Precast prestressed concrete (PC) construction provides numerous advantages over traditional reinforced concrete (RC) construction, in terms of speed of construction, better quality control, cost-effectiveness, better space utilization, and optimized production. Owing to these advantages, the use of PC construction in the built environment has increased significantly in recent decades. While the structural behavior of PC members is well understood at ambient temperatures, there is a lack of understanding on the evolution of fire induced restraint forces in PC beams and their effect on the fire resistance of PC beams. Further, the fire resistance of PC members is currently evaluated using prescriptive design approaches which do not account for all critical factors governing the fire response of PC beams, including realistic restraint conditions, and therefore, current fire resistance provisions may not provide realistic predictions of fire performance. Therefore, a detailed experimental and numerical study is conducted to evaluate the evolution of fire induced restraint forces and to quantify their effect on the fire response of PC beams. Fire resistance tests were conducted on four PC beams under restrained and unrestrained end conditions. Test variables included fire exposure, restraint conditions, load level, and concrete strength. The fire response of the beams was traced throughout the fire exposure duration by measuring sectional temperatures, beam deflections, and fire induced restraint forces. All four beams were designed as per current building code recommendations to have a fire resistance of 4 hours, however, all four beams attained failure within 2 hours of fire exposure. A numerical model was developed for tracing the fire response of PC beams under specified fire, loading, and restraint conditions. The model accounts for critical factors governing the fire response of PC beams including fire-induced restraint forces, cracking and crushing of concrete, spalling, material and geometric non-linearity, and geometry of the beam. For modeling fire-induced restraint forces a new efficient spring idealization framework for connections is implemented. Also, the cracking and crushing of concrete is captured by developing a new modified adaptive temperature-dependent failure envelope. The developed numerical model was validated by comparing response predictions from the model with measured data in fire tests. Results from these comparisons show that the model can capture the fire response of PC beams with reasonable accuracy in both thermal and structural domains. The validated numerical model is applied to carry out a series of parametric studies on the effect of fire-induced restraint forces on the response of PC beams. The effect of cross-sectional shape, support conditions, the gap in connection, level of prestress, and concrete cover thickness on the evolution of fire induced restraint forces is studied for PC and equivalent RC beams. Results from parametric studies show that current prescriptive codes and standards may over-predict fire resistance of PC beams by as high as 100%, PC beams develop 5% to 20% lower restraint forces than equivalent RC beams, and for PC beams with gaps of more than 50 mm experience minimal restraint forces. Also, the fire-induced restraint forces can be either beneficial or detrimental and can significantly alter the fire response of the PC beam, and therefore, should be included in the design process. Based on the results from the fire tests and parametric studies, simplified recommendations are proposed for evaluating the fire resistance of PC beams.
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