Finite Element Modeling of Tall Building Structures with Transfer Diaphragms Due to Offset Shear Walls

Abstract

Diaphragms are elements of the seismic force resisting system in charge of stabilizing structures by tying in vertical elements, transferring inertial forces from one vertical element of the seismic force-resisting system to another and transferring shear forces from one vertical element to another. When vertical elements of the seismic force resisting system are offset horizontally conditions, the diaphragm that connects them must transfer large story shears in what is known as a transfer condition. Transfer conditions are often unavoidable and remain one of the least understood and most consequential aspects of building response in an earthquake. Poor performance of transfer structures in recent earthquakes further underscores the importance of determining the true behavior of transfer structures under seismic loading. In order to begin to understand the behavior of transfer structures under seismic loading, finite element models were created for a 12 story building with offset shear walls. The models created represent one half of the building structure lumped into the two frames containing the shear walls in each half of the building. The first set of models (Phase 1) were developed starting from a nonlinear shear wall model. The model was incrementally made more complex, starting by adding a rigid, elastic transfer structure, and then elastic columns. Afterwards nonlinearity was added to the columns and finally typical floor diaphragms were added to the building. While results from these models showed similar curves for shear vs displacement and moment vs rotation for all models, issues with the fiber models used for the last model of this phase deemed the model too complex for the analyses required, particularly when modal analyses were performed on the structure. The next set of models was developed from two two-dimensional models, one for each frame. Incrementally, the model was made more complex, starting by modeling the frames in three dimensions, with diaphragms being modeled as beams initially and then as shells. Seismic design loads were applied to this model in order to determine the shear and moment profiles of the building, as well as the shear transfer occurring at every story. Nonlinearity was added to the base of the building, yielding similar results to the elastic models with respect to the shear and moment profiles, but very different results for the shear vs displacement and moment vs rotation curves for the Phase 1 models.