Study On The Bearing Capacity Of Circular Honeycomb Composite Beams Before And After Fir

The circular honeycomb composite beam with open holes is a new type of composite beam structure developed on the basis of the traditional steel-concrete composite beam with solid abdomen.It not only has the advantages of simple construction,good ductility and high stiffness of the traditional steel-concrete composite beam,but also has the holes in the web of the steel beam to reserve enough space for the equipment pipeline to pass through,reducing the pipe’s occupying of the building’s net height.With good technical and economic effects and social benefits,it is widely used inbridges,factories,office buildings and other projects.It is a promising structure form.Through in-depth analysis of the research results of honeycomb composite beams,it is found that the current research on honeycomb composite beams mainly focuses on the mechanical properties of honeycomb composite beams before fire and fire resistance,but lacks the research on mechanical properties of honeycomb composite beams after fire,which can not provide reliable basis for the evaluation of mechanical properties of honeycomb composite beams after fire.In this thesis,fire test,static loading test and finite element simulation were carried out to study the effects of fire,stiffener setting,opening rate,edge spacing and other factors on the failure form,bending stiffness and bearing capacity of circular honeycomb composite beams with open holes.On the basis of the existing research results,the calculation method of residual bearing capacity of honeycomb composite beams with circular openings after fire is derived.The main research results are as follows:(1)Under static loading,the circular honeycomb composite beams with open holes before and after fire all experienced four stages: the original cracks widened(the honeycomb composite beams did not experience this stage before fire),the concrete generated new cracks,the crack development,and the bending failure occurred.However,after fire,the circular honeycomb composite beams with open holes were affected by the coupling effect of high temperature and uniform load.The cracking load of concrete slab is low,and its ultimate bearing capacity and ductility both decrease in different amplitude.In this test,the ultimate bearing capacity and ductility of specimens decrease by 41.5% and 27.3%.(2)The opening rate is the main factor affecting the ultimate bearing capacity of circular honeycomb composite beams with open holes after fire,and reducing the opening rate can improve the bending stiffness and ultimate bearing capacity of circular honeycomb composite beams with open holes after fire.Compared with those without stiffeners,the ultimate bearing capacity of the honeycomb beam with circular holes after fire is increased by 6.9%.The mechanical properties of honeycomb composite beams with circular openings for 60 minutes under fire are slightly better than those of honeycomb composite beams with circular openings for 90 minutes under fire.The ductility of the circular honeycomb beam with open holes can be improved by setting stiffeners,reducing the opening rate and reducing the fire time.The stiffeners have the most obvious effect on improving the ductility of the specimens.(3)In the process of static loading,the deflection of circular honeycomb composite beam with open holes constantly changes with the increase of load.In the elastic working stage,the vertical displacement of each point is roughly equal and increases in equal proportion,and the bending deformation of the specimen is not obvious.When the load reached the yield load,the mid-span displacement was greater than the displacement at the two loading points,and the vertical displacement of each load increased with the increase of the load,and the specimen showed a bending deformation trend.When the load exceeds the ultimate load of the member,the bending deformation of the specimen is obvious,but the vertical displacement at the two loading points is asymmetrical,and the position of the larger displacement end is not fixed.(4)The deformation of concrete slabs of circular honeycomb composite beams is larger in the steel beam area,and the influence of steel beam co-deformation is the largest under the action of shear connectors.With the increase of distance from the position of shear connectors,the influence gradually decreases,and the deformation of concrete becomes smaller and smaller.(5)ABAQUS software was used to establish the finite element analysis model of circular honeycomb composite beams with open holes under static loading conditions before and after fire.The calculated load deflection curve and bearing capacity of specimens were compared with the existing test results,which proved that the finite element model was reliable.In addition,the model is used to analyze the mechanical properties of specimens with different edge spacing.It is found that the ultimate bearing capacity of circular honeycomb composite beams with open holes first increases and then decreases with the increase of edge spacing.(6)Based on the calculation formula of the bearing capacity of honeycomb composite beams before fire,a reduction factor is introduced to consider the effect of material deterioration on the bearing capacity of honeycomb composite beams with circular open holes after high temperature.A calculation formula for the residual bearing capacity of honeycomb composite beams with circular open holes after fire is established.The results show that the method has high precision.