Roof Destruction due to Storms
In recent and local news around Adelaide there have been a couple incidents of roofs not being able to withstand the stormy weather. In just the last month (4th Dec 2018) due to a heavy storm the jetty shelter that was built as part of a major upgrade, completed in August, at Henley Beach was affected as a corner of the corrugated iron roof of the jetty shelter unbolted. While a couple weeks before that another huge storm caused a residential apartment building in Glenelg to completely lose its roof, leaving dozens without a home till it’s repaired.
For these incidents to occur they were obviously not designed to withstand the high wind forces that they were hit with during the massive storm. This could have been due to several reasons including;
- The storm was stronger than the assumed design value.
- The engineers in charge with designing the tie-downs used incorrect estimations when choosing the design values in their wind analysis.
- It was not properly constructed to the designed details.
Regardless for who was at fault, clearly these structures were not properly built to withstand the harsh winds and shows how important it is to properly design roofing for stormy weather. Compared to other states in Australia, South Australia is within the ‘normal’ wind region, meaning that we don’t get tropical cyclones or severe weather conditions compared to the northern regions where cyclones are much more frequent of an occurrence and must be designed accordingly. However, just because we’re not prone to these conditions it doesn’t mean we can slack with our design process. As engineers it’s important to take all possible effects into consideration, starting with the region of the building.
Adelaide has varying wind design specifications depending on location, for the most part, if you are in a region close to the shore, or high in the hills, winds are generally considered much stronger than within the CBD or other suburbs located further from the beach or in valley regions. Design speeds within the Adelaide regions vary from between category N1 to category N3. This means the design speeds are between 28m/s to 41m/s, which means it still varies quite a bit even just between the small region of Adelaide’s suburbs.
Not only is location important but the shielding and topography, natural or artificial physical features as well as slope, of the area, can be of significance as well. Other buildings or natural barriers can help protect another building from wind pressure, whereas houses built with no protection will need to be designed for higher wind velocities. Therefore, it’s important to get the location surveyed by a specialist or even have the engineer check the site personally to ensure it is designed with all these things in mind.
So how do engineers help combat higher winds? Well when designing a building, once wind analysis is complete and a category has been selected the engineers in charge of designing the framing must take these numbers into consideration to design tie downs and bracing as appropriate. During the framing process, the beams are tied down to ensure it is kept strong and secure to withstand uplift and strong wind loads from any direction. Even at lower wind speeds sheet roofed houses will have net wind uplift forces which will need to be countered with the use of tie down connections. The higher the wind loads for the area the more specific these tie downs will need to be in order to counter the wind uplift force.
At Epic Projects and Consulting we have done numerous work that included having to complete wind analysis and tie down designs. We consider ourselves experts in the field and would love the opportunity to help you with your next project to ensure it is built to standards.