Numerical Study of Prestressed Concrete Joist Slab System Under Fire Conditions

Due to the large number of fire and the resulting financial and human costs, it is necessary to prevent the roof from collapsing as one of the key members of the building during a fire. Because concrete has an inherent weakness in tensile stress, cracking in the tensile area of slabs is inevitable. On the other hand, in a system of prestressed concrete joist slab system, the tensile area of the concrete is usually completely eliminated or effectively reduced, and the performance of the floor can be expected to improve under fire conditions. The critical temperature was obtained and compared in 9 samples that differed in the amount of prestressing force, strength and concrete cover as well as the span length. The thermal load was applied to the studied samples according to the ISO 834-1 standard protocol. The temperature distribution across the slab was applied uniformly and incrementally and the failure states in the model were predicted and verified with laboratory results. The results showed that the prestressing force has the greatest effect and the span length has the least effect on decreasing or increasing the critical temperature index. Changing the prestressing parameter from zero to 600 and 1120 MPa, increases the critical temperature index by 20 and 43% and changing the beam length parameter from the initial value of 4.5 m to 6 and 7.5 m, decreasing the critical temperature index by 1 and 2%, respectively. Changing the parameter of concrete cover from 20 mm to 10 and 30 mm has caused a decrease and increase of critical temperature index by 11 and 9%, respectively. Also, in all samples, the same failure mode occurred in the middle of the joist in a bending manner.