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1C O N C R E T E D E S I G N H A N D B O O K • T H I R D E D I T I O N

Chapter 13Tilt-Up Concrete Wall Panels

By Gerry Weiler

April 2006

2C O N C R E T E D E S I G N H A N D B O O K • T H I R D E D I T I O N

Effect of Recent Code Changeson Tilt-up Construction

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CSA A23.3-04 - Design ofConcrete 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

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CSA A23.3-04 - Chapter 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

P ∆

P e ∆

DeflectedShape

+

PanelLoading

PrimaryMoment

SecondaryMoment

CombinedMoment

W =

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CSA A23.3-04 Chapter 23Tilt-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

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Moment Magnifier MethodBasic equations:

Mmax = Mb + Pf ∆max

Mb = applied factored moment

Pf = factored axial load 5 l 2 Mmax∆max = –– –––––––48 Ec Icr

Mmax 48 Ec Icr–––– = ––––––– = Kbf∆max 5 l 2

Kbf = bending stiffness

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Moment Magnifier MethodPf Mmax

Mmax = Mb + ––––––Kbf

Rearranging:⎧ 1 ⎫

Mmax = Mb ⎨–––––––– ⎬ = Mb δb⎩ 1- Pf / Kbf⎭

1 δb = ––––––––

1 - Pf / Kbf

= moment magnification factor

Gives identical results to iteration method

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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:

π2 E I 9.87 E IKbf = Pcr = –––––– = ––––––

l 2 l 2

• For tilt-up panels the following is more representative:

48 E I 9.6 E IKbf = –––––– = ––––––

5 l 2 l 2

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Member Resistance Factor• Member resistance factor, φm is used to

reduce the calculated bending stiffness:⎧ 1 ⎫

Mmax = Mb ⎨ –––––––––– ⎬ = Mb δb⎩ 1- P /φm Kbf ⎭

φm = 0.75• φm 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

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Area of Reinforcement Modification (23.3.1.5)

φs As fy + PfAs eff = ––––––––––– (23-4)φs fy

• 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

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Clause 23.3.1.2Provides limit on axial compression

Pwf + Ptf–––––––– < 0.09 φc f´cAg

• 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

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Panel Height to Thickness Limitations (23.2.3)

Max l / tSingle mat of reinforcement 50(centered in panel)2 mats of reinforcement 65(25mm clear to each face)

• Primarily intended as practical limits• Panel thickness my be controlled by

service load deflections• Reduce the above limits by half for

cantilevers

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Maximum UnsupportedPanel Height

Panel ReinforcementThickness Single mat Double mat

140 mm 7.0 m 9.1 m160 mm 8.0 m 10.4 m190 mm 9.5 m 12.4 m260 mm 13.0 m 16.9 m

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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 tilt-up panel will often be in tension at ultimate loads

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New Clause 23.3.2 ServiceLoad Deflection Limitations

• Provisions expanded and now similar in format to strength calculations

• Stiffness reduction factor, φm 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

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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

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Loading Conditions on Tilt-up PanelsChanges in NBCC 2005 for “Principal Loads”

and “Companion Loads”Factored Resistance Principal Load Companion Load

1. φR 1.4D2. φR 1.25D + 1.5L + 0.5S or 0.4W3. φR 1.25D + 1.5S + 0.5L or 0.4W4. φR 1.25D + 1.4W + 0.5L or 0.5S5. φR + effect of 0.9D 1.4W or 1.5L or 1.5S6. φR 1.0D + 1.0E + 0.5L or 0.25S7. φR + effect of 1.0D 1.0E

# 4 and 6 usually control for out-of-plane bending# 5 and 7 apply to in-plane shear

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Lateral Wind Loads on Panels(NBCC 2005, Clause 4.1.7)

p = Iw q Ce (Cp Cg - Cpi Cgi)Iw = 1.0 for normal buildings (ULS)

= 0.75 for serviceability (SLS)q = 1 in 50 reference velocity pressureCp Cg = typically + 1.3 or - 1.5 for tilt-upCpi = + 0.30 or - 0.45 for buildings with

only a few small openingsCgi = internal gust factor fixed at 2.0 (?)Ce = exposure factor ranges from 0.7 to

1.0 for most low rise buildings

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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

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Wind Load Design ComparisonPanel Height 30 ftWidth 20 ftThickness 6.25"Self Weight 80 psfReinf 2 layers

d = 4.75" Roof DL = 500 plfRoof LL = 1000 plf

.

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Wind Loads on Tilt-upWall 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)

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Comparison of Wind LoadsDesign Wind Pressures:

NBCC 1995 NBCC 2005 ASCE 7-02+ve W 21.2 psf 25.5 psf 13.6 psf+ve Wf 31.8 psf 35.7 psf 21.8 psf

------ + 12% - 31 %-ve W 23.0 psf 24.1 psf 15.1 psf-ve Wf 34.5 psf 33.7 psf 24.2 psf

------ - 2% - 30%Total Panel Reinforcement:

A23.3-94 A23.3- 04 ACI 318-021473 lbs 1536 lbs 1100 lbs

Difference ------ + 4% - 25%

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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 AxSp = –––––––– where 0.7 ≤ Sp ≤ 4.0

Rpand

hxAx = 1 + 2 ––

hn

• hx usually be taken as the center of mass of the panel at each storey

• Rp is typically 2.5 for wall panels

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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

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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

DesignCrossSection

bd

/ 2

/ 2l

l

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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 widthPwf + Ptf–––––––– < 0.09 φc f´cAg

Beam Load

DesignCrossSection

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= Design Widthb d

bd

bd

/ 2

/ 2l

l

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Panel Self WeightR

2 ∆3

W

1

R2

c

Wc

Wcl

∆3

2 Wc ∆R1 = R2 = ––––––3 l

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

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Continuity and End Fixity• Panels extending below

floor slab• Effect of lateral soil

pressure below floor slab• Consider the

effectiveness of footing restraint

• Continuous multi storey panels

• Moments may be affected by lateral deflections at flexible supports

PrimaryMoment

P

WM

SecondaryMoment

1 M

M2M2

δ

δ

1

= Moment Magnifierδ

• Additional lateral loads from intermediate floors

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Continuity and End Fixity• Not specifically covered in CSA A23.3-04• 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

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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

bb

= Design WidthTypicalDesign Strip

t

bd

bt

b

=12 t maxbd

d

= Tributary Widthbt

bd

t = Panel Thickness

=12 t max

=12 t maxbd

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Isolated Footings and Pier Foundations

• 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

CriticalCrossSection2

1

Joist Load

bd

DesignStrip

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Stiffening Pilasters• Support large vertical

loads• Provide increased out-

of-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 supportedon Pilaster

Roof

Floor

Pilasterat edgeof opening

Header BeamOver Opening

Roof

Floor

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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

R

1

a

2

H

.

R

c

b

1R

R2

W/2

W/2

H

W

W W LoadDiagram

MomentDiagram

Deflection

x

l

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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

l

M

Wc

∆3

∆max

l W2.

∆2

∆ 1

Roof

Floor

l

W 1

1

1

∆3

∆ 2

22Fixedbase

c

W W

2c

c

Panel with Parapet

Cantilever Panel

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Cantilever Panelswf lc2

Mb = ––––2

Wc ∆ Wc Mf lc2Mf = Mb + –––– = ––– ––––

3 3 4 EIMf lc2 4 EI

∆max = ––––; Kbc = ––––4 EI lc2

Moment magnifier equation:1

–––––––––Mf = Mf δc where δc = Wc1 - ––––––

3 φm Kbc

l

M

Wc

∆3

∆max

Fixedbase

Wc

Cantilever Panel

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Panels Subjected to In-Plane Shear• Lateral wind or seismic forces resisted by tilt-

up 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 wallsIn- Plane Shear fromRoof or Floor Diaphragm

Panel to SlabConnector

In-Plane Shear Forces

Panel to PanelShear Connector

In- Plane Shear fromRoof or Floor Diaphragm

Panel to SlabConnector

In-Plane Shear Forces

Panel to PanelShear Connector

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In-Plane ShearDesign Considerations:• Panel overturning• Panel sliding• Concrete shear

stress• Axial load stability• Frame action• Seismic ductility

In- Plane Shear fromRoof or Floor Diaphragm

PanelWeight

Panel to PanelShear Force

Resisting Forceat Foundations

In-Plane Shear Forces

Panel Shear

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Resistance to Panel Overturning• Panel overturning taken

about an outside corner• Point of rotation is usually

near the corner• All applied forces are

factored• Forces and weights resisting

overturning are modified in accordance with NBCC

• Provide connections to adjacent panels or tie down anchors to foundations for increased overturning resistance

b

Vr

Wpanel

Vpanel

Vroof

floor

roof

panel

2nd Floor

Roof

Vfloor

Main Floor

Vr fdn

Main

main

C of G

R

Foundation

Panel Overturning Resistance

ll

l

l

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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

SOLID BARWELDED TOEMBED ANGLE

ANGLE WITHREBAR ANCHOR

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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

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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

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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

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Seismic Design for Frame Panels

New Clause 21.7.1.2Tilt-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.

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Seismic Design Requirements for Solid Shear Panels

New Clause 21.7.1.2The 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 φc √ f ´c

vc = 0.33 MPa (48 psi) for 30 MPa concrete

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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 ( 1.140 )• 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

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Rotational Demand fora Simple Panel

Rotational Demand =

∆

Vf

L

∆ L/

10M Ties

4-20M EF

Left Leg10M Ties

4-20M EF

Right Leg

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Typical Wall Elevation ofTilt-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

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Typical Elevation ofFrame 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

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Single Storey Office

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Panels with Openings

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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

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Frame Panel

• 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.

Roof VF

PlasticHinge

WP

VP

Floor VF

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Joint in Frame Panel

HookedLongitudinalReinforcement

Continuous Vertical Reinforcement

Closed Hoop Ties

ConcreteSpalling

HeaderStirrups

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Frame Panel Reinforcement

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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 restrictive6db or h/2 (4” or less)

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Panel Leg Cross Section

HoopTies

Cross Ties

HoopTies

Section withCross Ties

Section withoutCross Ties

• Axial Stress in vertical legs is usuallysmall and typically less than 0.10 f ’c

• Panel legs can often be detailed without cross ties

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Warehouse Panel Detailed forIn-Plane Shear Forces

Roof VF

WP

VP

Floor VR

Ties @ 6 o/c at Joint

"

Ties @ 12 o/cBelow Joint

"20M VerticalBars in Legs

2 - 15MHorizontalBars inHeaders

Ties @ 4 o/cabove Joint

’

10M @ 18Ea Face

"72

10M @ 10 Alt Face"

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3 Storey Tilt-Up

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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

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New Clause 21.7.4Squat 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

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Squat Shear Walls• Can apply to solid tilt-up panels where shear

stresses–––vf ≥ 0.1 φc √ 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 this clause by

using “Conventional Construction” withRd = 1.5 and Ro =1.3

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Connections for Tilt-up Panels

3 major categories:• Cast-in-place concrete infill

sections• Welded embedded metal• Drilled-in anchors

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Cast-In-Place Concrete In-fill

Exterior grade

HookeddowelFloor slab infillafter panelinstallation

Rebar pinsor weldedconnection

Strip Footing

Panel on Strip Footing

Extend rebarinto pilaster

Cast-in-placepilaster with ties

Chamferon outsideface

Cast-In-Place Pilaster

Extend panelrebar intoconnection

Cast-in-place infill sectionChamfer

on outsideface

Cast-In-Place Panel Infill

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Cast-In-Place ConcreteIn-fill Sections

• Usually very strong and can emulate cast-in-place concrete

• Good seismic ductility• Excessive restraint for concrete

shrinkage• Post construction cracking

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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

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Welded Embedded Metal

Steel Joist on Angle Seat

EM3embedplate

Edge angleSteel decking

Angle seatfield welded toembed plate

Edge AngleConnection

EM2embedplate

Edge angleSteel decking

Angle tiestrutsSteeljoist

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Welded Embedded Metal

Shear PlateConnection

Steel BeamConnection

Edge angle

EM2embedplate

EM4embedplate

Steelbeam

Bolts withslotted holesShear plate fieldwelded to embedplate

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Welded Embedded Metal

Exterior grade

Strip footing

Panel on Strip Footing

EM5 in floor slab

Weld to filler barand EM2 in panel

Solid barwelded toembed angle

Embed angle with studs

Panel Edge Connector (not recommended)

Potential crack

EM5 Angle with Rebar Anchor

Solid BarWelded toEmbed Angle

Panel Edge Connector(for seismic shear transfer)

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StandardizedConnections

V

T20

0

4 6 0

11

3 8 m m

VT

EM5 Edge Connector

20M Gr 400 Rebar Anchor

Vr = 125 kN, Tr = 100 kN

L 38 x 38 x 6 x 200mm

V

T

f

f

f

f

ff

V

Tf

f

EM3 Shear Plate PL 200 x 9.5 x 2004 - 16mm studs

V

Tf

f

EM4 Shear Plate PL 225 x 9.5 x 4608 - 16mm Studs

EM2 Shear Plate PL 150 x 9.5 x 2002 - 16mm studs 100mm 150mm Studs Vr = 65 kN 65 kNTr = 50 kN 95 kN

Studs

100mm 150mm Studs Vr = 110 kN 130 kNTr = 65 kN 130 kN

Studs

100mm 150mm Studs Vr = 170 kN 265 kNTr = 90 kN 200 kN

Studs

Standard Tilt-up Connectors

d = 1 0 0o r 1 5 0 m m

d = 100mm 150mm Vr = 110 kN 110 kNTr = 45 kN 70 kN

EM1 Joist Seat L89 x 89 x 6 x 300mm2 - 15M Gr 400 Rebar Anchors

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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

71

Panel Edge Connectorin form

72

Panel Edge Connector after Welding

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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

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Connections for Seismic Forces• Vp = 0.3 Fa Sa(0.2) IE Sp Wp

Cp Ar AxSp = ––––––– where 0.7 ≤ Sp ≤ 4.0

Rpand

hxAx = 1 + 2 ––hn

• hx usually be taken as the center of mass of the component being connected

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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 Sp

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Construction Requirements

• Includes recommended panel forming tolerances

• Concrete cover for reinforcement• Concrete strength and mix

recommendations• Reinforcing Steel

77

Design for Lifting andBracing of Panels

• The handbook provides only basic guidelines for lift design

• Refers to TCA Guide and British Columbia WCB regulations for panel bracing

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AcknowledgementsThe following provided assistance in reviewing 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 EngineeringAnd of course Rick McGrath and Andy Viser of CPCA

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End of Presentation