Study On Behavior Of Welded Circular Hollow Section(CHS) Tubular Joints At Elevated Temperature Based On Steady And Transient Heat Transfer Theories

Currently, steady state theory and transient theory are generally adopted for the investigation on static performance of tubular joints at elevated temperatures. The socalled steady state theory utilizes a scheme of loading the joints at individual high temperatures while in contrast the transient theory explores the joint behavior in an increasing temperature scenario with the brace member subjected to constant external load. The transient theory considers both the heating history and the steel properties degradation history over time. In addition, the initial deformation of the joints caused by service load is considered as well prior to the fire heating. It is therefore deemed that the transient theory could reflect a more reasonable response of the joints in fire condition against the steady state theory. However, the variation in analysis results between this two methods is not presented in the previous fire research. This thesis thus investigated the static behavior of welded steel circular tubular T-joints at elevated temperatures by using the two methods to address their applicability in fire analysis.Firstly, finite element models of the T-joints for the two methods were established respectively by the general used finite element analysis software ABAQUS. The simulation results were then compared with the laboratory testing data. The comparisons showed that the developed finite element models could replicate the fire behavior of the tubular joints accurately and therefore proved the reliability of the models. After that, parametric studies consisting of 37 models were conducted for each analysis method, separately. According to the numerical results for the steady state method, the ultimate static strength of the tubular joints at elevated temperatures were firstly extracted. The strength was then applied to the transient model as the constant external load. The critical temperatures were finally obtained from the transient models based on the above procedures. By comparing the critical temperatures obtained in the transient models and the field temperatures in the steady state models, the variation in fire analysis of this two methods and the influence of selected parameters in this variation were investigated on the static behavior of circular tubular T-joints. Finally, the ultimate capacities of circular tubular T-joints at elevated temperatures were estimated on the basis of elastic modulus and yield strength reduction. The estimated strength values were compared with those extracted from the steady state and transient analysis results to address their feasibilities in predicting the fire resistance of circular tubular T-joints. Additionally, the effect of selected parameters on the failure modes of the circular tubular T-joints at elevated temperatures was briefly discussed in this thesis based on the numerical results.It is shown from this research that: 1) the critical temperatures obtained from the transient analyses are lower than those from the steady state analyses without exception; 2) the parameters ?, ?, ?, ?, n and Tw whave influence on the variation of steady state and transient analyses, where the effect of ?, ?, ?, ? and n is relatively small and shows obvious varying law whereas the parameter Tw has significant influence but without clear law; 3) the method of predicting fire resistance of tubular joints adopting elastic modulus reduction has much higher accuracy but its safety is not stable; 4) three typical failure modes of the joints exist within the selected parameter ranges, namely the brace member failure, the chord member plasticity failure and the failure at brace/chord intersection; parameters ?, ?, ?, ? and n have obvious influence on the failure modes and this influence is dependent on each of the parameters.