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

Widespread occurrence of brittle beam-to-column connection fracture has bee n discovered in steel frame buildings after 1994 Northridge and 1995 Hyogok en-Nanbu earthquakes. To investigate the effects of connection failure on s tructural response of steel buildings under earthquakes, a smooth connectio n-fracture hysteresis model based on the Bouc-Wen model is developed. It ca n 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-col umn, weak-beam type steel buildings a conventional shear-building model ass uming rigid floor systems is not adequate. The response behavior of steel b uildings with connection fractures may only be moderately altered if there is some residual strength after the fracture. On the other hand, once conne ction fracture results in complete structural member failures, the response would increase drastically that collapse might occur. The methods of model ing and response analysis developed will be used in the reliability and red undancy study in the companion paper.