Beam Span...Rule of Thumb needed

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Old 11-02-07, 10:46 AM
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Beam Span...Rule of Thumb needed

Hi all,

Designing and building an addition to our main house. I have consulted the 2007 IRC for One and Two Family Dwellings.

The problem I have is that the IRC doesn't address built-up beams that are laminated with plywood or OSB. Can anyone cite a generally accepted Rule of Thumb that determines how much the span between column supports can be increased?

For example: Per '07 IRC, a 20' wide bldg with a built-up beam of 3-2x10 that supports one floor and ceiling can span 8'-9" between posts. If you sandwich two layers of plywood in this built-up beam, that span between posts can be greatly increased.

I know this is a tricky one to answer due to our sue happy society; but there is a proper rule out there somewhere.

thanx,
caryman
 
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Old 11-02-07, 12:02 PM
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Lvl

Why not just go to a lumber yard and get them to order the proper LVL beam to span whatever opening you want to span.

It sounds like you want to eliminate some support posts, however trying to rig up a homemade laminated beam may not work.

A proper sized LVL beam will ensure that everything above it is properly supported and in the long run any extra cost won't be that big of a deal in the cost of an entire addition.
 
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Old 11-02-07, 02:13 PM
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There is no rule of thumb, each situation should be approached on a individual basis. Its not a difficult thing to do and the structural integrity of your building is paramount. Loading conditions very from building to building, that is why there is no general rule. Hire an architect or engineer. Or if you want to bypass that step, at least give us an explanation or some drawings of the building and we can get you close.
 
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Old 11-02-07, 07:33 PM
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Ok, here are my specs.

Bldg Width = 24' (Perpendicular to ridge line)
Bldg Length = 36'
Foundation Walls = 12" block
One story structure over full Bsmnt.
1st story deck = 2x10 framing
1st story sub-floor = 3/4" T&G panels
Floor joists will span over a central beam
Span to beam = ~11'
Total Span = ~22'
Ceiling Framing = 2x10 construction

Per Code Book (IRC '07) Table R502.5(2) Headers and Girders Supporting One Floor:
.....Interpolated.....For 24' Bldg Width
A beam of 3 2x10's can span 8'-2"
A beam of 4 2x10's can span 9'-5"

Back in my day, before OSB and engineered wood, we routinely sandwiched 1/2" plywood between 2x10's because it was stronger and lighter than just adding another 2x10 to the beam.

Now I'm designing my own. I need a way to figure the maximum span for 2x dimensional lumber sandwiched with 1/2" plywood or OSB.

Thanx for replying and sharing your knowledge.
 
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Old 11-03-07, 06:06 PM
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Well sorry, but I really have no way of sizing the beam type your referring to (and am not aware of any generally accepted rule). I can't give you an decently solid answer unless your were going with a steel flitch beam or LVL.

I will say that I routinely put together CD's for 4500+ sq. ft., 2-story, block crawl foundation homes that utilize (3) 2x10's with about a 5-7 ft. span between each post and footing. I would say 6-7 ft. would be acceptable for you as well, but I am still unsure of the loading conditions based on your description.
 
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Old 11-04-07, 05:48 AM
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Thank you Ohio and SgtG for responding. There is suppose to be a table of allowable spans utilizing plywood and dimensional 2x lumber beams. In the meantime, I can safely design with the IRC Code Book I cited in my last post.

Great thing building on paper...it can be changed with the stroke of a pen.
 
 

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