Analysis and Design of Reinforced Cast-in-Place Concrete Diaphragms

Presented by Drew Kirkpatrick, Thornton Tomasetti, Inc.; and Leonard M. Joseph, J. Ola Johansson, and Karem Gulec, Thornton Tomasetti, Inc. The distribution of forces through floor diaphragms is critical to the overall behavior and performance of buildings during both wind and seismic events. Simplified methods commonly employed by design engineers establish approximate magnitudes and distributions of inertial and transfer forces within floor diaphragms. Such methods can be appropriate for regular low rise buildings without significant transfer forces. However, for design of complex structures with large stiffness discontinuities in vertical or horizontal directions a more detailed understanding and modeling of diaphragm behavior is important. Common situations in high-rise projects include a tower stack meeting a podium base with supplemental shear walls and the tower meeting a grade level slab enclosed by basement walls. Large diaphragm transfer forces typically occur at these levels of abrupt stiffness changes. Using examples from recent projects and parametric studies following Performance Based Structural design (PBSD) principles, this paper describes the use of strut and tie models in commercially available software (PERFORM-3D and SAP-2000) to provide a better understanding of complex diaphragm behavior. Results can aid the designer in making decisions regarding floor thickness and reinforcing layout including at chords and collectors. While the need for enhanced modeling techniques and understanding of diaphragm behavior has been highlighted by the increased use of Performance Based Structural Design (PBSD), the findings presented in this paper may be applicable to projects based on traditional (code based) approaches as well.