Residential floor beam - 24' LVL with 1" deflection 3

[ThorenO2]

Hello,

I'm designing a house where the architect wants "open concept" living for the owners. This has resulted in a 4-ply 16" LVL spanning 24' picking up 30psf LL from 2 upstairs bedrooms and part of a hallway. The LL on the beam amounts to an incredible 11,000 lbs, where I've justified a LL reduction - no way can they fit that much stuff in the space. HUD's Residential Design Guide; however, uses a formula limiting the reduction to 25%, which still leaves tons of weight to design for. My question is, what are the opinions on designing to a 1" total deflection (L/288 < L/240) and 0.6" LL deflection (L/476 < L/360), with my opinion wavering between "no sweat - the calc meets code" or "that's going to leave a low spot in the floor at the adjacent joists". I appreciate any insight - thank you.

 

[SlideRuleEra]

Why not use steel instead of an LVL?
Say, a W16x26 which would be both lighter and no larger than the 16", 4-ply, LVL.
I did a quick upper bound check on deflection (11 kips point load at the center of a 24', W16x26 simple beam) and got deflection of only 0.63 inches.

http://www.SlideRuleEra.net

 

[dik]

agree SRE. I wouldn't have thought of wood framing for that sort of span.

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[ThorenO2]

Thanks SlideRuleEra, “there’s always a solution with steel” as AISC says. I have W16 basement beams on concrete walls & steel columns, but elected to use wood for the upper levels.

 

[phamENG]

16 to 18 feet is where I start thinking about steel if I have a second floor above. Steel can be supported in wood.

What's the rest of the framing doing? Is it framing into this beam, or parallel? If parallel you have a problem.

Limiting reduction makes sense. Walls tend to attract load in houses. Line that wall over the beam with dressers, a large bed, etc., and you can build up some load fairly quickly.

Your numbers don't look terrible, but it will be noticeable. I can't cite a source, but I've always tried to limit deflections to 3/4" where there's a rigid ceiling and no floor topping. If you can adjust a drop ceiling or pour a lightweight leveling compound is not a big deal. But there's no hiding it with drywall and hardwood floors.

 

[ThorenO2]

Thank you Pham, it’s got an I-joist bay perpendicular and a partial bay parallel coupled with another LVL beam framing into the side using a high-capacity Simpson face-mount hanger (watching the side loaded fastening adjacent to point load). The loads you mentioned don’t necessarily add up to 11,000 lbs however. My interpretation of the residential code is 40 psf and 30 psf LL is considered by HUD’s publication as already “conservative” when compared with published studies on historical residential live loads which average much, much less. However, with the parallel condition, I agree with you that it will be problematic (the beam framing into the side supporting the parallel joists would all deflect in unison and the hallway will wind up with a low spot of 0.4” from DL alone. I will need to change this out for a W16 and rework the beam hanger. Thanks everyone,

 

[XR250]

I def. would not be spanning (4)16's that far. As others have suggested, use some steel. My experience has been that LVL's seem to sag more than the numbers would indicate.

 

[phamENG]

Thoren - I agree it won't add up to 11,000lbs, and you may be able to use some judgement in this case. I have a copy of that HUD document somewhere, but I haven't read through it in detail. If I remember correctly it's a pretty general document, and is probably more applicable to lower and mid-range tract home style construction. Not to say it isn't valuable here, though.

If you have 11,000lbs of live load with a live load of 30psf, that means you have a tributary area of 366.67sf. So for an interior beam with K_LL=2, you meet the influence area requirement for reduction. Using the live load reduction from ASCE 7, you'd only be able to reduce it by 19.6% anyway.

Compare that to this breakdown of the survey statistics: I have ASCE 7-10 on my desk, so that's what I'll reference here but I'm pretty sure 7-16 has something similar in the same place. The bottom of the last page of the chapter 4 commentary is Table C4-2 Typical Live Load Statistics. The average survey live load in a house is about 6psf. That's all spread out. The mean maximum load is 38psf for a 200sf area. So you have (366.67sf-200sf)*6psf+200sf(30psf)=8600lbs. 8600/11000=0.78, or a 22% reduction. (Note that this isn't an "approved" method for reduction - just a way I like to visualize the reductions allowed in the code.) You end up pretty close to the chapter 4 live load reduction formula, and still under the HUD upper limit on LLR.

 

[ThorenO2]

Pham, appreciate it greatly again - I like the second calc with the mean maximum loads for visualization. I linked to the HUD doc in the original post. I definitely made a bad decision to try to get wood to work and am working on the re-design. Structural engineering is a very humbling occupation for me. The HUD LL reduction actually uses "10.6" instead of the "15" which returns a value of 0.25 + 10.6/(SQRT(2*367)) = 0.64, but it is limited by 0.75. I found a nice weldable hanger, the Simpson HGLTV for the beam framing into the side and the joist hangers are straight forward. No issue with bearing a steel beam on a built-up (5) 2x6 post although I would like to have a positive attachment. Do you know of any means for this beam-column connection?

 

[phamENG]

My pleasure. I usually detail it so there's a steel plate welded to the bottom flange with bolt holes in it. Since you'll be sitting on the top plate and be fully flush, you can put your built up column directly under the web, then make the plate wide enough to have a pair of through bolts clear the column below. Don't forget to provide rotational restraint at bearing. If you're packing the web tight that's usually sufficient, but your talking about welded hangers so you'll probably need either an end plate or web stiffeners at bearing. If you don't detail it, no residential builder in the world will put it in there.

 

[ThorenO2]

Pham, great I like it - it's like a pair of extended ears with holes to bolt through the top plate. Definitely need the rotational restraint from the concentrated point load coming from the other beam.

 

[phamENG]

That and AISC 360 J10.7 Unframed Ends of Beams and Girders: "At unframed ends of beams and girders not otherwise restrained against rotation about their longitudinal axes, a pair of transverse stiffeners, extending the full depth of the web, shall be provided."