Effect of Aspect Ratio on Fire Resistance of Hollowcore Concrete Floors


Andrew H. Buchanan
Professor, Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand
Peter J. Moss
Associate Professor, Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand
Rajesh P. Dhakal
Senior Lecturer, Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand
Jeremy Chang
Assistant Lecturer, Department of Civil and Natural Resources Engineering, University of Canterbury, Christchurch, New Zealand

Proceedings of the 5th International Conference on Structures in Fire (SiF’08).

Previous studies have shown that the fire performance of hollowcore units is significantly affected by the end support conditions, but it has not been clear how the fire resistance of the overall floor system can be improved by providing side supports. The previous studies used beam grillage and shell elements to separately model the hollowcore units and the topping concrete slab using the platform of the non-linear finite element program SAFIR. The modelling method required a lot of computational resources and is not ideal to model a large floor area. This paper describes the effect of the side supports and the aspect ratio of the floor on the predicted fire resistance. It also compares the efficiencies of shell elements and short beam elements for finite element modelling of the topping concrete in fire conditions. The results show that integrating the topping concrete slab into the beam grillages reduces the complexity of the model and also provides satisfactory results. Side supports can increase the fire performance of hollowcore floor slabs provided that the spacing of the side supports does not greatly exceed the span length.

Precast, prestressed hollowcore concrete floors are very popular in multi-storey buildings because of their excellent structural performance in ambient conditions, high quality control and low on-site labour costs. Hollowcore concrete floors are designed as one-way slab systems, with the units sitting side-by-side, spanning between supporting walls or beams. Most hollowcore concrete floors have in-situ reinforced concrete topping. Structural behaviour of hollowcore concrete floors is dominated by action parallel to the units and their prestressing strands. Two-way action can sometimes occur in such slab systems, resulting from transverse structural behaviour of the topping concrete, depending on the vertical supports parallel to the hollowcore units.

The fire resistance of hollowcore concrete slab has not been outlined specifically in Eurocode 2. However, Eurocode 2 provides separate measures for the fire resistance of flat slabs and solid slabs. The tabulated data in Eurocode 2 relate the fire resistance of a flat slab or of a one way solid slab to the slab thickness and the axis distance of the reinforcements to the surface; they also associate the fire resistance of a two-way solid slab to the aspect ratio which serves as an additional parameter. The British Standard BS EN1168 “Precast Concrete Products – Hollow Core Slabs” suggests that the fire resistance of hollowcore concrete floors follows the table for flat slabs which does not include the effect of the vertical supports parallel to the hollowcore units. The New Zealand Standard NZS 3101 “The Design of Concrete Structures”, however, suggests the fire resistance of hollowcore concrete floors follows the table for solid slabs, which considers the influence of the two-way effect.

References

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