Holistic behaviour of post-tensioned concrete floors under fire conditions

This paper investigates the holistic structural response of post-tensioned concrete floor slab systems under fire conditions. The investigation was based on the results of twelve bonded and unbonded post-tensioned concrete slab fire tests, recently conducted by the authors, which allowed the development of a validated finite element model. The model was used to study the behaviour of a typical concrete floor slab system. The floor system was post-tensioned using bonded tendons and was supported on reinforced concrete columns. The overall objective of the study was to provide a realistic understanding into the structural response and modes of failure of this system under fire conditions. The mechanical and thermal material nonlinearities of the floor's components, consisting of the concrete, grout, prestressing tendon, and the anchorages, as well as the reinforced concrete columns, have been carefully inserted into the whole-building model. The interface between the tendon and grout was also considered, allowing the bond behaviour to be modelled and the tendon to retain its profile shape during the deformation of the floor. The model has been validated against published finite element results under normal temperature conditions. Based on the standard fire curve, and different types of natural fires, the temperature distributions throughout the floor slab and surrounding columns were predicted together with the developed displacement and stress during the duration of the chosen fire scenario. An extensive study was completed with the variables that influence the structural behaviour comprising different static load during fire, the use of non-tensioned reinforcement, as well as restraint conditions, investigated. The results of the parametric study were compared with the design values specified in current codes of practice, with conclusions presented.