Experimental and Fragility Analysis of Threaded Cast Iron Pipe Fittings Utilized in a Fire Suppression System

Cameron Rusnak ^1* Sherif Elfass ² Allen J Rivas ²

• ¹ Lincoln University, Jefferson City, United States
• ² University of Nevada, Reno, Reno, Nevada, United States

Fire suppression piping systems utilize threaded connections to join pipes with fittings, such as elbows and tees, enabling the distribution of water throughout a structure. These fittings are integral for directional changes in the piping system and are essential for navigating the intricacies of building design. While previous research has explored the mechanical behavior of threaded connections at the pipe-fittings interface, limited attention has been given to the mechanical performance of the threaded fittings themselves. This study, conducted at the University of Nevada, Reno, addresses this gap by investigating the mechanical behavior, rotational capacity, and failure modes of threaded cast iron fittings, specifically examining the response of the fittings. Twenty-seven tests were performed on fittings (elbows and tees) with diameters of 1-inch, 1.5-inches, and 2-inches under quasi-static loading to evaluate their structural performance. Results reveal that leakage at the pipe-fitting interface is the primary failure mode, even as significant rotation occurs within the fitting body. The rotational capacity to failure was approximately 28 kip-ft per radian within the fitting and 8 kip-ft per radian at the pipe/fitting interface. Larger diameter fittings demonstrated significantly reduced deformation compared to smaller fittings, with an average reduction of approximately 63% in capacity relative to the next smallest size. Damage states were defined, rotational capacities established, and seismic fragility models were developed for each fitting size based on the experimental data.