I had a similair issue with a job many years ago.
it was a 45'x10' Aluminium
sign on three 8" wide posts.
In the end I got my ground engineer friend to work out wind loadings etc. but this did not help for what happened next.
The
sign was sited on a small hill and on the day the
sign was fitted the weather was blowing a gale and torrential rain.
Let's just say it was the worst weather to fit this sort of
sign in, but needless to say the customer insisted it had to be errected that day!
The following morning I had a call to say the
sign had moved.
When getting back to site, yes it had moved 2' down the hill but had styed perfectly upright and level!
The issue turned out that due to the amount of water that came down that day and night had saturated the ground so much, the
sign just floated down hill!
Turned out the engineer report and wind load was not an issue the biggest issue was an inpatient customer wanting too much too fast.
SC