Investigation On Fire Resistance Of Prestressed Continuous Steel-concrete Composite Beams Considering Initial Defects Of Steel Beams

The new structural form of prestressed composite beams,which is composed of steel beam and concrete slab,are applied in many long-span bridges and some super-high-rise structures.In actual engineerings,steel beams are inevitably with initial defects due to manufacturing,installing and transporting,etc..Initial geometric defects and residual stresses have a greater impact on performance and bearing capacity of steel beams.Especially rapid degradation of material properties at high temperatures probably cause flexual stiffness of composite beam to decrease rapidly.Therefore,it is necessary to study the fire resistance of the prestressed composite beams with initial defects.The main investigation in the dissertation are as followings:Firstly,finite element model of prestressed composite beam were developed in this paper,which were verified by comparison with previous experiments.Secondly,failure modes and structural response of prestressed composite beams are investigated by parametric analysis.These parameters included: initial geometric defect form and size,load ratio,prestress ratio,different fire mode and position of fire-resistant steel arrangement location.Results show that initial bending-torsional buckling mode and distortion buckling mode have great effects on flexural capacity of composite beam at high temperature.Bending-torsional buckling mode increases the initial bending curvature of steel beam and reduces flexural stiffness of section.Distortion buckling reduction stiffness of steel at intermediate support of beam.load ratio and prestress have an influence on fire resistance of prestressed composite beams with initial geometrical defects;The smaller the load ratio,the better fire resistance of composite beams are.The effect of initial prestress on fire resistance of composite beams is mainly in the begining stage of temperature elevating,and the prestress ratio is higher,composite beams have better fire resistance,but as temperature increases,the influence of the prestress ratio decreases.Fire resistance of beams which bottom flange is made of refractory steel is better than those beams which web is composited of refractory steel;Application of refractory steel in bottom flange and web can improve fire resistance of composite beams significantly is composited of refractory steel.The larger the portion of refractory steel in the beam,the smaller the deflection and the higher the critical temperature.Then,fire performance of prestressed composite beams under high temperature were analyzed by using parametric analysis.These parameters included: four different residual stress modes,load ratio and prestress ratio.Structural response such as mid-span deflection,section stress distribution,prestressed tendon internal force,bearing reaction force,neutral axis height and beam curvature,with temperature were investigated.Results show that different residual stress modes have different effects on the behavior of composite beams.Residual stress mainly affects the section stiffness by varing the section moment of inertia and tangential modulus of steel beam,which affects the bending performance of composite beam.With the increase of temperature,magnitude of residual stress on the fire resistance performance of composite beam also changes.In later stage of heating,residual stress has slight effect on the fire resistance of composite beams.Finally,work in this paper was summarized,and work needing to be carried out further is prospected.