COP:structure; types-load

3.6 Types of Load 

Loads acting on roof cladding are generally classified into two types: point load and uniformly distributed Load (UDL)

Cladding reacts differently to a point load and a UDL. A point load is applied to a particular area, but a UDL impacts on the total area of the roof.

In many cases the point load will govern; it is often the most severe of the actions and will determine the purlin spacing of roof sheeting. Uniformly distributed loads vary over the roof area. They are greatest at the periphery and corners of a structure. Purlin spacing may have to be reduced, or the fastener frequency increased, to cope with local pressure factors.

Manufacturer's roof and wall cladding design load data should be published with both point and UDL performance values.

3.6.1 Point Load 

Most roofing profiles will resist far greater point loads when the load is applied to the pan of the profile rather than the rib. When the load is applied to the pan, the load is shared by the adjacent ribs and is applied to the flange under tension, rather than the flange under compression.

Testing loads may be applied to the pan or the rib depending of the profile shape and the design criteria. (See 3.6.1.1 Roof Traffic).

Roofs that may be accessed by foot traffic must be designed to withstand a point load which is representative of a workman with a bag of tools. It is calculated at 112 kg, which equals 1.1 kN force.

In the case of a superimposed load, such as an air conditioning unit which is supported directly by the roof cladding, the unit weight per support and area of contact is calculated to arrive at point loads.

A point load on a roof is always positive or downward (+).

 

 

 

3.6.1.1 Roof Traffic 

The designer must consider the degree and type of foot traffic that may be expected on a roof. The following requirements are subjective standards and must be considered in line with customer expectations, and building use and type.

More robust design than specified below (reducing purlin spacing or adding protection from mechanical action), is required for:
  • roofs that are regularly accessed; and
  • roofs used as staging by subsequent trades; or areas that are adjacent to access points, particularly step down access.
Type A (Unrestricted Access) will comply for roofs:
  • that need to be regularly traversed by the roofer for access during installation;
  • that will be accessed regularly by sub-trades;
  • that butt on to walls or windows that may require maintenance;
  • that have plant, chimneys, or solar installations requiring regular maintenance; or
  • that require regular access for clearing gutters or spouting of debris.
For Type A roofs, the cladding must resist the load of 1.1 kN applied to the pan or a single rib.
Type B (Restricted Access) will comply for roofs:
  • that are simple in design and do not have to be regularly traversed by the installer;
  • which are infrequently accessed by qualified tradesmen for maintenance; or
  • with a pitch of more than 35°.
For Type B, roofs the cladding must resist the load of 1.1 kN applied to the pan or over two ribs.
Type C (Non-trafficable) will comply for roofs:
  • where supports are required to be laid to support roof traffic;
  • which have a pitch of 60° or greater; and
  • including non-trafficable translucent roof sheeting.
For Type C roofs, the cladding must resist the minimum load of 0.5 kN applied to the pan or over two ribs.

3.6.2 Unformly Distributed Action (Wind or Induced Action) 

A UDL is commonly either a wind load or a snow load. These loads are variable and depend on factors such as the location, topography, and position on the structure, but do not often exceed 6 kPa. The most severe wind load is usually an uplift load, or negative (-), and snow load is a downward load or positive (+).