Fire Performance Of Reinforced Concrete T-beams And Beam-slab Assemblies

Most of previous studies and design have mainly concentrated on the fire performance ofisolated RC members and the restraints imposed by adjacent unheated elements are ignored. Ithas been recognized that the fire resistance of isolated structural members is obviouslydifferent from that in whole structures. In recent years, the fire performance of reinforcedconcrete beam and column with restraints at their ends was investigated by some researchersand it is a great progress. There are some longitudinal restraints expect for the restraints at theends, such as the restraints provided by the concrete slabs. The effect of concrete slab on theadjacent beam has not been reported. Therefore, through test, numerical simulation, parameteranalysis and practical calculation method, this paper investigates the fire resistance of thesimply supported T-beams, restrained T-beams and beam-slab assemblies. The main work andconclusions include as follows:1. Six full scale RC T-beams and two beams with rectangular cross section were tested infire. The influences of some parameters, such as load ratio and width of concrete slab ondeformations, damage characteristics and fire resistance of the specimens at high temperaturewere discussed. The experimental results show that:(a) the fire resistance of RC simplysupported T-beam increases with the increasing of the width of concrete slab when the loadratio is relatively larger;(b) the fire resistance of simply supported T-beam decreases with theincreasing of the load ratio when the width of concrete slab is the same.2. Using the computer program of SAFIR which was verified by previous researchersand experimental data, the influences of some parameters (i.e., load ratio, span-height ratio,longitudinal reinforcement ratio of beam, concrete cover of beam and slab, slab width,thickness of slab and longitudinal reinforcement ratio of slab, etc.) onfire-resistance-enhancement-coefficient (FREC) of RC simply supported T-beams exposed toISO834standard fire are analyzed. Based on the simulation results of1296cases, a practicalcalculation method for FREC of RC T-beams is proposed. Simulation results show that:(a)the FREC of simply supported T-beam increases with load ratio and span-height ratio;(b) theFREC decreases gradually with the increasing of longitudinal reinforcement ratio andconcrete cover of beam;(c) with an increase of width of concrete slab, the FREC increasesrapidly first and then keep constant; and (d) the FREC increases significantly with increasingof the concrete cover of slab, but the effects of thickness of slab and longitudinalreinforcement ratio of slab within an ordinary scope are little.3. The sway restraint stiffness and rotational restraint stiffness of restrained beam-slab assemblies in a space frame with concrete slab in fire are derived according to some rationalassumptions. This calculation and method were verified by comparison of the internal force inthe beam, such as the moment at the beam end and axial force, of the restrained beam-slabassemblies with those in the space frame. The results agree well with each other.4. Nine full scale restrained RC T-beams were tested in fire with cooling phase. Theinfluences of some parameters, such as the axial and rotational restraint at the beam ends, loadratio and width of concrete slab on deformations, damage characteristics and internal forces ofthe T-beams during heating and cooling phases were discussed. The experimental results showthat:(a) the residual axial force in the restrained T-beams is significant and the residual axialdeformation remains after the heating and cooling phases;(b) with the increasing of axial androtational restraint stiffness ratio at the same time, the peak vertical deflections of restrainedT-beam are little different in the overall;(c) when the load ratio is0.3, the peak axial forceincrease slightly with the increasing slab width, however, the difference of the peak axialforce is very little when the load ratio is0.5;(d) the change of slab width has little effect onthe bending moment at the end of T-beam and maximum bending moment ratio.5. Using the computer program of SAFIR, fire performances of restrained beam-slabassemblies with restraints at the end of beam and restraints at the end of slab wereinvestigated. The influence of some parameters (i.e., axial and rotational restraint ratio ofbeam, sway and rotational restraint ratio of slab and width of slab) on the deformation andinternal force of beam-slab assemblies at elevated temperature were analyzed. The resultsshows that:(a) the axial force of the beam increases with the axial/rotational restraints ofbeam and the sway/rotational restraints of slab;(b) the bending moment at the end of beamand the vertical deflection at mid-span of beam are affected by the rational restraint of beamand the axial restraint of beam, sway restraints and rotational of concrete slab have littleeffect;(c) the axial force decreases with the increasing of width of slab, however, bendingmoment at the beam end, vertical deflection at mid-span of beam and deflection of slabincreases due to the increasing of width of concrete slab.6. The restraint stiffness at the beam end which are subjected to fire is inconstant due tothe nonlinearity of the adjoining RC elements in real structures. As a preliminary study, thefire performance of the rectangular beam with inconstant restraints was investigated and thisstudy is the cornerstone of the beam-slab assemblies with inconstant restraints. The variationof the axial restraints stiffness and rotational restraints stiffness were analyzed and theinfluence of the inconstant restraints on the axial force and bending moment at beam end wasinvestigated during heating and cooling phases. The results show that:(a) the maximum axial force ratio in an inconstantly restrained beam at high temperature is less than that in aconstantly restrained beam, the less the initial value of axial restraint stiffness is, the largertheir difference is;(b) the maximum bending moment at beam end in an inconstantlyrestrained beam at high temperature is almost equal to that in a constantly restrained beam;and (c) the variations of axial force ratio and bending moment at beam end in an inconstantlyrestrained beam with heating and cooling time are almost the same as those in a constantlyrestrained beam.