Enhanced Fire Resistance of Double Web Angle Cleat Connections with Austenitic High-Strength Bolts during Furnace Fires

Natural fire incidents and standard fire tests have demonstrated that flexible and semirigid connections have suffered premature failure due to the temperature sensitivity of the quenched and tempered carbon steel bolts, leading to an underutilization of the potential strength and ductility of other connected components. However, austenitic and duplex stainless steel bolts exhibit pronounced ductility at ambient temperature and can better retain material stiffness and strength than carbon steel bolts beyond 600 degrees C. Accordingly, this paper documents how web angle cleat connections with austenitic high-strength bolts (A4L-80) behave according to experimental investigations at ambient and elevated temperatures, based on which, furthermore, their experimentally determined M-theta(R) response was also compared with the corresponding connections with carbon steel bolts (Grade 8.8), which have the same nominal strength as A4L-80. The experimental findings indicated that, despite the differences in measured ultimate strength between two sets of bolts, connections with austenitic bolts at elevated temperatures could robustly improve fire resistance compared with those with carbon steel bolts, whereas the former had comparable strength and ductility to the latter at ambient temperatures. At 650 degrees C, austenitic bolts maintained their structural integrity in the loaded connections, whereas carbon steel bolts failed prematurely due to tension and/or shear fracture, resulting in the loss of load-bearing capacity in the joints. To approximately replicate the tested M-theta(R) response, a modified component-based model is formulated for connections with austenitic or carbon steel bolts in conjunction with the statistically proposed reduction models of material stiffness and strength regarding the connected components at elevated temperatures. This model considers the prying action of the column flange, addressing the empirical limitations of previous models for web cleat connections. The comparative results illustrated that this model can be correlated with the measured curves at a conservative prediction level, contributing to the potential safety of structural fire resistance without making highly conservative prediction at the joint level.