An approach to account for tie configuration in predicting fire resistance of reinforced concrete columns

High strength concrete (HSC) columns exhibit lower fire resistance, as compared to conventional normal strength concrete columns, due to occurrence of fire induced spalling and faster degradation of strength and stiffness properties of concrete with temperature. Fire resistance tests on HSC columns have shown that fire performance of HSC columns can be enhanced through bending the ends of ties at 135° into the concrete core, instead of bending the ties at 90°. This paper presents an analytical approach to model the effect of tie configuration on fire resistance of reinforced concrete (RC) columns. The proposed approach is based on seismic design principles and involves calculation of force acting on ties by evaluating effective stresses resulting from pore pressure, mechanical strain and thermal expansion. The resulting force acting on ties is compared against temperature (time) dependent bond strength (at the tie-concrete interface) to evaluate the failure of ties. The proposed tie sub-model is built into an existing macroscopic finite element based fire resistance analysis computer program that is capable of tracing the fire response of reinforced concrete (RC) columns in the entire range of behavior. The predictions from the model are compared against the full-scale fire resistance tests on RC columns to demonstrate the validity of the proposed approach in evaluating the beneficial effect of 135° tie configuration. The validated model is applied, through case studies, to quantify the effect of tie configuration on fire resistance of reinforced concrete columns. Results from numerical studies clearly show that HSC columns with 135° bent ties exhibit higher fire resistance than those HSC columns with 90° bent ties.