Fragility functions for pre-Northridge welded steel moment-resisting beam-to-column connections

Fragility functions that permit the estimation of damage in pre-1994 welded flange bolted-web beam-to-column moment connections are developed. The fragility functions provide estimates of the probability of yielding and of experiencing fracture as a function of the peak interstory drift ratio imposed in the connection, hence are particularly suited for performance-based methodologies for assessing the seismic performance of existing steel moment resisting frame buildings. Fragility functions were developed based on experimental results of 51 beam-to-column subassemblies tested over the past 26 years in 10 different investigations. The fragility functions developed in this paper incorporate uncertainty arising from specimen-to-specimen variability as well as statistical uncertainty associated with the fact of being developed based on a small sample of experimental results. Results indicate that the variability in interstory drifts at which fracture was observed is significantly larger than the variability at which yielding occurs. Furthermore, for beam depths commonly used in engineering practice there is a probability between 70% and 80% of experiencing a fracture in older type of connections at an interstory drift ratio of 2% which is usually the maximum allowed in US current codes indicating that many steel moment resisting frame buildings built prior to 1994 could experience fractures in their beam-to-column connections in a design level earthquake if they are not retrofitted.