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Old 09-11-2009, 02:24 PM   #1
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Beam Construction


I need to replace a beam under my house.

The existing beam is made from two 2X8s. Over time this slender beam has started leaning in various places along its path. Since it needs to be replaced, I thought I would create a thicker beam of 2X8s. I am thinking of using three or four 2X8s. Not for strength, but for stability.

Here are the three problems I am aware of:

1) The beam needs to be built so that when finished it is perfectly flat on top and bottom. If it is not, it too will start leaning over time.

Will using three or four beams help with this problem since it offers a wider platform? Any ideas for getting it perfectly flat?

2) How does one go about connecting all the 2X8s together? My guess is to nail an "M" pattern of nails that penetrate all layers. And, of course, alter the splices over the piers.

I have used this technique with temporary jacking beams in the past. It works fine, but I am not sure about this approach for a long-term beam.

3) Would you recommend building a single 36 foot beam or building a few short ones?

A single beam appeals to me, but I worry about trying to keep everything flat while building it -- since it would be so long, it would need to be constructed under the house.


Any thoughts appreciated.

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Old 09-11-2009, 05:15 PM   #2
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Beam Construction


A new built-up 2x8 beam should be flat enough on the bottom that there should be no issues. If that's not flat enough you need to seriously evaluate what the beam is sitting on, as it might be insufficient. Some pics of what you've got might help us shed light on this. I'd also have some concerns about a beam under a home's floor system only being 2-2x8's unless the spans are very short.

The leaning and rolling that you're seeing is often a natural reaction of a beam under excessive load. When they can't deflect as much as they need to they'll start to lean and twist. This warrants looking into.

A 4-ply beam is pretty much always bolted together, because that'll make all the plies work together. Carriage bolts staggered about 12" apart top and bottom is fairly common.

You cannot build a 36' beam out of 2x8's. If you can buy 36' 2x dimension lumber you've got one heck of a special lumberyard! Staggering the ends of shorter pieces would not be acceptable practice in most circumstances. Go full length with all 4 plies from bearing point to bearing point. At 36' in length I presume you have at least two mid-span bearing points.

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Old 09-11-2009, 07:34 PM   #3
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Beam Construction


As Kc said, build them in sections from bearing to bearing. Yours sound over-spanned, compare to chart: http://ftp.resource.org/bsc.ca.gov/t...2_page0376.pdf

Be safe, Gary
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Old 09-14-2009, 04:02 PM   #4
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Beam Construction


Thanks for the input guys.

The existing 2-ply beam is built so that the breaks line up over the piers. But, there is no one solid break. The breaks in the 2x8s are lined up like siding on a house, where the breaks alternate from one pier to the next. I assumed there was some reason for this. The existing beam is also not bolted, but nailed.

The twisting of the existing beam was caused by two things: 1) piers with improperly sized footings and 2) poor grading of the land around the foundation. Both items are being corrected. I just wanted to upgrade the beam as well.

thekctermite:

I am fine with building short 4-ply beams with carriage bolts. Sounds better all around to me. I understand staggering the bolts top and bottom.

But, how far in from the top and bottom (1", 2", 3"?).
Would you use wood glue between the 2X8s?
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Old 09-14-2009, 10:26 PM   #5
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I can't provide the engineering support that you really should seek before doing this, but can tell you that your bolts should be a good 2" from the top and bottom edges, no closer. Using construction adhesive between the plies of wood is common practice.
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