Replacing an external beam with an internal beam
I have a living room that used to be 10 ft wide and 30 ft long, a previous owner enclosed the porch making the living room 20 ft x 30 ft, then added another porch. where the old exterior was was they added an interior 28 ft beam (2×12’s) supported with 3 columns, on top of the ceiling rafters is a 2×4 wall to the roof rafters. The house was stick built in 1964
I want to replace this interior beam with a hidden beam. I paid an engineer for a drawing and specs. He specified 3 = 1-1/2 x 16″ x 30 ft LVL beams, supported on both ends with 4 sheathed 2×4’s and 2×4’s from the beam to ceiling joist every 6 ft. and a 2 x 4 wall on top of the beam to the roof rafters.
I checked today and I cant order 1-1/2 LVL beams, they are 1-3/4. I could use them but I checked a 1-1/2 x 16″ LVL load and a 1-3/4 18″ LVL and it looks like I can carry more load with 2 = 18″ 1-3/4 than 3 = 1-1/2 and the weight is only 30 lbs more.
Questions:
Does anyone see any thing wrong with my logic?
I assume 4 = 2×4 sheathed supports mean plywood sheathing on the outsides. I cant fit that inside my 2×4 wallboard walls. Does it matter if I just nail the 2×4’s together and cover with wallboard?
I live outside the city so there are no codes but I don’t want to skimp.
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Replies
If you hired an engineer, all of your questions concerning the engineer's recommendations and any possible changes to the engineer's design should be directed to the engineer, not to this web forum.
-- Martin Holladay
Martin's right about talking to your engineer, but for general knowledge, the limiting factor for sizing most beams is deflection, which, all other things being equal, is governed by the moment of inertia--a way to describe the shape of the beam. The moment of inertia for a rectangular shape is (width x height³)/12. A 2-ply 18" beam has a 5% smaller moment of inertia than a 3-ply 16" beam, so it will deflect more under load.
The absolute largest load a beam can carry before catastrophic failure is often governed by how much stress is in the lowest face of the beam, and in that case the (2) 18" beam can indeed carry more than the (3) 16" beam (half the height of the beam divided by the moment of inertia), but you shouldn't be worried about when the beam will break--you want to limit how far it bends. You could look at it like this: a 3 1/2" x 18" beam is stronger, but a 5 1/4" x 16" beam is stiffer, and that's more important.
Load tables like this one show the same result: http://www.weyerhaeuser.com/woodproducts/document-library/tj-9000.
That's all assuming a perfectly centered, vertical load. If there is an eccentric load, meaning off-center, the equations change because the beam then needs to be prevented from twisting.
Sheathing the built-up column is to make the column "laterally supported," meaning it is braced against buckling. I'm not clear on what you are describing for the additional 2x4's at the beam.
None of this is meant to keep you from calling your engineer, but to explain why it's important to clarify with him or her what their intentions were.
I had always planned on calling him and I did, I also found that he made a mistake on the measurement. The wall to wall measurement is 30' He forgot to account for the inside of the wall 4" inches on each end. He also said I could sheath the 2x4's with wallboard. I just wanted other opinions.
He's also checking if I can use 2 x 18" beams as opposed to 3 x 16"