Study On The Response Of External Welded Flange-bolted Web Joints Exposed To Fire

As an important component of steel frame structural building, beam-to-column joint plays an important role in maintaining the capability of the steel structure. The improvement of the fire-resistant capability of the joint helps to ensure the building integrity, prolong the fire resistance duration of the building and protect the life and property. It is an important prerequisite and basis for fire-resistant design and fire safety assessment of steel structure to systematically and accurately understand the response of joint exposed to various fire conditions. The mechanical properties of Grade Q235B steel structure at elevated temperature were studied. Then systemic researches were conducted on the thermal&structural response and failure mechanism of the external welded flange-bolted web joints exposed to fire with full scale experiments and finite element method.The experimental researches on the mechanical properties of grade Q235B structure steel at elevated temperatures were conducted with steady state method. The reduction factors of yield strengths and elastic modulus of the structure steel studied were proposed. Furthermore, a simplified stress-strain model for the steel material at elevated temperatures was presented based on the results.A full scale test was carried out to obtain the moment-rotation relationship of the external welded flange-bolted web joint at ambient temperature. Other four tests were carried out to investigate the temperature distribution, the temperature-rotation relationships of the joints in different fire and load conditions. The effect of load ratio and fire model on the fire-resistant capability of the joint was analyzed.The fire tests show that a slower temperature rising fire will lead to a slower temperature rising and more uniform temperature distribution of joint, which will help to improve the fire resistance of the joint. The decrease of load ratio will lead to the increase of limited temperature under the same fire condition. Exposed to fire, the typical failure model of the external welded flange-bolted web joint is the local flexure deformation in the bottom flange of the beam and obvious distortion in the column at the joint zone.The test results also reveal that the weld between the top beam flange and column flange has great influence on the fire resistance of the joint. For the joint with bad weld quality, the weld will be the most damageable position that can lead to early failure of the joint in fire.Three-dimensional finite element technique to investigate the temperature field and structural behavior of the external welded flange-bolted web joints exposed to fire was developed. All the components of the joints were modeled with solid elements, considering the nonlinear material and nonlinear geometric effects. The contact effects of adjacent surfaces'interaction were modeled using contact pair elements, and the pretension element was used to simulate the preloads in bolts. Considering the temperature field introduces thermal strains in the structural field, while the structural strains generally have no obvious effect on the temperature distribution of the joint, a sequentially thermal-mechanical coupled analysis method was used to model the thermal- mechanical coupled effect on the joint under fire conditions. With this method, the nodal temperatures obtained from the thermal analysis were applied as "body force" loads in the subsequent structural analysis.Comparisons between the predicted results and the experimental results indicate that the FEM have an acceptable degree of accuracy to predict the behavior of this type of joints exposed to fire. Further analysis on the failure mechanism of the joint exposed to fire indicates that column web is the most vulnerable location.The moment-rotation-temperature curves were obtained from the analysis on the response of the joint in different load levels. Further parametric study indicates that the column web thickness and column flange thickness have obvious effect on the fire resistance of the joint, and no obvious effect was observed for plate thickness and beam web thickness. The relations between the critical temperature and relevant parameters were established with mathematical statistical methods. The influence of fire development process on the response of the joint under fire condition was also studied by defining different fire models.