COP:natural-light; loadings

10.6 Loadings 

10.6.1 Point Load. Walking Traffic 

All plastic roof lights are classified as brittle roofing and is not suitable for roof traffic, unless specifically tested under the point load test provisions of AS 4040.1, As/NZS 4040.4, and AS/NZS 1562.3: 2006.

AS/NZS 1562.3 :2006 requires the provision of safety mesh under all plastic sheeting subject to local statutory or national building code regulations. The HSE Act 1992 classifies accessible roof lighting as hazardous and requires the use of safety mesh under or above translucent sheets over 500 mm in width.

Although normal chopped strand GRP of sheet thickness greater than 1.7 mm can resist the impact load to demonstrate resistance to accidental fall, this strength is not expected to be retained for more than 5 –10 years, and the sheeting is therefore classified as brittle. GRP sheet that is reinforced with a woven glass mat, may remain trafficable for 20 years, but this should be proven by testing.

Safety mesh can damage plastic sheeting by expansion movement and walking traffic, and should be isolated at the purlin. See 13.8.6 Purlin Protection.

The mechanical properties of plastic roof lights differ from those of profiled metal cladding in that they are more flexible which allows them to deflect to a greater extent without damage. Foot-traffic and or access for maintenance should be considered at the design stage, so one sheet or a reduced width is provided so a workman may step over and not on, the roof light.

Safety mesh must be provided under all translucent sheeting which is accessible and wider than 500 mm.
A temporary walkway must be provided for installation where the plastic sheeting is more than one sheet width, and if access for maintenance is required the walkway must be permanent. See 12.6 Walking On Roofs.

10.6.2 U.D.L. Wind And Snow Loads 

 

The maximum performance of plastic roof sheeting for spanning capability and deflection under a uniformly distributed load depends on the section properties and type of material of the profile. The section properties depend on the number and depth of corrugations and the thickness of the profile. 3.5.6 Section Properties 
The uplift pull-through load performance depends on the number of fasteners per square metre and the type and size of the washer.

Profiles which have deeper ribs are more rigid and will deflect less, but will not provide any greater resistance to pull-over at the fixings, unless the sheet thickness is increased. Greater spans also require a thicker sheet. Additional fixings will increase resistance to pull-over failure at fixings, but will not limit deflection.

Roof lights located in the peripheral zones of high wind design load should have provision for the higher load in this area by the use of additional fixings, reduced purlin spacings or by increasing the roof light thickness. Deflection of plastic roof lighting due to UDL wind or snow loading should be limited to less than 1/30th of the span or 50 mm.

All plastic roof lighting should be tested to withstand wind loads and extrapolation is not acceptable as a statement of performance.

On buildings higher than 10 m or areas located in the peripheral zones of high wind design loads, near verges, eaves or ridges it is better practice for roof lights to be omitted.

Plastic roof sheeting must match the design load of the adjacent metal roof cladding. This can be achieved by using a mid-span support or by increasing the weight of the plastic sheet.

Excessive deflection due to long spans can open up side laps or cause failure in compression at the fixing points.