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 relationship moves into the near-horizontal portion of the bilinear curve without suffcient strength to resist the second-order P-D moments created by the rapid increase in defection. The general provisions in ACI 318-19, Chapter 6, limit the second-order moments and I e to a percentage of the frst-order moments and I g . The slender wall provisions do not have this limitation because I cr is specifcally used and the substitution of I e is not anticipated.
Wall panels experience stresses during lifting, which often create a cracked section in the panel before they are set vertically in place. Although this microcracking is diffcult to detect, it can have a signifcant impact on the wall’s behavior. Slender wall panels can also be subject to larger-than-anticipated construction tolerances (for example, reinforcing not placed within tolerances or joists or girders more eccentric than planned). Addi- tionally, most panels in place undergo continued thermal cycling throughout their lifetime or may at some time experience loads not entirely anticipated (for example, new canopy installed or truck impact) or displacements with design-level wind or seismic loads as the diaphragm defects. All of these can create a cracked condition during the service life of the wall panel, potentially causing its performance to be very different than the original intended design.ACI PRC-551.3 pdf download.