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ANSWERED on Fri 7 Dec 2007 - 4:49 pm UTC by redhoss

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Hello,

I have an old car wash building that has cinder block walls and a corrugated metal roof. The block bay walls are approx 15 feet apart. There are two 4 x 12 wood beams that span the 15 feet between each bay. I would like to replace the 4 x 12 wood beams with steel I-beams to hopefully gain some height clearance. The roof is quite low, and the two wood beams are the lowest obstacles.

Again, I am not looking to change or add to the structure, I just wnt to directly replace 15 foot sections of 4 x 12 wood beam with the smallest steel I-beam that will be of equal strength.

Thanks, John

Myoarin,

Thanks for the comment. I am hoping for Redhoss to grab my question. I found this site while trying to track him down.

The existing beams are regular wood, not laminate. The roof is just a slightly pitched corrugated metal roof, and I am in California, so no snow.

Thanks, John

Hello John, while Myoarin brings up some very good points, I think that I can give you what you want. You probably don't know the exact wood used in the beams. We will use some conservative values that should yield a safe replacement. I recommend using an allowable bending strength of 1,000 psi and a modulus of elasticity of 1.0 x 10^6 psi for the wood beams. Steel beams will have an allowable bending strength of 30,000 psi and a modulus of elasticity of 30 x 10^6 psi. You can see that in both bending strength and modulus the steel beam is about 30 times as strong. Next we need the moment of inertia (I) and the section modulus (S) for the 4 x 12 wood beam.

I = bd^3/12 = 4 x 12^3/12 = 576 in^4

S = bd^2/6 = 4 x 12^2?6 = 96 in^3

We can divive both these numbers by 30 to get the steel equivalent. So, we are looking for a steel beam with:

I = 19.2

S = 3.2

A good choice would be a W6x12 with:

I = 21.7

S = 7.25

Or, you could go with a W5x16 if you want a slightly heavier beam that will save another inch of head room. I don't find a good 4 inch deep beam. I believe that this will answer your question, but please ask for a clarification if you want to discuss further.

Redhoss

Hi Redhoss,

Thanks for the response! I am glad you answered! I am not very familiar with I-beam terminology. What are the overall dimensions of the two choices you gave? Like I said, I am trying to gain height clearance. Is the W6x12 6" tall? I tried to do a quick google search, but didn't see any specs come up.

Thanks, John

Redhoss,

Please ignore my last request. I found a site with the dimensions. I did see a W4x13. Is that in the ballpark as well or not even close?

Thanks, John

Sorry I left out one of the most important things. As you found, a W6x12 is 6" deep and weighs 12# per foot. The W4x13 does not meet your criteria. Is it ballpark, well maybe. It has these properties:

I = 11.3

S = 5.45

This tells me that it is adequate in bending strength but will deflect more that your 4x12 wood beam. Deflection would increase by 19.2/11.3 or 1.7 times what you see in the wood beams. If this is allowable for you, then you may be satisfied with the W4x13. As Myoarin suggests, the wood beams may be over designed.

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myoarin

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7 Dec 2007 11:42 UTCFri 7 Dec 2007 - 11:42 am UTC

Hi John, just a coment:

I hope Redhoss, the expert for this type of queastion, comes around soon.

I expect that he can find the I-beam dimensions equivalent to the present 4 x 12 wood beams, but he may want to know more about the present beams (one piece, laminated, whatever) and the roof structure and maximum load (snow?).

Since you want to maximize the headroom, if the present beams have a very generous safety factor, adequate steel replacements could possibly have smaller dimensions than those needed duplicate the strength of the wooden beams.

Let us know.

Good luck, Myoarin