Fire Resistance and Punching Shear Capacity after Fire of Reinforced Concrete Slabs
|關鍵字:||鋼筋混凝土版;貫穿剪力強度;高溫;耐火時效;reinforced concrete slab;punching shear strength;elevated temperature;fire resistance|
Flat slab systems are widely used in various buildings, such as shopping stores and malls, parking lot, and bridge deck because the flat slab systems can be built quickly, provide more spaces, and reduce floor height. However, flat slab systems under heavy loads usually fail in punching shear. Currently, research on punching shear behavior of reinforced concrete slabs limits at room temperature and emphasizes on conducting the tests, and developing empirical formula to predict punching strength of reinforced concrete slabs supported simply. Currently, domestic and international research of reinforced concrete structures in fire loading is highly focused on beam and column members; however, studies on the fire resistance of reinforced concrete slabs are still quite rare. The objective of this study is to investigate experimentally and analytically fire resistance and residual punching shear strength of reinforced concrete slab-column connections after elevated temperatures. Furthermore, analytical calculation is proposed to predict punching shear capacity of the reinforced concrete slabs after fire. An experimental approach is proposed to simulate fire behavior of reinforced concrete slab-column connections under elevated temperatures and loads. The parameters of this experiment included concrete compressive strength, ratio of tensile steel reinforcement, fire exposed face, and duration in fire. The specimens were exposed to elevated temperatures in accordance with the ASTM E119 standard fire curve. Appearance, failure mode, temperature distributions, and time-deflection relations of the specimens were recorded and used to assess effects of the parameters on fire resistance and residual punching shear capacity of the specimens. Different fire exposed faces were categorized to heating on the tension side of the slabs to represent a fire above the floor slab, and heating on the compression side of the slabs to represent a fire below the floor slab. The test results showed that the fire resistance of the specimens heated on the compression side or tension side clearly differed from each other. The mechanisms of concrete cracking and spalling also varied according to the concrete strength of the slabs and fire exposed faces. The deformation and failure mode of the specimens also differed. The test results of the fire resistance showed that slabs heated on the compression side did not fail up to eight hours. But, the slab with high-strength concrete heated on the compression side would spall. The slabs heated on the tension side, the slabs would fail at around four hours. Neither the normal-strength concrete specimens nor the high-strength concrete specimens spalled during elevated temperatures. The failed specimens appeared to have a cone shape. The residual punching shear capacity decreased with the increase of heating time. Finite element analysis was also utilized to obtain internal temperature distribution of the specimens and the results were simplified to an isotherm to predict residual punching shear capacity of the specimens after fire. Finite element analysis could accurately predict the temperature distributions of the specimens. A proposed procedure to calculate the residual punching shear capacity reasonably predicted the residual punching shear capacity of reinforced concrete slabs after elevated temperatures. The residual punching shear capacity decreased significantly when fire occurred on the tension side of the slab and, consequently, resulted in brittle punching shear failure which would be a serious threat to structural safety. Hence, this concept should be seriously taken into account during the structural design to achieve better fire safe design and structural retrofitting assessment.