Saturday, May 14, 2016

The power of the wind

We turned up at the site today to find that the site shed (a good 12m long, weighing in at somewhere around 3t) had shifted about half a meter at one end, and toppled off it's concrete blocks.


After much discussion, Dad and I spent about half a day very carefully jacking up the structure again and removing the blocks before re-stacking them. We cut out the soil underneath to tilt the blocks inwards, and Kyle shaped a block of hardwood to fit in the triangular gap to provide a flat surface for the joists to sit on, and a little shelf cut out so it couldn't shift side to side.


What we think happened is that in the strong gusting winds the end of the shed had shifted, milimeters at a time, and eventually ended up on the edge of the previous stack which caused it to topple over and move the whole thing such a substantial amount. We're hoping that the angle of the blocks here prevents that incremental movement in future so it can't get to the point of toppling again.

But, to be sure, we dug a hole about 45cm deep and used some leftover concrete we're glad we had lying around to sink half a sleeper right up against one of the main struts. Currently we only have one, as it was an awkward hole to dig (half under the structure) and so took us a long time. The aim was to get the building to a point that was stable and safe first, before ensuring it stayed that way longer term.


We're going to add another couple of these posts (we've got loads of sleepers leftover) until we have a few on each side, though the prevailing wind is strongly from the northwest so this first post went on the northeast end (the end that shifted).

The other possibility that occurred to us was that the building could roll. The wind is very strong, and it's close to side-on (we wanted a cross breeze in summer). We're not really sure how much of a risk this is, and have no way of knowing. My assumption is that the shed is bottom heavy (there are two very large steel girders, with substantial steel joists across, that make the base of the structure, whereas the wall joists are smaller and I think aluminium). However, while the chance of it tipping may be low the consequences if it did so would be catastrophic - I doubt the structure would survive, nor potentially would we if we were inside at the time. We'd certainly have no way of righting it again ourselves.

A few suggestions were made, everything from angle iron braces on either side, and guy wires. Eventually our favourite suggestion so far came up - webbing and ratchets (substantial ones) attached to hooks concreted into the ground on either side, strapped over the whole structure at solid points. We're still figuring out the details that will make it work, and may yet do something different. We have most of the stuff for this strategy already, and it would be fairly quick to implement, whereas any other way would require us to go buy a bunch of materials.

For the moment, crisis averted. But we'll watch this space very carefully for signs of movement.

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