Post Hole Foundations

[80smoviefan]

Hi all,

I'm wondering if any one has information on post hole footings, i.e. how they work (cantilever action?) and how to design them (reinforcement, overturning moment).

Any help is appreciated.

Thanks.

 

[TLHS]

What are you trying to build? A fencepost, equipment support, house footing and utility pole all have slightly different constraints and things to look out for. Reinforcement is also a question of how you've attached your post. Embedded fenceposts may not need reinforcement depending on how you've detailed them, for instance, but a concrete footing that you've bolted a post to the top of certainly will.

Your start point is likely going to be the IBC post foundation formulas, if you're looking at small scale items that aren't significantly deflection sensitive.

 

[msquared48]

The proper term is "Pole Footing". You will find what you need in the IBC as mentioned, but also the AITC and Google. Lots of information available. Depending on the nature of what you are doing, you may need at least a PE design and stamp too.

Mike McCann, PE, SE (WA)

 

[jayrod12]

Check out the Post-Frame Building Design Manual, also ANSI/ASAE EP486.1

 

[jrbaus]

Assuming these are analogous to "post footings" typically used in Australia where steel posts are cast into bored piers to support residential construction (i.e. clad frame buildings elevated above ground level) I will share what I have found from my research:
1) cantilever action can be relied on for posts up to 900mm in height (limitation given in the timber framing code AS1684.2)
2) for heights above 900mm cross bracing sets are required to provide racking resistance
3) footing depth is determined by considering two factors. First is how much lateral/uplift capacity is required to resist wind loads. Second is how deep the footing needs to be to resist predicted ground movement in reactive sites prone to shrinking/swelling soils.

For lateral resistance, I usually just design as normal piers by estimating the un-drained shear strength of the soil based on the classification given in the soil report(20-30kpa usually - but I would prefer a geotech to make a recommendation rather than having to estimate values for each job). I have not seen the IBC reference before, thanks TLSH.

Jake

 

[racookpe1978]

But little in Australia is written against frost depths, right?

Even the local building codes here (NW GA, US) get pretty restrictive on "post holes" for outdoor decks. You need to define your problem (your application and your country and state/province) much, much better.