Long Span (35') LVL Beam Design 8

[Brobocop]

I was contacted by a building owner that wanted to build out his attic space into offices, and needed to ensure that 2 installed beams (Beam were never previously Engineered) are adequate to handle the additional live and dead loads.

There are two beams in question, each comprised of Four 2x16 LVLs fastened together and 35 ft in length. They are supported at the ends by two 5.5"x5.5"x0.25" Square HSS. The rest of the ceiling is framed laterally with 2x10s 16" o.c.

Rather than Model the entire building, I modeled just column/beam frame (diagram attached), and am wanting to put the distributed dead, live and wind load on the beam for my analysis.

My STAAD model results are currently showing Fail, and also a mid-point Deflection of about 3" (L/140).

Looking at the roof framing, is the rafter bracing wall - that sits directly onto the beam - picking up much load? or does most of the roof load go to the exterior walls? I'm making sure I'm accurately calculating all loads before recommending a different solution (Being that the beam is currently installed).

35' is a heck of a span to begin with. Not many LVL manufacturers have spans listed that high in their literature.

I appreciate the insight.

 

[McSEpllc]

Brobocop, on your question whether and how much of the roof load is transferred into the LVL beam:

There are a couple aspects to this:

1. Is the roof structure without the LVL beam ok to support the roof loads?
That means checking the ridge beam and rafters for adequacy. If not by themselves add the beneficial aspects of collar-ties. If that still doesn’t work, add the knee wall as support and see whether the roof framing works. Gauging from the rafter spans and sizes in the photographs, that roof is not over-designed, and likely relies on the knee wall for support.
2. How much load is transferred from the roof structure into the LVL floor beam depends on the load combination, the stiffness of the roof structure and the stiffness of the LVL beams. If the roof structure as is does not require the support of the LVL beam, in my mind you can ignore the roof dead loads, but still have to consider transient roof loads (roof liven snow and wind loads.) As the roof framing deflects, it will transfer roof loads to the LVL floor beam. How much load is transferred depends on the relative stiffness of the LVL floor beam vs. stiffness of the roof. This is just like when two beams of different stiffnesses are sistered, the load is supported based on the relative stiffness of each. This case is a bit more complex though, because the collar-ties and the sloped roof diaphragm add stiffness to the roof. To figure the load transfer somewhat realistically, would take a 3-D model of the LVL floor beam, and roof structure. To simplify the model, I would do rafters at something like 6ft o/c to have less model members.
3. To strengthen, add or truss the LVL floor beam makes only sense, if the current HSS5x5 columns and footings are adequate for the total loads. If either or both are not adequate, I would either add one or two columns within the middle third of the LVL floor beam span. If the additional support has to be closer the beam end, check for uplift at the end of the beam for unbalanced live loads. The leverage can create quite a bit of uplift. One thing I have done in the past is to cut the top 3/4 of the LVL floor beam depth over the new support column, to change the behavior from continuous to simple span. Naturally the simple span condition needs to be checked for adequacy.
4. Creating a truss over the LVL would be very effective, but I agree the magnitude of member forces would be difficult to design connections for. I have designed and built a plywood box beam. They are very stiff and effective, but very cumbersome and time consuming in engineering and construction. If possible, the by far easiest is to add columns below the existing beam.


Doing a quick calculations on the LVL floor beam:

Assuming L=35-ft, Tw = 8-ft, DL=10 psf, LL=50psf, and (4) 1¾”x16” 1.9E LVL’s, the beam appears to be (over) stressed at between 126% and 133% (for braced and unbraced conditions), and total load deflection ratio is L/110. That is not considering the loads due to the attic and knee wall, or roof. I probably would figure the LVL floor beam to support 5/8 of the rafter span (check that the ridge beam can support its share), and use that load to figure additional support column and footing. The engineering fee to do a thorough analysis likely would not offset a potentially lesser column and footing size.