Experimental Investigation of Chevron Special Concentrically Braced Frames with a Yielding Beam Plastic Mechanism

Abstract

Chevron-braced frames are preferred structural systems by architects and contractors in low to mid-rise buildings for seismic design because they accommodate architectural elements while providing the necessary lateral stiffness and resistance. This system was more common prior to the advent of Special Concentrically Braced Frame (SCBF) seismic provisions based on capacity design in the late 1980’s, which require that the beam develop the idealized expected unbalanced capacities of full yielding of the tension brace and degraded capacity of the compression brace. This results in large and costly beams, which deter their use in construction. Previous experimental tests of chevron SCBFs with beam strengths that do not satisfy the theoretical unbalanced force prescribed by AISC SCBF Seismic Provisions result in a yielding beam plastic mechanism. These tests suggest that the current beam strength requirement is not necessary for assuring life safety and collapse prevention. Three single-story chevron SCBFs were tested at the University of Washington to further evaluate the beam yielding mechanism. One of the tested specimens had a beam weaker than any previously tested to establish a lower bound for comparison of seismic performance. A second specimen had A500 Gr. C braces to determine the impact of brace steel type on seismic performance. The third single-story specimen used a deeper beam to determine the effect of beam stiffness on frame resistance and ductility. Finally, a capstone 3-story chevron SCBF was tested at the National Center for Research on Earthquake Engineering in Taiwan to evaluate the system’s performance with a yielding beam. Results show that the beam yielding mechanism improved the deformability of the SCBF and the weaker beam did not compromise the capacity of the system if the beam was not excessively weak.