Seismic Analysis and Design Considerations of Braced Steel Structures.

Abstract

This study deals with dynamic analysis and earthquake resistance of concentrically braced steel structures. The main objectives are to assess the safety level and damage potential of buildings designed according to the current codes, and to improve current design procedures for better structural performance. The study is divided into two parts: structural modeling and analysis, and design implication study. The US-Japan test results are used to establish and refine the modeling of the Phase I test structure. In structural analysis, member modeling is primarily based on the test results. The secondary beams are ignored in the floor systems. The effective width of the concrete slab is determined according to ACI code. For girder-to-column connections, formulas are derived to include effect of shear forces in the columns. For bracing members, the effective length factors are back-calculated from the test results by using SSRC formulas. An empirical formula is proposed to calculate strength reductions of rectangular tubular bracing members for subsequent cycles. A procedure is developed to predict fracture life of bracing members. It has empirical basis and is refined later with an energy approach by using Jain's hysteresis model. The procedure is incorporated in the dynamic structural analysis using DRAIN-2D program. The Phase I test structure is analyzed for Miyagi-Ken-Oki accelerogram with maximum ground acceleration scaled to 65, 250 and 500 gals, respectively. In the design implication study, seismic behavior of structures designed according to current codes is investigated with emphasis on effects of column buckling and early fracture of bracing members. In order to improve seismic behavior, approaches of increasing structural strength by specifying larger design forces, and increasing member ductility and energy dissipation are studied. Four concentrically braced non-moment resisting and three moment resisting structures are designed according to the current Uniform Building Code and by the above two approaches. By using the refined modeling and analysis techniques the structures are analyzed for Miyagi-Ken-Oki and Taft earthquakes with maximum acceleration scaled to 500 gals. Based on the structural responses recommendations are drawn for improved seismic design of concentrically braced steel structures. (Abstract shortened with permission of author.)