ACI PRC-551.3-21 PITFALLS OF DEVIATING FROM ACI 31 8 SLENDER WALL PROVISIONS-TECHNOTE

ACI PRC-551.3-21 PITFALLS OF DEVIATING FROM ACI 31 8 SLENDER WALL PROVISIONS-TECHNOTE. ACI-SEAOSC Task Committee on Slender Walls, 1982, “Test Report on Slender Walls,” ACI-SEAOSC, Structural Engineers Association of Southern California, Whittier, CA. Bischoff, P., and Scanlon, A., 2007, “Effective Moment of Inertia for Calculating Defections of Concrete Members Containing Steel Reinforcement and Fiber-Reinforced Polymer Reinforcement,” ACI Structural Journal, V. 104, No. 1, Jan.-Feb, pp. 68-75. Bischoff, P., and Darabi, M., 2012, “Unifed Approach for Computing Defection of Steel and FRP Reinforced Concrete,” Andy Scanlon Symposium on Serviceability and Safety of Concrete Structures, SP-284, American Concrete Institute, Farmington Hills, MI. (CD-ROM) Ekwueme, C.; Lawson, J.; Pourzanjani, M.; Lai, J. S.; and Lyons, B., 2006, “UBC 97 and ACI 318-02 Code Comparison – Summary Report,” SEAOSC Slender Wall Task Group, Structural Engineers Association of Southern California, Whittier, CA, Jan. Gilbert, R. I., 1999, “Defection Calculations for Reinforced Concrete Structures – Why We Sometimes Get It Wrong,” ACI Structural Journal, V. 96, No. 6, Nov.-Dec., pp. 1027-1032. Lawson, J., 2007, “Defection Limits for Tilt-up Wall Serviceability,” Concrete International, V. 29, No. 9, Sept., pp. 33-38. With the right circumstances, it is entirely possible to have design conditions that result in an effective moment of inertia, I e , which either approaches or equals the gross moment of inertia, I g . This occurs when the panel is predicted to be uncracked and corresponds with the near-vertical portion of the bilinear moment-defection curve. In this region, the factored panel defection, D u , and resulting P-D moments are very small because the moment of inertia is based on the full gross concrete section that gives a very stiff panel. By using I g , the full thickness of the panel cross section is considered and, if the calculation is to hold true, there should then be respective tension and compression regions at the extreme fbers. However, in a slender element that is required to be tension-controlled, using the full contribution to the member’s tensile strength provided by the concrete should be neglected, and only the tensile contribution from the reinforcing steel should be considered. If this is not done, and the full tensile contribution of the concrete is considered for the strength capacity, this approach can result in unsafe panel designs that are signifcantly under-reinforced and have greater potential for sudden failure should a cracked section initiate. This situation occurs if the moment-defection...