11.9 positive reinforced fire rated slabs
Positive reinforced fire rated slabs are required where slabs are simply supported between supports. In this case the tensile strength of the tray floor is ignored and the total ultimate fire loading is taken by extra Fire Emergency Reinforcement (FER) placed in the pans of the trays.
Under ultimate fire loading:
U = G + Qu
In the calculation of moment capacity of the slab, two major items affect the amount of Reinforcing required to sustain the loading.
(a) Temperature
The yield strength of the FER is affected by temperature. HERA document R4-82 sets down a procedure which relates the location of rebar, relative to the bottom profile of the composite section, to the temperature associated with each particular FRR. This then enables the yield strength of of the bars to be calculated.
The ultimate compressive strength of the concrete is assumed to be at a distance from the fire not to be affected by temperature effects.
(b) Live Loading
As with any ultimate loading, reinforcing quantities are directly linked to the amount of total loading needing to be sustained.
In the design of positively reinforced slabs all loading is taken in positive flexure and as a consequence all reinforcement is required to be added as additional to that sustaining normal ultimate dead/live loading. Consequently, positively FER reinforced slabs are generally limited to slabs which are originally simply supported.
