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Main Support Beam question.

Posted by bigporch (My Page) on
Wed, Sep 3, 08 at 17:23

I have a 3-story house with a basement that has 2 main support beams consisting of tripled-up wood beams measuring about 2" x 7.5" each. These are about 30' each in length and have 3 fixed steel poles (per beam) spaced pretty evenly.

The beams are cracked in many spots and have sunk into the support pole due to water damage. I am replacing these and have got quotes for both LVL and Steel.

Most recently I received a quote to use 4 LVLs per beam and keep the existing poles/footers. These would be the same dimensions as the existing wood beams with the exception of adding one more (4 instead of 3). This was much cheaper than the steel I-beam route.

Logically, this seems to be fine since LVLs are stronger than traditional lumber and the existing beams have lasted over 100 years. Does anyone see an issue with using the same sized LVLs? Or do I need to be going down the route of steel I-beams with new poles and footers?

Any thoughts?
Thanks.


Follow-Up Postings:

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RE: Main Support Beam question.

You need either an engineer or a very experienced carpenter.

There is no way to tell if the existing beams meet code without a careful site examination.

Many older places have been built with inadequate beams and joists.
Many fail badly, others seem to hang on.

The type of wood used for the beams matters a great deal, even now.

Even the grade matters.


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RE: Main Support Beam question.

The dimensions of the beam aren't the only factor. You haven't said whether you identified the wood species and quality in the original beam, and without knowing those things you can't assume that the LVLs are stronger.

FWIW, the original beam on your three-story house is the same size, with similar spans, as the beam in my one-story house. Intuition suggests that either mine is way overkill or yours is substandard.

I completely agree with Brickeyee that you need to hire on-site help -- this is no place to guess.


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correction

And by "on-site help" I meant an engineer or expert carpenter.


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RE: Main Support Beam question.

If the beam span is really only 7.5 ft. (30/4), switching from triple dimension lumber members to the same number and size LVL's (2.0 E, 2850 Fb) would more than double the live load capacity of the beam (simply because the Fb more than doubles) although the deflection would be greater but acceptable since the true max. loading is probably much lower. Adding a fourth LVL would almost triple the live load capacity and reduce deflection. There would be no practical reason to consider steel beams for such a small span.

Of course, you may not have described the conditions well enough for my information to be reliable so you need someone to look at the whole structure and size the beam in order to avoid oversizing it as well as undersizing it.

I have been able to reinforce this condition with LVL's on each side with thru-bolts and new posts but only an engineer could determine if that is feasible for you.

If the loading and supports can be accurately described, any lumberyard can get the beam sized by their LVL supplier. Sizing a beam is a simple task if all of the conditions are known.


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