Analytical and Experimental Study of the Effect of Bracing Pattern in the Lateral Load Bearing Capacity of Concentrically Braced Steel Frames

Abstract eng:
Seismic design codes give various values for Response Modification Factor (RMF) depending on the lateral load bearing system of the building structure, for example, they give a value like 10 for ordinary moment frames and a value like 6 for Concentrically Braced Frames (CBFs). However, the value of RMF for CBFs in codes does not depend on the number of braced bays and their location, or even the overall pattern of bracing in the building. This is while, at least, the number of braced bays in a frame is important from the redundancy point of view. This paper presents the results of an analytical and experimental investigation performed on a series of 5-story CBFs with three bays of which one or two bays have been braced with different patterns. At first, some Push Over Analyses (POA) have been performed to find out the ultimate capacity of CBFs. Then, some 1/3 scale samples of frames have been built and tested subjected to lateral loads to verify the numerical calculations. Results show that the ultimate capacity of CBFs and their ductility factor strongly depend on the number and location of braced bays and the bracing pattern. The capacity can vary up to 100% from case to case, and the displacement ductility factor can be as low as 3.5 in some cases, which is much lower than the code suggested value. On this basis it can be said that the code suggested values of ‘response modification factors’ need essential modifications.

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