Diagonal Loaded LVL Sizing

[cfcadman]

Hi Everyone, Complete Newbie Here to the Forums!

I have a rather interesting situation sizing a roof support LVL and am hoping somone can help me out. This is a new design, not a retrofit and involves an LVL supporting a garage roof.

First off, the LVL is free span inside, the roof has a 1 in 12 slope, essentially a flat roof. Unfinished inside; shingled exterior with a 35lb max snow load.

The outline of the walls are 24' x 20' rectangular. The LVL will run diagonally from one corner to the opposite corner, a run of a little over 31', and rest on steel and concrete filled block. The stringers on one side of the LVL are 2x10's on 16" and extend perpendicular to the LVL and rest on both the "short" and "long" walls on that side; they also cantilever 2' to 4' to form an overhang.

The stringers on the other side of the LVL are 2x10's on 16" but extend from the LVL perpendicular to the opposite "long" wall they rest upon. They are parallel to the "short" wall on that side; no overhang. The short wall bears no load.

I have been quoted a 1 3/4" 4-ply 18" tall LVL will support this load but I have doubts. Two fastened together to achieve 3.5" x 18" x 32' sounds more realistic. Of course I want to play it safe rather than sorry. Does this sound adequate?

Thanks Guys,
Cadman

 

[boo1]

Simplified Formulas:
Bending Fb=Mc/I
Shear Fv=3V/2db
Defl =(5*w*L^4)/(384*E*I), For constant w

Most likely deflection will govern
Connections and ponding must be considered

 

[FSS]

I must be missing something. Why are you running the LVL beam from corner to corner? Are you trying to frame rafter directions around the cantilever shape?

 

[RHB51]

First; if you really want to run the lvl diagonally then
check with Trus-Joist; they have a wealth of information of the design of their lvl. They are also commonly found.

Second; if I understand you correctly why do you want to
run it on a diagonal? Have you considered other options?

 

[cfcadman]

Hi Guys,

The reason the beam is diagonally run is that while the entire roof surface is slightly sloped to one side, half the roof (about the diagonal) rises several inches ala butterfly roof, but draining to one corner. Like half the decklid of a 1959 Chevrolet. Additionally, I'm trying to keep the non-bearing short wall completely open below.

I'm intrigued by boo1's formulae. I have found the LVLs are 1.8E Tremacs with the following:
Fb=2575.3
EI=1530.90 x 10^6 lb-in^2
Max Shear=5250 lb
Max Moment=20,280 lb-ft

Am I correct in that L = 32' ; w = 20'
Thus (5*20*32^4)/(384*1530.90)
1.0485 x10^08 / 587865.6
= 178.37
Is this lb-ft?

 

[boo1]

Your units are incorrect
Calculate total load on the beam in pounds per linear foot (plf).
Convert units to inches
I=851 in^4

Consider finding someone with professional experience in your local area to help you. They know to check and specify: Snow Area 115%, Max defl=L/360, lateral bracing, bearing, uplift connection, beam fastener pattern

 

[UcfSE]

Cadman,

Are you asking for us to help you design this or asking us if a design someone has already done for you is reasonable? When I read your post it sounds like the latter to me, but reading these other posts sounds like they took it as the former.

Now when you say you have been quoted a 4-ply LVL, it seems you are taking that to mean the total width of the beam is 1.75 inches. A single ply LVL is 1.75 inches, so a 4-ply LVL would be 4*1.75 = 7 inches wide by 18 inches tall. Have you confirmed that you are actually being quoted a 7-inch beam (4-ply) instead of a 1.75-inch beam (single ply)? That might clear up a little of this for you.

At a span greater than 31 feet, I would expect the beam to have more of a deflection problem than a strength problem. You can ask the person who quoted the beam to provide you with how much the beam deflects. Standard numbers for maximum deflection range from 1/240 of the span to maybe 1/360 of the span, depending on ceiling construction, stability problems, or other things. If it makes you uncomfortable after you verified your information, you could ask for less deflection but that would of course mean adding another ply or switching to steel.

The deflection equation that boo gave you is only for a uniformly distributed load for a simple span. With the beam running diagonal, your loading is most likely not uniform so this equation is no longer valid. Just FYI, the w in that equation is the amount of load per unit length of the beam, so it is force/length units. The result is a deflection, so its unit is length. Also, the EI you gave us seems a bit low. 1530 ksi is more on the order of E by itself. My LVL catalog lists LVL's as having E of 1900 ksi, or 1.9*10^6 psi.

 

[HouseBoy]

Some thoughts:
-I assume you are trying to see if something lighter than the 4 ply 1-3/4"( 7"total thickness) by 18 inch LVL would be acceptable, trying to save on material costs. Because of the long span, a steel beam might actually be cheaper to buy and install. With wood blocking bolted to the web, it might be just as convenient to install the rafter framing as with the LVL. Of course, a lumber supplier wouldn't choose that first but I would expect he'd understand if you wanted to consider an alternate material for cost reasons.
-Shingle roofing on a 1:12 pitch seems like a problem.
-The loading cases you have are a triangular load on one side with a max at one end of about 320 plf and a "peaked" type loading (2 trianglar loads with the high point off center) max about 340 plf. You could superimpose these loads to design
-I'd expect deflection to be an important consideration and might govern the design.
-This is probably not the best forum to be getting free advise on this situation. Ask the designer why they think a 2 ply or possibly a 3 ply beam wouldn't work. If they tell you deflection, I'd believe it.
BTW, I'd guess 2 ply doesn't work for bending, much less deflection.

 

[cfcadman]

UcfSE,

Really I was asking if the quoted 4-ply LVL is reasonable, but at the same time I was curious to see how to prove it out for myself. Sometimes the prodding by folks who know what they're doing causes a bit of inspiration on my part, which has already occured here!

You're absolutely right about the misunderstanding with each ply being 1.75". After getting my hands on the specifications yesterday this is all much clearer; the EI was taken from it. BTW, I wrote Tremac but it's actually Tembec as many of you probably knew.

Now having the 1.8E spanchart, it appears 32' is on the extreme end for a 3-ply 18" beam with 4-ply being more appropriate for a traditional application. This is encouraging!

The loads I calculated for dead weight (assuming even distribution) also fall into the acceptable range according to their charts.

I thank you all for your time and by no means was looking for a handout, but if for some reason one of you sees fault with the design, by all means I would be appreciative to hear about it!

Cadman

 

[boo1]

Houseguy you are correct, you can't shingle 1/12 pitch and expect it to not leak (3/12 is the min). Modified or membrane roof coverings are recommended.