Modelling of unbonded post-tensioned concrete slabs under fire conditions

Abstract

a b s t r a c tThis paper investigates the structural behaviour of unbonded post-tensioned one-way spanning concrete slabs in fire conditions. The slabs were simply supported and reinforced with 15.7 mm nominal diameter seven-wire mono-strand tendons. A nonlinear finite element model for the analysis of posttensioned unbonded concrete slabs at elevated temperatures was developed. The mechanical and thermal material nonlinearities of the concrete, prestressing tendon and anchorages 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 slab. The temperature distribution throughout the slab, time-deflection behaviour, time-longitudinal expansion, time-stress behaviour in the tendon, and the failure modes were predicted by the model and verified against test data. The study has shown that the coefficients of thermal expansion currently used in the European Code for calcareous and siliceous concrete can lead to inaccurate predictions of the structural behaviour. A parametric study was conducted to investigate the effects on the global structural behaviour due to the change in the aggregate type, load ratio and boundary conditions. It was shown that by varying the boundary conditions the fire resistance was greatly affected. Although changing the aggregate type and load ratio affected the time-displacement response, the fire resistance defined by failure of the slab was not affected due to the splitting mode of failure above the tendon locations not being affected by these parameters. Comparison with the codes shows that the UK code BS8110 is generally unconservative, whereas the Eurocode EN1992-1-2 provides reasonable design rules.

Whole-building behaviour of bonded post-tensioned concrete floor plates exposed to fire

“…The same failure mode was previously reported in the small-scale unbonded and bonded fire tests conducted by Bailey and Ellobody [1,2]. Detailed explanation of the cause of the splitting was provided previously by Ellobody and Bailey [13]. Briefly, when the temperature of the tendons increases the tendon starts to expand longitudinally causing partial relief of the compressive stresses, which then allows the lateral expansion to cause splitting at the weaker sections of the slab that contain the tendons.…”

“…The stress-strain curves of the plastic and steel ducts were predicted based on data provided by the manufacturers. Following the finite element modelling presented in Reference [13], the measured values of thermal expansion coefficients for concrete with Limestone and Thames Gravel aggregates available in the literature [25,26] were used, instead of the values given in the code [15]. It was shown that the values for thermal expansion given in the code produced erroneous predictions for the structural behaviour.…”

“…Previous research work has been conducted to study the behaviour of post-tensioned concrete slabs under fire conditions. Detailed finite element models have been developed by Lee and Bailey [9,10] Ultimate strain of concrete in tension and Bailey [11][12][13][14]. The models have investigated the effects of different parameters affecting the behaviour of this type of construction and produced useful information regarding the temperature distribution in the slabs, time-displacement behaviour, time-tendon stress behaviour, time-axial expansion behaviour, fire resistance, and overall failure modes.…”

Unbonded post tensioned concrete in fire: A review of data from furnace tests and real fires

“…Because full-scale fire tests on actual or model UPT buildings, however badly needed, are unlikely to occur in the foreseeable future, research is currently restricted largely to using computational analysis tools [52,53] to study their response to fire. In general, these tools have not been validated against real fires in UPT concrete structures and their ability to accurately model various aspects of UPT concrete beams and slabs at elevated temperature is doubtful.…”

Finite Element Modeling of Post-Tensioned Two-Way Concrete Slabs under Flexural Loading

“…The developed numerical tool utilizes FE analysis with prestressing model employing tendon element. Ellobody and Bailey [9] studied the structural behaviour of unbonded post-tensioned one-way concrete slab at elevated temperature. A nonlinear finite element model was developed for the PT analysis of unbonded concrete slab in fire.…”