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I work with LVL beams frequently and see a lot of them placed under I-Joists to carry load of the floor. This is usually no big deal in 9' deep basements but in 8' basements an 11-7/8" triple LVL beam takes up a good amount of space. In terms of load, is there any difference in instead putting the LVL beam in the same plane as the I-Joists are, and setting I-Joist hangers on each side of the triple LVL 11-7/8" LVL beam so that the floor joists still use that beam for support, but now there is no "header" or "beam" to have steal some ceiling space? I think in one sense it may be stronger because the beam has a lot less change of twist factor to play where as when it's under the I-Joists it can actually twist one way or another.

I do realize this severely limits accessibility for plumbing, hvac, and wiring though if needing to get into those joist's cavities which is almost guaranteed, but I believe running a small plumbing/hvac/electrical chase down below the LVL beam will take less space than the 11-7/8".

Thoughts? This would not be altering any span ratings, just the method the I-Joists transfer load to the carrying beam.

This is all in question for a space in one basement where the stairs go down and there is a support wall on one side of the stairs once you hit basement level, but an open family room on the other. It would be really slick to have that right side family room have no joist support below the joists for that span for about 10' of the basement. The staircase wall would carry the rest of the floor joist load for the family room space.

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A multi span joist with intermediate bearing (3+ points of bearing) can clear span more than a simple/single span joist (2 points of bearing.) One span benefits from having the other span connected to it.

Your dropped LVL is called intermediate bearing. It is typically sized to L/360 live load deflection, and the floor tributary is 5/8 of the joist span because of the added load from continuity.

When you flush that LVL, you typically size it to L/480 live load deflection, but you only use 1/2 of the joist span as the floor tributary. Obviously you have to cut the joists, so they can't span as far as joists that are continuous over the LVL.

These small differences mean that sometimes it matters if you flush or drop the beam. Just make sure you check the joist span works as a simple span, and that the LVL deflects less than L/480 live so that you don't reduce the floor's performance.

In my experience, going from dropped to flush with the same size LVL (which is what you're doing) works 90% of the time. You still need to check it though.

Doing the opposite and going from flush to dropped can add 1/8 more floor tributary to it, so it's more likely to fail in moment or shear with the added load.

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  • Great to know the proper terms! Why do you size the flush beam for L/480 instead of traditional L/360? I can see a minimal factor being that the I-Joist is now not solid at the center of the house where the beam would be, but I still assume with joist hangers grabbing that 2 to 2.5" bearing required by manufactures, that the I Joist would be unnoticeably the same deflection. I'm just curious.
    – CCCBuilder
    Jun 16, 2019 at 12:54
  • @CCCBuilder Increasing the design deflection improves the subjective feel of the floor. If you do everything at L/360, most people will notice that the floor bounces/vibrates as you walk on it. When you design the floor to be a little bit stiffer, less people notice/complain about the floor. Some plans specify to do L/560 or L/600 because of tile flooring or that it's an expensive house and/or they want the floor to feel extra solid. The deflection changes more than you think it would with a multiple span I-joist vs a single span.
    – Dotes
    Jun 16, 2019 at 14:22
  • I have found that homes with a triple ply LVL beam that span say more than 16' on that beam seem much more springy than the part of the home that is supported under a basement load bearing wall. The common joist rating for 16" O.C. is that the I-Joist can span 16' 8" but I find that is too much. Good to know about the change in deflection for multi-span i joists.
    – CCCBuilder
    Jun 16, 2019 at 17:57
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Yes you can do that been done many times in my life. Flush beam, Some had post, and some went the whole span. Most on a one floor home like a ranch .

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