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Design Guide for Tilt-Up
Concrete Panels
Reported by ACI Committee 551
� (^) American Concrete Institute
� Licensee=Chongqing Institute of quality and Standardizationb 5990390 Not for Resale, 2015/10/28 23:22:
American Concrete Institute Always advancing
First Printing August 2015 ISBN: 978-1-942727-30-
Design Guide for Tilt-Up Concrete Panels
Copyright by the American Concrete Institute, Farmington Hills, MI. All rights reserved. This material may not be reproduced or copied, in whole or part, in any printed, mechanical, electronic, film, or other distribution and storage media, without the written consent of ACI.
The technical committees responsible for ACI committee reports and standards strive to avoid ambiguities, omissions, and errors in these documents. In spite of these efforts, the users of ACI documents occasionally find information or requirements that may be subject to more than one interpretation or may be incomplete or incorrect. Users who have suggestions for the improvement of ACI documents are requested to contact ACI via the errata website at http://concrete.org/Publications/ DocumentErrata.aspx. Proper use of this document includes periodically checking for errata for the most up-to-date revisions.
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8.2-Resistance to sliding, p. 18 8.3-Concrete shear resistance, p. 19 8.4-Seismic ductility, p. 19 8.5-In-plane frame design, p. 19 8.6-Lateral analysis of wall panels linked in-plane, p. 20
CHAPTER 9-CONN ECTIONS FOR TILT-U P PAN E LS, p. 20
CHAPTER 1 0-CONSTRUCTION REQUIREMENTS, p. 25 1 0. 1-Forming and construction tolerances, p. 25 10.2-Concrete for tilt-up panels, p. 25 1 0.3-Panel reinforcement, p. 26
CHAPTER 1 1 -DESIGN FOR LIFTING STRESSES, p. 26 1 1. 1-General lifting concepts, p. 26 1 1.2-Steps for performing a lifting design, p. 27 1 1 .3-Lifting considerations: building engineer of record, p. 27 1 1.4-Lifting design considerations: panel specialty engi neer, p. 28
CHAPTER 1 2-TEMPORARY PANEL BRACING, p. 29
CHAPTER 1 3-REFER ENCES, p. 30 Authored references, p. 30
APPEN DIX A-DERIVATION OF M n AND len p. 30 A. l-Derivation of M,, and fer based on rectangular stress block, p. 30 A.2-Derivation of M,, and fer based on triangular stress distribution, p. 3 1
APPENDIX 8-DESIGN EXAMPLES FOR OUT-OF P LANE FORCES, p. 31 B.l-Panel with no openings design example, p. 33 B. IM-Panel with no openings design example (metric), p. 35 B.2-Panel with a 10 x 15 ft door opening design example, p. 39 B.3-Panel with concentrated axial load design example, p. 44 B.4-Panel with concentrated lateral load design example, p. 48 B.5-Multi-story panel design example, p. 5 1 B.6-Panel with dock-high condition design example, p. 56 B. 7-Plain panel with fixed end design example, p. 61
B.8-Plain panel on isolated footing or pier design example, p. 65 B.9-Panel with stiffening pilasters and header design example, p. 68
CHAPTER 1 -INTRODUCTION Tilt-up concrete buildings have been constructed in North America for over 100 years, but it was not until the late 1990s that ACI 3 1 8 specifically addressed the requirements for design of slender concrete walls. ACI 3 18-1 1, 14.8, provides a method of analysis and covers only the basic requirements for evaluating the effects of vertical and transverse out-of plane loads. ACI 3 1 8-11, Chapter 10, may also be used to design slender walls, but the requirements are more general and should be applied with discretion. This guide expands on the provisions of ACI 3 1 8- 1 1 , Section 14.8, andASCE/SEI 7 and provides a comprehensive procedure for the design of these structural elements. This guide also provides design recommendations for various support and load conditions not specifically covered in ACI 3 1 8, and includes design guidelines for in-plane shear.
CHAPTER 2-NOTATION AND DEFINITIONS
2.1 -Notation
D d
ece F FP !c' f,. h GCp= GCP;= H h fe fer fe
gross area of concrete section, in.2 (mm2) area of tension reinforcement, in.2 (mm2) effective area of tension reinforcement, in.2 (mm2) area of shear reinforcement, in.2 (mm2) depth of equivalent rectangular stress block, in. (mm) design width, in. (mm) tributary width, in. (mm) width of the concrete section, in. (mm) distance from the extreme fiber to the neutral axis, in. (mm) dead load distance from the extreme concrete compression fiber to the centroid of tension reinforcement, or the effective depth of section, in. (mm) distance from the extreme compression fiber to centroid of extreme layer of longitudinal tension steel, in. (mm) loads due to seismic force concrete modulus of elasticity, psi (MPa) steel modulus of elasticity, psi (MPa) eccentricity of applied load(s), in. (mm) loads due to weight or pressure of fluids factored load specified compressive strength of concrete, psi (MPa) modulus of rupture, psi (MPa) reinforcement yield stress, psi (MPa) external pressure coefficient internal pressure coefficient horizontal line load or soil pressure panel thickness, in. (mm) importance factor cracked section moment of inertia, in.4 (mm4) effective moment of inertia, in.4 (mm4) Licensee=Chongqing Institute of quality and Standardizationb 5990390 American Concrete I nstitute - Copyright� @>fMate'l'llfl!Uli'II!/WIAA�ncrete.org
Kb bending stiffness, in.-lb/in. (N·mm/mm) Kd wind directionality factor Kz velocity pressure coefficient at height^ z Kz1 topographic factor L live load L,. roof live load e vertical span of member between support le cantilever height ejloor =^ distance from floor diaphragm to the bottom of panel, in. (mm) lmain=^ distance from^ main floor to^ bottom of^ panel, in. (mm) lpaneF distance from panel center of^ gravity to the bottom of panel, in. (mm) l,.00J =^ distance from roof diaphragm to the bottom of panel, in. (mm) lw width of^ concrete section, in. (mm) Ma maximum moment at midheight of wall due to service lateral and eccentric vertical loads, including P-!1 effects, in.-lb (N·mm) Me,. = moment causing flexural cracking of the concrete section, in.-lb (N·mm) M,nax=^ maximum moment occurring over the span of the panel due to uniform lateral loads, in.-lb (N·mm) M,, =^ nominal moment strength at the midheight cross section due to service lateral and eccentric vertical loads only, in.-lb (N·mm) M,, (^) = maximum factored combined bending moment, in.-lb (N·mm) M,w =^ maximum factored moment at midheight of wall due to lateral and eccentric vertical loads, not including P-!1 effects, in.-lb (N·mm) n modular ratio P applied axial load at top of panel P-!1 = secondary moment caused by axial load P acting on a deflected shape with displacement !1, in.-lb (N·mm)
R R R s s T
critical buckling load factored axial load effective velocity pressure at mean roofheight z, lb/ ft2 (N·m2) rain load in 4. vertical reaction at footing in 8. seismic response modification coefficient in 8. snow load short-period design spectral response acceleration maximum considered spectral response acceleration mapped short-period spectral acceleration spacing oftransverse shear reinforcement, in. (mm) cumulative effects of temperature, creep, shrinkage, settlement nominal shear strength of normalweight concrete, lb (N) Vfloor=^ floor diaphragm shear force V11 =^ total shear strength of^ the concrete section, lb (N) VpaneF panel shear force (seismic only VR main =^ resisting shear force at main floor V,·oof =^ roof^ diaphragm shear force Vs =^ nominal shear strength of^ the rejpforcement,lb{N)
w Wa We Wjlo01·= WpaneF W,·oo;= w
Wu
Pt
q, !1; !3.max = ! f1s^ =
wind load wind load based on serviceability wind speed panel self-weight weight of tributary floor structure weight of panel weight of tributary roof structure uniform lateral load factored self-weight of concrete wall panel above the base factored uniform lateral load on element mean roof height moment magnification factor unit weight of concrete, lb/ft3 (kg/m3) ratio of area of distributed longitudinal reinforce ment to gross concrete area perpendicular to that reinforcement ratio of area of distributed transverse reinforce ment to gross concrete area perpendicular to that reinforcement strength reduction factor initial deflection at midheight, in. (mm) maximum total deflection at midheight, in. (mm) maximum potential deflection at midheight, in. (mm) maximum out-of-plane deflection due to service loads, including P-!1 effects, in. (mm)
2.2-Defin itions ACI provides a comprehensive list of definitions through an online resource "ACI Concrete Terminology" at http:// www.concrete.org/store/productdetail.aspx ?ItemiD=CT 13. Definitions provided herein complement that resource. compressive strength-measured maximum resistance of a concrete specimen to axial compressive loading; expressed as force per unit cross-sectional area. compressive stress-stress directed toward the part on which it acts. connection-a region that joins two or more members. modulus of elasticity-ratio of normal stress to corre sponding strain for tensile or compressive stress below the proportional limit of the material; also called elastic modulus, Young's modulus, and Young's modulus of elas ticity; denoted by the symbol E. moment frame--frame in which members and joints resist forces through flexure, shear, and axial force. net tensile strain-tensile strain at nominal strength exclusive of strains due to effective prestress, creep, shrinkage, and temperature. seismic-force-resisting system-portion of the structure designed to resist earthquake design forces required by the legally adopted general building code using the applicable provisions and load combinations. tensile stress-stress directed away from the part on which it acts. tension-controlled section-cross section in which the net tensile strain in the extreme tension fiber at nominal strength is greater than or equal to 0.005. slender wall-wall, structural or otherwise, whose thickness-to-height ratio make it susceptible to secondary
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e
\ \ \ I I I I I I I I I II '' ''� II II II II II II I I
Uniform lateral load
Deflected shape
Mmax
Mmar
� 8
'
I ' '
I, 'I I \
! I L'l
' ' I I , I It, I It·I
Fig. 3. 2a-Maximum deflection due to lateral load only.
e
f---,
MY'""" M
,, ,, ,,
I I I (^) I II
II
11 Deflected 1 shape I M �v Constant moment Moment
1'
\ \ \ \ \ \ I I I I ' I I I I I I I I I 2
L'l= Ml 8EJe
Fig. 3. 2b-Maximum deflection due to constant lateral moment.
Where the wall element is subjected to axial load P, only as shown in Fig. 3.2c, plus a small initiating eccentricity, maximum deflection at midheight is given by
L1max
Mma./2^ Mma./ 1t2 EJe 9.87 EJe
in which
M,nax= Pllmax
American Concrete Institute
e
p
\
\ ^
\
\
I (^) I I (^) I I I Mmax I (^) I I I I (^) I
II
1 1 shape J
p Mmax =P-J'I, Axial load
Fig. 3. 2c-Deflection due to axial load only.
1'
\ \ \ \ \ \ I I I I I I I I I I I I I I 2
/').= Mmaxe 9.87EJe
Maximum moment in a tilt-up panel is usually the result of a combination of these loading conditions. Lateral load effects are often large compared with end moments. Traditional methods for analyzing tilt-up panel walls have adopted the first of the aforementioned relations for deflec tion calculations
L),max
Bending stiffuess (^) Kb for a slender wall is therefore defined as
K
_ 48EJe 9.6EJe b 5T Ji. 2 _
This will slightly overestimate the deflection and maximum bending moment of a slender wall subjected to the combined effects of lateral and axial load for all axial loads that produce P-L'l moments larger than the moments produced by lateral loads. K6^ is similar in value to the more familiar term for critical buckling load, Per
Critical buckling capacity is a by-product of the P-L'l analysis and represents the maximum axial load that can be sustained by a pin-ended slender column or wall in the absence of any other applied loads. The factor 11? (^) = 9. defines a sinus()idal, single-curvature deflected shape due to
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the effects of a concentric axial load only. The applicable units for Per are force (k or kip [N or Newton]) and for K6, bending moment per unit deflection (ft-kip/ft or in.-kip/in. [N-m/m or N-mm/mm]). Section stiffness Eefe in the preceding equation varies with both axial and lateral loadings, degree of curvature of the panel, and properties of the concrete section. At ultimate load conditions, the concrete section exhibits cracks over most of the panel height. Full-scale testing in California in the early 1980s (SEAOSC 1982) and analytical studies by the SEAOSC Slender Wall Task Group (Lai et a!. 2005) veri fied that a value ofEJe equal to the cracked section stiffness EJer correlates closely with the load-deflection character istic of the test results. Ultimate load deflections using the preceding l'lmax equations will likely be overestimated. The cracked section moment of inertia, len can be taken as
where c= a/ AJY a (^) =-�'-- 0.85fc'b 01 =^ 0.85 forfc'^ :S^ 4000 psi (28 MPa)
= 0.85- o.o5(1c' -4000) � 0.65 forf' > 4000 psi (in.-lb units) 1000 '
(!'- 28 ) = 0.85 - 0.05 c 7
_ 2 0.65 forfc'> 28 MPa (SI units)
n =E/Ec Rectangular stress block stiffness has been used because the panel is at the ultimate load state. The development of this relationship, and a comparison to fer for a triangular concrete stress distribution, is provided in Appendix A. Applied axial forces will counteract a portion of the flexural tension stresses in the concrete section, resulting in increased bending moment resistance. For small axial stresses less than 0.1 Ofc', this can be accounted for by a simple modification of the area of reinforcement as follows
A (^) se = A (^) s+ �� (}!_) Jy 2d
Ase can also be used to account for the increased bending stiffness when computing P-l'l deflections. The assumption that concrete section stiffness is equal to EJer and is constant over the entire height of the panel is considered valid for factored load conditions. The calcu lation for fer is based on the value of c for the rectangular concrete stress block that occurs at ultimate loads rather than kd for the triangular stress distribution that occurs at service loads, because the purpose is to compute deflections at ultimate loads. ACI 3 1 8 adopted this approach in 1999,
1000 ft-lb
3750 ft-lb
Panel loading
Primary moment
Secondary Combined moment moment
Fig. 3.3-Slender wall example calculation-suction force acting with eccentric axial load. (Note: 1 in.= 24.5 mm; 1ft=
3.3-lteration method for P-1:. effects As noted in the previous section, the maximum moment M,,.ax^ in a slender wall element typically occurs at or near the midheight section. It is the sum of the applied moment Ma and the P-l'l moment
The relation between maximum bending moment and deflection is
�max
The solution to the previous two equations can be obtained by a simple iterative procedure. The following example illustrates the method, assuming a 12 in. (300 mm) wide panel strip depicted in Fig. 3.3. The assumed parameters are: e = 20 ft (6. 10 m) w= 25 lb/ft2 (1.2 kPa) P (^) = 4000 plf (5400 N/m) at top of panel eec = 3 in. (76 mm) Eeler= 45 x 1 06 lb-in.2 (129 x 106 N-mm2)
K = 48EJa (^) = 48 45 X (^106) = (^7500) b in.-lb/in.^ (33^ kN-mm/mm) 5£2 5 202 X (^122)
M = 25x-+4000x-x-^202 3 I = 1250+500 = 1750 ft lb (25.5 kN-m) a 8 2 12 Start with:
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The term b will be used in place of ew for the remainder of this guide to avoid confusion with the term ec for wall height and the substitution of Ase defined by
A (^) se =^ A (^) s+ �� (}!_) Jy 2d
A factor of 0.75 is used to reduce the calculated bending stiffuess of the concrete section in accordance with ACI 3 1 8- 1 1 , Chapters 10 and 14. It is intended to account for vari ations in material properties and workmanship. Test results (SEAOSC 1982) and analytical studies (Lai et a!. 2005) have indicated that small variations in the position of reinforcement in the concrete section will have a significant effect on the strength of the wall panel and the bending stiffuess properties. A reduction in bending stiffness is not limited to the provi sions of ACI 3 1 8. It should also be employed where other methods of slender wall analysis are used. Historically, design methods for tilt-up panels have not specifically included this requirement. The 0.75 reduction factor in bending stiffness should be incorporated by all other alternate design methods to comply with the requirements of ACI 3 1 8. Nominal moment strength M,, is obtained in accordance with ACI 3 1 8- 1 1 , Chapter 1 0, requirements by using the modified area of reinforcement, Ase· Procedures outlined in ACI 3 1 8- 1 1 , 1 4.8.3, provide a simplified method for design of slender concrete walls commonly used in tilt-up construction. The following limi tations should be noted: (a) Section stiffness EJm as obtained from the rectangular stress block derivation using the modified area of reinforce ment, As-. as permitted by 14.8 of ACI 3 1 8-1 1 , is valid for small axial loads only. Section 1 4.8.2.6 requires a limit of 0.06f! for the vertical stress P, /Ag at the midheight section. (b) Using a constant value for panel bending stiffness Kb is valid for walls subjected to out-of-plane bending forces due primarily to uniform lateral loads. Where applied moments are primarily due to end moments from eccentric axial loads, a reduction in bending stiffness may be necessary. (c) The ACI 3 1 8 method is intended for simply supported wall elements only. It can, however, be adapted to panels with fixed-end conditions, or for those spanning over multiple supports, as will be demonstrated in Chapter 7 of this guide. (d) ACI 3 1 8-1 1 , 14.8.2.4, provides a minimum strength requirement for the concrete section of
( 14-2)
where
as outlined in ACI 3 1 8, 1 0.2.7 (refer to Appendix A for the deri vation ofthis equation using the rectangular stress block) and
Mer = (^) J,S (9-9)
where
bh2 I S = -^ =^ ...!..^ (uncracked section modulus) (^6) Y,
J; =^ 7.5'Afl: (^) (9- 1 0)
where /... is taken as 1 .0 for normalweight concrete. The purpose of this provision is to prevent a sudden increase in lateral deflection where panel cracking occurs as a result of a temporary overload condition. An ancillary benefit is that it also contributes to improved behavior of the panel during the lifting operation. In many cases, the rein forcement required to satisfy ACI 3 1 8- 1 1, 1 4.8.2.4, will be greater than minimum reinforcement. ACI 3 1 8- 1 1, 9.3.2.1 and 1 0.3.4, indicate that the value of q, for tension-controlled flexural members is equal to^ 0.9. This occurs when the steel tension strain is greater than or equal to 0.005 and when the concrete in compression reaches its assumed strain limit of 0.003. This is equivalent to cld1 being less than 0.375 (refer to the commentary ofACI 3 1 8, 9.3.2.2) and is required for tilt-up panels designed in accordance with the provisions of ACI 3 1 8- 1 1, 14.8.
3.6-Comparison t o 1 997 Un iform B u i ld i n g Code The procedure specified in the Uniform Building Code (International Code Council 1 997) was developed specifi cally for tilt-up concrete panels. Primary factored bending moments are calculated from the applied loading to the panel. Maximum potential deflection is computed for the condition where the panel section is assumed to be at ulti mate bending moment over the entire span. The secondary P-/',. moment resulting from axial loads acting over this deflected shape is then added to the primary moment. Where the combined effects are less than or equal to the ultimate resisting moment of the concrete section, strength design requirements are considered satisfied.
M,, =^ Ma^ +^ Pu/',.u^ :S^ q,M,,
where
Other forces contributing to the primary moment, such as lateral out-of-plane point loads, need to be included when computing Ma. The effect of panel self-weight should also be taken into account in the P-/',. calculations, although this is not specifically stated in the Uniform Building Code (Inter national Code Council 1997); it is discussed in 4.3. The potential midheight deflection is given by
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_^ 5M /!n_c^2 48 EJcr
where, for normalweight concrete
Ec = 57,000Jl: psi (in.-lb units)
Ec =^4700 Jl: MPa (SI units)
and
Modification in the area of reinforcement, as outlined previously, is used to partially account for the increased bending resistance due to applied axial loads.
This equation is applicable for a single layer of reinforce ment at the center of the panel only. Other applicable requirements specified in the Uniform Building Code (International Code Council 1997) include: (a) Vertical service load stress at the location of maximum moment does not exceed 0.04fc' (b) Sufficient reinforcement should be provided so that the nominal moment capacity times the factor � is greater than Me, (c) Midheight deflection Lls under service lateral and vertical loads (without load factors) shall be limited by the relation
(^11) s =^ .!.s_ 150
This is discussed further in Chapter 6. The bending moment obtained from the ACI 318 proce dure will be greater than that from the Uniform Building Code because ACI requires the application of the 0. stiffness reduction factor whereas Uniform Building Code does not. This difference is partially offset in the Uniform Building Code by the method of computing the maximum potential deflection based on moment M,, rather than �M,,.
3.7-Limitations on panel slenderness ACI 318- 1 1, 14.8, does not provide a specific limit for wall panel slenderness ratios. The strength design provisions are self-limiting, and arbitrary limits on panel structural thickness or maximum deflections due to factored loads are not required. There are, however, practical limits in height to-thickness ratios for slender conoretewalls. The following
American Concrete Institute
guidelines taken from CAN/CSA A23.3 may be useful to designers
Single mat of reinforcement (centered in the panel cross section) ............................... 50 Two mats of reinforcement ( 1 in. [25 mm] clear of each face) .................................. 65 Where panel height-to-thickness ratio exceeds these limits, quantities of reinforcement may not be economical, and a thicker panel should be considered. Section 14.8.2. of ACI 318- 1 1 effectively limits the amount of tension rein forcement in a panel by requiring the wall section to be tension-controlled. By meeting this criterion, impending failure of a wall section is observable through large deflec tions and cracking (refer also to ACI 318-11, R10.3.4). Panel thickness may also be controlled by limitations on service load deflections. Refer to Chapter 6 for more information regarding these limitations.
CHAPTER 4-LOADING CON DITIONS
4.1 -Lateral loads The effect of lateral loads on tilt-up panels is often the largest contribution to the total applied bending moment. Wind pressures, soil pressures, or seismic accelerations are usually applied to the wall panel as a distributed lateral load. 4.1.1 Wind loads-ASCEISEI 7 specifies wind pressure for low-rise buildings as the algebraic difference between suction or external and internal pressure. Wind exerts loads on walls either as a net inward or a net outward pressure. Applicable building standards should be consulted for the proper determination of these forces, including any modi fications or amendments adopted by the authority having jurisdiction.
where the velocity pressure qz is determined by Eq. 30.3- 1 in ASCE/SEI 7-10 as
where Kd = 0.85 for building components and cladding; Kz = 0.98 for z = 30 ft (9 m); K21 = 1 .0 for smooth terrain; and qz = 0.00256 X 0.98 X 1 .0 X 0.85 X 1 1 52 X 1 .0 (^) = 28.2 lb/ft (1 .35 kPa)
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Fig. 4. 3-Panel self-weight.
2 L'l 3
Further, axial load should not be reduced due to wind uplift on roof members. Where large concentrated loads are supported directly on the panel, the effective width bd of the design cross section should be limited, as indicated in Fig. 4.2b. Extra reinforce ment, where required, should be concentrated in this area of the panel. The maximum factored axial stress on the design width b" is limited to 0.06fc'.
4.3-Panel self-weight The effect of panel self-weight should be considered because it represents a significant contribution to P- moments in slender walls. It is sufficient to assume that the weight of the panel above the midheight section acts as an additional concentrated axial load with no eccentricity (that is, concentric to the panel centroid) applied at the midheight. This is illustrated in Fig. 4.3 and by the following derivation
R 1 =R =-- 2 c- 3£c
The midheight moment is
4.4-Load factors and combinations ACI 318-11 , 9 .2. 1, specifies the following factored load combinations
U= 1 .4(D + F) (9-1)
U = 1 .2(D + F + T) + 1 .6(L + H) + 0.5(L,. or S or R) (9-2)
U= 1 .2D + 1 .6(L,. or S or R) + (1 .0L or 0.5 W) (9-3)
U = 1.2D + l .OW + l .OL + 0.5(L,. or S or R) (9-4)
U = 1 .2D + l .OE + l .OL + 0.2S (9-5)
U= 0.9D + l .OW+ 1 .6H (9-6)
U= 0.9D + 1 .0£ + 1 .6H (9-7)
All of these load combinations should be checked for slender wall design. The reader can observe the following with respect to individual equations: (a) Equation (9-1) does not generally govern in slender wall design because it predominantly relates to structures resisting fluid pressures. (b) Equation (9-2) may control the design for walls supporting dead and live loads in combination with lateral soil pressures. (c) Equation (9-3) could govern the design of walls supporting large gravity loads. (d) Equation (9-4) often controls the design of slender wall panels in low to moderate seismic locations. (e) Equation (9-5) could control the design for panels in high seismic areas, but results from Eq. (9-3) and (9-4) should be compared to determine the controlling condition. (f) Equations (9-6) and (9-7) are intended for situations where higher dead loads reduce the effects of other loads. They do not govern the design for most tilt-up panel appli cations, except for panel overturning calculations due to in-plane lateral loads. For the common load case of large bending moments due to lateral forces combined with small axial loads, the crit ical section for bending will occur near panel midheight. As axial load and top end eccentricity increase, this point shifts upward. The wind load factor reflects the switch to strength-level (factored) loads in ASCE/SEI 7 as discussed in ACI 318- 1 1 , R9.2. 1(b). Use of service-level wind loads calculated from earlier versions of ASCE/SEI 7 is permitted by substituting 1 .6 Wand 0.8 Win the previous equations for 1 .0 Wand 0.5 W, respectively.
CHAPTER 5-MI N I M U M REINFORCEMENT
5.1 -General Due to their segmented nature, experience has shown that there are fewer problems associated with temperature changes and concrete shrinkage in tilt-up panels than with monolithic cast-in-place concrete structures. There are, however, some design techniques that should be considered. Tilt-up panels are often cast and lifted into place within a period of 1 to 2 weeks, and may not have sufficient time to fully cure. If connections to the panels are made immedi ately after panel erection, the restraint induced could cause a buildup of stresses in the concrete as it continues to undergo drying shrinkage. Minimum horizontal reinforcement based on 0.002Ag may be insufficient to limit cracking. For this American Concrete Institute Provided by IHS under license with ACI No reproduction or networking permitted without license from IHS
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reason, the erector should delay the completion of connec tions as long as practical. Alternatively, increased reinforce ment in the direction of restraint should be considered to counter the stresses that could be caused by making connec tions early. Buildings with tilt-up panels have the advantage that each joint can act as an expansion joint. It is possible to have continuous lengths of wall panels without any special provi sions for thermal expansion or shrinkage. Some designers, however, may specify connections along the vertical joint of all panels, even if it is not justified by design analysis. This can result in excessive restraint and vertical cracking. Variations in relative humidity or temperature between the inside and outside panel faces can induce warping. These effects are usually small and can be accounted for in design by including an initial deflection in the calculations (Chapter 6). Panel warping due to temperature differentials can result in splitting ofthe caulk along the joint at intersecting comers. A simple solution is to routinely connect the panels together at these comers by means of welded embedded metal connectors.
5.2-ACI 3 1 8 provisions If a tilt-up wall spans vertically, the horizontal reinforce ment could likely be governed by minimum shrinkage and temperature reinforcement. The designer is permitted to determine shrinkage and temperature requirements by means of a thorough analysis of the structure. The minimum wall reinforcement requirements need not be met if the structural analysis shows that the walls meet the require ments of ACI 318-11, 14.2.7. Designers pursuing this approach are cautioned to consider all load effects and boundary conditions as a function of time. While all the provisions for minimum reinforcement are important, only minimum vertical and horizontal reinforcement provisions are discussed herein. Tilt-up concrete construction is a unique form of precast concrete (ACI 318-11, R16. 1. 1). The general structural integrity requirements ofACI 318-11, 7. 13.3, reference 16. for precast concrete. There are several integrity provisions in ACI 318- 1 1 , 16.5 that apply to tilt-up walls and their connections. If a wall resists in-plane shear force, and factored shear exceeds one-half concrete shear design resistance, hori zontal and vertical shear reinforcement should be provided (ACI 318-11, 1 1.9.9). For relatively short walls with a low height-to-length ratio, the amount of vertical shear rein forcement will exceed the horizontal shear reinforcement. For relatively tall walls or walls with a high height-to-length ratio, the amount of horizontal shear reinforcement will exceed the vertical shear reinforcement. If shear reinforce ment is required, ACI 318-11, 1 1 .4.5 and 1 1 .4.6, provide minimum limits on steel area and spacing for both horizontal and vertical shear reinforcement. Tilt-up walls, therefore, can be subject to the minimum vertical and horizontal reinforcement provisions of ACI 318-1 1, 14.3. Note that if the wall requires in-plane shear reintorcement, the minimum shear reinforcement provisions
of ACI 318-11, 1 1.9.9, will govern over the minimum wall reinforcement provisions of 14.3. ACI 318-11, 14.3 also addresses the number of layers of reinforcement required, transverse ties for vertical bars, and special reinforcement required around openings. The minimum reinforcement for walls in ACI 318- 1 1 , 14.3.2 and 14.3.3 addresses shrinkage and temperature reinforcement. Section 14.3 addresses all walls, including continuous cast-in-place walls. It is expected that the temperature and shrinkage requirements could be reduced for walls with frequent joints, such as tilt-up walls that are not linked together in a way that causes restraint. Crack control in tilt-up panels is deemed to be satisfied when the reinforcement is sufficient to satisfy the deflection limits of ACI 318-11, 14.8.4. Crack control can be particu larly important in tilt-up construction where the exterior faces of the panels are exposed to the elements or interior faces to a corrosive environment. Note that the use of high strength steel to reduce total reinforcement provided could effectively increase cracking. The smaller, minimum reinforcement indicated in ACI 318- 1 1 , 16.4.2, is not recommended for tilt-up panels because tilt-up panels are generally wider than plant-cast, precast panels and subject to more curing restraint. For seismic design, walls are classified as one of the following seismic-force-resisting systems: (a) Ordinary structural walls (ACI 318-11, 1 1.9.9; no required provisions in Chapter 21) (b) Intermediate precast walls (ACI 318- 1 1 , 2 1 .4) (c) Special structural walls (ACI 318-11, 2 1 .9) (d) Special structural walls constructed using precast concrete (ACI 318-11, 2 1. 10) Intermediate precast structural walls are governed by ACI 318-11, 21.4. This system requires targeted yielding of components of the connections either between the wall panels, or between the wall panel and foundation. Wall piers in this system must be designed per the special structural walls section (ACI 318-11, 2 1. 9) or members not designated as part of the seismic-force-resisting system (ACI 318- 1 1 ,
CHAPTER 6-CONTROL OF DEFLECTIONS Limitations on lateral or out-of-plane deflections for slender walls have traditionally been a concern of building officials and code committees, not only because of the increased bending moments due to P-l'l effects, but also the potential for long-term bowing of these elements. Experi ence in actual buildings, however, suggests that long-term deflections have not been a serious problem. This is likely due to the fact that the lateral forces that cause bending in panels are largely transient, and that the effect of axial Licensee=Chongqing Institute of quality and Standardizationb 5990390 American Concrete I nstitute - Copyright� @>fMate'l'llfl!Uli'II!/WIAA�ncrete.org
The following deflection limits are recommended to avoid residual deformations and negative effects on nonstructural components, respectively: (a) Total deflection with wind: L/150 to L/240 (designer discretion to increase limit based on type of veneer and sensitive nonstructural components as appropriate) (b) Total deflection with seismic: L/ The following service load combinations for checking deflection are recommended: a) Wind effects-Use the ASCE/SEI 7 wind speed map according to the proper importance category for the structure and the selected mean recurrence interval (MRI). For typical structures, a 50-year MRI is common among practicing engineers, but a I 0-year MRI may be warranted for service ability checks after the engineer and building owner review all the considerations and risks associated with this lower level of wind. The commentary to ASCE/SEI 7, Appendix C provides a good discussion on this topic.
D + 0.5L + Wa
b) Seismic effects
D + 0.5L + 0.7E
where E is a strength level force as calculated by ASCE/SEI 7 (refer to ACI 318-11, R14.8.4) and service moments are calculated with P-f,. effects.
CHAPTER 7-PANEL DESIGN PROCEDURES This section covers several common design conditions for vertical and transverse loading that could occur in tilt-up panels. Computer spreadsheet programs greatly simplify the design procedure. Design examples in Appendix B provide a breakdown of the analysis for panels, including comparisons with single and double mats of reinforcement.
br
rt
bd = l ' h max
X
b1 = tributary width bd =des1gn w1dth h = panel thickness
b br bd = 1 2 h max
Typical design stnp
Fig. 7. 2a-Design strips at openings. 7.2-Panels with ope n i ngs
bd = 1 h max
�
)
The effect of openings for out-of-plane bending in tilt-up panels can be approximated by a simple, one-dimensional strip analysis that provides accuracy and economy for most designs. Where openings occur, the entire lateral and axial load, including self-weight above the critical section, is distrib uted to supporting legs or design strips at each side of the opening (Fig. 7.2a). The effective width of the strip should be limited to approximately 12 times the panel thickness to avoid localized stress concentrations along the edge of the opening. This limit is not mandated by ACI 318, but is included in this document as a practical guideline where the opening width is less than one-half the clear vertical span. In most cases, the tributary width for loads can be taken as the width of the strip plus one-half the width of adjacent open ings. The design strip should have constant properties full height and the reinforcement should not be cut offjust above or below the opening. Thickened vertical or horizontal sections can be provided with the panel where openings are large or where there are deep recesses on the exterior face (Fig. 7 .2b). Some condi tions may require ties around all vertical reinforcement bars in a vertical pilaster for the full height of the panel.
7.3-Concentrated axial loads The effect of a concentrated axial load, such as the reaction from a roof or floor girder connected directly to the panel, was introduced in 4.2. The two most important consider ations for design are:
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Roof
Floor
Header beam over opening
Pilaster at edge of opening
Fig. 7. 2b-Stiffening header and pilasters.
Roof
on pilaster
Floor
Fig. 7. 3-Pilaster supporting beam load.
Where loads are very large, which is greater than 0. 1 Ofc'bdh, consider pilasters as shown in Fig. 7.3. These provide greater bearing area at the connection and increase the stiffness for out-of-plane bending. Consider increased local stiffness in the distribution of applied lateral loads. Provide ties around the vertical reinforcement in accor dance with requirements of ACI 318-1 1 , Chapter 7. Axial stress from beams, however, is usually concentrated at the point of bearing and quickly dissipates into the panel such that ties may not be required for the full height. ACI 318 does not mandate how the load should be distributed, so the designer has a choice if a member has to be consid ered a column and, therefore, subject to the requirement for confinement ties. ACI 318-11, 14.3.6, provides guidance on tie requirements specifically for wall applications. Often, overall panel design is controlled by flexural tension in vertical reinforcement rather than compression, and ties are not necessary. Ties within 12 in. (305 mm) of the point of bearing are recommended to ensure the axial load is distributed into the panel.
7.4-Concentrated lateral loads Concentrated lateral loads can occur due to:
w w Load diagram
Moment diagram
Fig. 7.4-Suspended canopy on panel.
Deflection
(a) Suspended elements, such as canopies, as shown in Fig. 7. (b) End reactions from header beams over wide panel openings (c) Lateral wind or seismic forces from intermediate roofs or floors where independent lateral-force-resisting systems have not otherwise been provided The effect of these loads can be included in the analysis by superimposing the moment directly with the other primary bending moments. This is a simplistic approach that may be too conservative, as the algebraic sum of the maximum moments does not consider direction of the applied load(s). The designer may consider a more rigorous analysis of the panel to determine the correct combination of moments to include for reinforcement analysis. 7.4.1 Example: Canopy supported on panel W = canopy load R1 = end reaction = Wxl2lc = R H = horizontal line load = Wx/2b where the horizontal load is a point load, the effective panel design width should be limited to no more than 12 times the panel thickness at the application point, and the load should be distributed evenly across this width. Additional reinforce ment could be required in this localized area.
7.5-M u ltiple spans and effects of contin uity Most tilt-up panels are designed as simply supported vertical members spanning between the footing and roof structure. Where a panel is connected to both floor slab and the footing (Fig. 7.5), a degree of panel continuity can be considered. A panel could also be laterally supported by an intermediate floor, resulting in negative bending at interme diate supports and a reduction of positive bending between supports. It is difficult to properly analyze this condition and, at best, only approximate methods are practical. Some analysis problems and limitations include:
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Fixed
. base � M
Fig. 7. 7a-Cantilever panel.
Fig. 7. 7b-Panel with parapet.
7.7-Cantilever panels
2
+---
Roof
Floor
Tilt-up panels are sometimes required to function as vertical cantilevers. Typical examples include freestanding signs and screen walls, or parapets above the roof of a building (refer to Fig. 7.7a and 7.7b). When the cantilever is high, P-f,. effects will increase the bending moments on the panel. A simple but conservative way to analyze a fixed-end cantilever panel is to assume a simply supported panel with a height two times the cantilever height. The more correct method of analysis for a fixed base cantilever is
American Concrete Institute
M =^
w,.( ua 2
M - M + Wet-,. -^ M +^ w;,^
M,/·c u ua 3 ua (^3) 4£/
M, 4EI Kb
4EI Kb = (^) - (^2 -
The maximum moment can now be written as
where 0.75 is included as a stiffness reduction factor. The dynamic effects of wind buffeting or seismic accel erations might temporarily increase cantilever deflection because there may be little structural damping. This should be considered when selecting design forces. Where the cantilever is a high parapet, a more detaih;d analysis may be required. As illustrated in Fig. 7.7b, rota tion of the panel section at the roof connection can increase deflection and the associated P-f,. effects. ·.
CHAPTER 8-IN-PLANE SH EAR Design procedures for in-plane shear forces are distinctly different from methods used in design for out-of-plane bending. Forces from the roof or floor diaphragms acting parallel to the plane of the wall induce shear stresses and overturning moments in the panels (Fig. 8). In seismic areas and regions with high wind, in-plane shear requirements may control panel thickness and reinforcement design. The design considerations for tilt-up panels subjected to in-plane forces include: (a) Resistance to panel overturning
In-plane shear from roof or floor diaphragm
Resisting force at foundations
Fig. 8-In-plane shear forces.
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____::..
1 � Panel shear
Panel-to-panel shea r force
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(b) Resistance to sliding (c) Concrete shear resistance (d) Increased axial forces on portions of the panel (e) Load distribution to foundations (f) Frame action in panels with openings (g) Seismic ductility In regions of low seismicity (Seismic Design Categories [SDC] A and B), wind most likely controls the lateral anal ysis, and a target failure mode is not required. In regions of moderate and high seismicity (SDC C, D, E and F), a ductile failure mode is desired with overstrength to guard against brittle failure modes. Energy dissipation can be accom plished through repeated inelastic deformations or rocking. For wall panel in-plane shear, the applicable code sections inACI 318- 1 1 are 1 1 .9.9 (SDC A, B), 21.4 (SDC C), or 21. and 21.10 (SDC D, E, F). Other situations where inelastic deformations could occur include extreme events, such as blast design, and in shelters, like those for tornados and hurricanes, which are designed as areas of refuge.
Vroof
...
Vjloor ...
e it
eJl, or
Wpanel
epanel VR main
emain
Fig. 8. 1a-Panel overturning resistance.
�I'
Roof
2nd floor
C of G
Main floor
Foundation
Panel-to-slab connector
8.1 -Resistance to panel overturning When roof and floor diaphragm forces are applied parallel to the plane of the wall panels, overturning moments and in-plane shears are induced. Overturning moments are usually taken near an outside corner of the panel. Resis tance to overturning is obtained from a combination of panel weight, tributary roof or floor loads, panel edge connectors, and tie-down anchors to the foundations. The actual point of rotation will be close to the outside corner of the panel, at the center of the bearing area between the panel and the footing. In most cases, assume that the width of bearing is zero. Footing pressures beneath the footing and the footing design capacity should be checked for this concentration of force. For the panel shown in Fig. 8.1a, the overturning equation in a seismic event is written as: Factored overturning moment Mo :S resisting moment MR
where the resisting moment is given by
All applied shear forces contributing to overturning are factored. Forces and weights that resist overturning should be reduced in accordance with load combination factors in ACI 318-11, 9.2, which is outlined in 4.4 of this guide. No additional safety factor is required. Where there is insuffi cient overturning capacity, edge connections to an adjacent panel or tie-down anchors to the foundation can be added until the overturning equation is satisfied (Fig. 8.1 b). Depending on the requirements of the seismic-resisting system, connections to the foundation for resisting over turning may need to consider ductility requirements in accordance with ACI 318-11, 2 1 .4 or 2 1. 1 0. This additional moment resistance could be limited by the weight of the foundation or adjacent panel. Founda tions should be checked to ensure that the footing capacity or the soil-resisting pressure is not exceeded. The geotech nical engineer should be consulted for allowable increases in bearing pressure due to wind and seismic forces.
8.2-Resistance to sliding Resistance to sliding forces can be obtained by a combina tion of friction between the bottom of panel and the footing, and connections to the floor slab or foundation (ACI 318- 1 1 ,
In-plane shear from roof or floor diaphragm
Panel-to-panel shear connector
Fig. 8. 1b-Typical shear wall with connections to adjacent panels and foundations. American Co ete Ins Licensee=Chongqing Institute of quality and Standardizationb 5990390 Provided by I (^) o m t e with ACI (^) American Concrete I nstitute - Copyright� @>fMate'l'llfl!Uli'II!/WIAA�ncrete.org No reproduction or �or mg permitted without license from IHS
