Seismic Performance and Impact of Geometric Nonlinearity on 3D Steel Braced Frame Building Models

The objective of this paper is to examine the seismic performance of steel braced frame buildings, braced by either buckling restrained braces (BRBs) or concentrically braced frames (CBFs). A unique feature of the investigation is the creation of fully three-dimensional building models so that the interaction of the building diaphragm with the vertical lateral force resisting systems can be examined in detail. Few investigations of seismic performance have explored this interaction as models are typically two-dimensional. The work is conducted as a portion of the ongoing Steel Diaphragm Innovation Initiative -- and is of particular interest in better understanding the design and performance of building diaphragms. Another novel aspect of the modeling is a specific look at the impact of including or ignoring geometric nonlinearity (e.g., P-D and P-δ) in the building models. There are questions about the impact of geometric nonlinearity and its application both in equivalent lateral force designs and in nonlinear time history analysis. The building models are used as case studies to explicitly demonstrate the impact of geometric nonlinearity on overall building response. The building simulation studies are conducted in OpenSees and are currently ongoing. Completed work reported herein includes analysis of building archetypes designed to the current U.S. seismic standards and potential diaphragm design alternatives for a 1 and 4-story BRB and CBF steel frames. The results demonstrate the sensitivity of building diaphragm demands to the vertical lateral force resisting system and to the inclusion of geometric nonlinearity in the building response.

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