There is a strong tendency for shade providers to use pre-engineered ‘generic’ structures to both justify the initial costs of engineering and to simplify and streamline the fabrication and supply process. This ‘one-size-fits-all’ carries with it inherent dangers of non-compliance.
The behaviour of members and elements of ‘apparent’ similarity based on comparable ‘size’ can often be quite different. This misunderstanding of the true behaviour of tension structures can lead to misuse and subsequent failures.
Typical examples of this problem include the following:
- Some shade companies work with “pre-engineered” post and footing designs which were developed based on a perfect hypar of various sizes and heights. However these same tables are being incorrectly used for triangle or kite shaped sails which might have the same surface area but produce very different reactions.
- Hypars with insufficient twist, and with no proper control of applied pre-tensions.
- Incorrect or inappropriate site locations for assumed similar wind speeds. Often “regional” wind speeds are cited on generic drawings without considering site-specific factors such as shielding or topography etc. These structures are presumed to apply anywhere in a region, say anywhere in Melbourne, when plainly an exposed beachfront location will suffer more severe winds than say a well shielded school courtyard. Either one will be significantly over-designed, or the other significantly under-designed
If the drawing you receive from your supplier is generic it will not include or refer to your site address – you should insist on a site specific engineering certificate which will confirm that the structure is suitable to be used at the site.