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I'm removing a load bearing wall and the structural engineer recommended a W8x15 steel I-beam. It would save me a lot of effort if I could use a 6" beam instead. So I'm trying to calculate the deflection of the W8x15 beam and then find a W6x?? beam with similar deflection.

I've found the specs for various wide flange beams here, but I can't find a max deflection equation that uses the elastic section modulus provided in the specs.

So, can someone point me in the right direction?

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    Not an engineer, but if I read that chart correctly a W6-25 should be equally or more robust. You should talk to your engineer.
    – UnhandledExcepSean
    Jun 13, 2019 at 14:49
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    I suppose there was a reason you hired, and presumably paid, and engineer for his recommendation. Generally an engineer will specify the minimal-sized components to support the loads. So you now want to save money by using something smaller than what the engineer specified. That doesn't sound wise to me. Why don't you go back to your engineer and see if a W6 could be used instead of a W8?
    – jwh20
    Jun 13, 2019 at 14:50
  • Not sure what you are doing .Or how far a span you want open. Could you live with one center post. Or not a option . And if you go to the W-6 , do you loose ,head room. They do have a blind header. were the beam goes post to post. And the framing hangs on that,giving you a flush ceiling look and you do not see beam. Works on 2nd floor. with of beam goes into attic.
    – user101687
    Jun 13, 2019 at 15:14
  • What is the span?
    – Lee Sam
    Jun 13, 2019 at 15:26
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    there is a possibility that an insurance company could use the fact that you did not follow the engineer's design to void your claim if something happened to the house .... you may want to have the engineer certify the use of the new beam
    – jsotola
    Jun 13, 2019 at 19:19

1 Answer 1

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I think I figured it out. I was thrown off by the Elastic Section Modulus.

Anyway, the deflection is calculated using the following parameters:

W = load

L = length

E = Young's Modulus

I = moment of inertia

And the equation for max deflection is:

a

I'm trying to find a beam with the same maxdef or smaller. I can ignore W and L because they are the same regardless of I-beam. Young's Modulus E is a property of the steel used, so it is also the same regardless of I-beam and can be ignored (this is what I misunderstood). This means, the only variable I have to worry about is I. I can see in the equation that I is part of the denominator, so if I increases, deflection decreases. So I just need a W6 beam with a larger I than what the W8x15 beam has.

Final answer: The only W6 beam with a greater I than the W8x15 beam is the W6x25,

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  • If something happens it'll all be on you. The engineer didn't sign off on this. You still need to go back to him.
    – Nelson
    Jul 8, 2020 at 3:59
  • The responsibility is all on me anyway because I didn’t pay the engineer to produce proper drawings because it would triple the cost of the beam project. That was also his recommendation for a DIYer. I did confirm my changes and math with the engineer so this answer is the correct answer and the downvotes are wrong. I understand the comments questioning my judgement and I think they are healthy. But downvoting the correct answer just creates misinformation.
    – Joe Mac
    Jul 19, 2020 at 15:00

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