Using Wood Sheathed Wall As a "Deep Beam"

[cal91]

I have a 52'-0" long wall on a second story. It has 30'-0" of roof trib (12 psf DL). The owner needs the space below that to be open. I try supporting the wall with perpendicular joists but they get killed. I would need a W24X62 steel beam at the roof level, and I don't know if they can ship a 52'-0" long beam out anyways.

Can I not simply use the wall as its own beam? I'd find the max shear and design the sheathing accordingly. I'd design the top and bottom plate for the chords forces. Has anyone ever done this, or is this crazy sauce?

 

[JoshPlumSE]

I don't think it's totally crazy.... It's kind of like a truss, right? With the double top plate and double bottom plate as the chords.

That being said, I'd very much prefer to design a true truss instead. Where I was careful about the way I detail the splices, and where I have some diagonals rather than relying on the sheathing.

 

[cal91]

So could you design a 10'-0" deep truss out of, just an example, flat 2x6 or 4X6 members, with top and bottom chords being maybe 6X6, spliced with metal straps? The top and bottom chords would both laterally stabilized by the roof and second floor respectively. Not sure what I would use for the web to chord connections...

Or would you just get a floor truss fabricator to make a 10'-0" deep truss for prescribed loads?

 

[Toby43]

Not done it myself, but if you search authors "Taylor" and "Milner" you will find some research on walls clad with particleboard designed and tested as deep beams (not read through it in detail, but made it into my diaphragm library). Long-term affects would concern me - nail slip etc.. and whether future renovations would not recognise the importance of the wall system i.e. "lets chuck a little window here, re-clad over there..."
Regards
Toby

 

[Celt83]

I'd say if you already have wood trusses everywhere else, dictate this location as a special truss and give them the pertinent design data similar to process for a special bar joist.

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[BUGGAR]

Check the design requirements for plywood box beams. your idea is valid and has been done before.

 

[MikeHalloran]

I think you can ship a 60' beam by truck; the trick is finding one in stock.

USACE used heavy I-sections 133 feet long for pilings in LA, but they came on barges, not trucks.

In old buildings, I have seen very deep, long trusses made of double angles of modest dimension; does anyone do that these days? In more modern buildings, I see large trusses made of rectangular steel tubing (HSS?). At least the flat faces are potentially easier to sister to an existing wall.




Mike Halloran
Stratford, CT, USA

 

[Huevo]

APA has some information about this on their website.

Look for: Nailed Structural-Use Panel and Lumber Beams and Nailed Box Beam Design Example

I doubt this is practical for a 52 ft span.

 

[JAE]

Yes this will work great...until someone comes along in 5 years and cuts in a new door opening near the end of the span where shear is large.

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[MIStructE_IRE]

Just truss it out like this. Pretty cool solution! This has been used historically in Ireland and the UK for centuries. I’m still finding diagonals hidden away in old city centre buildings.

Of course, your truss will differ. The attached photo just shows the principle.

 

[XR250]

I agree with JAE. You have no control over what someone will do to this wall in the future. Beams are pretty obvious, but not sheathed walls.
Also, I would be concerned with eccentric forces on the beam if it was not sheathed on both sides.

 

[Ron247]

Beyond the actual structural issues of this modification, as stated by JAE and XR, you really can open up a future can of worms regardless of how much you tell the current Owner about how they can never tinker with this wall without an engineer. Do not rely on them to relay that information to a future buyer. Also, I make a moderate living off contractor's removing walls or making openings in walls that are obvious structural components without consulting anyone. They are worse when it is not obvious the wall is a key component. As an example, 3 months ago I had to go to a 2 story motel where they had removed a 1st floor wall that support 2x10 floor joists that spanned 15'. You could clearly see the joint in the 2x10s where this wall had been removed. The sheerocked wall above was serving as a beam that was nailed to the floor system. There was a slight sag in the floor but not alarming in magnitude. The floors were not loaded anywhere close to the design load and it had sagged. I was actually amazed it had not sagged a lot more. What helped most was that they apparently built the floor in sections and placed them in place. For that reason there was a 2x10 banding across the ends of the joists.

Secondly, the fire code is another issue. I assume that wall would need to be rated fairly high.

 

[JAE]

If there was a story-deep truss system then that might be a bit more obvious to people in the future that the large in-wall diagonals mean something special.

Story trusses are used a lot with steel framing - not sure I've seen it much with wood framing.

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[WARose]

I don't know if you have 'Wood Engineering and Construction Handbook' around the office or not, but, IIRC, they ran through a design example with a wall as a beam.

 

[jayrod12]

a 52' long beam is shippable. Think of the standard 53' semi trailers. Although it certainly can be accomplished, I don't think you want to go too far out of the box on this one. As others have alluded to, wood walls make people think they can remove any portion at any time. Don't plan just for the now, think about 15 years down the road when someone realizes a new picture window would be nice.

 

[cal91]

With the thought of someone modifying the wall afterwards, and the simplicity of the steel beem I'm electing to just go with the steel beam. If I didn't use either then the floor joists supporting that wall would have been 24" deep Red-S at 12" o.c (32'-0" long). With the steel beam I only need 24" Red-L at 24" o.c. Seems like a fair trade?

Thank you everyone for the references and the suggestions!

 

[KootK]

Any chance you could replace a few of the Red-L @ 24" o/c with LVL Pieces and support a structural LVL rim to pick up the wall and roof? I like the steel option but, if it's the only steel on the project, I'd be doing my best to eliminate it as a separate supplier. And the steel beam is going to be forcing the wall and joists to be flexing downwards anyhow unless you're providing a movement joint.

 

[cal91]

I would need (2) 18" X 32'-0" LVL at 24" o.c. for 52'-0" of wall. I'm not sure if that's much more economical than 24" Red-Ls at 24" o.c. plus the W24x62 x 52'-0" long plus steel posts. Maybe it is, I'm not sure.

I could do that, but the owner likes having the open webs for MEP. I have plenty of steel elsewhere on the project so I'm not worried about that. And yes I'm going to provide vertical slotted clips from the non-bearing wall to the bottom of the steel beam.

 

[KootK]

Nope, that doesn't sound too good. I was hoping it could be four 2ply 24" LVL or something with normal joists in between.

I'm not sure that I actually understand the framing system here. I've been envisioning this wall on top of a cantilevered joist system? If so, what's the cant?

 

[cal91]

I should've just shown a picture from the beginning. Here's whats going on.