Fire Performance And Design Method Of Prestressed Concrete Thin-web Girders Under Semi-open Environment

Transportation infrastructure is an important driving force for national economic development.In recent years,with increase of traffic volume,the threat of fire to bridges has become increasingly significant.Most bridge fires are caused by combustion of flammable materials resulting from vehicle collision or overturning.Such vehicle fires usually have characteristics of “deflagration”,and the temperature can reach more than 1000 ℃ within a few minutes.These fires may cause local component damage or even overall collapse of the bridge.Prestressed concrete(PC)girders are widely used in bridge construction due to their numerous advantages.However,fire will pose a serious threat to safety of PC thin-web girders(boxsection girders and T-section girders).The high temperature of fire penetrating section of thinweb girder causes temperature of prestressing strands to increase,and thus effective prestress declines significantly.The high temperature of fire may also cause serious cracking of girder body,concrete spalling,and direct fire exposure of prestressing strands,resulting in fracture of prestressing strands and sudden collapse of bridge girders.Further,the high temperature of fire causes the mechanical properties of materials to decline significantly.Under continuous action of fire exposure,the stiffness and strength of PC girders are almost lost,and deformation and bearing capacity reach limit state.Therefore,to ensure the full life cycle service of bridges and people’s lives and properties,it is necessary to fundamentally reveal the structural damage mechanism and bearing capacity decline law of bridge structures under fire conditions,clarify fire performance of bridge girders,and propose the fire resistance design method of bridge structures.Aiming at fire performance of PC thin-web girders under semi-open environment(fire under bridge),this thesis conducts investigation on fire response and bearing capacity decline of PC girders through experimental research,numerical simulation and theoretical calculation.The damage mechanism of PC thin-web girders under high strength deflagration fire exposure is revealed,and a practical fire resistance design method for PC bridge girders is proposed.The main research contents are as follows:(1)The temperature field of bridge fire in semi-open environment is simulated using field model.The temperature field of bridge fire with different clearances,different vehicle types and different fire locations is analyzed.The temperature field varying with horizontal distance,height from center of fire source and time is obtained.The temperature relationship of any point in space as a function of time is proposed,and temperature field model of bridge fire in semiopen environment is established.(2)Five large-scaled PC simply supported reduced scale model tested girders(3 boxshaped and 2 T-shaped section girders)were designed and manufactured.The localized fire tests of PC girder under heating condition of fuel fire were carried out.The relevant experimental data,such as temperature field of girder section,deflection variation,decay of effective prestress,crack development,high temperature spalling,and fire resistance,were obtained.The damage evolution process and failure mode of PC girders were explored.The influence of section type,load level,prestress ratio and diaphragm on fire behavior of PC girders was analyzed.(3)A CFD-FEM coupling analysis method for fire response of PC girders under fire exposure is proposed.This method involves sequential coupling analysis of fire dynamics,three-dimensional transient heat transfer and nonlinear structural response.The test results are used to verify the finite element analysis method,in which the temperature related material properties,material nonlinearity,geometric nonlinearity and boundary nonlinearity(bearing contact and girder end constraint)are considered.This analysis method can consider real boundary conditions of PC girders with simply supported,continuous,various complex sections,and non-uniform characteristics of actual fire temperature field.(4)Considering characteristics of real fire field of bridge,the CFD-FEM coupling analysis method is used to analyze deflection development,failure mode and prestress decay of PC simply supported girders under combined action of flexural and shear loads,and typical vehicle fire fields with different clearances and fire positions.At same time,the effect of load level is considered in the analysis.The influence of key factors on fire performance of PC simply supported girders is identified and quantified.The control factors and typical failure modes of PC simply supported girders under fire exposure are obtained.Based on load-displacement curves with different fire exposure time,the degradation laws of flexural and shear capacity of PC simply supported girders are obtained.The influence of girder end constraint on decay of bearing capacity in PC simply supported girders is analyzed.(5)The deflection development,failure mode and prestress decay law of PC continuous girders under typical vehicle fire conditions of mid-span in middle span,mid-span in side span and mid fulcrum are analyzed using CFD-FEM coupling analysis method.Considering different clearances and different flexural and shear load levels,the most unfavorable fire scenario of PC continuous girders is obtained.Based on load-displacement curves with different fire exposure time,the degradation laws of flexural and shear capacity of PC continuous girders are obtained.The redistribution law of internal force of PC continuous girders under fire conditions is studied,and the damage mechanism of PC continuous girders under vehicle fire exposure is revealed.(6)A practical fire resistance design method for bridge structures is proposed.Considering balance between economic performance and structural performance,two types of fire resistance design objectives for PC bridge girders are established.A fire resistance design principle of“repairable in case of moderate fire,and no collapse in case of severe fire” is proposed.Aiming at this principle,some measures,such as installing longitudinal limit bearing is proposed for PC bridge girders and setting standby prestressing strands,can effectively prevent bridges from collapsing under severe fire exposure.The application of fire-retardant coating on the inner side of strands corrugated pipe near the concrete surface can prevent high temperature loss of prestress,and therefore the bridge structure after fire exposure can be repaired as soon as possible.Based on damage equivalent section of concrete and damage equivalent strength of steel,a simplified calculation method for flexural and shear capacity of PC bridge girders under fire conditions is proposed and is verified by comparison with numerical calculation results.This method can provide a preliminary judgment basis for fire resistance design of bridges.