Seismic Performance of Chevron Braced Frames With Yielding Beams

Abstract eng:
Steel concentrically braced frames (CBFs) have been used as seismic force resisting systems for many years. In seismically active regions of the US, the chevron configuration was used extensively in older construction but is less common today due to the codification of stringent beam-strength requirements. Today, the AISC Seismic Provisions prescribes a post-buckling brace load case for SCBFs which induces large axial-flexural demands that the beam must resist in addition to gravity load effects. These provisions require deep and heavy beam sections which are not economical. An international research collaboration between the University of Washington and the National Center for Research on Earthquake Engineering in Taiwan has examined the use of beams in chevron CBFs that do not meet SCBF requirements and are expected to yield after brace buckling. The research examines the vulnerability of older chevron CBF infrastructure with yielding beams and explores the potential of yielding-beam SCBFs. The results show that brace yielding in tension is prevented due to beam deflection, but frame action mitigates the reduced brace resistance. In addition, yielding-beam frames can achieve drift capacities comparable to SCBF-compliant yielding-brace frames. Consequently, older CBFs with weaker beams may not require beam retrofit and the yielding-beam mechanism shows promise for new construction. This increases the viability for retrofit of older CBFs if the beam can be retained (allowing for focus on more impactful areas) and alleviates economic concerns for chevron SCBFs.

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