| In this paper,the composite beam-column structure with unequal span,which were comprised of three columns and two beams with bolt and weld rigid connection,was studied.Based on the alternate load path method,the anti-collapse performance of the composite beam-column substructure with asymmetric spans were studied by theoretical analysis,experimental research and numerical simulation.The effects of the combination of slabs and height-span ratio on the collapse resistance of substructures are discussed in depth.The main research contents of this paper are as follows:(1)Based on the alternate load path method and fully considering the composite effect of floor slabs,the composite beam-column substructure with unequal spans were simplified as explicit models of double-span composite beam under concentrated load,and it has been studied to deduce the formulas for load-deformation calculation in five different stages(elastic stage,elastic-plastic stage,plastic stage,transient stage and catenary stage)during progressive collapse in detail.The anti-collapse mechanism of composite beam-column substructure was emphatically analyzed.A numerical model was established by ABAQUS,the theoretical formulas were compared with numerical simulation under different span ratios.The results showed that the initial stiffness and ultimate bearing capacity of composite beam-column structure will decrease with the increase of span ratio of two-bay composite beams,which is not conducive to the exertion of arch action.And the results of the theoretical formulas agree well with the numerical analysis,which showed that the theoretical formulas have good generality and higher accuracy for progressive collapse analysis of composite beam-column substructure with asymmetric span.(2)The composite beam-column substructure(span is 1.4)with unequal span was studied.Through static loading test,the failure mode,bearing capacity,deformation capacity,internal force development and resistance mechanism of the specimen were studied.And analyze the effect of unequal span on the collapse resistance to the substructure.The results show that,the existence of slabs can significantly improve the resistance provided by the flexural in the small deformation stage,and the steel bar in the slab plays a key role in the catenary stage,that is,the combination of the slab and the steel beam makes an important contribution to the structural resistance to load.When the fracture of beam tension flange near the failure column,the bending moment of the corresponding beam end decreases sharply,resulting in a significant decrease in the bending resistance.At that time,the resistance of the catenary mechanism plays a major role and exceeds the contribution of the resistance of the flexural mechanism;The bending resistance of beams without damage is not obviously reduced,and the external load is mainly resisted by flexural mechanism and catenary mechanism,which shows the asynchronism of the resistance mechanism of two-bay beams.(3)The finite element analysis software ABAQUS was used to establish the refined numerical analysis model of the specimen with unequal span.Compared with the experimental results and the finite element analysis results,the model accurately reproduced the internal force development process and the final damage of the specimen under the failure condition of the middle column,which showed the simulation method has high precision and can be used as an important means for the mechanical mechanism of the composite beam-column structure.On this basis,the influence of the axial restraint of the peripheral members on the collapse resistance of the beam-column substructure is discussed.The results show that the axial stiffness provided by the peripheral members to the beam ends can make the catenary effect of the substructure fully exerted,and improving the vertical bearing capacity of the double-span beam.(4)The numerical simulation parameters analysis of the composite beam-column substructure with bolt and weld rigid connection was carried out,and the influence of the of the composite slab and unequal height-span ratios(beam height,span and other parameters)on the collapse resistance of the substructure were emphatically analyzed.The results show that the combined effect between the slab and the steel beam caneffectively improve the initial stiffness and bearing capacity of the structure,and the beneficial effect of the slab on the collapse resistance of the structure should be fully considered in practical engineering.The height-span ratio is the key parameter affecting the collapse resistance of the substructure,according to the height-span ratio of the two-bay beams connected to the failure column,it can be divided into two situations:(1)the beam height is unchanged,the span is not equal;(2)the span is unchanged,the beam height is is not equal.Based on the full-scale model,the proportion of the resistance mechanism of the composite beam-column substructure in these two cases was quantitatively analyzed.The results show that the contribution proportion of flexural in the whole loading process of all specimens is in the range of 60%~70%,and the catenary mechanism accounts for about 30%~40%.Based on this,the empirical formulas for the contribution of two-bays beams to the substructure catenary mechanism resistance in two cases are proposed,which can provide reference for design code for collapse resistance and engineering applications.Finally,the structural dynamic response of composite beam-column substructure specimen with unequal span under sudden external loads is investigated by using the principle of energy balance.The results show that the composite beam-column substructure has higher load-carrying capacity than the beam substructure under dynamic loads;In case(1),the maximum pseudo-static load increases continuously with the increase of the right beam’s height-span ratio,and the specimen with equal span has the largest displacement,indicating that the structure with equal span is more favorable to the development of the displacement under the dynamic load than the structure with unequal span.In case(2),the bearing capacity of composite beam-column structure under dynamic load can be significantly improved with the increase of the right span beam height,but it is not conducive to the development of structural deformation. |
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