A component model of fully welded connections in steel frame at elevated temperature

Abstract

The mechanical behavior of fully welded connections in steel frames (FWC-SF) is significantly affected by fire, whereas their high-temperature response remains insufficiently understood. This study conducted fire tests and finite element (FE) simulations to systematically investigate the temperature distribution, deformation characteristics, and failure mechanisms of FWC-SF joints. Experimental results revealed pronounced temperature gradients among the joint components. And joint failure was primarily governed by the formation of plastic hinges at beam ends, which induced excessive rotational deformations, characterized by local buckling of the beam bottom flange and web, as well as cracking of the concrete slab. Parametric analyses further demonstrated the sensitivity of the joint fire resistance to the load ratios, component dimensions, and thickness of fireproof coatings. The degradation of the mechanical properties of FWC-SF joints exhibits pronounced non-uniformity, with the residual rotational stiffness and flexural capacity retaining about 20 % and 50 % of their ambient-temperature values at 600 °C, respectively. A simplified component model was then developed to characterize the temperature–moment–rotation relationship of FWC-SF joints. The model accounts for the upward shift of the plastic neutral axis induced by material degradation and incorporates the nonlinear behavior of key components, including the beam flanges, web, and rebars. Comparisons with experimental and numerical results confirm that the proposed model can reliably reproduce the degradation of rotational stiffness and flexural capacity of FWC-SF joints exposed to fire, thereby providing an effective tool for fire resistance assessment and design.