Seismic Performance of Chevron Concentrically Braced Frames with Weak Beams

Abstract

Steel concentrically braced frames are lateral seismic force resisting systems that have been historically popular. Despite extensive deployment and continued service of these systems, little research has focused on the seismic performance and rehabilitation of older braced frames, termed non-seismic concentrically braced frames (NCBFs). Such frames were designed prior to the advent of capacity design standards and lack details and brace section geometry that promote ductility and sustain resistance, such as adequate gusset plate rotational clearance, minimum weld toughness, and brace compactness limits. Beams in NCBFs with braces in chevron configurations (V and inverted V) are commonly unable to develop the design brace unbalanced load specified in the 2010 AISC Seismic Provisions that arises after brace buckling. NCBFs with these weak beams are the primary subject of this research program. Two-story experimental specimens with braces in the inverted V configuration were tested at the National Center for Research on Earthquake Engineering in Taiwan to examine the seismic response of a prototypical existing NCBF and three rehabilitation schemes. The results show that poor brace compactness and connection details can be highly detrimental to system performance, but weak beam braced frames with ductile details can achieve good ductility while maintaining strength. Numerical models of the specimens, developed in Abaqus, simulate the observed response well and provide a validated basis for future analyses.