Fracture behavior of high-strength bolted steel connections at elevated temperatures

Abstract

In fire events, to utilize catenary effect and accommodate large deformation in beams demands a sufficient deformation capacity and ductility in steel bolted connections, which is not well considered in design practice. Therefore, for performance-based fire safety design, it becomes very essential to accurately predict the fracture behavior of steel bolted connections at elevated temperatures. However, experimentally predicting their fracture behavior is very costly at elevated temperatures and limited by the scale of structural components. Therefore, this paper proposes a numerical analysis approach and investigates the fracture behavior of high-strength bolted steel connections at elevated temperatures based on the validated numerical analysis approach. The temperature-dependent material properties including true stress-strain curves and fracture parameters in a three-stage fracture model (describing the pure tension, pure shear, and combined tension and shear fracture behavior) for high-strength ASTM A325 bolt and A572 Grade 50 steel are firstly calibrated from and validated against their tensile and shear test results. They are further validated against the test results on single-bolted and multi-bolted connections at elevated temperatures, which were not used in the calibration of these material properties. The validated material properties and fracture parameters are finally proposed for fracture prediction of different types of bolted connections under various loading conditions at elevated temperatures. This study shows the potential of reasonable prediction of the fracture behavior for the steel bolted connections at elevated temperatures and under complicated loading conditions using a numerical analysis approach. According to the numerical analysis results, the degradation of shear fracture resistance of A325 bolts at elevated temperatures is more significant than that of A572 Grade 50 steel plate, which may lead to a bolt shear failure mode for these connections with a large end distance at high temperatures (>400 °C). It is also found that the fracture failure mode can significantly influence the degradation of the load and deformation capacities of high-strength steel bolted connections at elevated temperatures. With the bolt shear failure mode, the degradation of load and deformation capacities of steel bolted connections are more significant than that with plate tearing-out failure mode.