PLF Calcs for LVL Beam

[CCCBuilder]

Hey all. I'm wanting to learn how to properly size an LVL beam when the specifics do not simply fall under the manufacture's generic span chart giving two options for premade spans. I believe this is the PLF rating? It seems a little too straightforward using the tech sheet so I want to make sure I am calculating correctly.

The situation is as follows. A LVL beam to be installed to support ceiling joists that are *not* continuous, as in they tie into the LVL beam. The ceiling joists are for two rooms, each room for this example has a length of 14' so the "Span Carried" would be 28' but technically when I run the PSF I would halve this I assume?

The ceiling joists have 5/8" drywall - let's say 2 layers - and then attic space above with blow in insulation. The ceiling joists are 2x6, 16" OC. I was going to use 20PSF for this rating. 5PSF for attic dead load of insulation and 2x6 framing (stick framed with rafters, ridge board, etc and not webbed truss), 5PSF for two layers of drywall/mud, and 10PSF for what would be live load of someone crawling around in attic... I'd say that is more than 10PSF but online searches seem to treat person in attic as 10PSF?

The LVL beam needs to span 22'.

For the first part of this, I am taking 20PSF x 14' (half the 28') to get my PLF of 280. I then divide this by 2 and 3 to find my individual PLF capacity needed for 2 and 3 ply of LVL EDIT:If using a chart for single 1-3/4" only. In this case, the Georgia Pacific GP Lam LVL tech sheet has 1, 2, 3, and 4 plys already built in. I scroll down to the Allowable Uniform Floor Loads because it uses at least L/360 in this case since it is drywall that would be sagging here. When I comb through the chart, I locate 22' for the span and go across on the "Two 1-3/4" GP Lam LVL" (and three) to find the first LVL that can do 280 with 2 ply is a 16" LVL. and the first LVL to do 280 with 3 ply 14".

Can someone please tell me if this is the correct process, assuming I had a correct PSF to start with? I'm just looking for confirmation that I am not missing something here. It seems too easy simple, and I'd also like to know of any quick formula perhaps to convert the PLF rating of the LVL for less deflection like L480 when their chart maxes on 360. I was considering using the percentage change from their roof PLF (240) to their floor PLF (360) and multiplying that ratio to the numbers but I'm sure that's just shooting in the dark.

 

[Luceid]

So wait, are you building an LVL out of multiple individual plies yourself? Please clarify

Like normal wood, LVL’s are also given allowable properties for bending, shear, etc so you can just run through the NDS capacity calculations like you wood for a normal wood beam. However if you are assembling an LVL yourself, you need to be careful about how the layers are attached to each other.

 

[phamENG]

I'm not familiar with the specific tech sheet/load table you're using, and you're loads are probably wrong (unless it's a really low attic space), but the approach is generally correct. No, no good way to extrapolate a different deflection ratio.

Big caveat - if you're not 100% sure you know how to read the load and span tables, the loading isn't simple and uniform, and/or you want some criteria specifically shown in the table, let the supplier size it for you or hire a structural engineer to do.

Luceid - fastening together multiple plies is the normal practice and is required once you reach a certain depth - 16" I think. Solid LVLs wider than 1.75" are pretty tough to come by.

 

[jayrod12]

Besides the cautions noted here, I want to specifically note that it appears you're dividing your total load by 3, to get the load in each ply, but then comparing that 1/3 load to the load ratings in the charts for a 2 ply or a 3 ply beam. That is incorrect. You would compare your total load going onto the beam, with the total allowable load specified in the table for a 2 ply or 3 ply beam.

When I've designed beams supporting attic only, 20 psf total load is where I'd be. however if it supports any roof structure at all, even via knee walls, then 20 PSF is essentially only half of the proper design loads.

 

[Luceid]

Ahh, good to know, thanks pham!

 

[CCCBuilder]

@Luceid - Building LVL beam consisting of multiple 1-3/4" LVL sorry if plys was confusing there. I traditionally follow the LVL manufacturer specs for fastening them together

@phamENG - I meant to link this ONE. I am a little more familiar with this companies' tech sheet. What do you mean by loads are probably wrong unless it's really low attic space? Please share! Are you talking the 10PSF live load factor? I'm here to learn and make sure I understand.

@Jayrod12 - That was my mistake in typing late and mixing the PDF's. The PDF I linked in this post for pham shows individual 1-3/4" only which is why I was originally doing the math and dividing by plys at an earlier time. So as a correction if using the GP Lam sheet from original, no dividing the 280PLF! In which the correction is two plies of 16" for their rating of 315PLF or 3 plies of 14" for their rating of 320PLF for span of 22' with L360 rating.

This is all just for the beam supporting the figure of 20PSF for attic load and not roof load. I would like to also be able to calculate for example if this beam did that load and then had some 2x4 braces coming off the ridge board on the older stick-framed roofs with rafters and the collar tie system. I'd like to calculate the dead roof load of shingles, sheathing, framing but more importantly snow load during those times it is wet and heavy. Our region is specced out for 20PSF snow load. However, the calculation seems much more complex but I believe I have a rough idea to figure it.

The roof is not relying solely on this one beam but the ridge board is getting some support/help from this beam during those snow load times. I assume I would take the horizontal (not sloped) depth/width of the roof, divide by 2 to gather what the ridge board could potentially be bearing (even though it's not a ridge beam). Then I take the length of the ridge board and divide it by the number of 2x4 braces coming off of it going on load bearing walls or beams, then multiply the roof dead load + live snow load by the number I got when dividing ridge length by number of braces.

For example, assume the house is 30' wide. The rafters are 2 or 3/12 slope so it's fairly flat. The ridge is down the center of the house, so taking 15' as the total amount of length the ridge board could possibly have weight from. The length of the ridge is 40', and assume it has 4 supports, one every 10 foot. One lands on this LVL beam, so taking 30' / 2 = 15 x (40/4) = 150 SQFT of roof area on that 2x4 support on the beam @ 20PSF Snow + 15PSF roofing material (I've read 12-15 is common?), so 150 x 35 = 5250 pounds of point load potential on the 2x4 brace? Which I come up with being possible assuming the 2x4 is both short and of high grade/quality. Am I anywhere near close - I realize the square foot calc isn't right because technically the roof is at a hypotenuse with the slope causing more surface area per lineal foot, however most 'Roof Beams' seem to only care about horizontal length not total length with the slope? I'd like to know how to convert that point load from the ridge board brace to PLF for sizing of a beam, but I'm assuming that isn't really possible.

 

[jerseyshore]

You have a lot going on here.

First you need to confirm the loads with whatever code you're using. Live, dead, snow. In most places I've done work, 10 psf attic LL is for areas without storage, 20 psf minimum LL with storage. 10 psf attic DL is pretty typical as well. I almost always assume storage unless there is no access because the reality is people love to throw crap up there, even when the height is low.

And it sounds like you want to king post the ridge down for support, is that right? I do see that a lot, especially with lower sloped roofs.

 

[CCCBuilder]

@jerseyshore

Yes indeed! It's not a storage attic whatsoever (aside from insulation and dust, lol). Around here we have basements people normally use for storage. The roof is so low slope you can barely climb around and the attic entry is tiny. So that said, if 10psf LL for human is good, I'll keep that. I saw 5/8 drywall was 2.2psf and to be safe I doubled that in case someone just attached another layer in the future. Around here they do that to avoid scraping off texture, etc. So 4.4 there plus giving the 2x6 ceiling framing and insulation/wire the other 5.6 to come up with 10DL.

I am not familiar with the term for that ridge 2x4. Some call them kickers because the fact is they sometimes are not very structural members, typically just scrap 2x4 that may even be spliced together when not long enough but in this case for this beam it's 1 solid 2x4 about 24"-30" and that could potentially shorten as the LVL beam grows in height. I ran the numbers on that single short unbraced 2x4 and come up with 5400# bearing capacity if it's #2 and Hem-Fir.

I'd like to see/learn how that 2x4 (king post) point load can be calculated to the beam. I didn't think ridge boards had king posts but instead just ridge beams. But I would believe it matters where the point load of the 'king post' lands on the beam in terms of how close to the beam's two end supports? But is there a common practice to just assume it lands in the worst spot possible such as the middle of the 22' LVL beam and work with some safe numbers from here? The ceiling PLF is easy, it's load spread evenly out on the LVL Beam. But this rafter king post/kicker is a point load applied to the LVL beam at a very high PSI.

The roof loads would be 2x6 rafters 16oc, 5/8plywood, 1 layer asphalt shingles, and 20PSF SL for that winter duration. The roof is low sloped, 3/12 so that is the concern for when that 2x4 king post has weight in winter months, not if.