Behavior And Vibration Characteristics Of Two-way Slabs Of Full-scale Steel-framed Structures Under Fire

Recently，Fire-resistant tests and research of building structures are mainlyconcentrated in the shrink-size structures and isolated full-scale structural elements.However, all structural elements in actual building structures are mutual constraint anddeform coordinately, therefore, it can’t be avoided that there are some discrepancybetween current building fire-resistant design code and the actual situation, and it’simportant and practically significant to test and research the elements in full-scalecomplete structures.The reinforced concrete floor slab is the main form of horizontal load-bearingstructure in modern architecture, the slab is also the most severely destroyed elementunder fire, this paper conducts a test on a3×3spans, three-storey, full-scale,steel-framed building, the floor slabs subjected to fire are the northwest corner of the2×2spans plates located in the second floor of the roof panels. During the test, theoccurrence time and location of crack on decks were detailed recorded, andtemperatures of concrete and steel bar in the cross section of concrete reinforced slabs,temperature of steel beam in the furnace, vertical deflection of slabs, in-planedisplacements of slabs, slab edge rotations and horizontal displacements of steelconstrained beam and steel constrained column were measured. Meanwhile, the test alsomonitored the acceleration vibration signals of slabs and top panel of steel beamssubjected to fire.Through the analysis of the experimental phenomena and data, it was concludedthat, reinforced concrete slabs subjected to fire produced a large number of cracks, andformed circular crack zone on the surface of slabs in the later period of test, andappeared the internal force redistribution phenomenon; there were large temperaturegradients inside the cross sections of reinforced concrete slabs, which caused uneveninflation and led to the deformation of slabs, steel beam and steel column. Along withthe increase of temperature inside the slabs, material performance of the reinforcedconcrete appeared degradation, and vertical deflection increased rapidly, the maximaldeflection was affected large by boundary constraints, the stronger the constraints, thesmaller the maximal deflection; During the cooling phase, the deflections of slabsrecovered in different degree. The vibration could be divided into three stagesthroughout the heating stage according to the intensity of vibration, and accelerationsignals mutated greatly, when cracks, relative motion between slabs and steel beam,bond failure between concrete and steel bar appeared, what’s more, the frequency ofslabs mutates when seriously damage of slabs occurs for the degradation of materialperformance, this phenomenon can be used to monitor the failure of slabs under fire.The stiffness of reinforced concrete slabs constantly decreased with the degradation of material performance of reinforced concrete and the boundary restraints, which led tothe decline of the basic frequency of slabs. The vibration of the slabs lasted a period oftime after fire, but the amplitude of vibration decreased obviously.