New Form INC

11.11 negative reinforcement calculation

Negative reinforcement over supports is required to satisfy two distinct load cases:

(a) Continuity conditions, under normal dead and Live Loading.

(b) Fire emergency design conditions.

The best economic design occurs where no extra reinforcing is required, over and above continuity requirements (which are required in any event) to satisfy fire resistance requirements. All tables included within this manual have been prepared on this basis.

The consequence of this includes:
• All negative reinforcement equates to that required for Continuity.
• No extra positive reinforcement is required to be placed within the trays.
• Positive reinforcement capacities are based on the top mesh capacity only, which is small as a consequence of its distance to the slab top surface.
• Maximum capacities included in table do not necessarily coincide with the maximum capacities available to that slab cross-section.
• Increased Fire Capacity can be obtained by increasing the amount of negative reinforcement, but is more likely to be achieved economically by placing positive reinforcement within the trays.
• Specified cover requirements must be strictly adhered to to achieve the ratings stated.
• Placing Extra negative reinforcement also adds to the reinforcement available to control cracking over supports.
• Negative Reinforcement requirements also include for shrinkage effects due to internal restraint from the tray floor.
• Where Negative span tables do not allow the desired Superimposed Loading and Fire FRR rating to be achieved, it is recommended that the designer select a thicker slab in preference to including a greater level of reinforcing in designs.
• Economies can be achieved by incorporating specific structural models within the building structure, minimizing required FRR.