Fragility Analysis of Threaded T-Joint Connections in Hospital Piping Systems

Abstract

The cost of damage to the non-structural systems in critical facilities like nuclear power plants and hospitals can exceed 80% of the total cost of damage during an earthquake. Studies assessing damage from the 1974 San Fernando and 1994 Northridge earthquakes reported a widespread failure of non-structural components like sprinkler piping systems (Ayer and Phillips, 1998). The failure of piping systems led to leakage of water and subsequent shut-down of hospitals immediately after the event. Consequently, probabilistic seismic fragility studies for these types of structural configurations have become necessary to mitigate the risk and to achieve reliable designs. This paper proposes a methodology to evaluate seismic fragility of threaded T-joint connections found in typical hospital floor piping systems. Numerous experiments on threaded T-joints of various sizes subjected to monotonic and cyclic loading conducted at University of Buffalo indicate that the “First Leak” damage state is observed predominantly due to excessive flexural deformations at the T-joint section. The results of the monotonic and cyclic loading tests help us evaluate the following characteristics for a given pipe size and material: (i) Maximum allowable value of rotational deformation at the T-joint section to prevent “First Leak” damage state; (ii) The force-displacement and moment-rotation relationships at the T-joint section. A non-linear finite element model for the T-joint system is formulated and validated with the experimental results. It is shown that the T-joint section can be satisfactorily modeled using non-linear rotational springs. The system-level fragility of the complete piping system corresponding to the “First Leak” damage state is determined from multiple time-history analyses using a Monte-Carlo simulation accounting for uncertainties in demand.