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I am building a new house. It is a simple 2016 ft2 rectangle. 42X48 feet. One single story above a basement. I need to maximize head room in the basement. Two steel beams will span the 42 foot dimension. The beams will sit on 12" filled concrete blocks on each end. Support posts are planned 12 feet from each end. So the longest free span would be the 18 feet in the center of each beam. The beams would be placed 16 feet apart. What is the smallest size H beam that would work for this project? I am hoping 8" or smaller. I appreciate your input.

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  • That's more an engineering question than one for a DIY site. Ask your supplier. No way would I build a house on the word of some schmoe on the internet.
    – isherwood
    Nov 9, 2023 at 20:58
  • You need an engineer to figure out what the actual load on this beam will be and from that derive size
    – keshlam
    Nov 9, 2023 at 20:59
  • I can say that it won't be 8". That's absurdly short.
    – isherwood
    Nov 9, 2023 at 20:59
  • It will be the size specified by an engineer, who you pay, and who is bonded against design failure.
    – Tiger Guy
    Nov 9, 2023 at 21:32
  • If building a house, it might be easier to have a higher basement, than worrying about smacking your head on support beams. Most places require engineer drawings for building permits, they will state minimum size of beams.
    – crip659
    Nov 9, 2023 at 23:08

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For a single floor's loading and assuming simply supported boundary conditions, there's a 6" deep wide-flange section, W6×25, that passes for bending strength, but it fails because of excessive deflection. I didn't bother with the typical compactness checks for bending because the W6×25 is such a beefy section and it ultimately failed.

For bending strength it had a demand to capacity ratio of 80.%, so that's nice. Under the live load's 40 psf, however, the W6×25 deflects 0.98" compared to the IRC's floor deflection limit of L/360 = 0.60". That's a fail.

You're looking for a 50 ksi steel section with a Zₓ (plastic modulus) of at least 15.2 in³ and an Iₓ (second moment of inertia) of at least 86.9 in⁴. These criteria are necessary conditions. They're not sufficient conditions. Any section that you find meeting your geometric constraints and these criteria still needs to be subjected to a gauntlet of other checks (transverse stiffeners may need welding at support positions, for instance). Here's a database of section properties. The W8×28 satisfies the two necessary conditions.

My simply supported boundary conditions could be pessimistic, though. This assumption implies that the beam is free to hinge at its supports. If each beam spans the whole structure with no bolted splices, then the W6×25 could possibly work. Even if the middle span had a chunk of beam sticking out a ways, the W6×25 could possibly work. CJP welds on a residential structure sounds pretty crazy, but the beam free of bolted splices is a potential solution to bring the W6×25 into spec.

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