Research on Fire Resistance of Two-way Reinforced Concrete Slabs with Different Edge Restraints
|School||Harbin Institute of Technology|
|Keywords||Fire full-scale test two-way reinforced concrete slab edge restraints the whole structure fire resistance|
Two-way reinforced concrete (RC) slabs bear complicated forces under fireconditons, and there are many affecting factors. For now, however, the relevantexperimental and theoretical researches are imperfect and insufficient. Fire testsperformed on single isolated structural members are the necessary first step tosupport the structural fire resistance design. However, such tests do not reflect thereal behaviour of the elements in the whole structure under fire conditions.Interactions between different structural elements in a whole structure can alter theloading and support conditions of any structural element. This alteration can lead tocompletely different structural behaviour from that based on the initial set ofloading and boundary conditions. Based on the above consideration, thisdissertation studies the fire-resistance properties of full-scale two-way simplysupported RC slab, two-way RC slab with edges clamped and two-way RC slabs ina complete building. The main contents are summarized as follows:(1) Two fire tests were conducted on full-scale two-way simply supported RCslab and full-scale two-way RC slab with edges clamped respectively. The verticaldeflections and horizontal displacements of the slabs, temperature distributionalong the slab depth and temperature variation of the steel reinforcement weremeasured during the tests. The variation of constrained force at the edges of theslab with edges clamped is investigated. The cracking and failure characteristic ofthe slabs are analyzed. The test results indicate that three main cracks parallel to theshort span direction occurred on the top surface of the two-way simply supportedslab, and they were located at approximately the mid-span and1/4span of the longspan of slab. Plastic hinge lines of ellipse shape formed on the top of the slab withedges clamped. The fire resistance of the two-way RC slab with edges clamped wasbetter than that of the two-way simply supported RC slab.(2) The influence of tensile membrane action on the load-carrying capacity ofRC slabs is not incorporated in the conventional yield line theory, and thecalculated result of the load-carrying capacity of RC slab is conservative. In thisdissertation, based on the conventional yield line theory, the tensile membraneaction is assumed to be provided by the vertical component of the steel forces at the section of yield lines or the plastic energy dissipation due to the extension ofreinforcements along yield lines. Then, a new model is proposed to estimate theload-carrying capacity considering the influence of tensile membrane action underfire conditions. Based on the test results, the load-carrying capacity of the two-waysimply supported RC slab and two-way RC slab with edges clamped is estimated byimproved segment equilibrium and energy method. Comparison between thedeveloped model and test results shows good correlation.(3) The specially designed furnaces were used, and two full-scale fire testswere conducted on a corner and an interior panel in a three-storey steel-framedbuilding. The structural fire behaviour is studied. It is shown that interactionsbetween different structural elements and the restraint provided by the adjacentstructural members had significant effect on the structural fire behavior of the slabin a whole structure. For the corner panel, main cracks which had significantinfluence on the structural behaviour occurred on the top of the slab, and they werelocated at approximately the1/4span near the interior edges of the slab (theposition where the top reinforcing bars were broken). For the interior panel, plastichinge lines of circular shape appeared on the top surface of the slab. Because of thesstructural continuity and interaction between structural members, regular cracksalso occurred on the top of the adjacent unheated panels. The two-way concrete slabin a whole structure had good fire performance.(4) Acoustic emission technique was used for monitoring the failure of two-way RC slabs in a whole structure under fire conditions. The changes of parametersof event, energy rate and b value are investigated. The relationship between theseparameters and the cracking of slabs, furnace temperatures and the verticaldeflections are analyzed. The results indicate that the early stage of cracking, thestage of extensive cracks occurring and the stage of cracks propagating slowlycould be distinguished accurately by analyzing these parameters. When theseparameters change abruptly, such as the sudden rise of energy rate and suddendecrease of b value, some structural failure may occur, and these moments shoud behighly concerned in fire.(5) The vibration properties of two-way RC slabs in a whole structure underfire conditions are studied. By means of the fast Fourier transform (FFT) technique,the frequency of the RC slabs in fire is analyzed. The results indicate that therestraint provided by the adjacent structural members had significant effect on the frequency of the two-way RC slab in a whole structure. The frequency of theinterior panel was higher than that of the corner panel. The frequency of the slabsdecreased to various degrees under fire conditions, and the frequency decreaseamplitude of the corner panel was higher than that of the interior panel.