Steel Transfer Beam supporting Wood Bearing Walls above

[gramy13]

I have some 30ft long span steel beams transferring out 3 stories of wood bearing walls above. Using, L/360 or L/480 i'm limited to 1" or 0.75" total deflection respectively. I'm leaning towards limiting the deflection to a tighter limit like 1/2", since it is supporting gypsum sheathed wood walls above - but the beam sizes are becoming rather large. What deflection criteria would you use?

 

[DaveAtkins]

I would use L/240 for TL, and L/360 for LL.

DaveAtkins

 

[dhengr]

Gramy13:
And, you could camber the stl. beams for most of the DL. Then, know too, that the framed and sheathed walls will act like deep beams to some extent, depending upon number and location of wall openings; and that it is unlikely that you will ever get full LL, from three stories on those stl. beams at the same time. While you may not want to (or be allowed to by code) apply a LL reduction on the stl. beams for their design, it may give you some confidence that the deflections will not be excessive.

 

[XR250]

I would be surprised if you even got 10 psf live load per floor, so I would not sweat the LL deflection too much.

 

[jfmann]

Deflection should be limited to code-specified limits.........even if only for your own (or your firms) liability.
Take full advantage of live load reduction allowed by code, which will likely reduce floor live load substantially (up to 50-percent), depending on total floor area being supported by beam.

John F Mann, PE

http://www.structural101.com

 

[gramy13]

Thanks for the responses. I am looking in to how much LL can be reduced per code, and will at least design to the code specified deflection limits. I know it won't see all of the load at once - but you have to design per the code. My concern with the steel beam deflecting too much, is that the drywall on the wood bearing walls above may crack? Isn't that a valid concern? The wood bottom plate of the wall above will attached to the top of the steel beam and the drywall will be nailed/screwed to the bottom plate. That's why I was looking to limit the total deflection to no greater than 0.75"...

 

[apsix]

Serviceability is always a valid concern.
However, as it's steel there will be no creep. Therefore you will see full dead load deflection by the job's end, so it's only LL deflection which will contribute to visible cracking. Any cracking due to DL will be repaired before completion.
For extra peace of mind you could specify the drywall immediately above the beam to be fixed last.

 

[SlideRuleEra]

gramy13 - You have been given excellent advice, suggest that you take it. Even a major drywall manufacturer, USG Corp., states:

"For drywall assemblies it is desirable to limit deflection to L/240 (L = length of the span) and to never exceed L/120 (L/180 in some codes). The preferred limit for veneer assemblies is L/360 and should not exceed L/240."

See the second page of this USG publication:

http://www.usg.com/content/dam/USG_...struction-handbook-ch2-framing-can-en-pdf.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[manstrom]

Don't forget that that beam needs to be "individually fire rated" since it supports more than 2 floors. The detail gets tricky with wood framing.

There is a good bet the architect won't call that out.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller