Chapte Chap terr 13 Tilt-Up Concrete Wall Panels By Gerry Weiler April 2006
C O N C R E T E D E S I G N H A N DB O O K •
T H I RD
E D I TI O N
1
Effect of Recent Code Changes on Tilt-up Construction
C O N C R E T E D E S I G N H A N DB O O K •
T H I RD
E D I TI O N
2
CSA A23 A23.3.3-04 04 - Des Design ign of Concrete Structures • • • •
Includes changes affecting Tilt-up Construction Minor revisions to Chapter 23, Tilt-up Wall Panels Comprehensive changes to Chapter 21, Special Provisions for Seismic Design Significant effect on seismic requirements for tilt-up and other low rise buildings
3
1
CSA A23 A23.3.3-04 04 - Cha Chapte pterr 23, 23, Tilt-up Wall Panels •
•
Provides a simplified method for analysis and design of slender concrete walls Based on flexural tension yielding of the longitudinal reinforcement
P
e
∆
Pe
W
+
P∆ =
Deflected Shape
Panel Loading
Primary Moment
Secondary Moment
Combined Moment
4
CSA A23.3-04 Chapter 23 Tilt-up Wall Panels • • • •
Covers only the basic aspects of tilt-up panel design Limited to vertical and out-of-plane lateral forces Design for in-plane shear forces not included in this chapter Chapter 13 of the handbook provides additional guidelines for the application of A23.3 5
Moment Magnifier Method Basic equations: Mmax =
Mb + Pf
Mb
=
appl ap plie ied d fac facto tore red d mom momen entt
Pf
=
factor fact ore ed axi axial al lo loa ad 2 5 l M –– –––––max –– 48 Ec Icr
max
=
M ––max –– = max
Kbf
=
max
48 E I –––––c –cr – = K bf 5l2 bend be ndin ing g sti tiff ffne nes ss 6
2
Moment Magnifier Method Mmax =
Pf Mmax Mb + –––––– Kbf
Rearranging: Mmax =
b
Mb
1 –––––––– 1- Pf / Kbf
= Mb
b
=
1 –––––––– 1 - Pf / Kbf
=
moment magnification factor
Gives identical results to iteration method 7
Bending Stiffness Bending stiffness, Kb is the maximum moment divided by maximum deflection It will vary, depending on support conditions, type of loading and properties of the cross section For pure axial load it is the same as the Euler buckling load:
• •
•
2 EI 9.87 E I Kbf = Pcr = –––––– = –––––– 2 l l2
For tilt-up panels the following is more representative:
•
48 E I 9.6 E I Kbf = –––––– = –––––– 5 l2 l2 8
Member Resistance Factor •
Member resistance factor, m is used to reduce the calculated bending stiffness: 1 Mmax = Mb –––––––––– = Mb 1- P / m Kbf m
•
• •
=
b
0.75
has been increased from 0.65 to 0.75 and is now consistent with CSA A23.3, Chapter 10 and ACI 318-2002 P-delta deflections decreased Designs will be slightly less conservative m
9
3
Area of Reinforcement Modification (23.3.1.5) s A s fy + P f As eff = ––––––––––– s fy • • •
(23-4)
Simulates increased strength due to axial load on the cross section Not specifically permitted for increasing bending stiffness in CSA A23.3-04 More conservative than ACI and UBC codes where stiffness modification is permitted
10
Clause 23.3.1.2
Provides limit on axial compression Pwf + Ptf
–––––––– Ag
• • •
< 0.09
c
f´c
Assumptions for bending stiffness and P-delta effects not valid with large axial loads Axial loads on most tilt-up panels are small Sometimes affects panels with large openings and narrow legs
11
Panel Height to Thickness Limitations (23.2.3) Max l / t Single mat of reinforcement (centered in panel) 2 mats of reinforcement (25mm clear to each face) • • •
50 65
Primarily intended as practical limits Panel thickness my be controlled by service load deflections Reduce the above limits by half for cantilevers 12
4
Maximum Unsupported Panel Height Panel Thickness 140 mm 160 mm 190 mm 260 mm
Reinforcement Single mat Double mat 7.0 m 8.0 m 9.5 m 13.0 m
9.1 m 10.4 m 12.4 m 16.9 m
13
New Clause 23.2.10 “Where vertical reinforcement is placed in two layers, the effect of compression reinforcement shall be ignored”
Reasons: • Assumptions for bending stiffness and P-delta effects are not valid with compression reinforcement • Reinforcement on the compression side of thin concrete cross sections in a tiltup panel will often be in tension at ultimate loads 14
New Clause 23.3.2 Service Load Deflection Limitations • •
• • •
Provisions expanded and now similar in format to strength calculations Stiffness reduction factor, m not included in deflection calculations span Service load limit of –––– is unchanged 100 Panel deflections are rarely problematic Recent studies suggest that the CSA / ACI methods for deflection of thin concrete members may be non-conservative 15
5
Creep and Initial Deflections • • •
•
The design should allow for initial deflections due to warping or uneven casting beds Differential shrinkage and thermal gradients may also be a factor Long term creep has not been a significant problem because axial loads are usually small Clause 23.3.1.4 requires a minimum initial deflection 0 = l / 400
16
Loading Conditions on Tilt-up Panels Changes in NBCC 2005 for “Principal Loads” and “Companion Loads” 1. 2. 3. 4. 5. 6. 7.
Factored Resistance Principal Load Companion Load R 1.4D R 1.25D + 1.5L + 0.5S or 0.4W R 1.25D + 1.5S + 0.5L or 0.4W R 1.25D + 1.4W + 0.5L or 0.5S R + effect of 0.9D 1.4W or 1.5L or 1.5S R 1.0D + 1.0E + 0.5L or 0.25S R + effect of 1.0D 1.0E
# 4 and 6 usually control for out-of-plane bending # 5 and 7 apply to in-plane shear 17
Lateral Wind Loads on Panels (NBCC 2005, Clause 4.1.7) p Iw q Cp Cg Cpi
= = = = = =
Cgi Ce
= =
Iw q Ce (Cp Cg - Cpi Cgi) 1.0 for normal buildings (ULS) 0.75 for serviceability (SLS) 1 in 50 reference velocity pressure typically + 1.3 or - 1.5 for tilt-up + 0.30 or - 0.45 for buildings with only a few small openings internal gust factor fixed at 2.0 (?) exposure factor ranges from 0.7 to 1.0 for most low rise buildings 18
6
Lateral Wind Loads on Panels • • • •
Effect of new wind load provisions are not a significant change for tilt-up design NBCC 2005 load factor reduced to 1.4 for wind Design wind pressures are typically greater compared to ASCE requirements Panels reinforcement for high, simply supported panels is directly proportional to wind loads
19
Wind Load Design Comparison Panel Height 30 ft Width 20 ft Thickness 6.25" Self Weight 80 psf Reinf 2 layers d = 4.75" Roof DL = 500 plf Roof LL = 1000 plf
.
20
Wind Loads on Tilt-up Wall Panels •
• •
100 kph (62 mph) was selected for the 1:30 reference wind speed in NBCC 1995. The reference pressure q would be 0.5 kPa (10.4 psf) Corresponding 1:50 wind speed for NBCC 2005 is 105 kph (65.2 mph), or q = 0.55 kPa (11.5 psf) Equivalent 1:50 wind speed for ASCE 7-02 is 85 mph (137 kph), q = 15.4 psf (0.74 kPa)
21
7
Comparison of Wind Loads Design Wind Pressures: +ve W +ve Wf -ve W -ve Wf
NBCC 1995 21.2 psf 31.8 psf -----23.0 psf 34.5 psf ------
NBCC 2005 25.5 psf 35.7 psf + 12% 24.1 psf 33.7 psf - 2%
ASCE 7-02 13.6 psf 21.8 psf - 31 % 15.1 psf 24.2 psf - 30%
Total Panel Reinforcement:
Difference
A23.3-94 1473 lbs ------
A23.3- 04 1536 lbs + 4%
ACI 318-02 1100 lbs - 25% 22
Lateral Seismic Loads on Panels • •
Obtained from NBCC 2005, Clause 4.1.8.17 Vp = 0.3 Fa Sa(0.2) IE Sp Wp Cp Ar Ax Sp = –––––––– Rp and hx Ax = 1 + 2 –– hn
where 0.7
Sp
4.0
•
hx usually be taken as the center of mass of the panel at each storey
•
Rp is typically 2.5 for wall panels 23
Lateral Seismic Loads on Panels • •
• •
Cp is the risk factor equal to 1.0 Ar is usually 1.0, but could be as high as 2.5 for if the fundamental period of the building is similar to that of the panel component Large warehouse buildings with tall panels may be affected Lateral seismic forces may be similar in magnitude to wind loads and both should be checked 24
8
Axial Loads • • •
•
•
Axial (vertical) loads from roof or floor members Assume uniform line load for multiple joists Apply minimum eccentricity of ½ panel thickness Effect of eccentricity should be additive to lateral load effect Do not use wind uplift to reduce axial load
Joist Load .
l/2
.
Design Cross Section
l/2
bd
25
Large Axial Loads •
• •
Large axial loads can sometimes be supported on the panel Restrictions on effective panel design width bd Check for axial stress in the design width Pwf + Ptf –––––––– Ag
Beam Load
l /2
2 1
Design Cross Section
l /2
bd
< 0.09
c
f´c
bd
bd = Design Width
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Panel Self Weight R1
2 Wc R1 = R2 = –––––– 3l
2∆ 3
Mid-height moment:
R1 l Wc Wc M = –––– + –––– = –––– 2 2x3 2 Wc = panel self weight Panel weight above the critical section acts as an additional axial load
l
Wc
∆
3 Wc
R2 Wc
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9
Continuity and End Fixity •
Panels extending below P floor slab Effect of lateral soil pressure below floor slab M Mδ Consider the W effectiveness of footing restraint Mδ M Continuous multi storey panels Primary Secondary Moments may be affected Moment Moment δ = Moment Magnifier by lateral deflections at flexible supports Additional lateral loads from intermediate floors
• •
1
•
2
•
•
1
2
28
Continuity and End Fixity • • • •
Not specifically covered in CSA A 23.3-04 Simplified, but conservative methods are often used Assume simple spans with a reduced effective length k l k should not be less than 0.9
29
Openings in Panels •
•
•
•
Effect of openings approximated by using vertical design strips Gives reasonable accuracy and economy for most designs Distribute entire axial and lateral load over the tributary width to the design strips each side of the opening Limit design width to 12t
bd
b bt b d =12 t bd =12 t max max
bt = Tributary Width bd = Design Width t = Panel Thickness
bt bd =12 t max
Typical Design Strip
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10
Isolated Footings and Pier Foundations • •
• •
•
Joist Load
Panels support at each end of panel Continuous lateral restraint at top (roof) and bottom (floor) Design strip bd limited to 12t Distribute all vertical loads, including self weight into the design strips. Lateral bending resisted by entire panel width
Design Strip
2 1
Critical Cross Section
bd
31
Stiffening Pilasters • •
• •
Support large vertical loads Provide increased outof-plane bending resistance at edges of large openings Provide ties at beam bearing points Compression ties otherwise not required with axial stresses less than 0.10f´c
Beam supported on Pilaster
Roof
Floor
Roof
Header Beam Over Opening Pilaster at edge of opening
Floor 32
Concentrated Lateral Loads •
•
•
•
End reactions from header beams in wide panels Lateral loads from wind or seismic on intermediate floors Lateral loads from cranes or other equipment Opposing lateral loads from suspended elements such as canopies
R1
R1
a
l
W/2 H
W
b
W/2 H
c
x
.
R2
R2 W
W Load Moment Deflection Diagram Diagram
33
11
Cantilever Panels Free standing signs and screen walls with cast-in-place concrete footings Parapets above the roof of a building Moment magnifier method can be used but Kb factor is different
•
• •
∆ max
∆1 ∆
Wc1
3
l
c
2
l1
W
Wc
∆1
W Roof
Fixed base
2 ∆2 3
M
Cantilever Panel
.
l
Wc2
2
Floor ∆2
Panel with Parapet
34
Cantilever Panels Mb
wf lc2 = –––– 2
∆ max ∆
3
Wc Wc Mf lc2 Mf = Mb + –––– = ––– –––– 3 3 4 EI Mf lc2 4 EI Kbc = –––– max = ––––; 4 EI lc2
l
c
W
Wc
Fixed base
M
Cantilever Panel
Moment magnifier equation: Mf
= Mf
c
where
c
1 ––––––––– Wc = 1 - –––––– 3 m Kbc 35
Panels Subjected to In-Plane Shear • • • •
Lateral wind or seismic forces resisted by tiltup panels around the building perimeter Sometimes, interior shear walls are required Concrete shear stresses are usually very small No specific design guidelines in Chapter 23 of CSA A23.3-04 for design of tilt-up shear walls In- Plane Shear from Roof or Floor Diaphragm
Panel to Slab Connector
Panel to Panel Shear Connector
In-Plane Shear Forces
36
12
In-Plane Shear Design Considerations: • Panel overturning • Panel sliding • Concrete shear stress • Axial load stability • Frame action Panel to Panel • Seismic ductility Shear Force
In- Plane Shear from Roof or Floor Diaphragm Panel Shear Panel Weight Resisting Force at Foundations
In-Plane Shear Forces
37
Resistance to Panel Overturning • • • •
•
Panel overturning taken Roof Vroof about an outside corner Point of rotation is usually near the corner Vfloor 2nd Floor All applied forces are Vpanel C of G l roof factored W l floor Forces and weights resisting l panel Vr main Main Floor overturning are modified in l Main accordance with NBCC Foundation Provide connections to Vr fdn R adjacent panels or tie down b anchors to foundations for Panel Overturning Resistance increased overturning resistance panel
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Panel Edge Connector (For Seismic Shear Transfer) • • • •
Embedded angle with rebar anchor Recess below surface Good for seismic ductility Resists overstress
ANGLE WITH REBAR ANCHOR
SOLID BAR WELDED TO EMBED ANGLE
39
13
Resistance to Sliding Friction between the panel and the foundation Direct bearing of panel to notch in floor slab Connections to foundations or floor slab Minimum 2 connections at base of panel is recommended Soils resistance to sliding should also be checked
• • • • •
40
Chapter 21, Special Provisions for Seismic Design • • •
•
Past requirements originally intended for monolithic concrete elements The unique aspects of tilt-up construction are not specifically addressed Seismic forces in tilt-up buildings often resisted by a series of individual wall panel elements Solid panels do not have a well defined mechanism for seismic energy dissipation
41
Section 21.7, Moderate Ductility • • • • •
Rd = 2.0 and Ro = 1.4 for moderately ductile shear walls Rd = 2.5 and Ro = 1.4 for moderately ductile frames Requires capacity design principles and a well defined energy absorbing mechanism Does not recognize panel rocking as a legitimate seismic energy absorbing system Dimensional limitations for solid shear walls are severe and impractical for tilt-up 42
14
Seismic Design for Frame Panels
New Clause 21.7.1.2 Tilt-up Wall Panels shall be designed to the requirements of Clause 23 except that the requirements of Clause 21.7.2 (Moderately Ductile Moment Resisting Frames) shall apply to wall panels with openings when the maximum inelastic rotational demand on any part of the panel exceeds 0.02 radians. However, the inelastic rotational demand shall not exceed 0.04 radians.
43
Seismic Design Requirements for Solid Shear Panels New Clause 21.7.1.2 The requirements of Clause 21.7.4 (Squat Shear Walls) shall apply to solid wall panels when the maximum in plane shear stress exceeds
vf
vc = 0.1
––– c
f ´c
vc = 0.33 MPa (48 psi) for 30 MPa concrete
44
Clause 21.7.2 - Moderately Ductile Moment Resisting Frames • • • • •
•
Rd = 2.5 and Ro = 1.4 Applies to frame panels, where joint rotational demand exceeds 0.02 radians ( 1.14 0 ) Provides a joint rotational limit of 0.04 radians Difficult to check rotational demand except for very simple and regular buildings Analysis of deflections and rotational demand for buildings with a mixture of stiff and flexible panel elements may be impractical Clause 21.8 Conventional Construction will be easier to apply to tilt-up 45
15
Rotational Demand for a Simple Panel Vf Rotational Demand = ∆/ L
L
∆
4-20M EF
10M Ties Left Leg
4-20M EF
10M Ties Right Leg
46
Typical Wall Elevation of Tilt-up Panels
• •
Most tilt-up warehouses consist of solid panels with high in-plane shear strength and stiffness Shear force capacity often limited by panel overturning 47
Typical Elevation of Frame Panels
• • •
Panel widths, thickness and opening configurations may vary in a wall line Panels may also be interrupted or offset within the wall line The provisions in Chapter 21 are difficult to apply 48
16
Single Storey Office
49
Panels with Openings 50
Panels with Openings • • • •
Large openings are common in tilt-up office buildings Panels designed as moment resisting frames to resist in-plane shear Plastic hinges may develop at some interior panel joints Panel overturning should be checked, but may not control the design
51
17
Frame Panel Roof VF
VP
Plastic Hinge
Floor VF WP
• •
Panel header beams are often deep compared to width of supporting legs Plastic hinging is more likely to occur in the legs rather than the headers. 52
Joint in Frame Panel Continuous Vertical Reinforcement Closed Hoop Ties Header Stirrups
Concrete Spalling
Hooked Longitudinal Reinforcement
53
Frame Panel Reinforcement
54
18
Clause 21.7.2 - Moderately Ductile Moment Resisting Frames • • • •
The provisions were modified to reflect changes to NBCC Close spacing of ties required to prevent buckling of longitudinal reinforcement Tie spacing of 8db or 6.25" (160mm) for 20M longitudinal bars in beams Tie spacing in columns is more restrictive 6db or h/2 (4” or less) 55
Panel Leg Cross Section Cross Ties
Hoop Ties
Hoop Ties
Section with Cross Ties • •
Section without Cross Ties
Axial Stress in vertical legs is usually small and typically less than 0.10 f ’c Panel legs can often be detailed without cross ties 56
Warehouse Panel Detailed for In-Plane Shear Forces Roof VF 2
7
"
10M @ 18 Ea Face
Ties @ 4’ o/c above Joint VP
"
10M @ 10 Alt Face
WP
2 - 15M Horizontal Bars in Headers 20M Vertical Bars in Legs
Floor VR
"
Ties @6 o/c at Joint "
Ties @12 o/c Below Joint
57
19
3 Storey Tilt-Up
58
Solid Panels • • • •
Panel overturning should be checked and often controls the panel design Edge connections added to resist overturning Panel hold down ties to foundations are rarely used Energy absorption achieved by deformation of edge connectors and panels rocking on foundations
59
New Clause 21.7.4 Squat Shear Walls •
•
The wall is required to yield and absorb seismic energy, but the diaphragm must remain elastic Allows the designer to opt out by designing the wall and diaphragm for Rd = R0 = 1.0, or about 2.8 times the prescribed earthquake force
60
20
Squat Shear Walls •
Can apply to solid tilt-up panels where shear stresses vf
•
• • •
0.1
–––
c
f ’c
Equivalent to a threshold in-plane shear force of 3400 plf for a 6” tilt-up panel for 4350 psi (30MPa) Permits Rd = 2.0 and Ro =1.4 Buildings with a mixture of stiff and flexible panels may fall into this category Designers will likely try to avoid t his clause by using “Conventional Construction” with Rd = 1.5 and Ro =1.3 61
Connections for Tilt-up Panels 3 major categories: • • •
Cast-in-place concrete infill sections Welded embedded metal Drilled-in anchors
62
Cast-In-Place Concrete In-fill Chamfer on outside face
Extend rebar into pilaster
Chamfer on outside face
Cast-in-place infill section Extend panel rebar into connection
Cast-in-place pilaster with ties
Cast-In-Place Pilaster
Cast-In-Place Panel Infill
Exterior grade
Hooked dowel Floor slab infill after panel installation Rebar pins or welded connection Strip Footing
Panel on Strip Footing
63
21
Cast-In-Place Concrete In-fill Sections • • •
Usually very strong and can emulate cast-in-place concrete Good seismic ductility Excessive restraint for concrete shrinkage
•
Post construction cracking
64
Welded Embedded Metal • • • •
Good strength and low to moderate ductility Adaptable to a variety of applications Edge distance is sometimes a problem Preferred by most designers and builders due a relative cost advantage
65
Welded Embedded Metal Edge angle Steel decking EM3 embed plate
EM2 embed plate
Angle seat field welded to embed plate
Steel Joist on Angle Seat
Edge angle Steel decking Angle tie struts Steel joist
Edge Angle Connection
66
22
Welded Embedded Metal EM2 embed plate
EM4 embed plate Steel beam
Edge angle
Bolts with slotted holes Shear plate field welded to embed plate
Shear Plate Connection
Steel Beam Connection
67
Welded Embedded Metal Solid bar welded to embed angle
Embed angle with studs Potential crack
Panel Edge Connector (not recommended)
Exterior grade
Solid Bar Welded to Embed Angle
EM5 Angle with Rebar Anchor
Panel Edge Connector (for seismic shear transfer)
Weld to filler bar and EM2 in panel EM5 in floor slab
Strip footing
Panel on Strip Footing 68
Vf Tf
EM1 Joist Seat L89 x 89 x 6 x 300mm 2 - 15MGr 400 Rebar Anchors d = 100mm 150mm Vr = 110 kN 110 kN Tr = 45 kN 70 kN
d = 100 or 150mm
Standardized Connections
EM2 Shear Plate PL150 x 9.5 x 200 2 - 16mmstuds
Vf
100mm 150mm Studs Studs Vr = 65 kN 65 kN Tr = 50 kN 95 kN
Tf
EM3 Shear Plate PL200 x 9.5 x 200 4 - 16mmstuds
Vf
100mm 150mm Studs Studs Vr = 110 kN 130 kN Tr = 65 kN 130 kN
Tf
EM4 Shear Plate PL225 x 9.5 x 460 8 - 16mmStuds
Vf
100mm 150mm Studs Studs Vr = 170 kN 265 kN Tr = 90 kN 200 kN
Tf
4 6 0
EM5 Edge Connector 38mm
0 0 2
1 1
Vf Tf
L38 x 38 x 6 x 200mm 20MGr 400 Rebar Anchor Vr = 125 kN, Tr = 100 kN
Standard Tilt-up Connectors
69
23
Standardized Connections • • • • • •
Developed by an SECBC Committee in Vancouver Testing Carried out at UBC by Kevin Lemieux Included monotonic and cyclic testing Includes 5 basic connector types Decreases cost of fabrication Provides load capacities for design
70
Panel Edge Connector in form
71
Panel Edge Connector after Welding
72
24
Drilled-in Anchors • • • • •
Includes expansion bolts, adhesive anchors and coil inserts Limited strength and ductility Readily available and inexpensive May be used where other connections are incorrectly installed Suitable for light architectural components
73
Connections for Seismic Forces •
Vp = 0.3 Fa Sa(0.2) IE Sp Wp Cp Ar Ax Sp = ––––––– Rp and hx Ax = 1 + 2 –– hn
•
where 0.7
Sp
4.0
hx usually be taken as the center of mass of the component being connected
74
Connections for Seismic Forces •
Cp = 1.0 for ductile connections = 2.0 for non ductile connections
•
Ar = 1.0 for rigid elements = 2.5 for flexible elements
•
Rp = 1.0 for non ductile connections = 2.5 for ductile connections Connection forces are less than previous code due to the limit of 4.0 on the S p
•
75
25
Construction Requirements • • • •
Includes recommended panel forming tolerances Concrete cover for reinforcement Concrete strength and mix recommendations Reinforcing Steel
76
Design for Lifting and Bracing of Panels • •
The handbook provides only basic guidelines for lift design Refers to TCA Guide and British Columbia WCB regulations for panel bracing
77
Acknowledgements The following provided assistance in r eviewing and checking this document: • Kevin Lemieux, Ben Benjamin, Brent Weerts; WSB Consultants • Andy Metten; Bush Bohlman • John Wallace, Pomeroy Engineering • Perry Adebar, Ken Elwood; UBC • Ron DuVall; RJC • Jim Mutrie; JKK • Walid Salmon, Sal Tabot, Calvin Schmitke; Krahn Engineering • Bill McKevitt; McKevitt Engineering And of course Rick McGrath and Andy Viser of CPCA 78
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