Effect of Thermal Creep on Circular Hollow Section Welded T-Joints Under In-Plane Bending Load

Abstract

Thermal creep of steel is a critical factor contributing to the failure of structures exposed to elevated temperatures for extended durations. Circular hollow section (CHS) steel members are widely used in infrastructure, offshore platforms, and industrial facilities. T-joints represent a typical connection type in such structures. While several studies have investigated the high-temperature behavior of CHS joints, the time-dependent effect of creep remains underexplored. This thesis presents the results of a numerical investigation of welded CHS T-joints under in-plane bending. Finite element (FE) models developed in Abaqus were validated against existing experimental data, and the creep effect was analyzed under varying load ratios, temperatures, and exposure durations. A two-step approach was adopted: short-term (time-independent) simulations were first conducted to establish baseline failure loads, followed by time-dependent creep simulations under constant temperature and load. Creep in the base material was found to have a substantial influence on joint deformation and failure, particularly at elevated temperatures and high load ratios. Creep in the weld material was also found to reduce failure time, especially at lower load ratios where its contribution to deformation is more pronounced. Isochronous curves highlighted a transition to creep-dominated behavior beyond 550 °C, with deformation accelerating sharply over time. Two brace diameters were tested for creep. The larger diameter increased short-term strength. However, it slightly reduced creep resistance. For design practice, joints expected to experience prolonged fire exposure, particularly at high temperatures (600°C – 700°C), should be designed to carry only a significantly reduced fraction of their time-independent capacity. These findings highlight the necessity of including both base material and weld creep effects in the fire-resistant design of welded CHS connections. Neglecting these effects may otherwise lead to non-conservative predictions of structural performance and premature connection failure in fire.