New Form INC

11.5 structural fire models

With careful consideration of structural model types, and selection of appropriate isolation joints which comply with both fire requirements and shrinkage restraint requirements, economic building solutions can be found for particular situations.

(a)    Model 1 - Fully Integrated Structure
Building floor acts as a diaphragm, directly connected to boundary members which support fire rated external walls. In this case, the floor must satisfy both “F” and “S” rating requirements and it is likely that a fire rating of between 60 - 240 minutes will be required.

(b)    Model 2 - Perimeter Integrated Structure.
The exterior span of a building floor acts as a diaphragm, directly connected to boundary members which support fire rated external walls. In this case, the internal spans are not required to provide lateral support to external walls. The perimeter diaphragm, constituting the tray floor in this location, along with primary structure, and associated boundary support members must be designed for the “S” rating (assuming “S” rating is greater than “F” ) Should internal tray floor spans collapse under fire, the designer should ensure that neither the building primary structure nor the perimeter diaphragm structure collapse prematurely.

The floor system internal to the perimeter system if isolated from the exterior structure need only be designed for the buildings “F” rating. If the internal section primary structure is required to form part of the overall primary structure supporting the external walls then they are governed to the “S” rating and must not collapse if the internal floor collapses.
The perimeter and any structure required to fulfil “S” rating requirements will have FRR’s between 60 - 240 minutes. The isolated internal tray floors require compliance with “F” ratings which lie between 0 - 60 minutes.

(c)    Model 3 - Articulated Structure.
The exterior wall system is completely independent of the floor and its associated support systems. In this case, if the floor/support system collapses under fire, the exterior walls remain standing and are capable of supporting the loadings laid down in NZS4203 “loading code” under fire loading. Examples of such structures are boundary tilt panels which are structurally isolated from the floor. In this case the “F” rating governs the complete floor system, which is likely to have a maximum FRR requirement of 0 - 60 minutes.

(d)    Model 4 - Intermediate Floors.
The floor forms part of an intermediate floor which under the acceptable solution has special requirement. In this case the FRR requirement is 15 minutes and since under most conditions tray floors have a inherent rating of 30 minutes, no special design is required. The designer is to ensure that if collapse of the intermediate floor occurs, that this collapse does not prematurely fail the exterior wall. (refer section 11.6)

(e) Model 5 - Household Buildings
Household units should include purpose groups SH and SR from the acceptable solution.
Multi-dwelling household units containing upper floors may be treated as single floor firecells (Clause 6.8). In this case a building may have adjacent firecells on the same floor level (taken as single level) and each firecell must have a minimum FRR of 30 minutes. Generally for such structures, the floor forms part of a diaphragm/shear wall structure in which the floors and walls rely on each other for structural support. As tray floors generally have an inherent FRR of 30 minutes, no further fire design is required.

The provision between units however do not override the requirements of the “S” ratings, which will vary from zero when exterior walls are permitted to be unprotected, through to a maximum of 90 minutes. It is likely however that with the provision of vertical openings in external walls in the form of windows that Av/Af will be approximately 10% and therefore the required rating will be approximately 60 minutes where “S” ratings govern.