Study On Mechanical Performance And Fire Resistance Design Of Cable Of Spatial Cable Suspension Bridge Under Fire

With the continuous improvement of China’s transport infrastructure and the rapid growth in traffic volume,the economic losses and adverse social impacts caused by bridge fire accidents cannot be ignored.Suspension bridges have an extremely strong spanning capacity and are one of the main bridge types for large span bridges.Suspension bridge cables are made up of parallel steel wires or strands,but their fire resistance is poor and will seriously affect the safety of the bridge structure after damage occurs under fire.In this paper,a space cable deck suspension bridge is used as the engineering background to carry out the analysis of the structural stress performance and fire resistance design of the bridge cable structure under the action of fire.The main contents and conclusions are as follows:(1)The current situation of tanker fires and cable fire resistance was investigated,steel performance parameters at high temperatures were summarised,a numerical simulation method for fires was determined based on heat transfer theory,heat release rate models for car fires were compared,and the 200 MW tanker fire was determined to be the most unfavourable fire condition for bridges under the squared heat release function.The transient thermal analysis of the 200 MW tanker fire was carried out by applying the fire numerical simulation software FDS.The transient temperature was higher than 300°C in the space range of about 15 m to the side of the fire source and about25 m above the fire source.(2)Based on the spatial temperature field law of 200 MW tanker fire,a spatial rope deck suspension bridge was used as the engineering background,and four most unfavourable fire scenarios were set up for the cables by combining the characteristics of the bridge structure,and the numerical simulation of each fire scenario was carried out by FDS.The fire-stricken cables in each fire scenario had different degrees of high temperature damage,and in fire scenario three there was a risk of breaking the cables within a short period of time.(3)Using ANSYS to establish a transient thermal analysis model of the cable plane,to obtain the internal heat transfer law and temperature field distribution of the cable.Combined with the heating law of each steel wire layer of the main cable section,the main cable "series-parallel" model was proposed,and the stress and strength change law of each steel wire layer of the main cable after the fire was calculated,the temperature difference of each steel wire layer of the main cable section was large,and the strength of each steel wire layer after the fire was reduced to different degrees,and the temperature difference between each steel wire layer would lead to stress redistribution.(4)Using ANSYS to build a spatial cable deck suspension bridge model,combined with the heating law of the cable surface in the fire scenario,based on the "series-parallel" model of the main cable,the mechanical properties of the bridge cable in each fire scenario were analysed,and the structural properties of the bridge cable after fire were obtained,the main cable will not break in each fire scenario,of which fire scenario 3 is the In the most unfavourable fire scenario,the fire-stricken cable breaks 5 minutes after the fire and the stresses in the adjacent cables increase dramatically,but no continuous cable breakage occurs.(5)Based on the most unfavourable fire scenario cable surface temperature,the basic principles of cable fire resistance design are proposed,combined with the 200 MW tanker fire space temperature field distribution and cable surface temperature at different fire scenarios,basalt fibre is used as the fire resistant material,the thickness of the cable fire resistant layer is not less than 6mm,the height of protection against cable failure fracture is 13 m,the height of protection against cable sheath breakage is 30 m,the protection distance of the main cable is 98.7m on each side of the mid-span span,and the distance between the side span main cable and the bridge deck intersection is 30 m.The protection distance is 98.7m on each side of the middle span and 30 m above the intersection of the main cable and the bridge deck on the side spans.