An analytical framework for the evaluation of the progressive collapse resistance of precast concrete hollow-core floors

Buildings may be subjected to extreme events such as earthquakes, tsunamis, explosions, fires, or terroristic attacks during their service life. The current codes and guidelines provide three methods for the structural robustness assessment of buildings subjected to unidentified accidental actions: (i) the Tie Force, (ii) the Alternate Load Path and (iii) the Key Element methods. In particular, the Alternate Load Path is a direct method based on the evaluation of the ability of the structure and/or sub-assembly to effectively redistribute the applied loads and develop alternate load paths after the loss of a load-bearing element. In the literature, several studies aimed to investigate the progressive collapse performance of monolithic reinforced concrete structures, with almost no attention paid to precast concrete structures. In this work, in the context of the Alternate Load Path methods, an analytical framework is proposed and validated for the assessment of the progressive collapse resistance of precast concrete floors composed of hollowcore units and precast concrete beams, where the role played by tying reinforcement is analysed. Firstly, the study focuses on double-span hollow-core floor units by providing: (i) the static pushdown response, (ii) the dynamic capacity curve as well as the dynamic amplification factor and (iii) a practical robustness indicator, based on geometrical and mechanical features through the calculation of the chord rotation capacity, a fundamental parameter to be adopted for progressive collapse resistance. Finally, the method is extended to an entire hollow-core floor system, where both distributed ties in slab units and concentrated ties in beams are considered in the system resistance. Due to its simplicity, the analytical framework is considered useful for practical purposes, as reported with an example in the Appendix.