Fire Resistance Of Composite Beams With Superposed Slabs In Frames

In order to protect the environment and improve the industrialization and commercialization of housing,the country is currently vigorously promoting housing industrialization and prefabricated structures.As a prefabricated component,the laminated composite beam has the mechanical properties of high bearing capacity and high rigidity.It reduces the mold supporting process during the construction process and has a good economic advantage.At present,many experimental studies have been carried out on the fire resistance of the composite beam structure,but most of them are limited to the test of individual components.There are few studies on the composite beams with superposed slabs in frames.Therefore,the structure of the composite beam of the composite plate has been studied.The fire resistance research can provide a basis for the fire resistance design of the composite beam structure of the laminated slab,thereby effectively reducing the loss of life and property.In this thesis,the test specimens of two-strand two-span composite beam frames are tested for the fire resistance performance under constant load in an environment that simulates the standard heating curve.The test phenomena and failure modes of the specimens are observed,and the temperature field distribution and displacement deformation of composite beams with superposed slabs are measured.Analyze the temperature development and deformation laws of composite beams with composite slabs,and explore the influence of node constraints,floor forms and fire times on the mechanical behavior of composite beams with composite slabs in frames.The finite element software ABAQUS was used to establish the finite element model of the composite beam in frames during the fire phase,and the simulated value was compared with the experimental value.Get the following conclusions:(1)The lower flange and web of the steel beam of the composite beam are twisted under the effect of thermal coupling.The degree of torsion is related to the time of exposure to fire.After the second fire,the degree of torsion of the steel beam increases significantly;the degree of torsion It is related to node constraints.When the welds of just-connected joints are exposed to fire,they will break and release part of the internal force of the steel beam,and the internal force will be redistributed inside the steel beam to avoid excessive torsion of the composite beam specimen.(2)The composite beam floor has a temperature gradient along the section height,and the superimposed interface between the precast bottom plate and the post-cast layer has no effect on the temperature transfer.The damage pattern of the floor is related to the floor form.The cracks at the joints of the prefabricated bottom plate of the separated composite beams are large,and the cracks develop from the side to the top of the floor,and the cracks at the joints of the integrated composite beams after pouring Smaller,more cracks.The temperature field in the cracked area on the bottom of the floor is higher,and the material performance degradation is more serious.(3)The ability of the composite beam to resist vertical deformation is related to the node constraint.During the same fire time,the mid-span displacement value of the rigidly constrained specimen is always smaller than that of the hinged constrained specimen.The rigidly constrained composite plate The composite beam has a stronger ability to resist vertical deformation;when the structure is exposed to a second fire,the growth rate of the mid-span displacement of the composite slab composite beam in frames does not change much compared with the first time.After the standard heating curve is heated for 55 minutes,the second time When the structure is exposed to fire,the ability of the composite beam to resist vertical deformation does not decrease,and the ability of the composite beam to resist vertical deformation in frames It is related to the performance of the bottom reinforcement;the form of the floor has little effect on the ability of the composite beam to resist vertical deformation in frames.(4)The vertical displacement value of the composite beam in frames is related to the boundary constraint.The greater the surrounding boundary constraint,the smaller the vertical displacement value.The development of cracks is related to boundary constraints.The floor near the middle support bears a large negative bending moment at the end.During the fire process,the transverse crack develops at the earliest time and the crack width is the largest,which seriously weakens the composite beams with superposed slabs in frames.For the fire resistance of the beam,it is recommended to add steel bars that resist negative bending moments here.(5)After cooling,the composite beams in the structure showed a certain degree of deformation recovery.The deformation recovery ratios of the two groups of composite beams in the same frame structure are close.Mutual influence.The deformation recovery ratio of the specimen is related to the fire time and the number of times.The material performance of the secondary fire specimen and the long-term fire specimen is seriously degraded,and the deformation recovery ability of the specimen is poor.(6)Using the finite element software ABAQUS,the finite element model of composite beams with superposed slabs in frames was established,and the sequential coupling method was used to analyze the temperature field and thermal-mechanical coupling of the model.By observing the laws of cloud diagrams and comparing the simulated values with the experimental values,it is found that the temperature field analysis results better reflect the temperature field distribution of the composite beams in the structure during fire,and the thermal-mechanical coupling analysis results better reflect the specimens However,the simulation of the law of displacement changes in the experiment requires further research.