Advanced Analysis of Prestressed Hollow Core Concrete Slabs Exposed to Different Fires

Abstract

This paper discusses the nonlinear structural analysis of full-scale prestressed hollow core one-way spanning concrete slabs in fire conditions. The slabs were simply-supported and reinforced with 9.3 mm nominal diameter seven-wire mono-strand tendons. Nonlinear 3-D finite element models for the analysis of prestressed hollow core concrete slabs at elevated temperatures were developed. The finite element models were based on a nonlinear finite element modelling approach adopted by the author over the last eight years for the analysis of different structures under fire conditions. The mechanical and thermal material nonlinearities of the concrete and prestressing tendons have been carefully inserted into the model. The interface between the tendon and surrounding concrete was also modelled, allowing the tendon to retain its profile shape during the deformation of the prestressed hollow core slab. In addition, gap elements were used to model the thermal behaviour of the hollow cores, which ensures, for the first time, correct temperature distributions in the regions surrounding the cores. The temperature distribution throughout the prestressed hollow core concrete slab, time-deflection behaviour, time-longitudinal expansion and failure modes of the slabs were predicted by the model and verified against test data. A parametric study was conducted to investigate the effects on the global structural behaviour due to the change in the prestressed hollow core slab geometries, hollow core shapes, load ratios during fire and different fire curves. It was shown that the failure of the prestressed hollow core concrete slabs investigated in this study was due to tensile splitting with longitudinal cracks along the slab directly above and inline with the tendons developed between 15–25 minutes from the start of heating. It was also shown that no shear failure was observed. The fire resistances of the prestressed hollow core concrete slabs obtained from the finite element analyses were compared with the design values obtained from the British Standards (BS) and European Code (EC) for concrete structures at elevated temperatures. The comparisons have shown that the BS were generally conservative for the prestressed hollow core concrete slabs investigated in the parametric study, except for the slabs heated using the standard fire curve under a higher load ratio of 0.7, while the EC predictions were conservative for all the slabs investigated in this study.