Studies On Fire Resistance Properties Of Full-scale Two-way Reinforced Concrete Slabs

Two-way reinforced concrete slabs are of great importance in the prevention of fireaccidents. Other than bearing the high temperature effect, they also need to prevent firespread and collapse of the building structure. The force interactions in two-wayreinforcement concrete slabs under fire conditions are complicated, and there are manyaffecting factors. The relevant experiment and theory are imperfect. Supported by theNational Natural Science Foundation Projects “Fire Behavior of Two-way ReinforcedConcrete Slabs in Whole Structure”(Grant Number:51178143) and “Study on AcousticEmission and Vibration Characteristics of Structural Members in Fire”(Grant Number:50878069), this dissertation studies the fire-resistance properties of full-scale two-wayreinforced concrete slabs, which include slabs with two edges being clamped and twoedges being simply supported, two-way RC slabs and flate-plate floors slabs. It canprovide data support and theoretical guidance for the design of two-way reinforcementconcrete slabs with fire resistance, and offers reference for fire fighting. My workincludes:(1) Experiment study. The fire resistance of two-way reinforced concrete slabscould be reflected accurately by full-scale experiment. In this dissertation, We testfull-scale slabs, slabs with two edges being clamped and two edges being simplysupported, two-way RC slabs and flate-plate floor slabs which are exposed to elevatingtemperature with constant load. Two samples were made for the three different slabs. Theedge horizontal deflection and vertical deflection of slabs under fire are measured, thevariation of constrained force is recorded, the temperature distribution of concrete andtemperature variation of rebars are analyzed. The experiment study indicates that afterfire test, the slabs with two edges being clamped and two edges being simply supportedform cracks on the top surface in a semi-elliptic pattern，A spherical crown shape withnotch upward failure mode is formed of the two-way RC slabs, and the top surface cracksformed in a hyperbolic pattern between every diagonal columns of the flate-plate floor.The temperature field distribution along vertical direction of the slab with single surfacebeing exposed to fire is nonlinear and the temperature variation increases quickly by time.There is a plateau in the temperature rise at100℃level attributed to the existence ofmoisture. Not only this, moisture also leads to spallings of concrete, which will furtherlead to slab destruction if its rate is too high.(2) Calculation of limit load carrying capacity of slabs. The classical yield linetheory does not consider the membrane effect in calculating the limit load carryingcapacity of slabs, and the result was conservated. We take into account the membraneeffect and present several new ways for different slabs in this dissertation: Based on theclassical yield line theory and the force equilibrium, we give a segment equilibrium method when dealing with two-way slabs with two edges being clamped and two edgesbeing simply supported, and based on work equilibrium of internal and external force, wealso give an energy method to flate-plate floor and two-way slabs with two edges beingclamped and two edges being simply supported. The membrane effect shows only afterthe formation of mechanism of plastic hinge lines of slabs in fire. The segmentequilibrium method thinks the tensile membrane to be caused by the vertical componentof tensile force of reinforcements at the yield line section, and the energy method thinksthe tensile membrane to be caused by the plastic work of reinforcements at the yield linesection. The results calculated from our methods agree with experiments very well. Archaction was formed under the effect of compressive membrane force, which leads thevertical displacement upwords. And the damage of the two-way RC slab is the crushingof concrete at the compressive membrane force sections next to the inner side of thebeam.(3) Research on acoustic emission properties of slabs. In order to monitor the failureproperties of slabs under fire conditions, we propose to use Acoustic Emission techniqueto monitor the failure of these three types of slabs at elevated temperature. The changesof parameters of event rate, energy rate and b value, are investigated. And the relationbetween these parameters and the crack of slabs, temperature of the furnace and thevertical deflection are analyzed. The results indicate that，using these parameters, we caneasily tell the occurrence of the first and the second crack of the three different two-wayreinforced concrete slabs. Because of the overlapping of the occurrence of new cracksand the extension of old cracks, it is not easy to distinguish every crack，but theseparameters can accurately tell the three stages of the cracks：the stage before cracks occur,the stage when extensive cracks occur and the stage of when cracks extend slowly.(4) Research on vibration properties of slabs. The vibration properties areself-characteristics of slabs in fire. So in this dissertation the vibration properties of thesethree types of slabs at elevated temperature are studied. By means of the Fast FourierTransform (FFT) technique and numerical calculation, the frequency of the reinforcedconcrete slabs in fire is analyzed. The results indicate that, there is a large decrease infrequency under fire conditions. The decrease amplitude after fire is39.7%,52.7%and42.5%for each type respectively. We analyze relations between frequency and thecentral vertical deflection of the three different slabs by regression method in thisdissertation.