LVL Flitch Beam utilizing WT Shape 1

[StructuralHokie03]

Has anyone ever specified or constructed an LVL flitch beam utilizing a WT shape with the flange on the bottom and 2 LVL's on each side? Is there any reason to avoid such madness?

Here is the situation. I have to provide a flush beam within an 11 7/8" "I" joist floor system. The beam spans approximately 21'-6", supports second floor joists flush framing into each side as well as a wall above that supports attic floor joists. I've run the analysis and I can get a traditional flitch beam to work using three (3) 11 7/8" LVL's with two (2) 1/2" x 11 3/4" steel plates. That equals approximately 40 plf of steel.

I then started looking at other alternatives, steel beams, channels, etc. I decided to look at a WT 12 x 31 installed with the flange along the bottom and bolting (2) 1 3/4" x 11 1/4" LVL's to each side. The total depth of the WT 12 x 31 is 11 7/8" to match the joists and the flange is 7" wide which is wide enough to bear on top of the wall top plates without crushing the wood. The weight of this steel member is only 31 plf, obviously lighter and cheaper than the previously discussed plates. It seems like a great practical solution but I've never seen WT shapes used in flitch beams before and I can't find anything on the Google machine. Anyone have any insight?

 

[GastonLagaffe]

I think I'd look at using just a W10 that passes before the flitch beam. Especially if this is for a new construction, I'd imagine the time savings of not dealing with flitch & fastening should more than offset the cost of going with a pure steel solution. Also with the price of lumber these days!

Going with a WT, keep in mind also that your neutral axes won't line up anymore and there will likely be a significant shear flow between the steel and the LVL. There'll also be the radius at the stem base which means you'll need a gap between the flange and the LVLs.

 

[KootK]

Speaking only to the technical aspects of the problem, I could get behind the WT if certain technical aspects were properly tended to:

1) For most fastener types, I don't believe in attempting to make steel truly composite with wood. I feel that there's to much slip available in the connections for that to be reliable and predictable. So I'd design the setup as though it was non-composite other than the for the demand on the fasteners.

2) I'm guessing that you would not carry the WT through all the way to the bearing? That being the case, the WT will attempt to pull away, in the downwards direction, from the LVL at the WT ends. I'd design for that and likely provide a gaggle of fasteners at the WT ends to get the job done.

I agree with Gaston's practical concern regarding cost etc.

 

[StructuralHokie03]

It's new construction but the first floor walls are already built. This is a last minute realization by the contractor that he needed a steel/flitch beam.

I was going to recommend that the bottom edge of the first ply of LVL against each side of the WT be cut on a bevel to allow the LVL's to be installed flush against the stem. Also, I get your point about the neutral axes not lining up, if we go this route I'll make sure to use plenty of bolts!

I could certainly find a W10 that would work, I'm just concerned about crushing of the top plates at each end. I would either need to use a significantly heavier piece of steel or install steel columns at each end. There are definitely other options to consider.

 

[GastonLagaffe]

KootK - good point about the slip in connection. Agreed on still checking for fasteners though!

With 11-7/8 joists and a W10 you should have enough space to fit at least a 1" thick flat bar over your top plate to spread the load - should be plenty to make sure your top plates don't crush.

 

[StructuralHokie03]

Good point, I had not considered using a steel bearing plate at each end of the W shape. Such a simple solution. Thanks for helping me kick this idea around guys!

 

[HouseBoy]

I get requests for flitch beam often enough but they are not nearly as efficient as wide flange beams

If the WT was going to be suitable to not crush the wood at the bearing location, how would a W10 not also be suitable?
The WT is 7" wide. Would you be using 2x8 studs (or 4 or 5 studs the other way)?

I assume the beam design is "controlled" by deflection. I usually find that a wide flange be is cheaper and more readily available. Attachment of wood blocking is much less cumbersome/critical than with flitch beams too.

 

[Agent666]

For some residential builders it comes down to them being able to simply buy a flat plate off the shelf of the right size and make the flitch beam up themselves and get it in place (manhandling it) vs having to get a structural steel trade involved and more difficulty in doing by themselves due to the (usually) heavier overall weight of a steel beam. I agree sometimes they are an expensive option, but builders tend to like them round these parts for the above reasons.

They are more familiar and comfortable with timber construction vs steel is sometimes all it comes down to.

Flitch beams sort of fill this niche at a certain span where they work much better for deflections than bare timber when structural depths are constrained or limited, but where you don't quite have enough strength demand to warrant a steel member. With the advent of higher spec LVL's (E>13-15+GPa) this niche is somewhat eroded provided you have the depth available to fit an LVL beam in. Although sometimes these are just as expensive as a steel beam, but again builders can kind of do it all themselves so it's preferred in some circles.