Concerned about how wind might affect your roof mounted product?... Not quite sure on how to calculate wind loads? Our latest content piece addresses some of the most common 'wind related' questions, and even includes tips on how to tie the family trampoline down!
How will wind affect my roof mount or plant maintenance work platform?
After a stretch of good weather, it is easy to forget about storms, cyclones and the potential trampoline flying effect of wind gusts. While the kids’ trampoline (me too when no one is watching!) is often one of the first to suffer in high winds, a lot of buildings and structures including roof mounted platforms can fail if not designed and engineered correctly.
Figure 1 The Trampoline, often the first to suffer in high winds
How do I allow for wind in the design of my roof platform or condenser mount?
All certified designs in New Zealand and Australia should be designed to the standard AS/NZS1170.2 Unless you are an engineer and love algebra, and Greek letters it will be all Greek to you so it is best to go by the engineers design certificate or producer statement showing they have confirmed the suitability of the product for that application. Standard Monkeytoe platforms and condenser mounts are pre-engineered with these for most applications while site specific designs and certificates can be provided when required.
How are wind loads calculated?
The standard AS/NZS1170.2 provides a map showing the different wind zones for areas of New Zealand and Australia. The most severe region in NZ is predictably the Wellington region while for Australia this is the North and North-eastern coastline beginning at Bundaberg and the highest is on the WA coast near Port Headland. To calculate the force the structure is required to withstand the maximum windspeeds based on cyclones is used to calculate it effect based on where it is located and the surrounding terrain and height above ground. For the speedfreaks out there the typical design windspeed in Darwin and Bundaberg is 237.6 Km/hr while in Wellington NZ it is 190.8 km/hr. Using typical calculation factors the wind load on a single condenser unit of 1.2m high by 1m wide is more than 500kg in Darwin/Bundaberg while in Wellington this is nearly 300kg. This means to make sure it doesn’t end up like a trampoline in the next storm it pays to have a proven certified product. Next time you see a large 3M high screen on a plant platform think about how much force this needs to withstand in severe weather. This example shows why the typical design limiting factor in Australia is wind loads while in NZ it is often seismic loads.
How could my roof mount or platform end up like the trampoline?
When the side of the trampoline (or HVAC unit or screen wall) receives pressure from the wind this is transferred through the bottom of the structure through its framing. On one side this will create a downforce and the other side will have uplift. Once this starts to overturn and move if it cannot withstand the force, as the bottom catches the wind it then turns into a kite and ends up on a nearby fence. This can also happen to HVAC units and maintenance platforms that are not tied down or properly designed and engineered.
Figure 2 How the trampoline got away
How does my Platform or condenser mount stay on the roof in these winds?
When a the side of a HVAC unit or screen wall receives pressure from the wind this is transferred through the structure, a lot of this load is transferred from the screen walls though the brace and then into the truss below diagram which shows a load of 10Kn or 1,000 kg. In the same way as the trampoline one side this will create a downforce and the other side will have uplift. These loads are then transferred to the roof fixing system which for our products is a Monkeytoe clip system.
Figure 3 Load transfer diagram for a standard MKT platform
How do roofing clips keep my platform or roof mount on the roof?
Monkeytoe developed its patented roof clip system over 10 years ago to create a safe weathertight mounting point that doesn’t damage the roof. When the platform or condenser mount coms under load the clips then transfer this to the building and in turn the roofing structure of the building. To ensure these work Monkeytoe carry out extensive testing of all the mounting systems, in fact to further increase the load capacity of our standard roof clip we have developed a new patent high uplift fixing system that will reduce the amount of fixings needed for windy regions by around 1/3rd. This will have a certified maximum pull-out force of 968kg, this is slightly higher than our resident fitness buff Shinda can bench press on a good day. The below picture shows a test for this with the test load simulating more than 1,000kg of wind force.
Figure 4 Standard RC01 Roof Clip
Figure 5 Testing Trusses and roof clips for wind loads one of many tests