| With the development of industrialization and urbanization,building fires are becoming more and more frequent.In the event of a fire,steel structures are inevitably exposed to high temperatures.Due to the redundant backup in the structural design,steel structures may not collapse.Therefore,it is critical to evaluate the remaining bearing capacity of steel structure components after fire,which determines whether the member can be used directly,repaired or replaced.Bolted angle steel brace is commonly used as tension and compression member in steel structures,and its joints are the key parts.Net section fracture and block shear failure are the common failure modes of bolted tension angles.At present,there are few achievements in the field of research on the net section fracture and block shear failure bearing capacity of bolted high strength steel angles after fire exposure.Based on this,this paper takes steel grade,out-of-plane eccentricity,bolt connection length,experienced temperature and cooling method as numerical simulation parameters to study the bearing capacity for net section fracture and block shear failure of bolted Q460 and Q690 steel angles after fire exposure.The main research contents include the following points :(1)The software ABAQUS was used to numerically simulate the testing results of net section fracture and block shear failure f or 6mm thick Q690 steel angles at room temperature in the literature [59,78].Three specimens were taken from the above literature respectively to obtain the failure mode and load-displacement curve of the numerical simulation.The numerical simulation results are compared with the experimental results to check the accuracy of the finite element model.(2)The bearing capacity for net section fracture of bolted Q460 and Q690 steel angles after fire were studied by finite element analysis.The failure mode,loaddisplacement curve and peak load of the component were o btained,and the effect of out-of-plane eccentricity,bolt connection length,experienced temperature and cooling method on the effective sectional coefficient was analyzed.The results calculated by the corresponding equations in the previous studies and relative code are compared with the ultimate load for net section fracture of the bolted angle steel obtained by simulation.According to the least square method,a formula for the effective sectional coefficient at normal temperature is proposed,and the influence coefficients of the two cooling methods on the residual bearing capacity of the members after fire are analyzed.(3)The block shear failure bearing capacity of bolted Q460 and Q690 steel angles after fire were studied by finite element analysis.The influences of out-of-plane eccentricity,bolt connection length,experienced temperature and cooling method were considered.The failure mode,load-displacement curve and peak load of the component were obtained.Through the comparison between the results calculated by the corresponding equations in the previous studies and relative code with the ultimate load for block shear failure of the bolted steel angles obtained by simulation,the accuracy of the existing block shear capacity formula was verified.(4)Based on the above numerical simulation database,the first order second moment method is used to analyze the reliability of the calculation formulas for the net section fracture and block shear failure of bolted Q460 and Q690 steel angles after fire.Appropriate resistance factors are recommended respectively for the calculation formulas corresponding to the two failure modes. |
