Behavior of bolted connections during and after a fire

After the collapse of World Trade Center Buildings, the safety of steel structures in fire and post fire started to draw more and more attention from structural engineer communities and researchers in United States. Efforts are being made on improving fire resistance of steel structures by new design methods, which is an alternative of the traditional passive (spray-on insulation) and active (automatic sprinkler) fire proofing system. In developing the new design methods, the behavior of steel structure at elevated temperature must be understood. Floor beams and bolted connections were studied with both experimental and analytical methods in this research program, which provide useful information on the strength of bolted connection at elevated temperature.

Shear strength of A325 and A490 high strength bolts were studied with double shear tests performed at different temperature levels, from ambient temperature to 1500°F. It was found that both types of bolts reduce their strength significantly between 600°F and 1300°F. Strength reduction factors are recommended for practical design. Residual strength of A325 and A490 bolts after being exposed to elevated temperature were studied with single shear tests and hardness tests. Strength losses were found on both bolts when they were exposed to a temperature higher than their tempering temperature. Bolt hardness was found to be an accurate method to estimate bolt residual strength and provide an efficient and economical way to assess residual strength of a high strength bolt. Single bolt connections were tested at different temperature. Significant capacity reduction was found on all the connections at elevated temperature. The failure mode changed from a bearing failure to a bolt shear failure when temperature was above 600°F. This phenomenon indicated that a high strength bolt was more sensitive to temperature than structural steel. Two bolts connections were tested to study block shear failure at different temperatures. Residual slip load on slip critical connections was also investigated with slip load tests on connections that had been heated and cooled to ambient temperature. Significant reduction of slip load occurred in connections which had been heated to above 800°F. Material properties of two different heats of Grade 50 structural steel plates used in the connection tests were determined at different temperature levels and strain rates. Finite Element Analysis was performed on steel beams and composite beams subjected to elevated temperature. Connection forces at different temperatures were obtained. Design recommendations are put forward based on the experimental results on connection strength and connection forces from the analysis.