Damage Evaluation of Steel Beam-to-column Connections Based on Flange Strain History

Abstract

Abstract The plastic deformation capacity of beam-to-column connections strongly affects the seismic capacity of steel moment frames in seismic design. Ductile fractures that occur after adequate plastic deformation and crack propagation is one of the most typical steel component failure modes during earthquakes. A key index that affects the fractures of materials is the strain amplitude at the critical cross-section. For most structural steels, the strain-life relationship can be expressed using a Manson–Coffin-type equation. The current study proves whether there is a similar Manson–Coffin relationship between the average flange strain amplitude at the critical cross-section and the number of loading cycles until fracture. Shop welded beam-to-column connections made of 400 N/mm2 class structural steel with conventional or improved types of weld access holes are discussed in this study. When these connections are subjected to cyclic loadings, ductile cracks initiated at the toes of the weld access holes propagate during loading, leading to the eventual fractures of the connections. Moreover, cyclic in-plane beam analysis is conducted using an experimental database of 10 beam-to-column connections that are loaded until ductile fractures occur to obtain the strain histories at the beam-end flanges. Based on the strain histories, it is found that, similar to the material level, the Manson-Coffin relationship between the average flange strain amplitude at the critical cross-section and the number of loading cycles until fracture is confirmed on the component level. The Manson–Coffin-type strain-life relationship equations are obtained for these connections with two types of weld access holes. In addition, the results of previous beam-to-column connections loaded under variable amplitudes were studied. To expand the size of the database, experiments were conducted on two beam-to-column connections corresponding to random loading histories. The accuracy of the proposed Manson–Coffin relationships is verified with the experimental results under random loading histories. The current study provides a possible solution for the damage evaluation of steel beam-to-column connections based on strain amplitudes through analysis and a limited number of experiments.