Structural designers for townhouse and unit developments can be conservative in their provision for bracing design because they are unfamiliar with the specific codes that govern them. Areas like these are where a good engineer earns their keep by designing for-purpose structures that are easy and cost effective to build.
OK. First things first. This insight article involves a fair amount of technical information, so if you’re interested in that stuff, great! Otherwise, it’s main purpose is as an illustration of why good engineering matters. Bottom line: good engineering saves you money. PS: we do good engineering.
Wind is a funny thing. Most of our personal experience of wind is when we are walking on a windy day and we need to brace ourselves against a strong gust of wind. Or maybe we are holding an umbrella and wind causes the umbrella to invert.
It is because of these personal experiences with wind that it is one of the most underestimated forces that structures have to resist. But, as the saying goes: it’s all fun and games until a strong gust of wind lifts the roof off your house*.
Drivers of Wind Classification
To make a complicated concept less complicated, the codes relating to wind applied to above-ground structures summarise the main drivers of wind classification as follows:
Geographical Region
Depending where you are in the country, there will inherently be different levels of wind. Generally speaking the closer you are to the coast, the stronger the wind. And the closer you are to the tropics, the stronger the wind.
The Australian Standards for wind divide Australia up into several indicative regions with base level wind speeds ranging from 26m/s to 55m/s (94km/h to 198km/h). And if you had to read those numbers twice, I don’t blame you, as I mentioned earlier: wind is one of the most underestimated forces that a building must resist.
Terrain, Topography and Shielding
The shape of the land (topography) and the amount and type of surrounding structures (terrain and shielding) influence the amount of wind that a structure will experience.
Generally, if you are amongst a sparsely populated environment, are on top of a hill, or do not have anything shielding you from the effects of wind, the effects of wind will be higher.
Building Shape and Height
Lastly, depending on the height and relative proportions of the specific structure being designed, wind will have varying effects. Some building shapes are more conducive to funnelling wind in different directions. And generally, the higher the structure, the stronger the wind.
Residential Bracing Design
Wind is the major lateral (sideways) force that residential structures need to account for in design. Because these forces are typically applying a horizontal force, and not a vertical force like most other forces that are due to gravity, they require a totally separate structural system to resist these forces.
In residential townhouse and unit developments, these lateral forces are absorbed by bracing walls. These are typically solid plywood or steel straps that are fixed to regular stud walls. Their purpose is to transfer horizontal forces back into the ground.
The amount of bracing elements in a structure should be determined by the determined lateral wind force being applied to the structure.
Setting the Standard: AS1170.2 v. AS4055
So here’s the interesting part (if the above hasn’t already been keeping you on the edge of your seat). The Australian Standards have two distinct codes for the determination of wind loading on structures: AS1170.2 Structural Design Actions – Wind Actions for general purpose wind loading and AS4055 Wind Loads for Housing, specifically for residential housing.
Both of these codes use the same philosophies and concepts. After all, the characteristics of wind remain the same regardless of what it’s blowing against. However, AS4055 Wind Loads for Housing provides more refined wind parameters specific for townhouse and unit developments that:
(a) are less than 8.5m high
(b) are not wider than 16m or longer than 5x the width
(c) have a roof pitch not exceeding 35 degrees.
While both can rightfully be applied, so can a sledge hammer be used to hammer in a nail. That is to say, that it makes more sense to use the right tool for the right job. And in the case of wind loading, the all-purpose code is generally more conservative and results in designs that are comparatively more expensive to build.
Townhouse Engineers for Townhouse Developments
Most all-purpose structural and civil engineering consultants use a generic template and design tools to design residential townhouse developments. However, the simple truth of the matter is that the construction details for a 10-storey apartment building are overkill for a residential townhouse.
At Tingmore Structures our focus is on developing cost-effective and buildable designs that save time and money for our townhouse and unit builders and developers. Our designs improve upon Code-sanctioned standard designs and deliver tangible value to our clients on every project.
If you are looking for structural and drainage design engineers for your next unit, townhouse or multi-dwelling development, please get in contact with us so we can provide you with a comprehensive fee proposal for our services. We look forward to partnering with you on your next project.
