Experimental Study On Fire Resistance Of Prestressed Composite Slabs With Plate End Restraint Steel Trusse

After the reform and opening up,with the vigorous development of the economy,China’s urbanization rate has achieved a historic growth.China has the largest number of high-rise buildings in the world,and high-rise buildings have become an important symbol of modern cities.In the past ten years,fire accidents in high-rise buildings have occurred frequently,with more than 30000 fire accidents in high-rise buildings in China,showing an increasing trend year by year.The damage and economic losses caused by fire are huge,so it is particularly important to study the fire resistance of the structure.As an important horizontal load-bearing member,steel truss prestressed concrete composite slab is common in various buildings,which plays a very important role in ensuring the overall stiffness and bearing capacity of buildings.Unlike the traditional cast-in-place slab,the laminated slab is made of prefabricated and cast-in-place composite.Under the action of high temperature under fire,its stress mechanism is more complex.When the building is in fire,the laminated slab must bear the upper load and the coupling effect under high temperature under fire for a long time.With the continuous rise of temperature,the mechanical properties of the material change,the internal force redistribution phenomenon continues to occur in the structure,and the member stiffness continues to decline,It is easy to cause the collapse of the whole structure.Therefore,it is of great practical significance to study the mechanical behavior of reinforced concrete composite slabs under the coupling action of fire and load.This thesis focuses on the fire resistance performance of four steel truss prestressed concrete composite plates under the action of dead load and temperature rise,and mainly analyzes the influence of different plate end restraint conditions,post-cast layer thickness and plate top reinforcement on the fire resistance performance of the specimens in the test.Observe the crack development,failure mode,deflection change,and crack of concrete at the bottom of the slab during the test.Based on the analysis of the test data,the influence of the plate end restraint mode,the thickness of the composite plate and other factors on the fire resistance and failure mechanism of the steel truss prestressed composite plate is summarized.The following conclusions are obtained through analysis:(1)Under the coupling of load and fire,the prestressed precast layer reinforced truss composite plate has low bonding strength on the composite surface.Under the combined action of load and temperature stress,horizontal cracks will occur on the combustion cross composite surface,and the precast layer will separate from the post cast layer composite layer.However,due to the installation of a steel bar truss,the prefabricated layer and the post cast layer are not completely separated,ensuring that the prefabricated layer and the post cast layer still have a certain overall working capacity.Therefore,the stress performance of the steel bar truss in the prestressed steel bar truss composite plate under fire plays an important role.(2)During a fire,the prestressed precast layer steel truss composite plate will experience severe cracking in the bottom precast layer,with the depth of the cracking reaching the post pouring layer.Severe vibration is generated during the explosion,further causing separation between the precast layer and the post cast layer.(3)Due to the explosion of the prefabricated layer,the cold-drawn steel wire prestressed steel bars at the bottom of the slab are exposed to the fire and directly subjected to fire,resulting in a significant reduction in the strength of the steel bars and a significant reduction in the bearing capacity of the slab.(4)The prefabricated layer will burst,and the stress mechanism of the slab will change.The stress mechanism of the slab will change from a concrete slab to a compression strut and a post cast concrete layer,and the tension strut will be a steel truss concrete composite slab with bottom reinforcement,so that the slab still has certain stress performance.But as the combustion time increases,the elastic modulus of the steel bar decreases,and the tensile deformation increases.The post cast composite layer will generate bending moments,resulting in vertical cracks.The negative bending moment of the cantilever end support will increase,resulting in internal force redistribution.The negative bending moment section of the support will extend outward.Under the combined action of negative bending moment and temperature stress,the end of the negative reinforcement at the top of the plate will generate transverse cracks that run through the plate surface.When the negative bending moment reinforcement is extended or arranged in full length,this type of crack can be avoided.(5)The pre cast layer experiences cracking,the thickness of the floor concrete decreases,and the post cast layer is directly exposed to the fire,reducing the fire resistance limit of the slab.(6)The fire resistance limit is related to the thickness of the plate.When the thickness of the prefabricated layer is the same,increasing the thickness of the post cast composite layer can not only improve the fire resistance limit of the specimen,but also effectively improve the bending stiffness of the specimen and reduce its deformation.(7)Under the action of constant load and temperature rise,the cantilever end of the specimen levers the fire span with the support as the fulcrum,causing upward displacement,reducing the downward deflection of the specimen,and thus having a certain restraining effect on the deformation of the specimen.