| description abstract | Steel concentrically braced frames designed prior to the implementation of capacity design principles in seismic design provisions may exhibit poor inelastic response under seismic excitation. These older, nonductile concentrically braced frames (NCBFs) used several configurations, with the chevron configuration being one of the most common. The response of chevron-configured NCBFs is unknown, as relatively large axial and flexural demands are imposed on the beam after brace buckling. Current code requirements for special concentrically braced frames (SCBFs) promote full yielding of the braces while the beam remains elastic, but NCBFs develop a mechanism in which the beam yields, deforms plastically, and limits tensile elongation of the brace. However, if ductile, this plastic mechanism may meet current performance limits and not require retrofitting. To examine this issue, four tests of two-story NCBFs were conducted at the National Center for Research on Earthquake Engineering in Taipei, Taiwan. The tests were designed to form a yielding-beam plastic mechanism as opposed to a yielding brace-elastic beam plastic mechanism. The results show the yielding-beam mechanism provided adequate lateral strength and deformation capacity and beams in a chevron configuration are a low priority with regard to retrofit, whereas retrofitting to mitigate brace and connection deficiencies is a high priority. The tests also demonstrate that modern CBFs can sustain lateral force and deformation demands if the yielding-beam, as opposed to yielding-brace, plastic mechanism is formed. The results support the modification of the stringent design requirements for chevron SCBFs. | |
