I-beam rather than lvl
I’m in the process of replacing floor joists for the second floor (1.5 story cape).
When built the top floor was just attic space and there were several room below. Many of the 2×8 joists were butted end to end and had 1x lapping them together.
I’ve gutted out the 3 rooms for a new kitchen and will be putting bath above. I’ve used 230n grade 9.5” ijoists to replace the 2×8’s, span is 14’.
1 of the joists picks up a door opening that carries the load from the ridge beam (2nd floor was dormered out on 1 side, two 14” lvl put under orig ridge to carry roof load). There is no path at this third of the ridge to carry roof load directly to earth, so i was going to use a pair of 9.5” lvl (the pair of 7.25” lvl’s already there have sagged near 1”).
After looking at it I’m thinking of using a 6” or 8” (S6x12.5 or S8x18.4) I-beam. I’ve no idea how to determine if the 6” is sufficient to exceed the pair of 9.5” lvls. I welcome any and all comments and suggestions. Thanks
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Replies
mech644,
Unfortunately no one here on GBA should be sizing or suggesting structural fixes beyond giving very generic advice, or suggesting where you could go for help obtaining it.
What Malcolm said.
This is a very specific engineering question that should be answered by a local, licensed engineer or architect.
A lot of 6" steel I-beam were used in Twin Cities homes built in the 1950's. The basements were only about 7' deep and the I-beams were used in supporting the floor joists, retaining the maximum basement head room. This is not scientific, the spans between supports were about 8' as I recall. Steel is strong and I have used it occasionally in new construction were wood did not suffice. Have married wood floor joists to a steel beam to create a wider open span. The engineered wood beams are quite astounding too but generally are quite wide and deep to carry the same load as a more shallow steel beam.
Doug
Malcolm and Peter already pointed out that you need an engineer here. I wanted to add a bit of the "why". All those loads you're transferring around are additive, and need to go all the way down to the foundation. When you put loads onto things like central beams that rest on columns (as is almost surely the case from what you're describing), you are concentrating the loads of a large area onto a small point at the base of the column. You're likely to need to beef up other things besides just the beams you already have, especially if your existing LVLs are already sagging -- which indicates a significant load! An engineer will be able to look at your specific situation and advise you on a solution that will be safe and won't just be moving a problem from one place to the other.
Note also that that it's common to use wide flange steel I beams in residential construction, which start with a "W". Ultimately the engineer will determine what you need. I do recommend you order your steel with primer pre-applied by the steel supplier though.
Bill
It's always interesting to see what people come up with as gut feelings for beam sizing. I have a BS in structural engineering and have worked as a carpenter or designer for 30 years, doing most of my own engineering over that time, including many steel beams, and occasionally working with licensed engineers. I would never size a steel beam without running the numbers, which is quite simple if you know what you're doing, and impossible if you don't. A licensed engineer should be able to design that for you pretty quickly.
If the only licensed engineer in this area that does residential would return a call I’d have already posed the question and paid for the answer. But my bad for not including that in my initial post.
The door frame (header of 2x 7.25 lvl sitting 2 posts that are 2x 2x4lsl) only has 1 post that is bearing on the joist in question, other post is bearing on framing that goes straight to interior conc bearing walls in basement. End of said joist is also more than adequately supported in the walls it bears on, direct path to earth on 2x 2x4lsl blocked to adjoining studs with 2x at 1/3 and 2/3 height.
How about this question, what size I-beam will double the L480 of two 9.5” lvl at 14’ span?
For deflection two things matter. The shape factor of the beam, referred to as moment of inertia, and how stiff the material is (modulus of elasticity).
You multiply the two together, gives you the flexural rigidity of the beam, and it be used for comparison of two beams.
For example a 1.8 LVL (so modulus of 1.8 x 10^6 psi) that is 1.75 x 14" tall has a polar moment of 400 in^4.
1.8*400=720
A W 8 x 15 beam has an moment of inertia 48 in^4 and 29 x 10^6 PSI modulus of elasticity.
48*29=1392
So a W8x15 steel beam is twice as stiff as that single ply 14" LVL. This means for the same load, it will deflect half as much.
You can do this math to compare other beams the same way.