Structural Behavior Of The New Assembled Waffle Slab System Exposed To Fire

Based on prefabricated technology,a novel type of assembled waffle slab has been proposed in this paper,which is composed of orthogonal two-way ribs and top wythe.The ribs are reinforced by steel trusses to improve the flexural rigidity rigidity of the floor,the welded rebar mesh is embedded in top concrete wythe for preventing cracking on the top,foamed concrete blocks are filled among the ribs with the purpose of heat insulation and fire resistance.A special template system with favourable construction efficiency is established and adopted for the installation and concrete cast of the slab.Compared with traditional waffle slab,there are unique structural configuration and obvious construction advantages in this floor system.In order to promote the application of this new type of assembled waffle,six new type of assembled waffle slab specimens are designed and made by special template system for experimental investigation in this paper.Static loading tests,fire tests and post-fire static loading tests are carried out,and the mechanical behavior of the new type of assembled waffle slab under ambient temperature,fire and post-fire conditions are explored.Based on the static loading test of four new-type assembled waffle slab at ambient temperature,the deformation characteristics,the strain development of reinforcement and concrete,the law of crack generation and the failure mechanism of the slab under under four-point bending load are studied.The influence induced by two different boundary conditions is analyzed.The test results show that the deflection curves of slab center can be divided into four stages: elastic stage,elastic-plastic stage,plastic stage and failure stage,with cracking load,yield load and ultimate load as the boundary points.When the slab reaches its ultimate bearing capacity,the slab does not completely lose its bearing capacity immediately,plastic internal forces redistribution is exhibited and plastic hinge lines at the end of rib beams and the central area of slab top is presented.The end beams of four-side simply supported slabs are mainly subjected to torsion and shear,and the crack width is small and the tensile reinforcements at the lower part are not yielded when the slabs are destroyed;the end beams of four-point simply supported slabs,as the main part of force,play an important role in the bending resistance of the slabs,when the slabs are destroyed,the tensile reinforcements at thelower part of the side beams reach yield,although the ultimate bearing capacity decreases,better deformation capacity is obtain.The failure mechanism is different for four-side simply supported and four-point simply supported slabs,but there is no essential difference in the cause of failure,which belongs to flexural failure.During the loading process of all slab specimens,there is no foam concrete blocks falling.The bonding performance between ribs and foamed concrete blocks is good,which can meet the safe requirements.ABAQUS is used to simulate the mechanical behavior of the new type of assembled waffle slab at ambient temperature.The numerical model is verified by experiments.The finite element simulation proves that the new type of assembled waffle slab possesses two-way mechanical characteristics.The plastic hinge theory is used to calculate the ultimate bearing capacity of the new type of assembled waffle slab under two kinds of support modes.The calculated results are in good agreement with the experimental results.Considering the flexural stiffness change of the slab in the plastic stage,the central deflection of the new assembled waffle slab with four simply supported edges is calculated by using the plastic deflection theory,and the elastic deflection theory of the solid plate is extended to the deflection calculation of the new assembled waffle slab with four simply supported angle in the plastic stage.After the slab ribs cracking,the actual stiffness is simplified to establish the elastic theory correction method.The calculation results are in good agreement with the test results.The two methods can be used to guide engineering application and deformation control design of new assembled waffle slab.The temperature field distribution under natural fire and post-fire mechanical performance is studied by means of constant load-heating fire tests and post-fire static loading tests on two new type of assembled waffle slabs.The test results show that two slabs do not exhibited any serious damage under 955 ? temperature for two hours.The new type of assembled waffle slab possesses good fire resistance performance and enough fire resistance time.The thermal resistance of foamed concrete blocks filled among ribs is excellent,so that the rib beams are in a state of unilateral fire,and the rib beam concrete has an obvious temperature gradient along the vertical section.The closer the location is to the floor,the larger the temperature gradient is;there is obvious cooling lag effect between concrete and reinforcement,and structural failure may also occurs in the stage of fire weakening;crackdevelopment and failure mechanism of the slab during post-fire static loading are similar to the slab specimens of static loading tests at ambient temperature;after fire,the ultimate bearing capacity of the slab does not decrease,but its anti-deformation ability is lower,the decrease is exhibited in the load-displacement curve of the slab center,where the limit displacement and deformation rate increase greatly after fire when the slab reaches ultimate bearing capacity.The main effect of fire on the new type of assembled waffle slab is to reduce the flexural rigidity.Based on the basic principle of flexural design of reinforced concrete beam and temperature field distribution of rib beams obtained from the fire tests,three-step model is adopted to calculate the flexural bearing capacity and design the cross-section of rib beams for the new type of assembled waffle slab under fire.It provides reference and guidance for the fire resistance design of the new type of assembled waffle slab.The equivalent fire floor live load is proposed as a reference index to evaluate the fire effect and serves as the basis for evaluating the residual performance of the slab after fire.This paper provides a broad scientific research basis and reliable theoretical guidance for the structural design,fire resistance design,post-fire evaluation and appraisal reinforcement of the new assembled waffle slab.