Modelling hollowcore concrete slabs subjected to fire

Precast, prestressed hollow core concrete slabs have been widely used in recent years. In a previous paper at ACMSM19 the authors presented a simple computational method that could be used in design for modelling the structural behaviour of hollowcore concrete slabs in fires. The model proposed at that time consisted of a grillage system using beam elements to include the thermal expansion in both directions and to simulate the vertical cracking in the flanges, with the topping concrete modelled using shell elements. The simulation showed good agreement with the experimental results. This present paper shows that it is possible to avoid using shell elements to model the topping concrete and instead model the topping concrete as part of the hollowcore slabs. This beam grillage method of modelling both hollowcore slabs and topping concrete gives similar results to using beam and shell elements, but requires less computational time. Figure 1. Discretisation of the cross section of hollowcore unit in the original method. Most importantly, the results from this modelling method showed good agreement with experimental results available in literature [Chang et al 2008a]. The model developed in the previous study worked well for small subassemblies. However, although the sections representing the topping in the beam elements needed in the thermal analysis (Fig. 1) do not contribute to the performance of the slab, they are modelled as non-load bearing material and still consume a lot of computer resources in the structural analysis. As a result, the model becomes too complicated when analysing subassemblies containing more than 4 parallel hollowcore units. Figure 2. Schematic drawing for the (a) original (b) second method to model hollowcore floor systems. Therefore, a different model was needed in order to study the effect of aspect ratio on the fire performance of hollowcore concrete floor systems (Chang et al 2008b). The schematic drawings of the two modelling methods are shown in Figure 2. It was found during the development of the original model that, when modelling the floor slab with only one hollowcore unit, simulating the topping slab as part of the beam elements or separately by the shell elements gave the same result (Chang 2007). Hence, instead of giving the section representing the topping in the beam elements zero strength and using shell elements to simulate the topping, the topping can be modelled as part of the beam elements and nearly all the shell elements can be removed from the model. This method worked well and was used for the studies reported in Chang et al (2008b), using 300 mm deep hollowcore units. Present studies involve 200 mm deep units which contain six hollow cores and need to be represented by seven beam elements interconnected by transverse elements to provide continuity within the hollowcore units. 3 MODELLING OF HOLLOWCORE SLABS IN SAFIR The details and dimensions of the hollowcore floor slab investigated are shown in Figure 3. A typical 200 mm deep prestressed hollowcore unit is shown in Figure 4 while the properties of such units are shown in Table 1. Figure 3. Dimensions and layout of the studied subassembly. Figure 4. Cross-section of a 200 mm hollowcore unit. Table 1. Properties of the hollowcore floor system.