Research On Fire-resistant Performance Of2×2Grid Continuous Concrete Slabs In A Steel-framed Structure

Reinforced concrete structures are often used in the buildings due to a numeberof advantages over other construction materials. However, the material properties ofconcrete have been singificantly affected by the high temperature, sometimes thestructure collapse will occur. In recent years, many researchers have paid wideattention to the fire-resistant performance of the concrete structures or members.Recently, to further understand the fire behavior of structural members, ourteam has conducted several fire tests on one panel in a full-scale steel-framedbuilding. However, this case does not represent a real fire situation in a buildingbecause the steel beams are awalys subjected to fire. Therefore, a fire test furnacewas built to heat the four panels (two by two) and steel beams, and all of thebeam-to-column connections and columns were protected from direct fire exposureand the supporting members at the perimeters of the furnace wall were placed toavert fire. Additionally, the cost of fire tests is very high as well as long period. It isvery important to have analytical and numerical methods that can predict thebehavior of reinforced concrete slabs in fire.The main contents are listed as follows:(1) Based on Borland C++6.0and the object-oriented method, the nonlinearfinite element model, in which the moisture was taken into account, has beendeveloped with a computer program, and several comparisons were carried out todemonstrate the efficiency of this program.(2) According to the industrial furnace principle, a special furnace wasdesigned to study the fire behavior of floor slab and steel beams in a buidling. Thispaper briefly presents the furnace, temperature measurement, deflectionmeasurement. The experimental phenomena were discussed in detail as well as thefloor crack characteristic, joint fracture and filled wall crack. The temperaturedistribution of structural elements (floor, steel beams and columns) were mainlyanalyzed, together with their displacements, the present results were also comparedwith the other testing results. The test indicates that the cracking patterns in theheated panels depend on the restraint conditions, the number and locations of theheated panels in the floor have a considerable influence on the cracking patterns ofunheated panels. Also, the steel beams show better fire-resistant performance thanthat indicated in standard fire tests depending on their structural integrity and theinteraction between structural members. Unlike the high-strength bolt connections,the welded-bolted connections did not cause local buckling of the steel beams in fire.Finally, the floor temperature was simulated by the nonlinear temperature program, and the predictions agreed well with the testing results.(3) According to the yield-hinge line theory, the vertical component of steelforce was considered in the developed model. Based on the reinforcement ratio ofreinforced concrete slabs, two failure criteria (deflection and stress) were proposedto determine the load and displacement of the slabs. The effectiveness of thedeveloped model was validated through satisfactory comparison with testing results.The results show that the calculated carrying capacity of the slab was agreementwith the tests, and the calculated limit displacement was relatively conservative.Addtionally, according to the mechanical analysis, the model also explained thefailure mechanism that the limit carrying capacity of the square slab was lower thanthat of the rectangular slab under the same deflection.The above method can easily be adapted for analyzing the ultimate loads of theconcrete slabs in fire, through incorporating the thermal effects on the materialproperties. To obtain the conservative results, it is suggested that the deflectionfailure criterion can be considered to be l/30.(4) Based on the shell elements (flat and three-demensional degenerated) andthe elasto-plastic theory, the nonlinear finite element model was developed tosimulate the structural behaviour of the reinforced concrete slabs at ambient andelevated temperatures.The proposed model has been used to predict the structural behavior ofreinforced concrete slabs at ambient temperature, the predicted bottom layer crackpatterns were discussed and distributions of the membrane tractions were presentedto illustrate the tensile membrane action. The paper discussed the stress-temperaturebehavior of restrainted concrete specimens, and this was used to demonstrate theimportance of the transient strain as well as its mechanism. According to the unaxialtransient strain, this paper proposed transient strain model of concrete with transientmodulus under the biaxial stress state. The propsed model was used to simulate thefire behavior of the slab, and the three-stage response mode was revealed as well asthe mechanism of each stage. The free thermal strain and the material degerationhave significant influence on the fire behavior of reinforced concrete slabs. Theconcrete tranisent strain, the steel creep strain and its crush strain have played animportant part in the later fire behaviour of the slabs.In all, the results show that the propsoed model can be used to analyze the firebehavior of the slabs and the simulation system offers an efficient platform toinvestigate the fire behavior of floor in the building.