Help with Beam Design
Help with Beam Design
(OP)
I am trying to design the steel beam shown in the attached sketch. The beam span is 34'-0" and is loaded with 300 pounds per foot from the floor (one side only) and with 700 pounds per foot on the top flange from the roof framing (snow loads included). Because the top flange is unbraced I am coming up with a W30x99 (Fy 50)!! I don't think I can get the floor diaphragm to brace the top flange because the connection would have to act in tension or compression depending on which way the beam wants to rotate. Any suggestions?






RE: Help with Beam Design
Alternatively you could use a lower beam with the top flange at floor height.
RE: Help with Beam Design
Are you implying that full height stiffeners will act as a brace?
RE: Help with Beam Design
RE: Help with Beam Design
RE: Help with Beam Design
RE: Help with Beam Design
RE: Help with Beam Design
I agree with UcfSE and would go with the unbraced length and keep it simple. But I did do a quick calc for this (ASD) with 1.1 k/ft and unbraced length = 34'. With W18x86 I get fb = 11.35 ksi, Fb = 13.67, and deflection ~ 3/4" (L/544). Are you trying to get less deflection? Concern for torsion?
RE: Help with Beam Design
You should be able to install a network of full height vertical and horizontal stiffeners on one side and weld straps periodically from the stiffeners to be nailed off to the plywood floor diaphragm. Depending on how often you install the vertical stiffeners and straps, that should establish the laterally unsupported length. With a 34'span and a 16" joist spacing, a max of 5'-4" should work.
I wonder how you will address (finishing)the flange width at the intersection to the top wood bearing wall of the W30 you mention which is 10.5" as opposed to a 5.5" wall with sheetrock/plywood? I think that you are going to have to comeupwith a beam with a smaller flange width for the Architect to buyoff on it.
Upon looking at the numbers you presented, the load is 1 kip per foot with a 34 foot span, generating a simple span moment of less than 145 kip feet. If you are braced at midspan, a W18X50 should work with a 7.5" flange, but have a 1.3" deflection (L/316)(AISC 13th Edition Table 3-10, page 3-125. Am I missing something here?
Mike McCann
McCann Engineering
RE: Help with Beam Design
RE: Help with Beam Design
And I'd worry that the vertical stiffners might someday get "mis-perceived" and removed, altered or otherwise made ineffective.
Then we'd have "fall-down-go-boom" mode of failure.
RE: Help with Beam Design
RE: Help with Beam Design
Not almost anymore!