Threaded cast iron fittings, including tees and elbows, are commonly used in fire suppression systems to connect piping components due to their simplicity, reliability, and compatibility with National Pipe Thread (NPT) standards. While extensively used in practice, their structural performance under seismic demands, particularly for large-diameter fittings, remains insufficiently understood. Previous studies have primarily focused on smaller-diameter fittings or have idealized the fitting body as rigid, overlooking the potential for internal deformation and rupture. This study addresses this knowledge gap through an experimental investigation of 3-inch and 4-inch gray cast iron threaded elbow and tee fittings subjected to loading. A total of 18 full-scale assemblies were tested across multiple configurations. Results showed that structural rupture of the fitting body was the dominant failure mode, with no specimens reaching the defined threshold for substantial leakage. Limited early-stage leakage was observed in select 3-inch tee configurations prior to rupture. Damage State 3 (DS3), defined as rupture, occurred at rotational capacities ranging from 0.0061 to 0.0179 radians. Fitting stiffness to failure averaged approximately 643 kip-ft/rad. Fragility models were developed within a probabilistic framework, incorporating variability due to material imperfections and assembly tolerances. The findings reveal a size-dependent shift in behavior: 3-inch fittings demonstrated greater pipe rotation prior to failure, while 4-inch fittings exhibited higher internal stiffness but reduced flexibility. These results provide moment–rotation relationships and fragility models to support analytical modeling and seismic vulnerability assessments, advancing the predictive understanding of large-diameter threaded cast iron fittings used in fire suppression systems.
Rusnak et al. (Wed,) studied this question.