New Form INC

11.10 negatively reinforced fire rated slabs

Negatively reinforced fire rated slabs occur when double and multiple span slabs are subjected to fire loading. As with the positively reinforced slabs, temperature and loading affect the ultimate design of the slab. However, due to the indeterminate nature of continuous slabs, economies can be obtained by utilizing reinforcing already required for continuity, for use in fire loading.

Temperature affects continuous slabs as follows:

(a) Temperature effects-Positive Regions.
Temperature effects the midsection of slab spans in the same manner as for positively reinforced fire rated slabs.

(b)   Temperature effects-negative Regions.
Converse to positively reinforced slabs, it is the compressive strength of concrete which is affected by temperature. Concrete strength closest to the soffit is reduced the greatest, with temperature effects diminishing with distance from the tray soffit. Hera document R4-82 sets out a procedure to relate the depth of compressive zone to resist ultimate fire loading.

It is assumed that the negative reinforcement, by nature of its distance from the soffit is not affected by temperature. This is assumed by this manual, but is reliant on such reinforcement being placed above the top most section of the steel tray. The British Standard, in section 4.5 requires a 25mm cover to the soffit under positively reinforced conditions. As a natural extension, 25mm cover should be placed between the topmost section of the tray floor and the negative reinforcing, to ensure that negative reinforcing is not affected by Temperature.

A practice which is sometimes found in placing reinforcing, is the placement of reinforcement directly onto the tray floor. As steel is an excellent conductor, thepractice nullifies the assumption that Temperature does not extend to FER reinforcing. Consequently, any fire tables are voided.

Due to moment redistribution, ultimate fire resistance can be provided by a combination of positive and negative moment capacities within the slab system. The general over-riding condition which must be complied with:

ø . (M+rc + ( M-rcl + M-rcr) /2 ) = (q* . L^2 ) /8
where : -
M+rc                 Positive region ultimate capacity.
M-rcl                 LHS Negative region ultimate capacity.
M-rcr                RHS negative region ultimate capacity.
ø                      ultimate capacity reduction factor.
q*                    ultimate fire load on slab expressed
                        as a UDL

When a end span is considered, both for discontinuous or integral slabs, either M-rcl or M-rcr is taken as zero.

Provided the above condition can be met, as the load is placed on the slab, the degree of distress inherent in the positive and negative regions will create redistribution to ensure that the overall stability is maintained. It is to be noted however that as a consequence, large rotations and deformations will likely manifest themselves within the slab. This is acceptable since NZS4203 requirements are ultimate limit state requirements and not serviceability limit state requirements