formulas 1.11-3, 1.11-4, or 1.11-5.” The AISC specification also gives setting tolerances for bolts used to anchor structural members; however, these tolerances are unsuitable for anchoring machinery. 6.2.4 American Society for Testing and Materials (ASTM) 6.2.4.1 Annual Book of Standards - Volume

04.07 contains test standard ASTM E 488, “Standard Test Methods for Strength of Anchors in Concrete and Masonry Elements.” This test standard describes procedures for determining the

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355.1R-67


355.1R-68


static, dynamic, and fatigue tensile and shear strengths of cast-in-place, chemical, grouted, and expansion anchors. Volume 15.08, Fasteners, contains various ASTM specifications for the steel used for bolts, including A 193, A 307, A 325, A 449, and A 490. 6.2.5 Construction Industry Research and Information Association (CIRA) (Great Britain). 6.2.5.1 Section and Use of Fixings in Concrete and Masonry (Guide 4) - CIRA Guide 4, is a

comprehensive guide on the selection and use of anchors installed in concrete. Three main categories of anchor types are covered. These include cast-in-place, expansion, and bonded anchors. The guide also covers behavior of fastener assemblies under load, design considerations, limitations, durability, testing, and practical considerations. 6.2.6 Institut fir Bautechnik (IfBT)(West Germany) 6.2.6.1 Tests to Evaluate the Strength of Metallic Expansion Bolts for Anchorage in Concrete with an SC of 20 MPa (2500 psi) or

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Greater-Approvals are based on results of tests carried out by licensed universities. In the tests the proper functioning of the anchors under extreme conditions are checked, and tests to evaluate allowable loads for design are performed. For evaluating allowable conditions of use (e.g., allowable loads, required edge distance, and spacing), a sufficient number of tests have to be performed to calculate a statistically reliable confidence level for the failure loads [5 percent fractile (or 95 percentile) of failure loads]. A safety factor of 3 is applied to the determined 5 percent fractile of the failure loads to account for the variations of the concrete tensile strength and of jobsite installation quality. For reasons of simplicity, one value for the allowable load is given per anchor size which is valid for all loading directions (tension, shear, combined tension, and shear). Expected displacements of anchors under allowable loads are given which should be taken into account in the design of the fastened element (when appropriate). 6.2.7 International Conference of Building Officials (ICBO) 6.2.7.1 U n i f o r m B u i l d i n g C o d e ( 1 9 8 5 Edition) -The Uniform Building Code (UBC),

Table 26-G sets forth allowable shear and tension loads for cast-in-place bolts of at least ASTM A


307 quality or better. The table assumes an anchor spacing of 12 anchor diameters. The spacing may be reduced down to 6 anchor diameters with a 50 percent reduction in allowable load values. A minimum edge distance of 6 anchor diameters is required. Edge distance may also be reduced up to 50 percent, provided that the listed values are reduced in equal proportion. Tension values listed in the table may be increased 100 percent when “special inspection” is provided. UBC Section 2719, on anchor bolts for steel column bases, does not provide design values for anchor bolts, but simply states that “Anchor bolts shall be designed to provide resistance to all conditions of tension and shear at the bases of columns.” The section on steel column anchorage does not refer to Table No. 26-G. Application of this table to steel column anchorage would greatly affect current design practice because of the requirement in Table No. 26-G of a minimum spacing of 6 anchor diameters. 6.2.8 Precast/Prestressed Concrete Institute (PCI) 6.2.8.1 PCI Design Handbook-The handbook gives equations for shear and tension load allowables for headed shear stud anchors. Combined loading, as well as required edge distances and anchor spacing for groups of anchors, are covered. Based on a review of past design methods and actual testing and modeling, the PCI Connection Details Committee recommends the use of a projected cone model to define the actual bolt tension at which concrete failure will occur. The PCI cone surface equation is: Pm = 2.8 hL@ [fi 7~ ld (li + da] (6.1) where iz = 1 = = ;= d,, = f’, = P = IlC

1.0 for normal weight concrete 0.85 for sand lightweight concrete 0.75 for all lightweight concrete embedment, in. diameter of anchor or stud head, in. specified 28-day compressive strength of concrete, psi nominal tensile capacity of anchor as governed by concrete failure

In anchor bolt design where the concrete does not fail, the anchor bolt fails via a combination of The PCI equation for tension and shear.



combined tension and shear strength is:

where 4 = strength reduction factor Pu = applied factored tension load P = nominal tension strength of anchor vu = applied factored shear load Kc = nominal shear strength of anchor as governed by steel failure In-depth discussions of these equations may be found in Klingner and Mendonca (1982) and Shaikh and Yi (1985). 6.2.9 The Agrbnent Board (Great Britain) YlC

6.2.9.1 The Assessment of Torque-Expanded Anchor Bolts When Used in Dense Aggregate Concrete (M.O.A. T. No. 19:1981) -This document --`,``,````,,,`,``,``,,,,,```,`-`-`,,`,,`,`,,`---

presents the procedures for deriving design information and classifies ten different types of expansion anchors according to the mechanism for achieving expansion. It considers the effects of different types of loading conditions and typically requires a minimum of 277 tests (for six different anchor diameters) to calculate safe working loads as the lower of: a. The 5 percent exclusion value (or 95th percentile, calculated by regression analysis or other statistical techniques), then divided by three or, b. The mean of the loads determined at a displacement of 0.1 mm (0.004 in.) under direct tension or, c. The mean of the loads determined at a displacement of 1.0 mm (0.039 in.) under direct shear. 6.2.10 UEAtc (Union European of Agrbment) The UEAtc Directives for the Assessment of Anchor Bolts (December, 1986) is a European code for the assessment and approval of anchor bolts. The document has been adopted by the Common Market Countries of Germany, U.K., France, Austria, Italy, Spain, Ireland, Netherlands, Portugal, Denmark, and Belgium. 6.2.11 Nuclear Regulatory Commission (NRC) Bulletin 79-02 and 79-14).

Anchor bolt design methods have been revised based on the United States NRC Office of Inspection and Enforcement Bulletins No. 79-02


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and 79-14. Only Class I piping (piping used to safely shut down a nuclear power plant) was impacted by Bulletins 79-02 and 79-14. The NRC requires that during anchor bolt design, the following must be considered: baseplate flexibility, (i.e., baseplate prying action that increases anchor bolt loading), performance of anchors due to cyclic loading, anchor performance in masonry walls, the effect of pipe support loads on masonry walls, and the maximum support load considered for anchor bolt design. Concrete expansion anchors must have the following minimum factor of safety between the bolt design load and the bolt ultimate capacity determined from static load tests, (e.g., published data from the anchor bolt manufacturer) which simulate the installation conditions, (i.e., type of concrete and its strength properties): (1) a safety factor of 4:1 - for wedgeand sleeve-type anchor bolts, (2) a safety factor of 5:l - for shell-type anchor bolts. The bolt ultimate capacity should account for the effects of shear and tension interaction, minimum edge distance, and proper bolt spacing. A summary of the USNRC criteria is found in USNRC “Anchor Bolt Study Data Survey and Dynamic Testing” by the Hanford Engineering Development Laboratory. 6.2.12 Draft 1 Regulatory Guide MS 129-4 “Anchoring Component and Structural Supports in Concrete”

This draft guide from the U.S. Nuclear Regulatory Commission provides the criteria for acceptance, qualification, design, installation, and inspection for steel embedments anchored in concrete. It also provides information on the acceptability for NRC licensing actions in accordance with Appendix B, of ACI 349-80. 6.3 -Application and development of codes

ASTM E 488 is the only existing American standard exclusively and specifically concerned with testing to determine the performance of all types of concrete anchors. It is not intended to describe design procedures for anchorage connections, nor to identify characteristics which affect performance in conditions other than astested. ICBO has also published a limited test standard for expansion anchors only. ACI 349, Appendix B, specifies anchorage design and applies ultimate strength design philosophy to all types of anchorages. Other American codes limit their consideration to cast-


355.1R-70


in-place or grouted anchorages. The Uniform Building Code (UBC) allows for alternative devices as specified in the code, generally applying the same conditions as specified for cast-in-place anchors. American codes generally base recommended design procedures on ultimate strength data. European codes recommend the criterion of displacement (slip) for post-concreting anchors, supported by ultimate strength data derived by regression analysis of other statistically reliable techniques. Codes cannot address all the conditions applicable to a particular design or absolve the designer of the responsibility to check the relevance of code data for a given design. New and technically reliable information will inevitably be developed between publication dates of amendments to existing codes. Designers are encouraged to maintain familiarity with ongoing research and other developments and to supplement the provisions of governing codes with such information as it becomes available.

“Prtifungen zur Beurteilung d e r Tragfghigkeit v o n zwangsweise s p r e i z e n d e n Diibeln aus MetaIl nach d e r Verankerung in Normalbeton 1 Bn 250” (Tests to Evaluate the Load Capacity of Metal Expansion Anchors Fastened into Normal Concrete, r Bn250), Institute for Construction (IfBT), Berlin, West Germany, January 1974.

Shaikh, A.P., Yi, W., “In-Place Strength of Welded Headed Studs,” PCI Journal, V.30, No. 2, March-April, 1985. “Standard Specification for Highway Bridges”, Twelfth Edition, American Association of State Highway Transportation Officials, 1977. “Standard Test Methods for Strength of Anchors in Concrete and Masonry Elements”, (ASTM E488-88), 1988 Annual Book of ASTM Standards, Volume 04.07, American Society for Testing and Materials, Philadelphia, PA, October, 1988. “The Assessment of Torque-Expanded Anchor Bolts when used in Dense Aggregate Concrete”, M.O.A.T. No. 19:1981, Agrkment Board, Watford, Herts., England, January, 1981. “UEAtc directives for the Assessment of Anchor Bolts”, M.O.A.T. No. 42:1986, European Union of AgrCment; December, 1986. Uniform Building Code, International Conference of Building Officials, Whittier, CA. 1985.

6.4 - References ACI Committee 318,1989, “Building Code Requirements for Reinforced Concrete (ACI 318-89) and Commentary - ACI 318R89, American Concrete Institute, Detroit, MI, November.

USNRC “Anchor Bolt Study Data Survey and Dynamic Testing”, Hanford Engineering Development Laboratory, NUREG/CR-2999, December, 1982.

ACI Committee 349, 1990, “Code Requirements for Nuclear Safety Related Concrete Structures (ACI 349-90) and Commentary - ACI 349R-90, American Concrete Institute, Detroit, MI, March. Fasteners. 1988 Annual Book of Standards, Volume 15.08, American Society for Testing and Materials, Philadelphia, PA, January, 1988. Klingner, R.E. and Mendonca, J.A., (1982a) “Tensile Capacity of Short anchor Bolts and Welded Studs: A Literature Review,” ACI Journal, Proceedings, V. 79, No. 1, July-August. Manual of Steel Construction. Eight Edition, American Institute of Steel Construction, Inc., New York, NY, 1980. Paterson, W.S., “Selection and Use of Fixings in Concrete and Masonry”, CIRA Guide 4, Construction Industry Research and Information Association, London, England, October, 1977.

PCI Design Handbook, Third Edition, Prestressed Concrete Institute, Chicago, IL, 1980.

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APPENDIX A-CONVERSION FACTORS: INCH-POUND TO SI By

Multiply

To obtain

Length

ft

3.048 x 10-l

Area

ft2

9.290 x 10-2

m2

Volume

ft3

2.832 x 1O-2

m3

Velocity

ft/s

3.048 x 10-l

m/s

Acceleration

ft/s2

3.048 x 10-l

m/s2

Mass

“An

4.536 x 10-l

kg

Force and Weight

lb f

4.448

N

Pressure and Stress

lb$ft2 psi psi

4.788 x 101 6.895 x lo3 6.895 x 1O-3

Pa or N/m2 Pa or N/m2 N/mm2

Work and Energy

ft-lbf

1.356

J

Mass Density

lb,/ft3

1,602 x 10

kg/m3

Weight Density

lbf/ft3

1.571 x lo2

N/m3

n = number of anchors N = factor which takes into account steel shear strength, usually 0.6 to 0.7 pnc = bolt tension load at which concrete failure will occur P, = applied factored tension load S = spreading force, as from expansion sleeves of an expansion anchor, also anchor spacing T, = applied tension load T,,, = allowable anchor tensile load T,, = ultimate wedge-splitting capacity of a singlt bolt r, = ultimate tensile load, also design tensile load V, = applied shear load V, = nominal shear strength of anchor as governed by steel failure = shear strength, ultimate shear load, or applied factored Yl shear load W = crack width usually measured at the concrete surface (Y = included angle of concrete spa11 cone measured from the axis of the anchor to the failure cone surface p = coefficient of friction 4 = strength reduction factor x = chi factor which represents a partial influencing factor such as a load capacity reduction based on anchor spacing interaction (x,), edge distance influence (x,), etc.

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This report was submitted to letter ballot of the committee was approved according to Institute procedures.

APPENDIX B-NOTATION = distance between center of anchors = summation of projected areas of individual stress cones, in.2 = net bearing area of head of embedded anchorage, in.’ Ab A, = tensile stress area, psi C’ = clear cover to bolt, in. d/I = head diameter of headed stud or bolt D = anchor diameter DUJ = diameter of anchorage device such as embedded washer, in. E, = elastic modulus of concrete, psi f, = compressive strength of concrete measured by cylinders psi or N/mm2 fee = compressive slrength of concrete measured by cubes, psi or N/mm2 = yield stress of anchor or bolt, psi fY F,, = ultimate strength or capacity, lb or N F,, = ultimate tensile stress of steel, psi h = member thickness 1, = embedment depth of anchor = distance from anchor centerline to free unsupported edge 1?1 0

A


355.1R-71


ACI 355.1R.pdf

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