Evaluation of pre-Northridge low-rise steel buildings. I: Modeling

Widespread occurrence of brittle beam-to-column connection fracture has been discovered in steel frame buildings after 1994 Northridge and 1995 Hyogoken-Nanbu earthquakes. To investigate the effects of connection failure on structural response of steel buildings under earthquakes, a smooth connection-fracture hysteresis model based on the Bouc-Wen model is developed. It can reproduce the hysteretic characteristics found in cyclic loading tests of the FEMA/SAC (SEAOC, ATC, CUREe) steel project. To account for the effects of flexible floor systems, biaxial interaction, and torsional motion, a 3D inelastic structural analysis model developed by the writers is used. From a numerical study, it was found that for response evaluation of strong-column, weak-beam type steel buildings a conventional shear-building model assuming rigid floor systems is not adequate. The response behavior of steel buildings with connection fractures may only be moderately altered if there is some residual strength after the fracture. On the other hand, once connection fracture results in complete structural member failures, the response would increase drastically that collapse might occur. The methods of modeling and response analysis developed will be used in the reliability and redundancy study in the companion paper.