Experimental Study On Fire Resistance Of Composite Beams With Cast-In-Place Concrete Slabs On Concrete Slab With Steel Bar Truss

With the vigorous advocacy of assembly building in china,composite beams with cast-in-place concrete slabs on concrete slabs with steel bar truss are widely used.It is composed of cast-in-place concrete slabs on concrete slabs with steel bar truss,studs and steel beam.The composite beam has the advantages of excellent mechanical properties,fast construction speed and high degree of industrialization.While bearing the vertical load from the floor,the composite beam also transmits load to the vertical force component.So,the composite beam is an important force-bearing and force-transferring components in the assembled structures.When building fires occur,its fire resistance is an important factor affecting the fire resistance of assembled structures,so it is necessary to study the fire resistance of composite beams with composite concrete slabs.In this paper,through reviewing the domestic research status of bearing capacity and fire resistance of composite beams,the author found that there is no research on fire resistance of this kind of composite beams.Based on this,the mechanical properties,fire resistance and residual load-bearing capacity of fired composite beam are experimentally studied.Also,the temperature field model of composite beam with composite slab is established by using finite element software ABAQUS.The following conclusions are drawn:(1)Because of the shear resistance of steel bar truss and roughening treatment of prefabricated slab surface,the horizontal overlap surface of composite beams with prefabricated slab did not crack in fire resistance test and two-point loading test,but the vertical overlap surface of the joints appeared cracks.(2)The existence of overlapping surfaces have no obvious influence on the temperature field,and when a fire occurs,the laws of temperature field of the composite beam with prefabricated slabs are similar to that of the composite beam with cast-in-situ slab.There is a large temperature gradient in the section of steel beam and flange plate,which leads to the generation of temperature stress.(3)Because the concrete curing time is shorter and the moisture content is higher than that of the prefabricated floor,the surface of the composite beam is loose and gray-white after fire,and the damage is more serious than that of the prefabricated floor.So the crack distribution of the composite beam is different from that of the composite beam with cast-in-situ slab.The cracks of the composite beam with prefabricated slabs are mainly concentrated in the joints of the latter pouring,and the cracks are wider and less,but the cracks of the composite beam with cast-in-situ slab are evenly and densely distributed on the side of the slab,with fine and dense cracks,and there are no obvious cracks on the bottom of the slab.(4)Under the coupled action of heat and force,the composite beams with prefabricated slabs and composite beam with cast-in-situ slab underwent downward deflection,and the deformation amounts and laws are basically similar.The deformation recovery ability of beams is similar in the cooling stage.Although the damages of composite beams with prefabricated slabs are different from that of composite beam with cast-in-situ slab,their macroscopic failure characteristics are similar.Their deformation law and the macroscopic failure characteristics are similar,which shows that the integrity of the composite beam with prefabricated slabs is basically the same as that of the composite beam with cast-in-situ slab,and the fire resistance of the two kinds of composite beams is roughly same.(5)Under the same external load,when the composite beams with prefabricated slabs are designed as a complete shear connection,the main factor affecting its fire resistance is the thickness of post-poured layer,the shelf length of prefabricated floor and the space of bolts have little effect on its fire resistance.(6)In the bearing capacity test,the composite beams without exposed to fire and exposed fire occurred bending failure when reaching the ultimate bearing capacity.The bending moment-deflection curves of composite beams in mid-span are all in the form of double-fold lines,but there are still some differences in the failure process.Due to the poor ductility after fire damage,the composite beams quickly reach the ultimate bearing capacity after yielding of the lower flange of the steel beams,while the ultimate load of the composite beam without exposed to fire is much higher than that of the yielding load,and the ductility of normal composite beam is better.(7)By comparing the fire-free and fire-exposed specimens with the same parameters,it is found that the damage caused by fire to ductility of composite beams with prefabricated slabs is more serious than the bearing capacity.(8)The main parameter affecting the residual bearing capacity of composite beams with cast-in-place concrete slabs on concrete slabs with steel bar truss after fire is the spacing of studs.The smaller the spacing of studs,the residual bearing capacity is larger after fire.When the thickness of post-poured layer increases,the cracking moment increases,but the yield moment and ultimate moment decrease.The shelf length of prefabricated bottom slab of truss reinforced concrete composite slab beam is recommended to be 40 mm.(9)After the fire,the composite beams with prefabricated slabs and composite beam with cast-in-situ slab both suffer bending failure in the bearing capacity test,which has similar failure characteristics,but the residual bearing capacity of composite beams with prefabricated slabs is larger than that of composite beam with cast-in-situ slab.Therefore,after the fire,the bearing capacity of composite beams with prefabricated slabs is better than that of composite beam with cast-in-situ slab.(10)The finite element software ABAQUS is used to establish the model of composite beam with prefabricated slab to calculate the temperature field.Then the correctness of the temperature field model is verified by comparing the calculated value with the measured value.The model can provide a calculation model for temperature field analysis of composite beams with cast-in-place concrete slabs on concrete slabs with steel bar truss.