Monday, 14 September 2015

Should sail-shades, be treated as cable-nets and tension membranes?

{Previously posted in LinkedIn group I created: Pre-Engineered manufactured structural Building products, and have since shut down. It having a membership of 30 people. Most of these people also in my group Pre-Engineered Manufactured Building Systems Group which has over 2000 members.}

Builders of sail-shades hold standard calculations for sail-shades. These calculations are trivial, basically one page: as far as the ones I've seen. Assume elevation of cable, determine horizontal component of wind loading, apply to post and determine moment in post. Job Done!

Treated as a cable-net suggests the pretension would snap the posts. Since this doesn't happen in practice, pretensions are not high. Which then suggests that the sail-shade is not tensioned, and that also not able to support load of kids climbing onto the shade. Also one verandah builder told me story, that he had seen failed sail-shade with the cables and shackles whipping up and down damaging several very expensive cars.

So seems more rigorous assessment is required. But councils appear to grant approval for the simple approach on regular basis. Checking as cable-net/tension membrane takes considerable longer.