Don't forget foot traffic when designing a roof

The design of roofs using the limit-state method must take account of serviceability as well as strength. When designers set serviceability criteria, they often overlook foot traffic.

Serviceability refers to the various functions that the design (in this case, the roof) will be expected to perform during its life. The strength limit-state is the point beyond which the roof would collapse or blow off. The serviceability limit-state is the level at which full functionality would be maintained throughout the intended life of the building. This level varies with the type of structure but is obviously lower than the strength limit-state.

The serviceability limit-state for roof sheeting is the longest span at which no permanent product distortion will occur and any deflection will be kept to an acceptable level.

Most Australian Standards for structural building products set serviceability limits, but these are minimum levels only. It is the responsibility of the designer to establish whether the appropriate performance levels should be higher than these minima.

In setting these performance levels, loads other than wind may need to be considered. When designing large flat roofs, in particular, foot traffic loadings must be taken into account, since they can have a major effect on the appearance of the roof.

All roofs are subject to some foot traffic during their installed lives. This may be from fixers during installation or at times of routine maintenance of the roof or other equipment located on or projecting through the roof.

The Australian Standard AS1562 sets minimum serviceability limits for foot traffic on roof sheeting. These would be appropriate for an agricultural shed, for example, where limited sheet deformation and occasional leakage in very heavy rain may not be an issue.

However, most industrial and probably all commercial buildings need to remain not only leak-free but dent-free. Accordingly, roofing spans recommended by Stramit Building Products are set at higher serviceability limit-states than the Standard.

When designing a roof, refer to the Stramit Foot Traffic Design Guide, which provides recommended spans for Stramit(r) roofing products over a range of foot traffic loadings. These include "high" loads of 1.8kN (for a roof with high maintenance traffic), "normal" loads of 1.1kN (for moderate foot traffic) and "controlled" loads of 0.9kN (where foot traffic will be from experienced personnel only).

For further information, talk to your local Stramit Technical Services Manager or contact your nearest Stramit office for a copy of the Stramit Foot Traffic Design Guide.