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Old 07-05-2012, 09:02 AM   #1
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Beam Question


Let me start by saying I know I should ask an engineer but I cannot afford one.

I am replacing a double 2x10 joist in my house that the ends have rotted off due to a leak. The leak has been repaired. I am in the process of removing them and it seems to be more challenging than I thought it to be. I am having issues due to plumbing and HCAC ductwork in the area.

My question is could I replace a double 2x10 on a 12' span with a triple 2x8 and get the same strength?

All opinions will be appreciated. Thanks.

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Old 07-05-2012, 09:12 AM   #2
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No, it would not be the same strength, not even very close it.

Sorry for the bad news.

Andy.

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Old 07-05-2012, 09:19 AM   #3
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No prob. Just thought if it was I would save myself the aggravation of trying to squeeze new ones in. Well on with the 2x10's. Thanks.
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Old 07-05-2012, 10:47 AM   #4
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Originally Posted by AndyGump View Post
not even very close it.

.

Andy.
Andy, are you quite sure about that?

For a given bending moment, the stress in the beam will be proportional to the section modulus.
For the 2 2x10s, S = approx 45 in 3.

For the 3 2x8s, S = approx 42in 3, so not a great deal of loss of strength (say 5%).

Deflection will be a lttle greater, being dependent on I, but I would guess only by a factor of around 1.3.

If his existing beams are loaded to near their limit, then obviously he should stick to that size, but of course we don't know one way or the other.

Last edited by tony.g; 07-05-2012 at 01:12 PM.
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Old 07-07-2012, 01:36 PM   #5
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Without seeing the situation or knowing what this beam supports, I would make every attempt to replace with the same size beams. We are talking about a structural support beam here as opposed to plumbing and A/C which should be considered as secondary in importance.
I find a good Sawzaw with a dual metal/wood blade will aid in it's removal.
Time should be taken in figuring out how to support what is above before any attempt is made at demolition

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Old 07-07-2012, 03:42 PM   #6
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Tony G is correct, the difference in strength between 3 2x8 and 2 2x10 is about 6 percent lower for the 2x8. This may not make any difference in this particular case, only analysis can tell for sure. Deflection is about 20 percent worse for the 3 2x8 case, again this may not make any difference. This assumes identical species and grade of lumber for each condition, and assumes that the 2x8 is a true 1.5 inches by 7.5 inches, and that the 2x10 is 1.5 inches by 9.5 inches.
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Old 07-07-2012, 06:14 PM   #7
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Our 2x8 is 7.25 inches and 2x10 is 9.25"

I'd be safe and replace with the saem.
Can you remove the HCAC (Hvac?) and plumbing, put in your new 2x10 beside the old (sistering it) Then reinstall what you removed?
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Old 07-07-2012, 06:44 PM   #8
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If by chance his pipework and ductwork is causing problems near the end of the beam (say within a foot or 18" of the bearing) he could use the two 10" beams and just notch out the underside of the beams by the necessary 2".
This will make practically no difference to the bending stress in the beam. At the support, bending stess is normally nil; the notch would slightly increase the shear stress but that is rarely an issue with domestic loadings.

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Old 07-07-2012, 08:09 PM   #9
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For the case of 2 1.5"x9.25" versus 3 1.5"x7.25" beams, the three 2x8's are about 8 percent weaker than the 2 2x10's, and about 15 percent less stiff. There is no reason to be "safe" and replace with same size beam, it is simply necessary to perform the proper computations to determine if a triple 2x8 is sufficient.
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Old 07-07-2012, 09:07 PM   #10
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Originally Posted by Daniel Holzman View Post
For the case of 2 1.5"x9.25" versus 3 1.5"x7.25" beams, the three 2x8's are about 8 percent weaker than the 2 2x10's, and about 15 percent less stiff. There is no reason to be "safe" and replace with same size beam, it is simply necessary to perform the proper computations to determine if a triple 2x8 is sufficient.
Daniel,

Would (2) 1-3/4" x 7-1/4" lvl's work at that 12' span?
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Old 07-07-2012, 09:58 PM   #11
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Quote:
Originally Posted by Daniel Holzman View Post
For the case of 2 1.5"x9.25" versus 3 1.5"x7.25" beams, the three 2x8's are about 8 percent weaker than the 2 2x10's, and about 15 percent less stiff. There is no reason to be "safe" and replace with same size beam, it is simply necessary to perform the proper computations to determine if a triple 2x8 is sufficient.

Thank you Daniel, I had thought that they would be quite a bit weaker that what you stated.
I stand corrected (again) by Daniel.

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Old 07-08-2012, 12:01 AM   #12
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I appreciate all of your responses.

I will be replacing the current 2 2x10's with 2 new 2x10's for two reasons. 1) it is the right thing to do 2) I think that the current standard of lumber is not as good a grade as the original lumber the house was built with 60 years ago so I do not want to increase the deflection of the floor that much.

I cannot post pictures but i will try to explain the situation as best i can just for your info with a small drawing

This is a one story ranch built in 1950 in the suburb of Pittsburgh, PA.

The house has 2x10 floor joists, 2x4 walls and 2x6 ceiling joists with a framed roof of 2x6's all on 16" centers. Spans are 12' from center beam to sill or outside wall.

This beam is part of the living room section of the house. The room has a fireplace and this is all part of the problem. The roof around the chimney has been leaking for several years and this lead to the wall studs and the ends of the floor joists rotting off. (apparently, alot longer than several years) Anyways the area in front of the fireplace is a poured concrete hearth which makes the joists that run accross under are headed off in front of it by a double 2x10 that goes for about 8' carrying the joists of that part of the floor. This small beam of course is tied at each end to a double 2x10 that runs from the steel i beam to the sill. The ones I am replacing are the ones from the i beam to the sill as the end on the sill is rotted. I have supported the cross beam with a 4x4 post to a jack that is on a 1'x1' 3/4" piece of plywood on the floor. It is kind of weird because the joist space next to it is about a 20".
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Old 07-08-2012, 12:11 AM   #13
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Here is the crude drawing I made
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Old 07-08-2012, 08:18 AM   #14
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Quote:
Originally Posted by bcgfdc3 View Post
I think that the current standard of lumber is not as good a grade as the original lumber the house was built with 60 years ago
Good point. In the end, you have to do what you are happiest with.

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