Wood Connections

American Forest & Paper Association / American Wood Council APA - The Engineered Wood Association Wood Truss Council of America Canadian Wood Council

DES110: The Wood Connection Session

A F & P A®

®

Copyright © 2001, 2007 American Forest & Paper Association Inc., APA - The Engineered Wood Association, Inc., Wood Truss Council of America Inc., Canadian Wood Council, Inc. All rights reserved.

For many building designers, designing sound wood connections can be a daunting task. Yet connections are the most critical item to tend to in any structure and require a good understanding if one is to develop sound, aesthetic, cost-competitive solutions. It should be emphasized that slides and information provided in this program are proprietary in nature and that if, for instance, someone is using an I-joist manufactured by Manufacturer A, they can not use the literature for installation or for sizing that material as published by Manufacturer B.

Copyright © 2001, 2007 American Forest & Paper Association Inc., APA - The Engineered Wood Association, Inc., Wood Truss Council of America Inc., Canadian Wood Council, Inc. All rights reserved. For permission to reprint contact AF&PA at 1-800 AWC-AFPA.

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Copyright of Materials This presentation is protected by US and International copyright laws. Reproduction, distribution, display and use of the presentation without written permission of the American Forest & Paper Association / American Wood Council, is prohibited. Copyright © 2001, 2007 American Forest & Paper Association Inc., APA - The Engineered Wood Association, Inc., Wood Truss Council of America Inc., Canadian Wood Council, Inc. All rights reserved.

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DES110: Learning Outcomes •

By the end of this eCourse, you will be: 1. Familiar with current wood connection design philosophy, behavior, and serviceability issues 2. Familiar with design techniques for small and large wood members, panel products, and wood assemblies 3. Familiar with dowel-type and specialized components, and adhesives 4. Briefly introduced to connection design software solutions

This eCourse presents current wood connection design philosophy, behavior, serviceability issues, and connection design techniques for small and large wood members, panel products, and wood assemblies, using dowel-type and specialized components. Glued connections will also be discussed along with a brief introduction to connection design software.

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Outline • • • • • • • •

wood connection design philosophy connection behavior serviceability issues connection hardware and fastening systems glues and adhesive-based connections connection techniques design software where to get more information

This seminar presents current wood connection design philosophy, behavior, serviceability issues, and connection design techniques for small and large members, panel products, and wood assemblies, using dowel-type and specialized components. Glued connections will also be discussed along with a brief introduction to connection design software. Let’s begin with a few basic ideas on wood-connection behavior.

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Outline • • • • • • • •


This seminar presents current wood connection design philosophy, behavior, serviceability issues, and connection design techniques for small and large members, panel products, and wood assemblies, using dowel-type and specialized components. Glued connections will also be discussed along with a brief introduction to connection design software. Let’s begin with a few basic ideas on wood-connection behavior.

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Connecting Wood • wood likes compression parallel to grain – makes connecting wood very easy

The first fact is that wood likes load applied as compression parallel to the grain. This is the strongest mode of wood. Structural designs that capitalize on this idea are very economical, attractive, consistent with wood’s heritage the tree in the forest. Moreover, compression connections in wood are very easy to design and detail.

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Connecting Wood • wood and compression perpendicular to grain – compare wood cells to a bundle of straws – bundle crushes under perpendicular load

Here’s a simple illustration of this. Let’s model the cellular nature of wood with a group of straws. When compression is applied, the straw bundle is strong and connecting the ends is very simple. Applying tension also develops considerable tensile strength in the straw bundle, but hanging onto the ends becomes more of a challenge in designing a suitable connection. If load is applied perpendicular to the longitudinal axis of the straws, the straws crush because of the much weaker radial alignment orientation of the cellular walls. This illustrates the anisotropic nature of wood - different strength properties in three different directions: longitudinal (strong), tangential (weaker), and radial (weakest).

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Connecting Wood • the Hankinson Formula – used to resolve wood bearing strength at any angle to grain θ

Zθ =

Z Q

PQ P sin 2 θ + Q cos 2 θ

P

Many connections rely on the bearing resistance properties of the wood for strength. As we have seen, wood has different strength properties parallel and perpendicular to the grain. The shape of the shaded ellipse in the sketch relates to the strength magnitude in the wood as a result of an applied force. The wood resistance Z at any angle to the grain θ can be computed using the Hankinson Formula shown here, where P is the wood bearing strength in compression parallel to the grain, and Q is the compliment strength perpendicular to the grain.

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Connecting Wood • wood bearing strength – – – – –

sawn wood glulam OSB plywood structural composite (SCL)

Wood products have different strength values and relative perpendicular to parallel bearing strength ratios. This partial list is ordered from basic to highest strength in bearing. This may become important if a design is connection constrained - switching to a product with higher wood bearing strength may yield a more satisfactory solution.

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Connecting Wood • wood likes to take on load spread over its surface

The second idea about wood connections is that wood likes to see load spread out. Concentrated load should be avoided as it could easily exceed the bearing capabilities of the wood. Spreading the load out also builds in a degree of redundancy.

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Connecting Wood • but…wait a minute...

Here’s an interesting example of a connection found in the Library of the Forintek Canada Corp Laboratory, Vancouver, BC. The column pairs are made of 8”x18” x 60 ft PSL and feature a fan of PSL members loaded in compression. The fan meets at one bolt, or so it seems...

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Connecting Wood • looks can be deceiving...

What may seem like a violation of the second idea, is actually resolved with a clever combination of hidden steel plates and timber rivets which serve to spread out the load transferred through the very large bolt. The plates and rivets cannot be noticed by the observer below.

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Connecting Wood • wood and tension perpendicular to grain – the evil of wood connections initiators: • notches • large diameter fasteners • hanging loads

The third idea is wood’s weakest link: tension perpendicular to the grain. Tension-perp often leads to sudden catastrophic failures and should be avoided at all costs. Awareness of how the wood is being loaded is all that is needed to avoid this issue. Large diameter connectors can also initiate this weak strength mode.

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Notching of Sawn Lumber Not recommended OUTER THIRD OF SPAN ONLY

– outer third of span only – avoid tension edge

DEPTH, MAX. 1/4 JOIST

1/3 JOIST DEPTH, MAX.

1/6 JOIST DEPTH, MAX.

Another tension-perp initiator are notches because of the stress raisers at the internal corners. Notches can be problematic if not addressed properly. And, it is important to understand that there are different notching recommendations depending on the material, i.e. sawn lumber, glulam and LVL.

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Notching Problem

Solution

Here’s a case in point. Notching as shown can lead to a combination of tension perpendicular to grain and horizontal shear stresses resulting in the horizontal split shown. The better solution is not to notch at all - but, provide full bearing under the end of the section.

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Tension Perpendicular to Grain

These are no-no’s! Loading wood members perpendicular to the grain without sufficient “compression” wood may also initiate splits to form in the red-shaded zones shown here. There are better alternatives to these configurations.

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Hanger to Beam Lower half of beam – may cause splits – not recommended

Split

Here’s a problem example...

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Hanger to Beam Upper half of beam Full wrap sling option

– extended plates puts wood in compression when loaded

compression

…and here’s a solution. Sufficient “compression” wood exists to develop the full bearing resistance to the hanging load. A better solution would be to wrap a sling over the top of the beam placing the top of the beam in compression, rather than some interior portion of it.

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Beam to Concrete Notched Beam Bearing – may cause splitting – not recommended

Split

…another common problem...

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Beam to Concrete Notched Bearing Wall – alternate to beam notch

…with a better solution to avoid splits from stress raisers.

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Beam to Concrete Sloped Beam – – – –

not fully supported may split exposes end grain not recommended

Split

Sloped beams at partial end bearing often initiate end splitting.

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Beam to Concrete Sloped Beam – notched concrete wall – alternate to beam notch

Providing full end bearing will prevent splits from developing.

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Connecting Wood • wood, like other hygroscopic materials, moves in varying environments

A fourth idea is that wood moves in response to varying environmental conditions just like other building materials. The main driver for wood is moisture. Allowances must be made to accommodate this movement, particularly in connections. More later….

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Connecting Wood • fastener selection is key to connection ductility, strength, performance

The fifth idea is in selecting connectors properly to do the job. Connecting wood products can be done directly with fastening systems, or in combination with stock or custom-made hardware. Connections typically test the designer’s skill to arrive at safe aesthetic solution. Choice of connector/system is critical to the connection’s ductility, strength, and performance in service.

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Connecting Wood • mechanical fasteners – keep ‘em small – use lots of them issue is scale of fastener relative to wood member size

A key point in connector choice is scale relative to the wood product being connected. Remember that wood likes to see load spread out; so, lots of fasteners is a good idea. Often, this will automatically impose that the fastener be small. More on this, later...

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Connecting Wood Quick point summary • • • •

wood likes compression parallel to grain wood likes to take on load spread over its surface wood and tension perpendicular to grain is a no-no wood, like other materials, moves in varying environments • fastener selection is key to connection ductility, strength, performance – keep ‘em small; use lots of them

Here’s a quick summary of main ideas in connection design to keep in the back of your mind. This list will often lead you to designing a safe appealing solution.

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Next... • • • • • • • •


This topic can get incredibly complex and protracted because of the shear variety of wood connectors and techniques in the market. Some of these we’ll cover later, but for now we’ll just look at important basics.

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Connection Behavior • strength • ductility

Load

high strength, poor ductility

good strength, good ductility

– desirable to have good balance of strength and ductility for overall connection

low strength, good ductility

Displacement Strength and ductility…all you need for good solid connections. Strength behavior is understood for many connections, but ductility is more subtle and sometimes difficult to assess. Good ductility assures warning and structural resiliency to lateral loads such as seismic or wind.

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Next... • • • • • • • •


Incumbent with good connection structural behavior is performance in service.

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Connection Serviceability • temperature • humidity and moisture – ambient conditions – contact with cementitious materials

In-service performance implies some structure : ambient environment interaction. Two major drivers to consider are temperature and humidity / moisture.

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Day-to-day temperature change doesn’t play much of a role to disrupt a connection’s life - however, extreme change in short periods of time can, especially if there is a lot of metal hardware in the connection. Metal and wood have very different thermal expansion coefficients, and this difference can cause some grief if not taken into account for extreme conditions. Moreover, wood and metal respond very differently to moisture gain/loss which can also lead to interesting behavior.

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Connection Serviceability – ambient conditions and wood EMC

Ambient conditions drive the equilibrium moisture content (EMC) of the wood. The map taken from the USDA Wood Handbook shows EMC for wood in various parts of the country. The table also from the same source shows the variation by month of EMC for wood exposed to the outdoors for various locations in the country. From the table, more humid climates display less EWC variation than dry climates (see Montana for example). Changes in EMC translate into issues than must be accommodated, such as dimensional change.

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Connection Serviceability – wood EMC at installation is important

To assure connection stability, it is important to fasten the materials during construction at the EMC they will have in service. This table gives some guidance on the typical installation EMC’s to target for good practice and minimal problems down the road.

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Connection Serviceability moisture effects

What kind of moisture issues are we talking about? Dimensional change. Wood is very sensitive to moisture change, and alters its cross-section dimensions in response to the gain/loss of moisture in the wood cell walls. Cross-section shrinkage can loosen bolted connections, or increase instability in hangers. Swelling can deform connection hardware and cause other geometric problems. This is why small ambient variation in humidity is desirable - otherwise it needs to be accommodated.

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Connection Serviceability Issue: direct water ingress • water is absorbed most quickly through wood end grain

No end caps or flashing

Great looking cantilevers? Wood takes up moisture fastest through the end grain, and also dries out the quickest there. It doesn’t take many cycles of this to create the end splits that are evident in these glulams. And these are located in Palm Springs, CA, a relatively dry environment. What could you have done?

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Connection Serviceability Issue: direct water ingress • re-direct the water flow around the connection

end caps and flashing

Protect the ends - through cap flashings, sill flashings, to direct water around the connection, away from the ends of the wood members. And, be sure to provide an air space between the flashing/end caps and the wood members.

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Connection Serviceability Issue: direct water ingress • or, let water out if it gets in...

Moisture trap No weep vents

If water does get in - let it out - and let the end section breathe. Trapped moisture quickly leads to fungal growth and decay under the right conditions. Include weeps and vents in shoe-type connection hardware to facilitate the venting. This example is in Tucson, AZ, another relatively dry environment.

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1997 and 2001 NDS Provisions Wet Service Factor, CM for connection Z values Saturated

19% MC fabrication MC in-service MC

• • • • •

bolts drift pins drift bolts lag screws wood screws

Dry

CM

1.0 1.0

0.7 0.7

0.4 Lateral Load 1.0 Withdrawal Load (lag & wood screws only)

Connection strength also varies with wood EMC, and the NDS has provisions to this effect, the Wet Service Factor CM that affects connection Z values. Two conditions of EMC at fabrication and in-service are important: <19% and >19%. The latter condition includes both continuous or occasional exposure at moisture levels greater than 19%. The designer must assess the environmental situation to see which occurs when. At MC levels above 19%, wood is more elastic, and wood strength properties reduce somewhat. When wood connections are fabricated using wood with high MC’s over 19%, and MC levels are expected to drop to final values below 19% in service, considerable shrinkage takes place around the fasteners, and grouped fasteners are especially vulnerable in initiating tension perp failures; hence the low value of CM = 0.4. A design penalty? Perhaps. But there is a workaround...

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1997 and 2001 NDS Provisions Wet Service Factor, CM for connection Z values CM = 1.0 if:

Saturated

1 fastener

19% MC fabrication MC in-service MC

2+ fasteners Dry

CM

0.4 Lateral Load

!!?

split splice plates

The NDS has a detailing provision for the 0.4 value on bolt and lag screw connections that can provide full fastener capacity (CM = 1.0). Use: - one fastener only, or - two or more fasteners placed in a single row parallel to grain, or - use fasteners placed in two or more rows parallel to grain with separate splice plates for each row. Minimum distances between fasteners, and fasteners and edges still need to be maintained. This detailing allows the wood to change shape across the grain on drying without being hung up on the fasteners - the fasteners can move with the wood.

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Beam to Column Full-depth side plates – may cause splitting – wood shrinkage

Full-depth side plates. It is sometimes easier to fabricate connections for deep beams from large steel plates rather than having to keep track of more pieces. Lack of attention to wood’s dimensional changes as it “breathes” may lead to splits. Full-depth side plates may appear to be a good connection option. Unfortunately, the side plates will remain fastened while the wood shrinks over the first heating season. Since it is restrained by the side plates, the beam may split. THIS IS NOT A RECOMMENDED DETAIL!

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Beam to Column Smaller side plates – transmit force – allow wood movement

As an alternative, smaller plates will transmit forces, but they do not restrain the wood from its natural movements.

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Beam to Column Problem – shrinkage – tension perp

Hanger to side of beam See full-depth side plates discussion. Deep beam hangers that have fasteners installed in the side plates toward the top of the supported beam may promote splits at the fastener group should the wood member shrink and lift from the bottom of the beam hanger because of the support provided by the fastener group. THIS DETAIL IS NOT RECOMMENDED!

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Beam to Wall Solution – bolts near bottom – minimizes effect of shrinkage

Slotted hardware

Alternate to previous detail. Slotted hardware permits dimensional change in the wood without restriction.

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Beam to Beam Split

Beam hangers – fasteners in top of supported beam – wood shrinkage – may split – not recommended Gap under beam

Same problem just discussed….

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Beam to Beam Beam hangers – fasteners in bottom of supported beam – wood shrinkage allowed – prevents lateral movement

…with a solution. Note that the compression edge of the beam is still laterally supported but no bolt has been used at the top.

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Beam to Beam Face mounted – fastener penetration – avoid interference – nails or rivets

Face-mounted hangers are commonly used in beam to beam connections. In a “cross” junction special attention is required to fastener penetration length into the carrying beam (to avoid interference from other side).

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Beam to Beam Weld bracket – bucket -style – dapped support beam

Bucket-style welded bracket at a “cross” junction. The top of the support beam is sometimes dapped to accommodate the thickness of the steel. If this occurs on a beam cantilevered over a support, the top will be in tension and dapping is not recommended.

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Beam to Beam Deep members – – – –

through bolted shear plates clip angles resist rotation allow shrink / swell movement

Deep members may be supported by fairly shallow hangers — in this case, through-bolted with shear plates. Clip angles are used to prevent rotation of the top of the suspended beam. Note that the clip angles are not connected to the suspended beam — doing so would restrain a deep beam from its natural across-the-grain shrinking and swelling cycles and would lead to splits.

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Beam to Beam Concealed – kerf must accommodate steel and weld – dowel hole plugged

Concealed connections. The suspended beam may be dapped on the bottom for a flush connection. The pin may be slightly narrower than the suspended beam, permitting plugging of the holes after the pin is installed. Note that the kerf in the suspended beam must accommodate not only the width of the steel plate, but also the increased width at the fillet welds. The insulating detail symbol here is used to designate a connection that is applicable in Heavy Timber fire-resistive designs. Burying the metal underneath the insulative layer of the wood prevents the metal from overheating into a plastic state during a fire event. See AF&PA’s TR10 for details (free download).

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Another moisture generator is cementious materials. These materials harbor moisture within their material matrix and transfer it to other materials in contact. Wood should always be separated from these types of materials so that it does not wick up moisture that could lead to early decay.

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Beam to Concrete Beam on Shelf – prevent contact with concrete – provide lateral resistance and uplift

Concrete is porous and "wicks" moisture. Good detailing using separators permits wood to be in direct contact with concrete. Beam on shelf in wall. The bearing plate distributes load and keeps the beam from direct contact with the concrete. Steel angles provide uplift resistance and can also provide some lateral resistance. The end of the beam should not be in direct contact with the concrete.

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Beam to Concrete Beam on Wall – bearing plate under beam only – prevent contact with concrete – provide lateral and uplift resistance

Similar to previous detail with steel bearing plate only under the beam.

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Beam to Concrete Beam on Wall – prevent contact with concrete – provide lateral resistance and uplift – slotted to allow longitudinal movement – typical for sloped beam

Similar to previous detail with slotted holes to accommodate slight lateral movement of the beam under load. This detail is more commonly used when the beam is sloped, rather than flat.

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Beam to Masonry Application

Need 1/2” air gap between wood and masonry Same provisions for concrete, must be made for masonry.

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Beam to Masonry Application – bearing plate under beam – prevent contact with masonry by maintaining 1/2” space at end of beam

Here is an example of a good solution for a sloped beam bearing connection.

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Column to Base Problem – no weep holes in closed shoe – moisture entrapped – decay can result

Column to Base Connections Since this is the bottom of the structure, it is conceivable that moisture from some source might run down the column. Experience has shown that base plate details in which a steel “shoe” is present can collect moisture that leads to decay in the column. Bucket-style hardware also causes a lot of grief for wood connections if they are not properly vented or drained to let moisture and water escape.

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Column to Base Embedded bracket – bearing plate

Bracket carries uplift and gravity load while separating the wood from moisture or cementitious materials.

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Column to Base Bearing plate – anchor bolts in bearing plate – slotted column end

An alternate form…column bottom is slotted to allow space for anchor bolt fastenings.

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Column to Base Angle brackets – anchor bolts in brackets

A variant with external anchorage.

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Column to Base Simple steel dowel – bearing plate – shear transfer

Simple steel dowel for shear transfer, with a bearing plate (separator) added.

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Column to Base

Where’s the plate?

…’nuf said. And, by the way, the designer detailed a loose steel plate but the contractor felt a grout would work just as well. Unfortunately, not here.

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Hidden Column Base Floor slab poured over connection – will cause decay – not recommended

Hidden column base It is sometimes preferable architecturally to conceal the connection at the base of the column. In any case it is crucial to detail this connection to minimize decay potential. This detail is similar to other details already seen, but with floor slab poured over the top of the connection. THIS WILL CAUSE DECAY AND IS NOT A RECOMMENDED DETAIL

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Column to Base Floor slab poured below connection

Here’s an alternate that helps keep the wood safe from moisture intrusion.

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Arch Base to Support

For very long spans or other cases such as arches where large rotations must be accommodated, a true hinge connection may be required. And be sure the base connection can drain. These have a closed shoe. Decay occurred as shown in next slides. This project is located in Tuscon, AZ – a very dry climate. No locale is immune.

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Arch Base to Support Hinge – long spans – large rotation – weep slot

If bucket, or shoe -type solutions are proposed, the hardware must permit free breathing and drainage of the wood, ...

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Arch Base to Support Problem – no weep slots – moisture trap – decay

…otherwise you get this: early decay of the wood.

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Arch Base to Support Problem – end grain sitting in puddle – moisture uptake – decay

One might think that this solution works, however allowing the wood to contact standing water is not encouraging to its welfare.

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Arch Base to Support

The better solution is to slope the bearing surface to discourage the formation of standing water near the wood (this design actually improves the flow of reaction forces into the foundation)...

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Arch Base to Support Good connection – avoids tension perp – avoids decay

…and use connecting hardware that breaths / drains. Notice the cap flashing on the beam top edge to discourage rain water from being absorbed into the top of the wood section, and direct water away from the wood. The connection base is totally open, the hinge bolts holes close together. This connection was designed by Tom Williamson.

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Next... • • • • • • • •


What can we use to connect wood members together? Let’s look first at mechanical approaches.

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Mechanical Connectors

Here are some examples of mechanical connections made in variety of ways: some using only wood, some with hardware, some with fasteners.

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Traditional Connectors the all-wood solution • time tested • practical • extreme efficiencies available with CNC machining www.tfguild.org www.timberframe.org

If the all-wood connection option appeals to you, there is guidance available from the Timber Framer’s Guild web site and others. Modern European experience has developed this technique to a very high level using automated CNC milling technology to machine wood joints and pre-drill holes to very high tolerances for connections. Rapid erection with a perfect field fit is assured. This connection area has a huge history and detailing / performance data is still available in books over 100 years old in many public libraries. Many techniques and advances were made during the railway building years (trestles) during the late 1800’s.

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Traditional Connectors Wood dowel connection design technology now available

Schmidt, R.J. (2006): Timber Pegs – Considerations for Mortise and Tenon Joint Design, Structure Magazine, March 2006, NCSEA, 13(3):44-47.

Recently, wood dowel connection design technology was published in the March 2006 issue of Structure Magazine.

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Mechanical Connections Friction or bearing -based • dowel-type fasteners • specialized fasteners • new concealed connectors • hardware • mechanical systems

Mechanical connections are primarily either friction or bearing -based, usually involving some kind of dowel shaped connector with or without intermediary connection hardware.

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Mechanical Connections Nails and nomenclature • short • box nail • ring nail • common nail • sinker • power-driven • roofing The simplest of the dowel-connectors is the nail. Unfortunately, there are many variations of a nail as shown here, with a variety of names, even variations in the way they are installed. Nail capacities are tabulated for only some of them, such as box and common nails since these are standardized to some degree based on shank diameter - the driver of the capacity tables. Other nail types are not standardized so unless covered by an NER, capacities are difficult to establish or do not exist.

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Nail Types and Designations New nail capacity tables in 2001 NDS

Same-designation box, common, and sinker nails are NOT necessarily the same: a 6D common is similar to an 8D box, for example. Shank diameters differ among same-designation nail types. This table is an excerpt from the new 2001 NDS nail capacity tables that shows side by side designation comparisons of common, box and sinker nails based on shank diameter. What is important in nail capacity determination is nail shank diameter as seen in the capacity formulae on which the table is based. APA has similar tables 8.11A and 8.11B in the APA Engineered Wood Handbook. These are really handy tables for a lot of good reasons.

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Fastener Values Included in U.S. design literature Fastener Type

Reference

Bolts

NDS

Lag Screws

NDS

Wood Screws

NDS

Nails & Spikes

NDS or NER

Split Ring Connectors

NDS

Shear Plate Connectors

NDS

Drift Bolts & Drift Pins

NDS

Metal Plate Connectors

NER

Hangers & Framing Anchors

NER

Staples

NER

Design values for connections loaded in single and double shear tabulated in the NDS Chapters 8, 9, 11 and 12 are based on the fastener bending yield strengths, Fyb, given in the footnotes of the respective tables. Other fastener bending yield strengths may be used with the yield mode equations in these Chapters to calculate design values for the connections involved. However, bolts, lag screws and wood screws must conform to the applicable ANSI/ASME Standard referenced for these fasteners in 8.1.1, 9.1.1 and 11.1.1; and nails and spikes must meet the requirements specified in 12.1.2. Bending yield strength of nails and spikes may be determined in accordance with ASTM F1575-95 (see Appendix I of the NDS).

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Fastener Bending Yield Test Center-Point Bending Test Load

Fasteners need to be resistant to static and repetitive bending to be effective in transferring load. Static fastener capacities are determined from a centerpoint bending yield test....

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Yield Limit Equations Fastener Bending Yield Values

Fastener Type

Equation

Bolts

0.5(Fy +Fu)

Common Wire Nails

130,400 - 213,900 D

…that results in the following relationships for bolts and common wire nails.

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Yield Limit Equations 4 Modes 6 equations

Reduction term Rd

Equations have been developed (now part of the 2001 NDS) for four possible yield modes that dowel fasteners can take on.

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Yield Limit Equations Reduction Factors, Rd Fastener Type

I II

III IV

Parallel Perpendicular

Bolts Mode I

4.0

5.0

Mode II

3.6

4.5

Mode IV

3.2

4.0

Mode I

4.0

5.0

Mode II

2.8

3.5

Mode III

3.0

3.75

Lag Screws

The NDS considers six yield limit equations for dowel connectors. Reduction terms, appearing in the denominator of the NDS yield equations, vary by dowel type. To facilitate a general format for the six yield limit equations, reduction terms have been separated from the yield equations and are shown here for bolts and lag screws loaded parallel and perpendicular to the grain....

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Yield Limit Equations Reduction Factors, Rd Wood Screws & Nails D ≤ 0.17" 0.17" < D < 0.25" D ≥ 0.25"

Factors 2.2 10D + 0.5 3

….and for wood screws and nails.

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Fastener Penetration Lag Screws, Wood Screws, and Nails Fastener Type

Full

Reduced

Lag Screws

8D

4D

Wood Screws

7D

4D

Nails & Spikes

12D

6D

To be effective in holding and to develop its full capacity, fasteners must achieve a minimum penetration depth into the holding member as indicated in the table.

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Mechanical Connections Nail installation – correct toe nailing

The NDS provides guidance here: 12.1.3 Nominal design values apply to nailed and spike connections either with or without bored holes. When a bored hole is desired to prevent splitting of wood, the diameter of the bored hole shall not exceed 90% of the nail or spike diameter for wood with G > 0.6, nor 75% of the nail or spike diameter for wood with G ≤ 0.6.

It is important to understand that toe nails only resist loads in certain directions and thus are not recommended when the load application can be from several directions as shown.

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Mechanical Connections Nail installation – overdriving reduces performance

Overdriving fasteners can reduce performance and must be field checked to assure design intentions are carried through.

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Mechanical Connections Overdriven Nails – APA Recommendations – Prescriptive • If < 20% fasteners overdriven by <1/8”, then they may be ignored. • If > 20% fasteners overdriven by >1/8”, then add 1 additional fastener for every 2 overdriven.

Here are APA’s prescriptive recommendations on dealing with an overdriven nail problem with structural panels such as plywood and OSB...

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Mechanical Connections Overdriven Nails – APA Recommendations - Mechanics Based (engineered) • If < 20% fasteners overdriven by <1/8”, then they may be ignored. • Otherwise, – re-analyze capacity based on average thickness of panel measured from the bottom of the nail head. (5/8” panel with fasteners overdriven by 1/8” = capacity of ½” panel.) – Adjust nailing schedule accordingly.

If you choose to calculate it out, then APA’s recommendations are these in the slide to preserve capacity.

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Mechanical Connections Nail installation – if ya miss ‘em, well….

…and if the studs are missed entirely, then Cd is zero (no capacity!) In the trade, these are commonly referred to as “shiners”.

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Power Driven Fasteners Four important considerations: • nail nomenclature • contact • thin galvanizing • overdriving

Power driving fasteners is not new and has its own unique concerns.

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Power Driven Fasteners Nail nomenclature: • There is no control over nail nomenclature! Manufacturers can and will call fasteners anything that they want. • 8d does not equal 8d!! (8d box = 8d slightly longer power driven) – NER 272 (not included)

On nails: There is no standard - name, size, metal, or otherwise. Systems manufacturers have their own tables, etc. which vary among manufacturers.

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Power Driven Fasteners Contact: • Power driven fasteners rely on velocity to drive fasteners and not mass. They do not have the “clamping” action that the last swing of a hammer provides.

There is a difference in capacity between hand driven and power driven nails.

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Power Driven Fasteners Thin Galvanizing • power driven fasteners that are “galvanized” are thinly coated to prevent rusting in the box. The protection is scraped off of the fastener during driving. • generally not recommended where long-term performance against corrosion or staining is desired, which normally requires thicker coatings. – Anecdotal reports

Power driven fasteners that are "galvanized" are likely to be thinly coated. Galvanized power driven fasteners can loose their protective coatings on driving. It's clear that thin plating offers the minimum protection against corrosion. The result is that wherever corrosion resistance is required, more than the minimum protection is recommended. In most case this would be hot-dipped or hot-tumbled galvanized fasteners. Therefore, thinly galvanized fasteners are generally not recommended where long-term performance against corrosion or staining is desired, since these fasteners offer minimum protection.

93

Power Driven Fastener Considerations Overdriving: • If the “gun” is improperly adjusted, overdriven fasteners can be expected. Adjusting air pressure is NOT the correct way to prevent over-driven fasteners. – APA White Paper

Proper gun adjustment is CRITICAL. Calibrating for driving into D.Fir., then using the same gun to fasten sheathing to SPF will result in overdriving the entire assembly. Gun must be properly calibrated for the wood species being nailed, and size of fasteners driven.

94

Mechanical Connections Fastener corrosion resistance • galvanized • stainless steel • epoxy-coated • ... what should be used?

What about corrosion-resistant fasteners? AF&PA has a policy paper out on this subject: AMERICAN FOREST & PAPER ASSOCIATION POLICY ON NATURALLY DURABLE AND PRESERVATIVE-TREATED WOOD (Revised 8/00) which breaks down the subject like this...

95

Mechanical Connections see American Forest & Paper Association Policy on Naturally Durable and Preservative-treated Wood (Revised 8/00)

• naturally durable and preservative-treated wood – fasteners shall be resistant to corrosion or be protected to resist corrosion.

• treated wood foundations – fasteners shall be as required in AF&PA's The Permanent Wood Foundation System - Technical Report No. 7.

• fire retardant treated wood – fasteners shall be resistant to corrosion or be protected to resist corrosion.

“Fasteners for naturally durable and preservative-treated wood shall be resistant to corrosion or be protected to resist corrosion. Where sacrificial coatings are applied to fasteners, a minimum coating thickness capable of protecting the fastener for the expected service life of the structure shall be provided.” Fasteners of zinc-coated galvanized with sufficient thickness, stainless steel, silicon bronze, and copper have demonstrated performance to normal exposures. Fasteners for treated wood foundations shall be as required in AF&PA's The Permanent Wood Foundation System - Technical Report No. 7. Fasteners for fire retardant treated wood shall be resistant to corrosion or be protected to resist corrosion. Where sacrificial coatings are applied to fasteners, a minimum coating thickness capable of protecting the fastener for the expected service life of the structure shall be provided.”

96

Mechanical Connections ...sacrificial coatings applied to fasteners – a minimum coating thickness capable of protecting the fastener for the expected service life of the structure

• fasteners of: – – – –

zinc-coated galvanized with sufficient thickness, stainless steel, silicon bronze, and copper,

have demonstrated performance to normal exposures.

97

Mechanical Connections Larger fasteners • group action factor – NDS tables – equation calculation • accounts for load distribution within the connection • tabulated values still exist in the NDS • can calculate your own group factor if outside the tabulated table range

The Group Action Factor provided in the NDS for connections involving large diameter fasteners often causes a lot of confusion. Nominal lateral design values for split ring connectors, shear plate connectors, bolts with D less than or equal to 1”, or lag screws in a row are multiplied by Cg. There are two ways to determine Cg: tables and calculation.

98

Mechanical Connections Cg definitions: • row of fasteners: – 2 or more split ring or shear plate connector units aligned in the direction of load – 2 or more bolts of same diameter loaded in direction of load – 2 or more lag screws of same type and size loaded in direction of load

Let’s first review Cg terms.

99

Mechanical Connections What is a row?

Determining numbers of rows can be tricky…here are some diagrams to assist.

100

Group Action Factor, Cg • Equation method

⎡ m(1− m2n ) Cg = ⎢ 2n n ⎣ n (1+ REAm )(1+ m) −1+ m

[

where:

REA = the lessor of

⎤⎡1+ REA ⎤ ⎥⎢ ⎥ ⎦⎣ 1 − m ⎦

]

E s As E A or m m E m Am E s As

m = u − u2 −1

u =1+ γ

s⎡ 1 1 ⎤ + ⎢ ⎥ 2 ⎣ E m Am E s As ⎦

This is the calculation equation for Cg.

101

Group Action Factor, Cg • Load / slip modulus, γ (lb/in) D = diameter of bolt of lag screw (in) γ (lb/in) Bolts, lag screws: wood-to-metal connections Bolts, lag screws: wood-to-wood connections (wood-to-concrete connections) 2 ½” split ring 2 5/8” shear plate 4” split ring 4” shear plate

(270,000)(D1.5)

(180,000)(D1.5) 400,000 500,000

The calculation depends to a degree on the load-slip relationship between the fastener and the holding material(s). The NDS tabulates the load-slip modulus for various installations as shown here. For fasteners into concrete, wood-to-wood values are used as a reasonably conservative approach.

102

Group Action Factor, Cg • Equation method Example Find Cg for two rows of 1” diameter bolts spaced 4” apart in a wood-towood double shear splice connection using 2x12’s for main and side members. Wood Data

Em := 1400000psi

Es := 1400000psi

A m := 1.5in⋅ 11.25in

A s := 2⋅ 1.5in⋅ 11.25in

Am As

= 0.5

2

A m = 16.875in

Fastener Data

s := 4in

n := 10

D := 1in

Load / Slip

γ := 180000

lbf 2.5

in

1.5

⋅D

5

γ = 1.8 × 10

lbf in

Here is an example of a calculation run for Cg. The problem set-up and material data are featured here.

103

Group Action Factor, Cg • Equation method Example Cg equation

1 ⎞ 1 s + u := 1 + γ ⋅ ⋅ ⎛⎜ ⎟ Em⋅ A m ⎠ 2 ⎝ Es⋅ A s 2

m = 0.808

m := u − u − 1

⎛

u = 1.023

⎞ ⎟ REA = 0.5 Em⋅ A m ⎠ ⎝ Es⋅ A s 2 ⋅n ⎡ ⎤⎥ ⎛ 1 + REA ⎞ m⋅ ( 1 − m ) Cg := ⎢ ⋅⎜ ⎟ Cg = 0.669 ⎢ n⋅ ⎡ 1 + REA⋅ mn ⋅ ( 1 + m) − 1 + m2 ⋅ n⎤ ⎥ ⎝ 1 − m ⎠ ⎣ ⎣ ⎦⎦ REA := min⎜ Em⋅

(

Am

, Es⋅

As

)

…then the equation is run for a Cg of 0.669.

104

Group Action Factor, Cg • Table method Am = gross x-sectional area of main member, in2 As = sum of gross x-sectional areas of all side members , in2

We can use the table method for the same problem since criteria fits the bounds of the tables in the NDS. If the bounds are exceeded, then calculation is the only approach.

105

Group Action Factor, Cg • Table method Example • As/Am > 1.0, so use Am/As = 0.5 to enter column 1 of the table • also, use Am for column 2 according to Note 1 • (Am = 16.875 in2) • read across to column for 10 fasteners in a row • interpolate

Cg = 0.665

TableCg := 0.61 + ( 0.70 − 0.61) ⋅

(A

)

2

m

− 12in

(20in

2

2

− 12in

)

TableCg = 0.665

The steps here are explained in the slide. The table provides a Cg result of 0.665, consistent with what we calculated.

106

Group Action Factor Not applicable here - unit loads act along the length of the member and loads are not axial Anchor Bolts and Washers as required Bottom Plate

The Group Action Factor does not apply to sill plates because such loads are not necessarily axial with the plate.

107

Mechanical Connections Larger fasteners • bolts

Bolted connections are attractive with either hidden or exposed connection hardware. One consideration often forgotten is to allow erectors enough room to install and tighten the bolts / nuts, especially in junctions at tight angles or close proximity to other members.

108

Mechanical Connections Larger fasteners • bolts in wood bearing must be no larger than 1 inch diameter!!!

Bolts and other dowel-type fasteners should always be no bigger than 1” diameter. Studies show that dowel diameters larger than 1” have the capability to initiate high tension-perp stresses on the bolt hole that can induce splitting of the wood.

109

Spacing, End, & Edge Distances Parallel to Grain

Detailing provisions are given diagrammatically in the NDS for grouped fastener connections. Anything outside of these conditions will result require a factor calculation for edge distances. BREAKING NEWS! New information has recently been generated as a result of research at the US Forest products Lab on connections using multiple bolts. New failure modes will be added for the designer to check. Thisinformation will be included in a new Appendix E of the 2001 NDS, and detailed information will be broadly available shortly. Watch for the announcement on www.awc.org.

110

Spacing, End, & Edge Distances Perpendicular to Grain

This diagram applies for perpendicular to grain loading.

111

Local Stresses in Fastener Groups • Closely spaced fasteners – brittle failure – lower capacity wood failure mechanisms need to be considered in design

Where a fastener group is composed of closely-spaced fasteners loaded parallel to grain, the capacity of the fastener group may be limited by wood failure at the net section or tear-out around the fasteners caused by local stresses.

112

Local Stresses in Fastener Groups • Properly spaced fasteners – increased ductility – higher capacity spread out the fasteners!

By increasing the spacing between the fasteners, much higher capacity and ductility is achieved, even with fewer fasteners! The 2001 Edition of the National Design Specification ® (NDS ® ) for Wood Construction contains editorially clarified provisions for checking stresses in members at connections. The following requirements, included in the 2001 NDS, are also applicable to all prior editions of the NDS: Stresses in Members at Connections - Structural members shall be checked for load carrying capacity at connections in accordance with all applicable provisions of the NDS. Local stresses in connections using multiple fasteners shall be checked in accordance with principles of engineering mechanics.

One method for determining these stresses is provided in Appendix E from the 2001 NDS, which is also available free from www.awc.org. All referenced sections and design values used in sample solutions of this Addendum are based on information in the 2001 NDS.

113

Local Stresses in Fastener Groups • Appendix E NDS Expressions – Net tension: ' Z NT = Ft' Anet

– Row tear-out: ' Z RT = ni Fv'tsmin i

nrow

' ' Z RT = ∑ Z RT i =1

i

Tabulated nominal design values for timber rivet connections in Chapter 13 account for local stress effects and do not require further modification by procedures outlined in Appendix E. The capacity of connections with closely-spaced, large diameter bolts has been shown to be limited by the capacity of the wood surrounding the connection. Connections with groups of smaller diameter fasteners, such as typical nailed connections in wood-frame construction, may not be limited by wood capacity. Appendix E leads the designer through the stress checks for three failure modes: net tension capacity of the wood through the cross-section, row tear-out, and...

114

Local Stresses in Fastener Groups • Appendix E NDS Expressions – Group tear-out ' = Z GT

' Z RT −top

2

+

' Z RT −bottom + Ft' Agroup − net 2

applicable to ALL editions of the NDS Appendix E available free from www.awc.org

…group tear-out. Modification of fastener placement within a fastener group can be used to increase row tear-out and group tear-out capacity limited by local stresses around the fastener group. Increased spacing between fasteners in a row is one way to increase row tear-out capacity. Increased spacing between rows of fasteners is one way to increase group tear-out capacity. Footnote 2 to Table 11.5.1D(2001 NDS) limits the spacing between outer rows of fasteners paralleling the member on a single splice plate to 5 inches. This requirement is imposed to limit local stresses resulting from shrinkage of wood members. When special detailing is used to address shrinkage,such as the use of slotted holes, the 5 inch limit can be adjusted. These provisions apply to the 2001 NDS and ALL PRIOR EDITIONS. The example calculations provided in Appendix E use design values from the 2001 NDS. Appendix E in its entirety is available as a free PDF download from www.awc.org.

115

Timber rivet connections have been used in Canada for several decades. The new NDS design criteria introduced in Chapter 13 of the NDS apply to joints with steel side plates for either Southern Pine or Western Species glued laminated timber. The term "timber rivet" was chosen to accommodate future application to sawn lumber as well. Provisions of the Specification are applicable only to timber rivets that are hot-dipped galvanized. Rivets are made with fixed shank cross-section and head dimensions (Appendix M) and vary only as to length. Because of the species test results and property values used to develop the rivet bending and wood capacity equations, use of design values based on provisions of 13.2.2 should be limited to Douglas g fir-Larch and southern the p pine glued laminated timber. The NDS presently limits use of timber rivets to attachment of steel side plates to glued laminated timber.

116

Timber Rivet System timber rivet cross section

Provisions of the Specification are applicable only to timber rivets that are hot-dipped galvanized. Rivets are made with fixed shank cross-section and head dimensions (Appendix M) and vary only as to length.

118

Timber Rivet System perforated steel plates

Plates also have a fixed hole pattern geometry. Hole sizes are chosen deliberately to firmly hold and lock the head of the rivet in position, preventing the rivet from rotating next to the plate, to fully develop a cantilever action for the rivet shank embedded in the wood.

119

Timber Rivet System one or two -sided connection

b/2 p

p

b Metal side plates

Rivet connections can be made from one or both sides of a member.

120

Timber Rivet System loading to grain Load parallel to grain

Load perpendicular to grain

sq

aq

es ep

sp

ep

es es es

ep

sp

sq

ap

eq

Rivit rows

Rivit rows

P

Q

Similar rules apply as before in properly and safely loading the wood.

121

Timber Rivet System Load at angle to grain

angle to grain values

sp

e

p

sq

e q

a

q

Metal side plates

Rivit rows

N

Angle to grain capacity values are also provided in the NDS.

122

Timber Rivet System behavior

Timber rivets were invented by Professor Borg Madsen at the University of British Columbia for use with glulam. They have been popular in Canada for years because of their high strength, ease of use, and economy. The 1997 NDS referenced the use of the rivet in glulam, and since then the Canadian design standard CSA O86 has approved the rivet for use in sawn lumber as well. Timber rivets permit greater load transfer per unit contact area than most other fasteners thus providing more ductility in the connection. For more information: http://www.clevelandsteel.com 1-800-251-8351

123

Specialized Connectors • Shear plates

Shear plates have a long history and are still used, especially for large section glulam applications. In glulams which can achieve depths up to 84” or more, the scale of shear plates resembles that of nails in a 2x12. Shear plates sit flush with the wood surface in pre-cut grooves.

124


Split rings require special tooling of the wood for fit which can be performed off site for immediate erection on arrival. These were featured in the PSL frame joints the the Forintek Canada Corp laboratory Library frame shown here. Their discrete finish provides a very clean-looking connection. Bolt heads could be countersunk to make the connectors completely disappear. Shear plates are also suitable for joining wood to dissimilar materials such as steel or concrete (separator must be used between wood and concrete) since only 1 shell needs to be installed in the wood. Examples: FERIC Building, PSL column quads, glulam beams, Vancouver Canada Forintec Canada Corp Building Library, 60 ft PSL columns, PSL beam pairs, Vancouver Canada Architect: The Hulbert Group (Rick Hulbert) Vancouver Canada

125


More examples: South Surrey Ice Arena erection, 4-hinged counterbalanced PSL truss frame, near White Rock, BC Canada Architect: Lubor Trubka

126

Specialized Connectors • Split rings

Split rings are primarily wood-to-wood connectors requiring special tooling before sending to the site for erection. Split rings fit into pre-cut grooves in both wood surfaces being joined. They get their name from a tongue and groove split in the ring that allows it to deform slightly under load or changing wood MC conditions ensuring that all contact areas distribute load. In scale, they are treated much like shear plates, although many have been found in pin-jointed wood truss applications in buildings dating back to pre-1940.

127

Specialized Connectors which is which?

Split rings and shear plates are used with bolts or lag screws to improve structural efficiency by enlarging the area of wood over which the load is distributed. The connectors transfer shear either between the faces of two timber members or between a timber and steel plate. Bolts or lag screws need to be properly sized since they effectively clamp the connection assembly together. Aesthetically, can you tell from the outside which was used? Design values and provisions for timber rivet, split ring, and shear plate connections appear in the current NDS. For more information: http://www.clevelandsteel.com 1-800-251-8351

128

Concealed Connectors Proprietary Systems

New recent proprietary concealed connection systems, two from Europe and two from North America, offer new options in structural efficiency and aesthetics, and gradually designers are discovering these and newer ones as they come to market. Many of them already carry ICC-ES and/or state/municipal building code approvals. Designers value connection systems that produce predictable failure modes, and that fail in the steel components where homogeneity and lower material variability lead to more accurate strength calculations. When steel failure governs, ductility can be included in the connection design – a preferable quality for structures in seismic regions. The cost of these connector types vary and should be considered with the understanding that they allow more options for using timber, often in situations where steel may have been, heretofore, the best economic choice. In this sense, the new connectors provide economy and high reliability at the joint. To learn more see: Moses, D.; Malczyk, R. (2004): New Concealed Connectors Bring More Options for Timber Structures, Wood Design & Building, Winter 2004/2005, Janam Publications, Inc., 30:40-41.

129

Concealed Connectors SFS Intec

SFS intec manufactures two types of connectors for heavy timber construction – a selftapping tight-fit dowel for steel-wood-steel connections and a long, threaded screw for woodwood connections. The self-tapping dowel, or WS-T connector, has a built-in drill bit at the tip of the dowel can cal drill through up to three 1/8-in. steel plates in addition to the timber member. The dowel portion of the connector is smooth and is the same diameter as the drill bit, resulting in a tight-fit connection. This means no slipping of the connection compared to standard bolts which have oversized holes. As such, these qualify as tight-fit pins (friction pins), per European standards. There is a limit on 4 pins per row, but you can have many rows. Apparently, the pins are small enough that European designers generally neglect the wood shrinkage problem. Canadian designers still design to limit the 8" (or so) maximum array width across a member. Also, since friction pins are normally used in glulam connections, so kiln-dried material from glulam factory to in-service minimizes the potential for cross-grain movement due to changing moisture or humidity conditions. The WS-T is approximately ¼-in. diameter and available in lengths up to 9 in. long. When massed in large groups, it produces high strength connections for hangers, trusses, and other applications. The screw connector, known as WT-T, has various diameters and lengths up to 12 in. It is threaded over its full length except for about 1-in. in the middle that helps to pull the two wooden members together as the screw is tightened. The result is a strong, easy to install connection. For more information: http://www.sfsintecusa.com SFS intec, Inc. Wyomissing, PA tel: 800-234-4533 (head office) Waterdown, ON Canada tel: 905-847-5400

130

Concealed Connectors BVD Connector

The BVD connector consists of a shaped steel shaft that is inserted into the end of the timber member leaving a flush-mounted threaded connection exposed. The shaft is criss-crossed by tight-fit steel pins or dowels that form an interlock between the shaft and the timber. A non-shrink group fills the voids between the steel and the timber. The very solid connection forces the failure mode into the steel components so that the full tensile capacity of the timber can be reached unlike other conventional systems. BVD connectors can be designed for the allowable tensile strengths as high as 65 kips. Such tensile resistance means that true moment connections can be developed. For more information: email: [email protected] 1-541-683-5878

131

Concealed Connectors Stavebolt®

The Stavebolt® connector is used in post and beam construction in both sawn and round log applications. Testing has shown the connector to be five to ten times stronger than pinned mortise and tenon connections, with high ductility to resist wind and seismic loads. The Stavebolt® is intended to carry only tensile loads parallel to its axis, and not shear load perpendicular to its axis. Thus, in beam-to-column connections, the beam should rest on a shoulder. The connector consist of of 2-in. (approx) diameter steel pipe that has a receiving thread at one end for a ¾-in. diameter tie bolt that allows for fastening of one timber to another. Two holes at the other end permit the insertion of stitch bolts that lock the pipe into the end of the receiving member. Split rings or shear plates may be added to improve joint strength if necessary. For more information: http://www.loghelp.com/fastener.html#anchor_stavebolt 1-800-359-6614

132

Concealed Connectors Timberlinx

Timberlinx consists of a hollow steel connection tube inserted equal distance in both members of the joint and linked by two expanding cross pins that fit through 1-1/8 in. diameter holes in the connecting tube. Installation requires only an electric drill and jig, and appears similar to a mortise and tenon joint when plugged with a wood dowel. Engineering tests have shown it to be significantly stringer that the traditional mortise and tenon. The connection can also be tightened in service by removing the dowel plugs and tightening the cross pins with an Allen key. The hollow steel connection tube comes in various lengths, and can be modified to handle applications such as timber column anchorage to concrete, or as angled clusters for conical roof apexes. Shear resistance can be boosted by incorporating standard split ring connectors. For more information: http://www.timberlinx.com 1-877-900-3111

133

Next... • • • • • • • •


Glues can be a sticky subject…but they are used all the time to bond wood together either into manufactured components, or between components of an assembly. Glues and adhesives are sometimes broadly classed as bond -based connectors.

134

Glued Connections Bond -based connectors • mobilizes shear resistance at the bond line – construct composite systems which economize material usage – building components can be pre-assembled – increases rigidity of the joint and enables full utilization of material strengths

Bond -based connectors mobilize shear resistance at the bond line, thus allowing a number of productive things to happen...

135

Glued Connections Bond -based connectors • some adhesives are sensitive to changing environmental conditions (temperature and moisture) • epoxies lose strength above 150 deg F • some adhesives are not moisture resistant

• adhesives must be carefully chosen to suit expected conditions • glued joints are probably the most unpredictable Care must be exercised in selection the right adhesive for the task at hand. Moisture exposure considerations often play a significant role. In performance, glued joint capacities are very hard to predict. Failure modes are often brittle and sudden.

136

Glued Connections Adhesives classed based on application: • manufactured components • field construction • repair for complete discussion on adhesives and uses, see: Wood Engineering and Construction Handbook, McGrawHill, Chapter 12.

Chapter 12 of the Wood Engineering and Construction Handbook on Adhesives written by R. Richard Avent, PhD, PE provides an excellent description of wood adhesives and their application.

137

Glued Connections • gluing is not recommended for bonding siding or roof sheathing to framing • APA glued floor system

In typical building construction, gluing is often done of floor assemblies, but never of wall or roof assemblies for ductility reasons. Floor assemblies benefit from development of T-beam action between the subfloor sheathing and the joists to increase floor stiffness and reduce squeaks among components. APA provides a full description of this in the APA Design / Construction Guide - Residential & Commercial.

138

Next... • • • • • • • •


Here is some how-to on a variety of wood connection subjects...

139

Connection Techniques • must evaluate: – – – –

forces present environmental effects material effects aesthetics

Whenever we talk connection design, we must consider these four items, regardless of the fastening device chosen.

140

Connection Techniques • small linear members – nails, screws – truss plates

Design metal plate connections using the latest edition of ANSI/TPI 1

We’ve already seen nails. The truss plate is a device that can also be used very effectively for connecting small dimension linear members. ANSI/TPI 1 is the reference design standard for truss plate connections.

141

Connection Techniques • large linear members (heavy timber) – – – –

dowels lag screws bolts specialty connectors

with or without intermediate hardware

Larger size members mean larger size connectors capable of transferring higher loads.

142

Connection Techniques wood bolts in all-wood structure

Here is a modern wood trestle bridge connected completely with only wood bolts. The wood bolts were actually a laminated beech product. This was constructed by the US Air Force for a test stand in Albuquerque NM.

143

Connection Techniques steel bolts in columns

Hidden kerf plates

Bolts can be used to build-up members such as these columns - an excellent example of wood working in compression. Note that the base and branch connections to the ceiling members are very simple. Hidden kerf plates at the base also add a measure of fire resistance to the connection in that the steel plates are buried in the wood which acts as an insulator. Countersinking and plugging (not shown here) has the same additional effect.

144

Connection Techniques wood bolts in beam to column (hidden)

…another discrete hidden connection. Again, these are partially concealed steel bolts.

145

Connection Techniques wood bolts in heavy trusses

On the other hand, heavy truss joints are expressed through these bolted plate steel connections.

146

Connection Techniques • large linear members (heavy timber) • hardware – proprietary solutions: well-suited to many standard solid sawn and EWP applications • consult manufacturers’ literature

Heavy timber design often brings on the use of proprietary and customfabricated hardware in the connection.

147

Pre-engineered Connectors Post to Beam Beam to Beam

Pre-engineered connectors are manufactured by a number of component manufacturers that have exhaustive catalogs of their products that suit a variety of innovative applications. Many components are fabricated of formed sheet metal, increasing in thickness as loads increase. Specific nailing and fastening requirements are given and must be rigorously followed in order to develop full connection capacity. NER documents often underlie the component capacities found in the catalogs.

148

Pre-engineered Connectors Joist to Beam (Hanger)

Joist hangers are very useful products and save considerable connection construction time.

149

Pre-engineered Connectors I-joist web stiffeners

web stiffeners

none here

With I-joists especially, many hanger products require web stiffener blocks to prevent web buckling and joint rotation in the hanger (lateral torsional stability).

150

Pre-engineered Connectors Panelized roof connectors

So discrete, pre-engineered connectors were used in this panelized roof system.

151

Pre-engineered Connectors Truss hardware

Here are some well thought-out and design solutions using combinations of pre-engineered connectors, truss plates, and bolt patterns to secure trusses.

152

Pre-engineered Connectors Field creativity

…and one with some field creativity!

153

Pre-engineered Connectors Watch for those loose or popped-off plates!

Loose or missing truss plates usually result from rough transportation to or handling at the job site. Plates must be snug and installed correctly in order to properly transfer load. Consult the truss manufacturer should deficiencies be discovered.

154

Connection Techniques • large linear members (heavy timber) • hardware – proprietary solutions: well suited to many standard solid sawn and EWP applications • consult manufacturers’ literature

– custom solutions: one-off designs • for guidance, see: Structural Design in Wood, Stalnaker & Harris, Kluwer Academic Publishers

Heavy timber connections often require one-off innovative connection solutions to arrive at the sensitive balance between safety, efficiency, serviceability and aesthetics. It can be the meeting place of the artistic and technical design professionals and sometimes can test the skills of both of them in seeking a solution. One-off designs are typically custom-fabricated to suit the connection. Designing one-off connections is a respected skill and can be time-consuming. Guidance for new designers is provided in only a few publications. There are a few standard connections that frequently arise...

155

Custom Hardware Multiple beam connector with slotted holes

The hardware here is as much art as it is function. Slotted holes allow for wood movement. Design of the hardware would need to conform to the latest applicable steel standard.

156

Custom Hardware Difficult situations made easy

Skewed connections are typical of one-off designs needed. Getting sufficient room for constructing the connection, installing and tightening fasteners can be a struggle - and should not be forgotten!

157

Custom Hardware A blend of art and technology

Here is an example of a well-thought holistic solution.

158

Arch Base to Support Welded Shoe – – – –

more rigid little arch rotation transfers thrust weep slot

Arch Base to Support We’ve already seen some arch base connections earlier. Arches transmit thrust into the supporting structure. The foundation may be designed to resist this thrust (thrust blocks already seen) or tie rods may be used. The base detail should be designed to accommodate the amount of rotation anticipated in the arch base under various loading conditions. Elastomeric bearing pads can assist somewhat in distributing stresses. As noted earlier, the connection should be designed to minimize any perpendicular to grain stresses during the deformation of the structure under load. This more rigid connection is suitable for spans where arch rotation at the base is small enough to not require the rotational movement permitted in detail 25. Note that, although the shoe is “boxed” a weep slot is provided at the inside face.

159

Arch Base to Support Welded Shoe – transfers thrust – open to prevent moisture collection

Here, the welded shoe transmits thrust from arch to support. Note that inside edge of shoe is left open to prevent collection of moisture.

160

Arch Base to Support Welded Shoe – steel tie beam – transfers thrust – open to prevent moisture collection

Arch base is fastened directly to a steel tie beam in a shoe-type connection.

161

Beam to Column Concealed – steel plate in kerf – bolted or pinned

Beam to Column Connections Design Concepts. All connections in the group must hold a beam in place on top of a column. This shear transfer is reasonably easy to achieve. Some connections must also resist some beam uplift. Finally, for cases in which the beam is spliced, rather than continuous over the column, transfer of forces across the splice may be required. Here is a concealed connection in which a steel plate is inserted into a kerf in both beam and column. Transverse pins or bolts complete the connection.

162

Beam to Column Simple steel dowel – shear transfer

Another is a simple steel dowel for shear transfer.

163

Beam to Column Custom welded column caps – transfer shear, uplift, and splice forces – allows different widths and bearing areas

External options include this custom welded column cap which can be designed to transfer shear, uplift, and splice forces. Note design variations to provide sufficient bearing area for each of the beams and differing plate widths to accommodate differences between the column and the beam widths.

164

Beam to Column Beam seat on steel column – very common – transfer shear, uplift, and lateral loads

Here is a very common connection: beam seat welded to the top of a steel column.

165

Beam to Column Combination steel angle and straps – bolts and lag screws – transfer shear, uplift, and splice forces

Combinations of steel angles and straps, bolted and screwed, to transfer forces.

166

Beam to Column Continuous column – recess bolt heads, or – slot beam ends

When both beams and columns are continuous and the connection must remain in-plane, either the beam or the column must be spliced at the connection. In this detail the column continuity is maintained. Optional shear plates may be used to transfer higher loads. Note that, unless the bolt heads are completely recessed into the back of the bracket, the beam end will likely require slotting. In a building with many bays, it may be difficult to maintain dimensions in the beam direction when using this connection.

167

Arch Peak Steep arch – rod and shear plate

Lets looks at a few solutions at the top of a structure. Arch Peak Connections Steep arches connected with a rod and shear plates.

168

Arch Peak Steep arch – rod and 2 shear plates

Similar to the previous one, with added shear plate.

169

Arch Peak Low-slope arch – shear plate – bolted side plates

Similar to the first one, but for low slope arches. Side plates replace the threaded rod.

170

Beam to Beam Welded Bracket – bucket -style – lower capacity

Beam crossings are often interesting especially crossing beams in plane. Two example connections of this type were seen earlier. Here is a similar detail with somewhat lower load capacity.

171

Beam to Beam Clip angle – bolts & lag screws – connects cross beam

Layered beam crossings are a little easier to handle. Here clip angles are used to connect a crossing beam.

172

Beam to Beam Clip angle – bolts & lag screws – connect ridge purlin to sloped member – connect purlin to peak of arch members

Here is a special detail to connect the ridge purlin to sloped members or to the beak of arch members.

173

Beam to Beam Welded Brackets – through bolts – connect ridge purlin to sloped member – purlins flush with other framing

…and an in-plane variant of the crossing, with through-bolts….

174

Beam to Beam Welded Brackets – nails or rivets – connect ridge purlin to sloped member – purlins flush with other framing

…and without; using nails or rivets instead (depending on scale).

175

Beam to Beam cantilever hinge connector

Gerber-style systems are often efficiently used in commercial timber building construction. Connecting beams near zero moment points facilitate the use of simple shear connections using simple hardware. This slide shows is what we refer to as a panelized roof system. The hinges are not necessarily at points of zero moment but just at the end of the cantilevered beams which are designed to balance positive and negative moments. This is a Simpson hinge connector.

176

Beam to Beam cantilever hinge connector with tension tie

Hinge connectors transfer load without the need to slope-cut member ends. Beams are often dapped top and bottom for a flush fit.

177

Moment Splice Steel plates – top / bottom transfer axial force – pressure plates transfer thrust – shear plates transfer shear

Moment Splice Design Concepts Moment splices must transmit axial tension, axial compression, and shear. They must serve these functions in an area of the structure where structural movement may be significant — thus, they must not introduce cross-grain forces if they are to function properly. In the connection shown here, separate pieces of steel each provide a specific function. Top and bottom plate transfer axial force, pressure plates transfer direct thrust, and shear plates transmit shear. This is a situation where the designer should try to place the moment connection at a point of low moment.

178

Moment Splice Steel plates – side plates transfer axial – easier installation – pressure plates transfer thrust – shear plates transfer shear

This version is similar to the previous detail. Connectors on side faces may be easier to install, but forces are higher because moment arm between steel straps is less than in the previous details.

179

Connection Techniques • multi-ply linear members and inter-ply shear/load transfer

Multi-ply sections usually need some type of connection if the plies are to act in composite. This is particularly important for beams. Side loaded beams should typically be specified with a solid wood section, which will overcome the typical issue of overloading the outer ply of a multi-ply edge loaded beam.

180

Connection Techniques • multi-ply beams – prescriptive provisions found in AF&PA WCD 1

AF&PA’s Wood Construction Data 1 Details for Wood Frame Construction has some prescriptive provisions for multi-ply beams. The provisions apply to beams and girders of solid timber or built-up construction in which multiple pieces of nominal 2-inch thick lumber are nailed together with the wide faces vertical. Such pieces are nailed with two rows of 20d nails-one row near the top edge and the other near the bottom edge. Nails in each row are spaced 32 inches apart. End joints of the nailed lumber should occur over the supporting column or pier. End joints in adjacent pieces should be at least 16 inches apart, Figure 15. Glued-laminated members are also used. Beams and girders that are not continuous are tied together across supports. Bearing of at least 4 inches is required at supports.

181

Connection Techniques • multi-ply columns – guidance provided in NDS for: • spaced columns • nailed or bolted laminated columns

Provisions for built-up columns are found in AF&PA’s National Design Specification Chapter 15.

NDS section 15.3 contains provisions for designing nailed or bolted built-up columns with 2 to 5 laminations. These provisions allow the column to be treated as a solid section, with column stability coefficients, Kf, which reduce the capacity 25% and 40% for bolted and nailed columns, respectively.

182

Connection Techniques • nailed solutions

Nailing provisions are given in NDS 15.3.3: The provisions in 15.3.1 and 15.3.2 apply to nailed built-up columns (see Figure 15C) in which: (a) adjacent nails are driven from opposite sides of the column (b) all nails penetrate at least 3/4 of the thickness of the last lamination (c) 15D ≤ end distance ≤ 18D (d) 20D ≤ spacing between adjacent nails in a row ≤ 6tmin (e) 10D ≤ spacing between rows of nails ≤ 20D (f) 5D ≤ edge distance ≤ 20D (g) 2 or more longitudinal rows of nails are provided when d > 3tmin where D = nail diameter d = depth (face width) of individual lamination t min = thickness of thinnest lamination When only one longitudinal row of nails is required, adjacent nails shall be staggered (see Figure 15C). When 3 or more longitudinal rows of nails are used, nails in adjacent rows shall be staggered.

183

Connection Techniques • bolted solutions

• spaced solutions

Figure 15D Typical Bolting Schedules for Built-up Columns

15.3.4.1 The provisions in 15.3.1 and 15.3.2 apply to bolted built-up columns in which: (a) a metal plate or washer is provided between the wood and the bolt head, and between the wood and the nut (b) nuts are tightened to ensure that faces of adjacent laminations are in contact (c) for softwoods: 7D ≤ end distance ≤ 8.4D for hardwoods: 5D ≤ end distance ≤ 6D (d) 4D ≤ spacing between adjacent bolts in a row ≤ 6tmin (e) 1.5D ≤ spacing between rows of bolts ≤ 10D (f) 1.5D ≤ edge distance ≤ 10D (g) 2 or more longitudinal rows of bolts are provided when d > 3tmin where D = bolt diameter d = depth (face width) of individual lamination t min = thickness of thinnest lamination 15.3.4.2 Figure 15D provides an example of a bolting schedule which meets the preceding connection requirements.

184

Connection Techniques • panel to linear members gap panel edges 1/8” to allow for expansion

Connecting EWP’s can be done directly with fastening systems, or in combination with stock or custom-made hardware. Connections typically test the designer’s skill to arrive at safe aesthetic solution.

185

Connection Techniques • panel to linear members – nailing schedules in codes

The codes contain fastener tables which are similar to what you see here, which is a reproduction of the first few lines of the CABO fastener table. Note that the table talks about the connection to be made -- that is, what pieces are being joined together and how (face nail, toe nail, etc). It then talks about what type of fastener is to be use -- nail or staple. And then it stipulates how the fastener is to be applied.

186

Connection Techniques • wall panel systems – make sure assemblies are properly connected to each other

You might ask, what's so important about the CABO table? Everyone fastens the pieces together or else nothing would stand up. Hurricane Andrew showed us some very good examples of what happens when the wrong number of fasteners are used or when they're not applied correctly. We found no situations in which individual members of the framing failed. Failure was always caused when fasteners weren't correct. Here you see two examples of common damage in buildings that experienced Andrew. Notice how individual members such as wall studs and elements of truss are still intact, but how assemblies such as walls and trusses have been damaged. As you see here it wasn't uncommon for large portions of the building to separate from other portions. And trusses collapsed when the roof sheathing that held them in place was blown away.

187

Connection Techniques • roof panel systems exterior

Courtesy: Hurricane Andrew

interior

If you're not located in a high-wind area you may wonder why this discussion should be of importance to you. Generally speaking, it's important to keep in mind that the purpose fasteners serve is to transfer loads from one member of an assembly to another. If those loads exceed the capacity of the connection because the connection isn't correct, failure can occur, regardless of what the load may be. And staying with the wind discussion a moment longer, remember that many areas of the country that aren't in high-wind areas are subjected to strong straight-line winds in thunderstorms. Those winds often destroy outbuildings, patio covers, fences,and similar structures. They also strip roof coverings from buildings and can even remove panels of roof sheathing. Even though the roof framing stays in place because of the presence of the remaining sheathing, missing panels leave an opening into the attic during high winds and driving rains which can result in tremendous damage to the interior of the building. The example shown here is from Hurricane Andrew again, and while it may be more extreme than what you might see in thunderstorms, the damage that may result could be similar to what you see here.

188

Uplift Recommendations New construction WFCM 2001

Here are new tables that will appear in the 2001 edition of AF&PA’s Wood Frame Construction Manual that provide the roof suction loads for various 3sec gust wind speeds at a variety of locations on a structure. APA has as similar publication that was developed in response to Hurricane Andrew.

189

Uplift Recommendations WFCM 2001

Using the appropriate wind speed and panel location on the structure (structure zone), the correct nailing can be obtained.

190


Nailing detail for overhangs and rakes are also tabulated based on 3-sec gust wind speed.

191


Nailing tables for wall sheathing and cladding attachment as a function of 3sec gust wind speed are also provided.

192

Connection Techniques • floor systems

Floor systems usually incorporate connection hardware into their assemblies. Much of the hardware is of the pre-engineered variety that is chosen from manufacturer’s catalogs based on application and loading capacity.

193

Connection Techniques • glued floor construction

Elastomeric adhesives are often used in addition to fasteners to bond wood subfloor to joist assemblies. Glue helps eliminate floor squeaks while adding a measure of increased stiffness to the floor system. The glue bond is so strong that floor and joists behave like integral T-beam units. Consult APA’s Design/Construction Guide - Residential and Commercial for details and description on The APA Glued Floor System. Only adhesives conforming with Performance Specification AFG-01 developed by APA are recommended for use with the Glued Floor System. A number of brands meeting this specification are available from building supply dealers. For a list of qualified adhesives, write to APA. If OSB panels with sealed surfaces and edges are to be used, use only solventbased glues; check with panel supplier. Always follow the specific application recommendations of the glue manufacturer.

194

Connection Techniques • lateral force resisting systems

Nails with let-in corner bracing

This is one of the simplest ways of providing lateral resistance to a wall assembly. However, let-in braces require a perfect and well connected fit in order to work properly, which is often difficult to achieve. And, they cannot provide the same capacity as a properly constructed wood panel shear wall as shown in the next slide.

195

Lateral Force Resisting Systems • shearwall systems

A more convenient method is the use of shearwall systems: panels, or walls. Shearwalls feature special nailing and hold-down connections designed to resist applied lateral loads in shear and overturning. Minimum wall aspect ratios apply in order to develop “shearwall action” as opposed to “cantilever beam action” when the wall panel aspect ratios become very slim. Typically, the closer to the minimum aspect ratio for a shearwall, the more dense the nail perimeter nail spacing. In shearwalls, it is the perimeter nailing that is the most effective in resolving the transferred applied forces. Be sure to catch the seminar on designing for lateral loads for much more information on shearwalls and diaphragms.

196

Lateral Force Resisting Systems • hold-down hardware

Hold-downs are required to prevent the wall panel from overturning. Holddowns may also be used elsewhere to prevent uplift, and to tie the structure load path together to the foundation. Typical calculations are provided for hold-down connections in AF&PA’s LRFD Manual, Example 7.7-1.2.

197

Lateral Force Resisting Systems • diaphragm systems VFD

diaphragm panel diaphragm

Diaphragms are usually horizontal surfaces that resist in-plane shear forces. Nailing is more dense where the shears are highest.

198

Lateral Force Resisting Systems • tension tie drag strut in panelized roof

Here is a connection made to tie diaphragm framing members together to that the load transfer is maintained. This connection is in tension parallel to the member axis.

199

Lateral Force Resisting Systems Additional diaphragm perimeter nailing

8d toenails at 6" o.c. max.

Additional diaphragm perimeter nailing 8d toenails at 6" o.c. max.

B1

Wall or foundation below

B2

Shear transfer plate (Plate capacity selected to transfer diaphragm shear) 8d toenails 6" o.c. max.

• Shear transfer around floors B3

Wall or foundation below

B4

Floor diaphragm members must also be properly secured at the diaphragm periphery to properly transfer shear forces. These figures show details at the foundation...

200

Lateral Force Resisting Systems Nail into wood blocking

Nail into wood framing or

or

Panel joint

Panel

joint

Stapled sheet metal blocking or

• Shear transfer around floors

Nail into wood blocking

Panel joint or

Shear transfe r plate

Verify fastening limitations with I-joist manufacturer prior to use with LVL flanges.

…while these apply to intermediate stories.

201

Lateral Load Connection Details for Low-Slope Roof Diaphragms • Truss to wall Diaphragm perimeter angles/lumber chord not shown for clarity wood structural Angle corbel with gussets panel sheathing Inserts to provide approx. 10K

Weld to develop tension capacity

Wood truss purlin @ 8' o.c.

On bigger buildings and tilt-up structures, the same detailing attention for shear force transfer applies - for trusses....

202

Lateral Load Connection Details for Low-Slope Roof Diaphragms • Beam to wall 6" wide tension tie embossed to go over hanger

wood structural panel sheathing

Inserts to provide approx. 10K

Top-mount hanger

Glulam purlin

Full length steel channel

Elevation …and for beams.

203

Lateral Load Connection Details for Low-Slope Roof Diaphragms • Purlin straps Purlin

Strap installed over sheathing (not shown)

Subpurlin

Plan

Ties are often applied over top of framing members for load path continuity.

204

Lateral Load Connection Details for Low-Slope Roof Diaphragms • Beam to beam continuity

wood structural panel sheathing

Purlin (Typ.)

Wood structural panel sheathing not shown for clarity

….and when the forces get large, the connecting hardware gets more interesting.

205

Lateral Load Connection Details for Low-Slope Roof Diaphragms • Beam to beam continuity Wood structural panel sheathing not shown for clarity

Girder (glulam beam shown)

Hanger Tension ties on both sides of girder

50,000 lbs. Use (10) 3/4" diameter bolts 75,000 lbs. Use (12) 1" diameter bolts

Here is a glulam beam tension connection good for up to 75 kips.

206

Connection Techniques • connecting other frame materials : Steel

Wood can easily frame into other materials using standard fasteners.

207

Connection Techniques • connecting other frame materials : Concrete

Connection hardware is often used with concrete/masonry to transfer load and separate the wood away from cementitious materials.

208

Connection Techniques • connecting other frame materials : Masonry

Bolted wood ledgers to masonry are often used on lighter structures to facilitate connection of trusses.

209

Connection Techniques • connecting other frame materials : Dead Trees

Wood can connect to anything - even itself.

210

Next... • • • • • • • •


Designing wood connections day-to-day can be very tedious and timeconsuming. Thankfully automated tools have come along to alleviate the designer of this task. Here is a brief introduction to one such product dedicated to the wood connection designer.

211

Software solutions exist

WoodWorks Design Office is a suite of products for the wood designer from structural members, shearwalls, to connections. See www.awc.org for more info.

212

Software solutions exist Connections design software

Connections Design Software For designing new connections: • • •

Analyze a series of design alternatives. Leave all of the parameters set to 'unknown' and Connections will fill in the blanks producing a fully dimensioned diagram that shows the finished connection in addition to a complete report with full materials list. Permits full graphic input in entering geometry through an intuitive and easy-to-use Windows interface The current version does not accept imported CAD drawings (only Shearwalls does in the current version).

For checking the capacity of existing connections: • Specify every design parameter and have Connections check the joint capacity. • Connections leads you step-by-step through the design process for the following types of connections: • Beams may be horizontal, sloped or have an oblique angle • Beam-to-beam • Beam-to-column • Column-to-foundation • Splices • Decide on the best post-and-beam fastener with a quick 'what-if' analysis. Connections uses the following types of fasteners: • nails • bolts • shear plates • top-mount hangers

213

Next... • • • • • • • •


If you need to get information sources mentioned in this presentation, or you just want to browse, ...

214

Web sites... www.awc.org www.apawood.org www.woodtruss.com www.cwc.ca www.structuralcomponentdistributors.com www.aitc-glulam.org www.southernpine.com www.beconstructive.com www.tfguild.org www.timberframe.org Go online to any of these web sites for connection information….

215

Details Downloadable On-line • www.woodtruss.com

scroll down on home page to: WTCA ARCHITECTURAL DETAILS: download free …or get downloadable details from WTCA,...

216

Details Downloadable On-line • www.apawood.org

…or get downloadable details from APA The Engineered Wood People...

217

Info Resources

Or obtain any of these publications from AITC, APA or AF&PA ...

218

Heavy Timber Construction Details WCD #5 – – – –

framing members floor decks roof decks walls

AF&PA’s WCD #5 has been a popular resource for traditional heavy timber designers and builders for many years.

219

Heavy Timber Construction Details WCD #5 – graphic details

It contains many time-tested graphic details meant to be used as drawing details for buildings using heavy timber design.

220

LRFD Manual Chapter 7 – 40+ details – applicable beyond LRFD

The LRFD Manual is an all-inclusive document containing the ASCE Standard as well as other useful background, example, and technical information. Chapter 7 contains information and recommendations for use on over 40 standard connection details.

221

Take home messages... • transfer loads in compression / bearing whenever possible • allow for dimensional changes in the wood due to potential in-service moisture cycling • avoid the use of details which induce tension perp stresses in the wood • avoid moisture entrapment in connections • separate wood from direct contact with masonry or concrete • avoid eccentricity in joint details • minimize exposure of end grain

These are the key points to take home from this presentation. They will guide you to achieving the right connection solution for wood structures.

222

Murphy’s Law No matter how well it is designed...

?

And if you design connections really well…the weakest link may be somewhere else!

223

American Wood Council Engineered and Traditional Wood Products

LRFD and ASD Connection Design A F & P A®

® Copyright © 2001 American Forest & Paper Association, Inc. All rights reserved.

This section contains ASD and LRFD worked problem examples of connections. At least one example is all inclusive: member and connection design.

224

LRFD Problem / Solutions Manual • 40 examples • prepared by: – Dr. Steve Cramer, P.E. University of Wisconsin, Madison

– Dr. Dan Wheat, P.E. University of Texas at Austin

To aid the designer, a Worked Problems Manual for LRFD has been produced by AF&PA to guide the designer through 40 typical design problems. The manual is formatted for ease of use, filled with graphics, explanatory notes, and arranged in an easy-to-follow approach to process.

225

LRFD Problem / Solutions Manual • targeted to Universities • industry sponsored: – AF&PA – SFPA – WTCA

Sponsored by these associations of industry, the Manual can be easily incorporated as a teaching aid into any wood/timber design curriculum offered by universities or colleges around the country.

226

LRFD Connections Design Contents • a varied series of short examples on LRFD and ASD design of bolted and nailed connections

The examples presented are varied, and most contain both ASD and LRFD worked solutions.

227


Example 1: Bolted Joint Design A F & P A®


228

Example 1: Bolt Connection double shear tension splice • 1/8” steel side plates (A36 steel) • 3-1/8”x 12” Douglas fir-larch glued laminated timber main member (parallel to grain) • 3/4” diameter A307 bolts • 10,000 lb. lateral load (wind)

how many bolts? Consider this double shear tension splice made with exposed steel plates and A307 bolts that needs to resist 10 kips from wind.

229

Mode Im Equations (Bolt) NDS:

Dt m Fem Z= 4 Kθ

LRFD:

0.83Dt m Fem Z= Kθ

Before we get into this example, let’s review some of the Yield Mode equations for ASD and LRFD. These are the forms for Mode Im (wood crushing in main member) for bolts.

230

Mode IIIs Equations (Bolt) NDS:

Z=

k3 Dt s Fem . ( 2 + Re ) Kθ 16

LRFD:

Z=

. k3 Dt s Fem 208 ( 2 + Re ) Kθ

…and for Mode IIIs (bolt yield at one hinge)...

231

Mode IV Equations (Bolt) NDS:

2Fem Fyb D2 Z= . Kθ 3(1 + Re ) 16

. D 2 2Fem Fyb 208 LRFD: Z = 3(1 + Re ) Kθ

…and for Mode IV (full bolt yield).

232

Bolt Connection Parameters NDS

LRFD

Dowel bearing strength to wood main member

Fem

5600 psi

5.6 ksi

Dowel bearing strength of wood side member

Fes

58,000 psi

58 ksi

45,000 psi

45 ksi

Fastener bending yield strength

Fyb

The connection material parameters are given here.

233

Bolt Connection Parameters D = 0.75 in. tm = 3.125 in. ts = 0.125 in. NDS CD = 1.6

LRFD λ = 1.0 φ = 0.65

From the problem specifications, we assemble the basic data, including the load duration and LRFD factors applicable.

234

Bolt Connection Results (Z) NDS

LRFD

Mode Im

3281 lbs.

10.89 k

Mode IIIs

3078 lbs.

10.24 k

Mode IV

4352 lbs.

14.48 k

Running this through the three Yield Mode equations provides the following results. The lowest unit capacity governs, which is Mode IIIs (in white).

235

Bolt Connection (NDS) W < n Z CD 10,000 < (n) (3078) (1.6) n = 2.03 Must use 3 bolts (3/4” diameter) or try 2 larger diameter bolts

Inputting the NDS unit capacity requires 3 bolts (3/4”) to satisfy the connection (marginally over 2).

236

Bolt Connection (LRFD) 1.3 W < n λ φ Z (1.3) (10.0) < (n) (1.0) (0.65) (10.24) n =1.95 Must use 2 bolts (3/4” diameter)

LRFD requires only 2 bolts (max’d out).

237


Example 2: Bolted Splice Joint Check A F & P A®


238

Bolted Splice Joint Check What can this hold in tension?

1″ φ bolts

3−5/8″ P

P

4″

3−5/8″

62" P

P

Main and side members are No. 2 Southern Pine

In this example, we check the capacity of this specified connection using both NDS and LRFD processes.

239

Bolted Splice Joint Check Adjustment Factors • CMT = 1.0 lumber Ft • CME = 0.90 lumber E • CMZ = 0.70 • Cg = 0.85 tabulated • Cg = 0.84 calculated

Here are the wood adjustment factors for this situation, which are the same for both ASD and LRFD.

240

Bolted Splice Joint Check

3−5/8″ P

4″

Placement • Edge distance = 3 5/8” OK • Pitch = 4” OK • Gage = 1.5” OK • End dist. = 4” << 7 “ min NG need end distance reduction 1″ φ bolts CΔ = 0.57 P

3−5/8″

Using the proximity rules, the placement checks out except for end distance. An end distance reduction will be required to deal with the lower amount below minimum which calculates out to 0.57.

241

Bolted Splice Joint Check Yield Modes • Im controls • Is • IIIs • IV

Running the bolt through the yield mode equations shows that Mode Im controls the unit capacity of the fastener.

242

Bolted Splice Joint Check Bolt Capacity Z’ = n Z Cg CΔ CM = 30.9 kips Factored Capacity = λ φ Z’ λ = 0.8 φ = 0.65 λ φ Z’ = 16.1 kips Using LRFD process, the factored capacity comes out to 16.1 kips on the basis of 12 fasteners on each side of the splice.

243

Bolted Splice Joint Check Lumber Capacity • net section check λ φt Ft’ Anet = 12.6 kips net section controls

Reduce to 10 bolts λ φt Ft’ Anet = 13.4 kips net section still controls

P

P

Main and side members are No. 2 Southern Pine Since the connection is in tension, we must check the net section limit state for the wood. Working this produces a capacity of 12.6 kips; lower than the 12 fastener capacity, so the wood controls. Reducing the number of bolts will economize the connection. If we reduce to 10 bolts (down from 12), then the fasteners capacity drops to 13.4 kips, just above the wood capacity at 12.6 kips. So, the connection can be rated for 12.6 kips.

244


Example 3: Glulam Frame and Joints A F & P A®


Here is a complete worked example using LRFD only which includes member and connection designs.

245

Example 3: Glulam Frame Joints

Beam Portion A

Beam Portion B

Column A Size and specify all Glulam beams, column and connection.

Consider this frame situation not unlike one you could find in a residential basement (discontinuous 2-span beam supported by foundation walls and an interior column). The loading is prescribed as shown. Size the beams, column, and all connections.

246

Glulam Beam A P

8 ft P

8 ft P

8 ft P

8 ft P P = 7.60 kips

19.0 kips

19.0 kips

11.4 kips 3.80 kips

V -3.80 kips 1,459 in-kips

-11.4 kips

1,094 in-kips

M

Let’s size the members first beginning with Beam A. Here is the shear and moment diagram based on the prescribed loading for this portion.

247

Glulam Beam A P = 1.2 D + 1.6 S = 7.6 kips 1,094 in-kips

M

1,459 in-kips

Try 5 1/8” x 20 5/8” 24F-V1 Southern Pine Mu ≥ λ φ Mx’ NG

Using LRFD process, we arrive at the factored moment demand. Trying the first section shown doesn’t work on the basis of bending strength.

248

Glulam Beam A Try 5” x 22” ; adjustment factors: CV = 0.95 CL = 0.976 Bending Mu ≥ λ φb Mx’ OK Shear Vu ≥ λ φv V’ OK Bearing Pu ≥ λ φc Pperp’ OK

Going deeper satisfies bending, shear, and bearing capacities needed.

249

Glulam Beam B

Moving on now to Beam B: demand moment and shear, on the basis of prescribed loading.

250

Glulam Beam B Use 20F-E2 Western Species Demand: 1.2D + 1.6 L Sreq’d = 47.19 in3 Areq’d = 14.04 in2

Factoring the moment and shear up allows us to pick a section that meets the factored demands.

251

Glulam Beam B Try 5” x 9” ; adjustment factors: CV = 1.0 CL = 0.995 Bending Mu ≥ λ φb Mx’ OK Shear Vu ≥ λ φv V’ OK Bearing Pu ≥ λ φc Pperp’ OK

Here we try a 5x9 GLB that appears to work.

252

Glulam Column

Beam Portion A

Beam Portion B Column A

We now have sections chosen for both beam portions. We go now for the column.

253

Glulam Column Beam A (factored) = 19 kips

Use E-rated Southern Pine #54 Beam B (factored) = 2.496 kips

Load Combinations: 1.4 D 1.2 D + 1.6 L + 0.5 S 1.2 D + 1.6 S + 0.5 L controls = 21.5 kips Eccentric Loads

At the same time, we’re thinking about how to join all these member together at one point that is consistent with our pinned-end reaction design assumptions. We choose this connector with bolts - more later. We need to worry about unequal moment at the top of the column since the reactions from Beam A and Beam B may not be the same. These beam reactions would applied to the bolt locations in the connector where the beams connect. The reactions (column top factored moments) must be calculated that correspond to the load combinations listed here. If the eccentric distances between the column centerline and the beam reaction bolts are small, then the eccentric moments will be small, and their difference even smaller. Axial capacity will likely drive this design. Back to the column: we figure the factored load combinations to determine the controlling case….

254

Glulam Column Try 5” x 6 7/8”

…and so we try a 5 x 6 7/8 GL section.

255

Glulam Column Try 5” x 6 7/8” Adjustment Factors: CV = 1.0 CP = 0.27 CL = 0.994

…and the applicable adjustment values are these.

256

Glulam Column Combine Bending-Axial: 0.928 < 1.0 OK

Pu λφ cP'

2

+

Mmx λφ bM x'

+

Mmy λφ b M y'

< 1.0

Inputting to the beam-column capacity interaction equation suggests that only 92.8% of the section capacity is used. What about the column base?...

257

Glulam Column End Grain Bearing: PU ≤ 0.75 (λ φC Pg’) OK

Looking good…and hopefully someone remembers to separate the wood from the concrete. Members are sized…on to the connection.

258

Bolt Connection Design

Client design change - new configuration and loads! How many bolts needed in this connection? The client was not overly enthused about our previous connection solution, even our frame design for that matter, and wants something a little more discrete. A bunch of other changes he has in mind also changes the applied loading to lighter values. The frame geometry is the same, but the lighter loading allows the use of a continuous 6x10 hem-fir beam, and 2x6 column pairs with a bolted joint. This design dispenses with eccentric moments and the 2x6’s were found to be satisfactory in axial compression and bearing. Above are the new lighter specified loads at the connection.

259

Bolt Connection Design Demand: 1.2D + 1.6 S = 11.2 kips Try: 1” φ bolts Mode Im controls

Need: 3.3 bolts

Factoring up the load gives us 11.2 kips. Trying 1” diameter bolts (the largest available) we find that Mode Im controls in the requirement for 4 bolts.

260

Bolt Connection Design Try: Four 7/8” φ bolts Mode Im controls

Capacity: λ φ Z’ = 12.6 kips > 11.2 kips demand OK Use: Two rows of 2 bolts

Here is the LRFD calculation for four 7/8” diameter bolts that satisfies the factored demand.

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Bolt Connection Design Net section check: λ φt Ft’ Anet = 16.9 kips > 11.2 kips demand OK

Checking net section in the column pairs proves satisfactory.

262

Bolt Connection Design 2x6 Side member detailing: Edge distance = 1.3” Spacing = 3” 3” + 2(1.3”) = 5.6” > 5.5” NG

How about the column pairs and clearances? Applying the clearance detailing rules for the column section width reveals to us that we have run out of wood for this bolt size. So….change bolt size to...

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Bolt Connection Design Try: 5/8” bolts nf = ZU / λ φ Z’ Cg = 5.38 use: 6 bolts

5/8” which requires us to use 6 bolts and meets the width clearance rules. How about end distance?

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Bolt Connection Design 2x6 Side member detailing: End distance = 7D minimum = 4.37” Use 4.5”, so CΔ = 1.0 Recheck capacity: nf λ φ Z’ Cg = 11.5 kips > 11.2 kips OK

The end distance clearance rules are OK, so the adjusted factored capacity is 11.2 kips with six 5/8” diameter bolts OK!

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Example 4: Nailed Joint Design A F & P A®


A single shear nailed connection in ASD and LRFD process.

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Example 4: Nail Connection single shear connection • 2 x 6 S-P-F side member • 6 x 6 Southern pine main member • 16d common wire nails • 400 lb. (dead) + 1200 lb. (snow) How many nails are required? We need to connect a 6x6 and a 2x6 together is a simple single shear connection to withstand the given loads.

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Mode Is Equations (Nail) NDS:

LRFD:

Dt s Fes Z= KD Z=

3.3Dt s Fes KD

Recall the ASD and LRFD yield capacity equations for nails in Mode Is….

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Mode IIIm Equations (Nail) NDS:

Z=

k1DpFem (1+ 2Re ) KD

LRFD:

Z=

3.3k1DpFem (1+ 2Re ) KD

…those for IIIm...

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Mode IIIs Equations (Nail) NDS:

Z=

k2 Dt s Fem ( 2 + Re ) KD

LRFD:

Z=

3.3k2 Dt s Fem ( 2 + Re ) KD

...those for Mode IIIs….

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Mode IV Equations (Nail) NDS:

LRFD:

D 2 2Fem Fyb Z= KD 3(1 + Re ) 3.3D 2 2Fem Fyb Z= KD 3(1 + Re )

…and those for Mode IV. One these is going to govern. Find out right after this next commercial...

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Nail Connection Parameters NDS

LRFD

Dowel bearing strength to wood main member

Fem

5550 psi

5.55 ksi

Dowel bearing strength of wood side member

Fes

3350 psi

3.35 ksi

90,000 psi

90 ksi

Fastener bending yield strength

Fyb

The connection material values are given here.

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Nail Connection Parameters D = 0.162 in. ts = 1.5 in. L = 3.5 in. (p = 2.0 in.) NDS CD = 1.15

LRFD λ = 0.8 φ = 0.65

Here is the relevant data from the problem statement for the connection including design process factors for ASD and LRFD, which we enter into the previous Yield Mode equations to get...

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Nail Connection Results (Z) NDS

LRFD

Mode Is

370 lbs.

1.221 k

Mode IIIm

260 lbs.

0.857 k

Mode IIIs

153 lbs.

0.506 k

Mode IV

134 lbs.

0.441 k

….these results, showing that Mode IV (in white) governs. It is the lowest number.

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Nail Connection (NDS) D + S < n Z CD 400 + 1200 < (n) (134) (1.15) n = 10.4 Must use 11 nails (16d)

The ASD version of the capacity equation requires the use of 11 nails...

275

Nail Connection (LRFD) 1.2 D + 1.6 S < n λ φ Z (1.2)(0.4) + (1.6)(1.2) < (n) (0.8)(0.65)(0.441) n = 10.5 Must use 11 nails (16d)

….and so does the LRFD capacity equation: 11 (16d) nails installed with clearances sufficient to prevent splitting of the wood (NDS 12.4.1).

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Example 5: Nailed Splice Check A F & P A®


This is a short example worked in LRFD only to demonstrate a capacity check for a nailed splice connection.

277

Nailed Splice Joint Check 2 X 6 Southern Pine, No. 1 Pu(1.2D + 1.6S) 18 – 8d common nails per side

½-in Douglas-fir OSB (24/0 sheathing) Pu What is the tensile capacity of this connection? A 2x6 butt connection made with double OSB side plates and 18 - 8d nails per side.

278

Nailed Splice Joint Check Single shear p > 6D into 2x6’s 2.5”-15/32” > 6(0.131”) Cd = 1.0 15/32-in Douglas-fir OSB (24/0 sheathing)

Pu

Penetration into the 2x6’s is > 6D, so Cd = 1.0.

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Nailed Splice Joint Check Yield Modes • Is • IIIm • IIIs controls • IV

Running the numbers through the Mode equations reveals that Mode IIIs is critical (lowest number governs)...

280

Nailed Splice Joint Check Factored Capacity = λ φ Z’ λ = 0.8 φ = 0.65 Nail Capacity Z’ = n ZIIIs Cd = (18)(0.253 kips)(1.0) Factored Capacity = 2.4 kips …which we feed into the factored capacity equation to get 2.4 kips from the nails.

281

Nailed Splice Joint Check OSB Capacity = 3.6 kips 2x6 Capacity = 12.8 kips Nails control at 2.4 kips

Checking the material capacities in tension shows higher values than the nails (the OSB next critical at 3.6 kips) and this is good. We could optimize further to raise the entire connection capacity to the level of the OSB by adding nails until we approach about 3.5 kips; but the rating on this specified connection is 2.4 kips.

282

Nailed Splice Joint Check Placement • pitch spacing = 1.5” • gage spacing = 2.0” • edge distance = 1.75”

Pu

Checking placement rules reveals no problems.

283


Example 6: Shear Wall Chord Ties with Nails A F & P A®


A shear wall chord tie using nails, in LRFD.

284

Nailed Tension Tie How many nails for this connection? Design connection ties between first and second floor shear wall chords. Floor framing consists of 9.5” deep pre-fabricated wood I-joists. Walls are 2x6, dry Douglas Fir-Larch studs spaced at 16” OC. The factored wind overturning force is 2.4 kips.

The first practical consideration faced by a designer in this case is to choose a fastener type. Many proprietary pre-fabricated metal connectors are available to make this connection, (see AF&PA Guideline for Pre-Engineered Metal Connectors). However, a connection can be designed that will use commonly available, non-proprietary, components.

285

Nailed Tension Tie Try: • ASTM A446 Grade metal strap 16 gage x 2.5” wide • 2 rows staggered 10d common nails

Ts D Ip Fyb Fem Fes

= 0.06” = 0.148” = 3.0” = 90 ksi = 4.65 ksi = 45 ksi

Adjustment Factor: Penetration actual p = 3.0” > 12D Cd = 1.0

Re

= Fem / Fes = 4.65 / 45 = 0.103

Material design parameters are listed here. Since the strap is so thin, the penetration adjustment factor produces a value of 1.0.

286

Nailed Tension Tie Mode IIIs controls; factored lateral strength λφzZ’: Unfactored unit capacity: k 2 = −1 +

= −1 +

k 2 = 12.66

2(1 + Re ) 2Fby (2 + Re )D + 2 Re 3Fem t s

2

2(1.103) 2(90)(2.103)(0.148) 2 + 0.103 3(4.65)(0.06) 2

Z= =

3.3k 2 Dt s Fem K D ( 2 + Re ) 3.3(12.66)(0.148)(0.06)(4.65) 2.2(2.103)

Z = 0.373 kips

First, calculate the Unfactored unit capacity Z of the nail from Mode IIIs (AF&PA / ASCE 16-95 equation 7.4-3)...

287

Nailed Tension Tie Mode IIIs controls; factored lateral strength λφzZ’: Factored unit capacity:

λφ z ZCd = 1.0(0.65)(0.373)(1.0) = 0.242 kips Factored demand:

α otWot = 1.5(2.4) = 3.6 kips

Number of nails:

n=

3.6 = 14.9 → 15 nails 0.242

Use 15 - 10d nails per side, or 2 rows of 8 each. …then factor it using (AF&PA / ASCE 16-95 equation 7.1-1) to get the factored resistance of one nail. Determine the factored demand on the tie from wind overturning using the appropriate load factor. Divide the demand into the resistance (both factored) to arrive at the number of nails required: 15 per side of the joint, in this case. Increase to 16 (2 rows of 8 each) for ease of installation.

288

More examples? These examples and more can be found in these AF&PA publications

More worked problems can be found in these AF&PA publications.

289

Connections …and you thought connecting wood was complicated!

…of course you can design connections for other materials: Picture courtesy: SMI-Owen Steel Company “This 12 Ton multi-directional bracing connection node was used in the Mellon Bank high rise building in Philadelphia and was detailed by hand by Mr. John Alonzo of Steel Graphics."

290

Questions? • www.awc.org – Online eCourses – FAQ’s

• HelpDesk – [email protected] – (202) 463-4713 or (800) 292-2372

• Comments – [email protected] This concludes this approved continuing education program.

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Wood Connections

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