Acceptance Criteria for Unbonded Post-Tensioned Concrete Exposed to Travelling and Traditional Design Fires

Abstract

Modern architecture is striving for large open spaces, which has resulted in the development of design methodologies such as the Improved Travelling Fires Methodology (iTFM). This methodology is only applicable in large open spaces where flashover may not occur. This paper studies the effect of various design fires, including traditional uniform fires and for the first time iTFM on flat-plate unbonded post-tensioned (UPT) concrete, a structural system which allows long flooring spans (> 8 m). The predictions of a structural model of high temperature unbonded prestressing tendon relaxation that is available in the literature are compared for the first time with previously published fire experiments thus validating the model. A novel case study based on a real building was then analysed using the model to propose an acceptance criterion based on the longest tendon length where rupture and extreme stress relaxation is prevented. Two types of steel, fabricated according to BS 5896 and AS/NZ4672, were analysed for various unbonded lengths (between 8.15 m and 65.2 m). Emphasis is on the unique deformation mechanisms present when prestressing steel is heated locally. The relaxation model has been validated for negligible slab deflection, allowing for discussion on the difference between the impact of travelling and uniform fires on the design of UPT concrete. The tendon rupture and relaxation analysis have novelty in showing that slow moving fires are more onerous structurally than fast moving or uniform fires and therefore are important to consider when defining critical design thermal boundaries in a design context. Results also demonstrate that tendons in similar designs require consideration to be limited in unbonded length. The development of new acceptance criteria herein is the first step in producing a generalized criteria for better informed structural fire design of UPT concrete exposed to fire.