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Supporting timber-frame posts

Q: I am building a 1-3/4 story timber frame and would like to mount the posts in connectors atop a 3-ft. high concrete-block wall. Are the point-load compression capabilities of concrete block adequate for this arrangement? Are there concerns with lateral strengths or stability I should address?

Jerry Snodgrass, Chico, CA

A: Robert L. Brungraber of Benson Woodworking Co., Alstead Center, New Hampshire, responds: Although you say that the building is a 1-3/4 story timber frame, without knowing anything more about those stories or the spacing of the posts, there’s no way to assess the vertical load in those timber posts. Similarly, there are different kinds of concrete blocks, so I can only offer some generalities in my answer.

Even if the concrete-block manufacturer claims that its blocks can handle the 15,000-lb. point load a post might exert, I wouldn’t put point loads on an unreinforced, 8-in. thick concrete-block wall. The post connector is intended to be cast into and tied down to something heavier than the single block it sits in. In your part of the country, you’ll need a serious connection. Therefore, for both up and down loads, you should hook the connector onto horizontal rebar within a concrete bond beam placed atop a block wall where every block is filled with grout. You should also place vertical rebar in the blocks; this rebar ties into the rebar in the bond beam and into more rebar embedded in the footing. Using grout with steel reinforcement gives the block greater load-bearing capacity and anchors the post against the vertical accelerations of an earthquake.

If you’re going to backfill against that 3-ft. block wall, the vertical rebar tied into the rebar in the footing will prevent the wall from sliding, overturning and breaking.

You’ll also have to protect the end grain on the bottom of the posts from direct contact with the steel post connectors. Materials denser than wood can act as ongoing sources of condensed moisture: a situation certain to promote decay. I treat open end grain with a waterborne preservative and/or separate the two materials with a waterproof gasket.

From Fine Homebuilding 83, pp. 18 September 1, 1993