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05-25-2010, 06:57 AM   #1
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## wood vs steel I-beam

I bought an early 1900 cottage style 2 floor house. In the basement the joists are sagging and I just want to add some support for the joists. There is not a lot of head room. On each side of the stairway, which splits the basement in half, I have a 9 foot span. Right now there are 3 2x6's nailed together for a beam on lolly columns in that 9 foot span across the joists. It is still sagging a little.

My question is - would a 4" x 4 1/8" steel I beam be stronger than the 3 nailed together 2x6's?

05-25-2010, 08:54 AM   #2
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Quote:
 Originally Posted by newfeet1 My question is - would a 4" x 4 1/8" steel I beam be stronger than the 3 nailed together 2x6's?
The area moment of inertia for the steel is ~1/10th that of the wood and the E is ~18x, so steel is only ~1.8x stiffer.

This is pretty close so more info on the I beam and the wood grade/type is needed.

 05-25-2010, 12:16 PM #3 Civil Engineer   Join Date: Mar 2009 Location: Boston Posts: 5,640 Rewards Points: 4,860 Steel beams are not listed in the format you described, which I assume is depth x width. The closest shape I have in my tables is a W4x13, which is 3.83 inches deep x 4.16 inches wide, with a moment of inertia about the XX axis of 11.3 in^4. As mentioned by Yoyzit, the relative stiffness of the beam is a function of the modulus of elasticity of the steel, which is approximately 29 million, versus about 1.8 million for wood (depends on species). The relative STRENGTH of a steel I beam versus wood is a function of the yield strength of the steel, which is typically a minimum of 36 million psi for structural steel, but may also be 60 million psi. So to determine if the I beam has greater or lesser capacity than the wood, you need to know the yield strength of the wood, the exact dimensions of the wood, and the yield strength of the steel. All that said, are you sure you even want to consider replacing existing joists that are sagging with steel? That would be a large job, and unless there is a real structural problem, may be an unncessary expense.

 05-25-2010, 12:21 PM #4 Member   Join Date: Nov 2009 Location: central virginia mountains Posts: 1,857 Rewards Points: 1,000 daniel i think just the 2x6 beam is in the mix __________________ The older I get the better I was
05-25-2010, 02:13 PM   #5
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Quote:
 Originally Posted by Daniel Holzman Steel beams are not listed in the format you described, which I assume is depth x width. The closest shape I have in my tables is a W4x13, which is 3.83 inches deep x 4.16 inches wide, with a moment of inertia about the XX axis of 11.3 in^4. As mentioned by Yoyzit, the relative stiffness of the beam is a function of the modulus of elasticity of the steel, which is approximately 29 million, versus about 1.8 million for wood (depends on species). The relative STRENGTH of a steel I beam versus wood is a function of the yield strength of the steel, which is typically a minimum of 36 million psi for structural steel, but may also be 60 million psi. So to determine if the I beam has greater or lesser capacity than the wood, you need to know the yield strength of the wood, the exact dimensions of the wood, and the yield strength of the steel. All that said, are you sure you even want to consider replacing existing joists that are sagging with steel? That would be a large job, and unless there is a real structural problem, may be an unncessary expense.
in such a small application wouldn't an engineered wood product be better / less expensive? assuming that the existing structure is indeed replaced that is.

i hated taking structures...

05-27-2010, 10:46 AM   #6
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I am not replacing the joists. This is just for extra support. The previous owner only had a 4x4 pine beam. I just wanted to know if the steel I-beam (which Daniel clarified the size) would be stronger than the (one) beam I made up of 3 2x6's nailed together. By the way, thank you everyone for helping out with all this info.

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