Wind Loads

pv wind
 

The Microgeneration standard 3002 requires "the contractor shall ensure that the roof structure is capable of withstanding the loads (static and wind loads) that will be imposed by the PV modules and their mounting arrangements. If there is any doubt, a structural engineer must be consulted. Guidance on the mechanical installation and wind loads are given in BRE Digests 495 and 489 or BS 6399."

the software incorporates a wind load calculator which complies with BRE digest 489. A wind load calculations required for each PV system. The BRE digests defines three types of system.

  • Photovoltaic array mounted above a pitched roof - this is the common installation method.
  • Small format photovoltaic array mounted on a pitched roof as tiles
  • Small format photovoltaic array mounted on a pitched roof on battens.

To create the upward and downward force on the panel and the mounting frame the steps are as follows.

  • Select a site topography and wind zone, estimate the wind zone from the location and the map provided.
  • Enter the height of the site and the height of the building
  • For each PV system enter the area of the array and select the system type ie array mounted above the pitch roof.
  • The upward and downward force is calculated and displayed on screen

Mounting kits

To calculate the correct number of mountings for the solar array first enter the total area of the panels on the frame. In our example we have 12 Sanyo panels each with an area of 1.38sqm this gives a total area of 16.6sqm. The panels are installed in three rows on a pitched roof  using four Tric A  rails.

  • Calculated wind load is 18.5Kn upward and 14Kn downward.
  • Each Tric A mounting bracket has a strength of 1113N downward and 714N  upward.
  • Therefore worst case is upward so 18.5 divided by 0.7 = 26 brackets - the software displays an incompatibility warning until the recommended number of brackets is chosen. In this test case 28 brackets were used in the actual installation.