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Re: Wood post limitations
- To: seaint(--nospam--at)seaint.org
- Subject: Re: Wood post limitations
- From: bcainse(--nospam--at)aol.com
- Date: Wed, 19 Jul 2006 14:50:12 -0400
I assume you mean for exterior use.
They are used frequently here in Northern California, usually with Simpson or USP bases and caps. The Simpson and USP designs typically control load eccentricities that exacerbate the twisting tendencies. If they are sloppy wet when installed, they do warp, twist and split but not any more than the large sawn beams that used to be used. And certainly not any more than 4x4 P.T. posts often used as deck supports. Moisture control on the wood provided is important for good performance. Limitations on use are listed in the National Design Specification (NDS), which account for in place prpoerties, and do a reasonable job controlling performance. The L/d ratios specified limit the lengths allowed. Careful fabrication should be followed as with any material. I've seen steel fabricated as poorly as you describe for your situation with the wood so that is no guarantee.
As far as treatment goes, that needs to be selected for the conditions encountered. For example, a post embedded in concrete or soil should be treated for ground contact and means provided to freely drain water from collecting at the base. Also, clients should be informed of the durability limitations for wood, but then no material is permanent.
Bill Cain, S.E.
Sent: Wed, 19 Jul 2006 11:07 AM
Subject: Wood post limitations
I occasionally see 6x6 treated posts used as free-standing main carrying posts in residential and light commercial construction. They are arguably sound at this moment in time but I worry about future splitting, twisting and even eventual deterioration. Today, I found some in a new country store type of building where they extend through the slab to the footing. (The tops weren't even cut level so the supported glulam was listing about 1/2" to the side, but that's another matter...)
Are there any practical limitations to these members such as maximum height, required connections top and bottom, etc?
Since I only see them already in place, I', looking for legitimate reasons to direct owners/contractors to remove and replace them with steel.
Jim Wilson, PE