Static Strength And Hysteretic Behaviour Of Doubler Plate Reinforced Tubular T-joints After Fire Exposure

Steel structures have been widely used in large-span, high-rise and huge buildings due to their advantages of high strength, low weight and excellent deformation ability. However, steel material is very sensitive to high temperatures, which renders the fire safety a hot issue in the structural design. According to the temperature variation, a whole fire could be normally divided into four stages: ambient temperature, heating stage, cooling stage and post-fire stage. Most of previous research is focused on the structural response during heating stage, but current fire controlling systems in big cities have the capability to extinguish the structural fire within several minutes. The safety of buildings surviving from fires is difficult to assess and judge because the research concerning this topic is far from enough. Therefore, it is of great importance to investigate the mechanical behaviour of steel structures after fire exposure.In this thesis, the static strength and hysteretic behaviour of doubler plate reinforced tubular T-joints were experimentally and numerically investigated. Firstly, four specimens with identical geometric properties were fabricated. Two of them were tested under monotonic loading to obtain the ultimate strength and the others were tested under cyclic loading to study the hysteretic behaviour. In the static strength test, load-deformation curves of joints at ambient temperature and during post-fire stage were compared and the results indicated that the strength as well stiffness of joints did not deteriorate to a noticeable extent after fire exposure. In the cyclic loading test, the crack development, hysteretic curves, skeleton curves and energy dissipations of joints at ambient temperature and after high temperatures were analysed. The results revealed that the crack of joints after fire developed more quickly and severely and the area enclosed by post-fire hysteretic loop was less.Furthermore, based on the sequentially coupled thermal-stress analysis, the finite element(FE) models of doubler plate reinforced tubular T-joints during whole fire stages were built in FE software ABAQUS. In these models, the constitutive relationship of steel material during ambient-temperature, heating, cooling and post-fire stages were involved and the contact between doubler plate and chord was defined. The FE models were validated against the experimental data obtained from the static strength test, cyclic loading test and previous experiments conducted by other scholars.After the verification of FE models, a comprehensive parametric study was conducted to investigate the influences of various parameters on post-fire ultimate strength and hysteretic behaviour. The abovementioned parameters included chord/length radius ratio, brace/chord diameter ratio, brace/chord wall thickness ratio and the maximum temperature. Based on the data collected from parametric study, the criteria that could be used to determine the ultimate strength of reinforced tubular joints were discussed and a simply and useful method to predict the post-fire strength was developed.