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Flitch Beam Torsion

Flitch Beam Torsion

Flitch Beam Torsion

(OP)
Wanted to see if anyone has analyzed and designed a flitch beam (wood steel wood sandwich) for torsional loads? I am working on a project where an architect wants to hang a 4-seater porch swing from the rafter framing above. The porch rafter is unbraced at the bottom of the beam but has 2x decking nailed to the top. I'm currently proposing a 1" thick steel plate to be clad with 2x material. The porch swing is hung from an eyebolt that will be welded to the bottom of the steel plate. Therefore, I'm assuming that the seat can swing and load the rafter at an angle. To resolve this I'm designing the steel plate currently to take all of the load in strong-axis bending, weak axis-bending, and torsion. The architect is looking to thin out the plate section and hence I'm wondering if there is any benefit to using the wood.

My first reaction is it wouldn't help a whole lot. I know I would need to design the bolts for the shear flow through the section (which should be possible for weak-axis bending, but I haven't given much thought yet to how that would work with torsion). I'm also not convinced the wood would do a whole lot do to the difference in "E" and I've never designed wood in torsion before so not sure if it has very much strength and if cross-grain bending comes into play.

Just seeing if anyone else has gone down this path before I give it too much thought. Given it's a porch swing, I'm also starting to wondering if I'm overthinking the whole thing as there have obviously been lots of these hung from just typical porch framing.

RE: Flitch Beam Torsion

Thoughts:

- My gut feel is that your lamination fasteners wouldn't be stiff enough to produce a composite section torsional response. Bolt slip and such. You'd get something closer to three sections twisting independently.

- To some degree, I suspect that the beam will rotate such that its strong axis remains more or less aligned with the load for modest angles of swing. Of course, that may gradually tear your beam bearing connections apart depending on the torsional give available between the hangers and the bearing supports. What are your torsional supports at the end of the rafters anyhow? Blocking?

- I'd feel better about some snug blocking either side of the support beam, either side of each hanger to deal with weak axis bending and torsion.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Flitch Beam Torsion

So this is an exposed rafter ceiling and you are proposing a flitch beam for one of the rafters?
If so, seems like you need to add some rafter to rafter blocking to prevent the rotation. I am sure the Arch. will love this.

RE: Flitch Beam Torsion

(OP)
Blocking is unfortunately a non-starter architecturally. The rest of the rafters are 3x8 @ 30" oc spanning ~12'. Where the swing will be hung we will be using a 4x8 finished dimension flitch beam with a steel plate reveal at the bottom (matches some other architectural elements).

I would much prefer blocking, but the architect is really pushing hard against it.

To prevent rotation at the end there is blocking above a dropped WT clad porch beam. The flitch beam plate could potentially be fillet welded along the bottom. The top would be restrained by nailing of the 2x decking above to the flitch wood nailers. In a perfect world architecturally, we would not see any bolts between the steel plate and the wood nailers. The nailers would essentially be architectural furring that get epoxied on to the steel plate with threaded studs embedded into holes filled with glue.

I believe I can get a 1" thick x 6 1/2" deep steel plate to take all of the loading in strong axis, weak axis and torsion. The nailers would have to be assumed to brace the plate from buckling. I'm going to abandon the idea of trying to make the beam behave compositely and instead look at the steel section alone.

RE: Flitch Beam Torsion

Is there some reason you're determined to use 1" plate instead of a I or T section?

Seems like you could get a clad T section to perform as well or better, with less weight and the same finished appearance.

RE: Flitch Beam Torsion

(OP)
I section we can't use due to projection at the bottom. We could use a T-Section. My concern was that the T could potentially get in the way of the nailing of between the decking and the web-infill, although I'm sure there is an answer in detailing to deal with it.

Perhaps, the way I was planning on dealing with torsion and weak-axis bending is flawed, but my thought was to form a moment couple between the steel plate and the sheathing above (which is plywood over decking) to resolve the torsion. We are not talking enormous loads as it is just a porch swing so I was thinking the sheathing could take the load. In this load path, I didn't think the WT section added a whole lot of benefit. The plate is more than adequate in strong-axis bending, the weak axis / torsion governs the design.

RE: Flitch Beam Torsion

Perhaps a HSS would be best.

BA

RE: Flitch Beam Torsion

(OP)
I agree an HSS would be best, but I have significant pressure from the architect to thin out the steel component. They originally wanted this to be a 3x8 finished dimension, we've moved to 4x8 in order to fit in a thin steel section. An HSS would change the architectural intent. It is a relatively small loading (just a hung porch swing) so I would like a plate to work if I can get it to work.

RE: Flitch Beam Torsion

The vertical load may be small, but the lateral load is eccentric and is not small, particularly if four hefty people start swinging crazily back and forth as they might do during a party. HSS 4x8 may be sufficient without the wood side plates.

BA

RE: Flitch Beam Torsion

Wwhat about attaching the swing to the 2x decking using some forged eyebolts and then dealing with the bolt head in the roof covering?
You could even put a big steel plate on top of the decking with some rod couplings welded to it that penetrate down to the underside of the deck. You could then thread you eyebolts into that.

RE: Flitch Beam Torsion

(OP)
Thanks for all of the input and help guys. I've attached my preliminary calculations showing my approach. The plate dimensions have changed a little, but the concept remains the same. Let me know if any of you disagree with my approach.

I am currently designing the 4'x9' sofa for 40 psf live load which equates to 1500#. This seemed reasonable (and quite conservative) for 4 "hefty" people. I am assuming that with the swinging motion that 2 cables could potentially be slack. I didn't increase the load by a "dynamic" effect above the 1500# as I felt assuming 2 cables were slack and taking the force at an angle was a reasonable design load. I've assumed that 30 degree angle from the vertical is the highest I would design for. That to me seems like a pretty steep angle for a swing. 45 seemed to be too conservative.

The 9' direction is inline with the 12' direction of the rafter span. Therefore, the swing is only ~2' from each support point. The strong-axis bending moment is therefore not very large (2 point loads close to support). As I mentioned previously, I broke up the lateral force into a moment couple between the sheathing / decking and the beam and designed the beam in weak-axis for the resulting moment couple.

I would prefer to use a 4x8 HSS but this won't fly architecturally. It's a $15 mil house and architectural intent is extremely important. The HSS would break up the look of the other rafters. I like XR250's idea of attaching to the decking but we have 5/4 African Mahogany decking which I'm nervous to use as it's not full 2x and I don't have any design values for it. Additionally, the architect will likely not like the look of hanging the swing from the decking. I think I am stuck architecturally with the eyebolt on the bottom of the beam unless I tell them "there is no way it can work structurally"

I feel like the 7/8" x 7 1/4" plate (or possibly 1" x 6 1/2") plate calcs out.

Does anyone disagree with my approach? Do you feel a WT section (which would be possible) would significantly increase the torsional capacity of the section? With the bracing from the sheathing I felt it wouldn't add a lot.

The architect is accepting of our current design, but likely would be resistant to other larger structural sections unless absolutely necessary. He seems to think the porch swing should be able to be attached to a typical rafter as he has seen it 100s of times.

RE: Flitch Beam Torsion

I agree with BAretired. I don't think you have a good way to resolve the torsion and, if the house is 15 mil, I wouldn't use that detail. Without blocking to adjacent joists, I wouldn't swing in that chair. I'd push for a tube or other closed shape that can handle the torsion outright (without resolving the forces into the sheathing, that is then furred around with the mahogany decking. It is what it is and if everyone wants to be saved from a future lawsuit, then there needs to be an alternate way to support the swing. In the end, there has to be compromise from the architect since it's not safe structurally.

RE: Flitch Beam Torsion

(OP)
Thanks for the input Mike. Although, I am having a hard time seeing how it is "not safe structurally". The reaction to the sheathing would be ~625# in this torsion scenario. The sheathing would be nailed directly to the side nailers to lock it in and there is also 5/4 decking which could serve to help take the compression force.

I'm open to going back to them and saying it doesn't work, but I'm not sure I'm quite convinced.

RE: Flitch Beam Torsion

- I'm satisfied with your current approach.

- In my heart of hearts, I agree with the architect. I don't feel that heaven and earth should have to be moved to hang a swing.

- Realistically, I think that the consequences of failure would be pretty minor.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Flitch Beam Torsion

(OP)
Thanks KootK. In running some quick numbers it looks like the steel plate would be able to handle the torsion even without considering the sheathing. It is a solid 1" x 6.5" bar after all. I attached some scribbles of calcs that are probably really hard to follow without pictures. I also somewhat awkwardly transitioned from using stresses to following the strength code equations in AISC. I likely didn't combine stresses properly with all the stresses coming in (didn't think too closely about it) but it works at a 0.9 DCR with just adding them all together and 0.71 if I square the shear components after directly adding them, similar to as shown in equation H1-3. This does assume that plate buckling is constrained by the wood nailers but considering the largest torsional stresses are caused in the weak direction that wouldn't be as prone to buckling I'm ok with that assumption.

I was just trying this as a gut check to my other calc which I will use for submittal.

RE: Flitch Beam Torsion

It doesn't appear to be a problem when you look at the calculations. I thought the torsion would produce a lot of rotation, but there is only a 2.5' length at each end which is subject to torsion. The middle 9' has no torsion acting on it, so the total torsional rotation is only about 0.0029 radians.

BA

RE: Flitch Beam Torsion

(OP)
Thanks for taking a 2nd look BAretired.

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