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Deck Support Column Design

Question: I'm building a low deck with a beam that will be about 12" above grade. I'm planning to take the concrete footing tubes to the height of the deck beam as shown in Figure 2 of your web page: Deck Support Column Design & Size

The total concrete footing and column height, from the bottom of the footing, will be about 48 inches. The beam will be securely anchored to the concrete columns.

It occurs to me that the wood beam (2-2X10) will expand and contract very slightly lengthwise (parallel with the grain) due to temperature and moisture variations. This may cause a very slight lateral movement of the top of footing columns. Is this something I should be concerned about, and does it require any special beam-to-column attachment which compensates for lengthwise expansion and contraction of the beam?

This is obviously not an issue when the beam is supported by a wood post. But I don't know if it becomes an issue when the beam is attached directly to the rigid concrete footing column.

I would very much appreciate any feedback you can provide. Thank you.

Answer: Actually the wood will expand and contract less along the horizontal axis than it will tangentially – with the rings, this is what causes most wood to warp and bow.

I took a quick look at the temperature and humidity coefficient tables for fir lumber and over a 10 ft. length with 100 degree F variants and humidity variants of 10 to 90%, the horizontal dimension change was negligible.

I would not be concerned about connecting the beam directly to the concrete footings. Make sure that you do not rest the wood directly on the concrete as that will create a moisture problem and the wood will start to rot at that point. Use a piece of roofing felt between the concrete and the wood.

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