Study On The Post-fire Residual Mechanical Properties Of Bolted Spherical Joint Space Grids Structure

The fire resistance of steel structure has been an urgent concern in engineering,and the fire resistance of large span steel structure is different from that of small space steel structure.In order to study the mechanical properties of the large span steel structure after the fire,this paper took the typical space grids of the large span structure as an example,then the residual mechanics performance after fire was studied,which provided the basis for the safety evaluation of the grid structure after fire.First of all,the mechanical properties of high strength bolted joints after fire were studied.It was found that the slip coefficient,anti-skid bearing capacity,shear capacity and deformation capacity of high-strength bolted connections under 500 ? did not change much.While the capacity above 600 ? showed a significant trend.And the formula of the changing coefficient of bearing capacity after fire was estimated.The residual mechanical properties of bolted spherical joints after fire were studied.The numerical simulation was used to analyze the compressive joints and tensile bolted sphercial joints under fire and after fire.The bearing capacity of the bolted spherical joints decreased with the increase of the over-temperature,and the failure of the steel ball did not occur in the test.The reliability of the bearing capacity was then analyzed,and a resistance sub-factor of 1.45 was proposed.It can be used to evaluate the mechanical performance of post-fire bolted spherical joints.The ultimate fire-resistance temperature of the bolted spherical joints area decreased with the increase of the stress ratio of the steel tube,and the relationship between the ultimate fire-resistance temperature and the pipe diameter,the bolt diameter and the thickness of the sealing plate are not evident.The influence of the fire process on the bolted spherical joints of the tensile bolted spherical bolts was more obvious than that of compressive joints.Through the bolt pull-out test and the numerical analysis on the screw model,the stress distribution of the screw at normal temperature and during the fire was obtained,and the bearing capacity after fire-cooling process was not less than 0.55 times of that at ambient temperature.In the analysis of the whole structure duiring the fire,the nonuniform temperature field of the large space building structure under fire was introduced.Based on the analysis,the residual mechanical properties under the fire were analyzed.It was found that the bearing reaction force during the fire was much larger than the bearing reaction force at normal temperature,and the bearing reaction force would change its direction during the cooling process,which would seriously endanger the structure.Then it was found that setting the elastic support can be a better solution to this problem.In summary,the mechanics performance of bolted joint,bolted spherical joints and grids structure after fire were fully studied,and the post-fire mechanics performance of them was fully understood,which is of great significance for the post-fire assessment of the grids structure.