Calculation for holding down of fence posts 3

[Edu09]

I work at a UK company and I've been asked to come up with some calculations for a simple timber truss structure supporting a sign. I've done the majority of this, sizing the timber, checking permissible stress etc. I've built a few things from timber and I know for burying it in the ground there are rules of thumb such as 1/3 of the height should be buried but I am wondering how I can prove through calculation that this will withstand the overturning moment from the wind.

Can anyone more experienced point me in the right direction?

Any and all help will be much appreciated,

Thanks

Ed

 

[bridgebuster]

Your assumption of 1/3 is in the ballpark. I generally assume 40% as a starting point. In essence you have a shaft/pile with lateral load. In the US, the International Building Code, AASHTO, ASAE, among others have prescriptive measures for calculating embedment. I'm sure there's something similar in the UK.

It comes down to active and passive pressure. Some posts are designed assuming a constraint at the ground surface.

 

[oldestguy]

Look up "Sign Foundations" on Google. I found a link to a USA DOT site for that subject that may get you started. See below. You also might post on Geotechnical forum under Foundations. Rules of Thumb are not always suitable for all projects.

http://www.wsdot.wa.gov/publications/manuals/fulltext/m46-03/chapter17.pdf

 

[Edu09]

Thanks gents, I appreciate the help.

 

[a2mfk]

http://highways.transportation.org/Documents/Sec13-LTS-5TRev.pdf

 

[azcats]

You mention 'truss' in your original post. Is this a truss or cantilevered member that is fixed at grade?