Study On The Bearing Capacity Of Oblique Section Of UHPC Beam After Fir

Ultra-high performance concrete(UHPC)is a new type of cement-based composite material with ultra-high strength,high durability,high seismic energy dissipation,low permeability and good volume stability.Under fire,the thermal conductivity of UHPC is about1.25 ～ 1.40 times that of ordinary concrete,and the specific heat is about 0.75 ～ 0.85 times that of ordinary concrete,so the section temperature of UHPC is relatively higher than that of ordinary concrete under the same fire condition.The residual shear bearing capacity of UHPC beams after fire has a positive correlation with the temperature of the fire section under fire.Compared with ordinary concrete,the residual bearing capacity of UHPC beams after fire and the residual shear bearing capacity under fire may deteriorate more.In view of the fact that the study of the residual shear bearing capacity of UHPC beams after fire is an important basis for evaluating the reinforcement and repair of fire-damaged components,this thesis mainly carried out the following research work in order to provide some reference for the study of the safety performance of UHPC after fire :(1)In order to study the residual bearing capacity of UHPC beams after fire,12 static load tests of UHPC beams after fire were carried out,and the failure mode,crack development and load-displacement curve of UHPC beams after fire were obtained.The main diagonal cracks of the specimens from the loading point to the support occurred.When the ultimate load was reached,the failure showed brittle characteristics.The steel fiber was pulled out,and the concrete in the shear compression zone was crushed.The deflection deformation of the specimens increased after the fire,and the shear bearing capacity decreased.The influence law is roughly the same as that at room temperature.(2)The fire time,shear span ratio,load level and stirrup ratio have significant effects on the shear bearing capacity of UHPC beams after fire.With the increase of fire time and load level,the internal damage of the specimen increases and the shear bearing capacity decreases after fire.The shear span ratio has a significant effect on the shear capacity of UHPC beams after fire.When the shear span ratio increases from 2.0 to 3.5,the shear capacity can be reduced by more than 40%.The shear failure mode changes from ’ shear compression failure ’ to ’ cablestayed failure ’.The load level is negatively correlated with the residual bearing capacity.When the stirrup ratio increases from 0 to 0.27%,the shear capacity of UHPC beams after fire can be increased by more than 15%.(3)In order to study the influence of high temperature on the bearing capacity of specimens after fire,the calculated value of shear capacity was compared with the static load test value after fire,and the key influencing factors of the weakening degree of shear capacity of UHPC beams after fire were explored.The crack development and burst of the test beam under fire have a certain influence on the weakening degree of shear bearing capacity of UHPC beam after fire.The local burst beam is weakened by about 15%;the crack weakens the shear bearing capacity of the beam by 15-25%,and the influence of the penetrating diagonal crack is the greatest,and the shear bearing capacity of the beam can exceed 35%.(4)Using ABAQUS finite element numerical analysis software,a finite element analysis model of the shear performance of UHPC beams after fire was established.The effects of fire time and shear span ratio on the residual bearing capacity after fire were analyzed.Based on the results of finite element analysis,the temperature field of the beam with stirrups under fire and the load-displacement curve and failure mode after fire are obtained,and the evolution law of flexural bearing damage of the inclined section of the beam with stirrups after fire is revealed.When the fire time increased from 90 min to 30 min to 180 min,the shear capacity decreased by 25.6%,16.2% and 10.7% respectively.The shear capacity decreased by 17.5%,29.8% and37.5% respectively when the shear span ratio increased from 2.0 to 3.5.