Risk evaluation of steel frames with welded connections under earthquake

Numerous failures in welded connections in steel moment-resisting building frames (SMRF)were observed when buildings were inspected after the 1994 Northridge Earthquake. These observationsraised concerns about the effectiveness of such frames for resisting strong earthquake ground motions.The behavior of SMRFs during an earthquake must be assessed using nonlinear dynamic analysis, andsuch assessments must permit the deterioration in connection strength to capture the behavior of theframe. The uncertainties that underlie both structural and dynamic loading also need to be included in theanalysis process. This paper describes the analysis of one of approximately 200 SMRFs that suffereddamage to its welded beam-to-column connections from the Northridge Earthquake is evaluated.Nonlinear static and dynamic analysis of this SMRF in the time domain is performed using groundmotions representing the Northridge Earthquake. Subsequently, a detailed uncertainty analysis is conductedfor the building using an ensemble of earthquake ground motions. Probability distributions fordeformation-related limit states, described in terms of maximum roof displacement or interstory drift, areconstructed. Building fragilities that are useful for condition assessment of damaged building structuresand for performance-based design are developed from these distributions.