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Re: Beam Spans

From: info@builderswebsource.com
Category: Structural
Remote Name: 63.198.181.43
Date: 12 May 2001
Time: 12:12 AM

Comments

Thank you for your additional clarification. First, even though you believe your actual loads may be less than normal, the US model building Codes (BOCA, SBC and UBC) all have minimum required floor live loads (PSF). The 3 Codes use 40 PSF floor LL for residential, 50 PSF floor live load for offices and a 125 PSF floor live load for light storage.

So, we must use 40 PSF for the live load to meet minimum building codes.

Next, let's compute the dead load using the following minimum assumptions. All figures are in PSF:

Carpet and Pad = 3.0
3/4" Plywood = 2.2
2x10@16" o.c. = 2.8
Floor insulation = 2.5
Ceiling sheetrock (1/2") on underneath side of floor = 2.0
(we assume the basement could be finished, so we account for this added weight the dead load calculation, although this doesn't change the overall calculation by much even if it's eliminated).

Total Dead Load = 12.5 PSF

This sounds about right, since most floor dead loads figure between 10-15 PSF for typical residential construction.

Assuming, then, that we must use a 40 PSF LL for the floor and 12.5 PSF DL (accounting for the plywood flooring, carpeting, etc.), then the beam would be calculated with the following load parameters:

LL=40 PSF
DL=12.5 PSF
LL+DL=52.5 PSF

Furthermore, we assume that all load from the roof is transferred directly to the foundation via the external framing, so we will not include any additional roof or ceiling loads.

Span = approx. 16 feet (accounts for at least 1-1/2" min. bearing on each end).

Tributary Width = 11.5' (approx. 1/2 of 24' span, accounting for slight span reduction due to thickness of end walls and beam width itself).

Floor live load design deflection 1/360 span, 1/240 total load.

Of course, we need to know the wood species and grade. For our calculation and based on your location on the East Coast, we're assuming it's an "Eastern Species" grade #1 (best quality). Using the 1991 NDS specifications for wood, you need at least an 8x14 to meet these requirements. Even going to an 8x12 (which is a poor approximation of your proposed (5) 2x12), this fails all critical tests, including bending stress and deflection limits. The problem with "built up beams" is that their strength can vary considerably depending on the type and amount of glue and pressure used, the moisture content, the type of lumber, etc. For this reason, glulams are built to exacting engineering standards under controlled conditions, making it possible to better understand their true structural properties.

Your instincts are probably correct that your proposed built-up beam would not likely result in a building failure since in actuality your live loads will likely be less than the 40 PSF. However, it comes down to basic principles of residential construction. Either you can design according to codes (which is always recommended)...or you can take your chances (never recommended). Studies of collapses reveal that older design rules were insufficient and failed to account for changes in strength properties of modern woods. This is why the NDS changed their guidance during 1991 and again during 1997 (the latter has yet to be adopted by some building codes). Furthermore, you have no control over how the structure could be used after you sell it. Therefore, following codes is essential to ensuring occupant safety even after you're long gone.

If you want to squeak by with something less than our recommendations and if you don't have access to glulams or microlams due to cost or availability in your area, we recommend you consult a licensed structural engineer who can run a specific calculation for you based on the exact species of wood as well as any other variances that might lessen the requirements. We understand you feel our calculation results in a beam that's "overkill," but this is what the numbers show in order to meet minimum building codes based on the parameters you have provided us. Therefore, we stand by our recommendations herein. If you receive different advice from a structural engineer, we would be interested to see the load calculations and assumptions for further study. We will post an amendment to our recommendation upon further analysis or change in assumptions (such as wood species, tributary load, allowable deflection, etc.).

Finally, your choice of 2x10's on 16" centers for the floor joists checks out just fine. This is the minimum recommended size to meet building codes based on the loads provided above. Good luck and please let us know what you finally decide to do...and whether you receive different advice from an licensed structural engineer (not a contractor).

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