LVL Design Question

[mfstructural]

I'm designing an LVL to support floor joists and remove a bearing wall. There are several openings they want to make and have the ceiling open. the contractor wants to install one long LVL (18' long). The opening is 115", then there is a closet which the beam will sit on, and then span the opening over the entrance which is 32 inches. I started a thread yesterday regarding sistering the joists. This post is regarding the LVL. The other post can be found here:

https://www.eng-tips.com/viewthread.cfm?qid=469026.

When I run numbers on a s.s. beam for the 115" span they show I need 3 plies of 7.25" LVL (they don't want to go taller than that with LVL since the joists will be 2x8s). By increasing the E to 2.0 and Fb to 2850, I can get the beam to work. The interaction ratio is 1.0 for bending. The deflection is my concern then. Since the beam is not simply supported in reality...it's a 115" span with the second span partially over a closet (bearing for 48") and partially spanning an opening (approximately 32"). Because of this my thought is that the deflection in reality is not really 5wl/384EI, but could be 1wl/185EI because it more resembles a two span beam with one span loaded...I know it's a gray area and judgement, but I'm trying to get a 2 ply beam to work.

The last question is that I called around some lumber yards to just see if they have 2.0E LVLs and what the bending strength is. When I asked bending strength they all said they don't know they just know the E. So many lumber yards have LVLs with 2.0E, but how can you be sure what the Fb is? I got the feeling when I spec out a 2850 how do I know that's what the Fb really is? I know Fb can be 2850, 2900, or 3100 for 2.0E? If I specified Fb = 3100 the 2 ply beam would work for sure.

The last thing is that the attic above has a gable roof so there won't be any load near the two long walls of the residence as there is no head room. For purposes of design, I don't think you'll ever have 40 psf there, especially if they building a knee wall and wall it off.

I included a picture for reference

 

[XR250]

mf,

Are your sure you did your moment or stress calc correctly? I don't think I have ever had fb be a limit state for an LVL.

 

[mfstructural]

I am pretty sure I did. I checked it several times. It's a spreadsheet I created and validated with a hand calculation. Isn't Fb being the limit state a function of the beam strength and size? I am trying to keep the LVL as shallow as possible and therefore am running into this? Correct me if I'm wrong. I can post screen shots of my spreadsheet but no equations are shown. With my interaction coefficient at 1.0, deflection for a s.s. beam is .52, or L/215, which exceeds allowable. Using deflection equation for a 2 span beam, the maximum deflection is .21, or L/519.

 

[XR250]

Actually, I just ran a calc assuming 15ft. span joists on each side @ 40 psf total and do get it bumping up against the Fb. I personally would not worry about 10% diff in Fb on an LVL - it ain't gonna fail -especially in an attic w/o full height.
What is your distributed load?

 

[bpstruct]

I also rarely run into bending stress problems with LVL's before I have deflection issues. I could see it for a deep LVL that has a long unbraced length. What is the room being used for? Can you get away with using 30 psf?

 

[mfstructural]

It is a small single story house with a gable roof. It will be unfinished for now but in the future they may put a bedroom up there. With it being just a bedroom and the walls pitched at 8:12 I think I can use 30 psf. Or alternatively, I can use 40 but the 3 or 4' of the joists near the exterior walls won't see any load because of the slope of the roof.

As for the diff in Fb, I'm just curious why Fb is never mentioned when LVL's are ordered since we use Fb for design. In this case, it works at 1.0 for 2850, but not for 2650. Of course the E values are different. But if you designed for 3100 Fb and it didn't work for 2900, that would be an issue I guess. I know it's a small difference.

 

[XR250]

I'm just curious why Fb is never mentioned when LVL's are ordered since we use Fb for design.

Most likely because it is rare that Fb actually controls the design. If it does control, then I would specify the Fb. I would have no problem backing the loads off in your case to make it work.

 

[bpstruct]

I do specify the Fb and E on the drawings. I try to give myself a little wiggle room when doing the calcs in case a lower strength LVL is delivered. If I am right at the allowable stress, I would consider bumping up the size. Rarely an issue, as I am normally increasing the size due to deflection.

I have never seen an LVL on-site lower than 2800 psi. I normally see that stamped on the side just like the E. Others may have a different experience.

 

[JAE]

I've typically seen Fb and E stamped.