Load/Span tables
In practice, pre-finished steel cladding and secondary steelwork must be designed to withstand both positive and negative (wind-suction) loads. To help the building designer, roof and wall cladding system manufacturers publish load/span tables for both positive and negative loads, giving the maximum allowable spans for specific profiles under given design loads. Two limiting loads determine the load-bearing capacity of profiled steel cladding. These are the serviceability or deflection limit load, and the ultimate, or the failure, limit load. Both of these limits are taken into account by the system manufacturers in producing load/span tables. The design loads to be used in load-span tables are:
Roofs - negative (wind suction) loads
For each of the cases above, a temperature factor must be added to the loads stipulated to take account of the effect of temperature differentials for composite panels.
The mechanism of failure of a pre-finished steel cladding sheet depends on the specific fixing technique and the total span of the sheet. Load/span tables stipulate different permissible spans, depending on the total spanning condition of the sheet. The difference in spanning capabilities between these cases is due to the degrees of freedom at the sheetings steelwork connection. The designed spans given by the load/span tables are only applicable if the fixing of the cladding on site is to the manufacturer’s specification. Cladding system manufacturers may use different load span calculations and ratios for the different factors influencing the design. It is important to check with individual system manufacturers for the detail of these.
The negative design load is the calculated (negative) wind load plus the dead load of the cladding system.
For a built-up system, the design load should be the greater of the snow load, imposed (positive) wind load or the concentrated load (0.9 kN for roofs without access, 1.8 kN for roofs with access) plus the dead load of the cladding system.
