Test And Analysis Of The Fire Behaviors Of Reinforced Concrete Beam-Slab Assemblies

Boundary restraint has a great influence on the fire resistance of concrete members,so the fire behaviors of reinforced concrete(RC)beams,slabs or columns with boundary restraints have been studied in different degrees.However,in real fire scenarios,RC beams and slabs are often exposed to fire simultaneously,and may interact with each other.Consequently,the fire behaviors of the RC beam-slab assembly are tested and analyzed in this paper.The main woks and conclusions of this paper are as follows:1.To further improve the accuracy of boundary restraint stiffness determination of RC beam-slab assembly in fire,the formula of rotational restraint stiffness at beam end proposed by researchers has been improved,and then,the finite element software ABAQUS is employed to establish the beam-slab assembly model.The numerical study reveals the distribution of rotational and lateral restraint stiffness at slab edge,and two practical formulas are proposed for calculating the restraint stiffness at slab edge.It can be found that:(1)The rotational restraint stiffness at beam end increases about 1.2%~13.3% when considering the torsional effect of the orthogonal beams;(2)Both rotational and lateral distributed restraint stiffness at the slab edge show a slow change in the middle range and a sharp increase at ends;(3)With a decrease of the slab length and an increase of the slab thickness and the beam cross section,the rotational distributed restraint stiffness at slab edge increases gradually.(4)The lateral distributed restraint stiffness at slab edge obviously decreases with an increasing the slab length,and the speed of decrease overall appears first quick back slow trend,while with an increase of the slab thickness and beam cross section,it generally increases to some extent.2.Based on the formula of boundary restraint stiffness of the beam-slab assembly proposed in this paper,the numerical model of RC beam-slab assembly considering boundary restraints is established using ABAQUS,the influence of slab thickness,beam cross section,rotational(axial)restraint stiffness ratio at beam end and distributed restraint stiffness modes at slab edge on deformation of the beam-slab assembly under fire has been discussed.The results show that:(1)The restraint at beam end and slab edge have an obvious impact on reducing the central deflection of the slab;(2)When considering the lateral distributed restraint at slab edge,an inflection point will appear in the central deflection of the slab at the earlier of heating,and occurrence of the inflection point will delay as the slab thickness increases;(3)In case of a thicker slab,an inflection point of mid-span deflection of the beam will also appear at the initial stage of heating,and it disappears gradually with an increase of the section height of beam;(4)Increasing the section height of beam has little effect on the fire resistance of the slab.Meanwhile,the change of the lateral distributed restraint stiffness modes at slab edge has a limited effect on the central deflection of the slab,but it has an obvious effect compared with those without restraint stiffness at slab edge.3.A fire test program involving 3 concrete beam-slab assemblies has been conducted,the effects of different boundary restraint on high temperature deformation of the beam-slab assembly are investigated.Then,a mechanical test of the restraint device is carried out,and a numerical model of the restraint device is preliminarily established based on the test results.Meanwhile,numerical analysis is implemented for each specimen in accordance with the valid model.It is found that:(1)Two longitudinal cracks develop along the border of the beam and slab on the top surface of the beam-slab assembly.However,oblique cracks with the angle of about 45° appear on the top surface of the slab corner when slab restraints exist;(2)For the beam-slab assembly without slab restraints,several transverse cracks,close to the middle span of the beam,occur at the bottom surface of the slab under fire loading.While,for the beam-slab assembly with slab restraints,transverse cracks can hardly be observed in the same position,which implies that the slab restraints make the transverse cracking problem relieve;(3)Boundary restraints at beam end and slab edge obviously block the development of slab central deflection at the later stage of heating;(4)The established finite element model of the restraint device and the beam-slab assembly in this chapter can better predict the deformation process in fire.