Theoretical Investigation On Catenary Actions Of Restrained Cellular Beams In A Fire

The catenary action of a restrained cellular steel beam in a fire refers to the phenomenon that with the elevation of temperature,axial force is induced in the steel beam due to the thermal expansion is restrained;with the further elevation of temperature,a large deflection is developed in the steel beam and correspondingly the axial force in the beam turns from compression into tension to resist the external load.The potential of the structure could be fulfilled by considering the catenary action developed in the restrained steel beam and it is likely to improve the critical temperature of the structural members.Current research work on catenary action of steel beams is concentrated in numerical investigation and is based on the assumption of uniform temperature distribution across the section,however,and there is no reliable theoretical analysis method yet.First,the catenary action mechanism of restrained cellular steel beam in a fire was analyzed using validated finite element model.Results showed that the steel beam experienced three stages in a fire:compressive bending stage,post-buckling stage and catenary action stage.The bearing mechanism of steel beam turns from beam bending mechanism into catenary action mechanism with the elevation of temperature.The deflection of steel beam was increased due to the effect of thermal bowing induced by non-uniform temperature distribution across the section.Compared with solid web beam,the web of a cellular steel beam played a small role which can be neglected.After that,the effects of different parameters on the catenary action of restrained cellular steel beam in a fire were investigated.Parameters include temperature gradient,load ratio,axial restraint stiffness ratio,span to height ratio,opening ratio,web-post width and opening eccentricity.Results showed that parameters including load ratio,axial restraint stiffness ratio as well as span to height ratio had a significant influence on the catenary action behavior of restrained cellular steel beam in a fire,while the other parameters including opening ratio,web-post width and opening eccentricity had a limited influence on the structural performance of cellular steel beam in a fire.Finally,based on equilibrium equation,constitutive equation and geometric equation,a Simplified Analysis Method(SAM)for catenary actions of restrained Cellular Steel Beams(CSB)in a fire was proposed.Contributions to axial force and bending moment by the cellular web above and below web-openings were considered.Through comparing the results predicted by the simplified analysis method and Finite Element Analysis(FEA),the accuracy of the proposed method was assessed from three aspects:axial restrained force in the beam,mid-span deflection and feature temperature.Comparison results showed that catenary actions predicted by the simplified analysis method agreed well with those obtained through finite element analysis and it could be used to predict the behavior of the beam subjected to non-uniform temperature distribution across the section in the latter stage of a fire,which cannot be obtained from the finite element analysis due to convergence problem..A lower compressive force and catenary tension were predicted when the web is not considered.And the ignorance of strain reversal could lead to a larger deflection prediction.