Details to accomodate shrinkage for multi-storey wood balconies

[skeletron]

This is more of an open discussion topic on accommodating shrinkage at balconies. What detail has proven to be the best/worst at minimizing the shrinkage for multi-storey wood structures?

The current situation is a four-storey building. We've used TJI joists and PSL beams/posts at the balcony on a previous project, but still deal with the shrinkage at the exterior wall plates/studs. Current project may be switching to commodity lumber for cost driven concerns. There has been some talk about providing a "levelling nut" type detail at the base that can allow compensation before the building is closed in. I believe this is a common detail for log homes, which is why I'm unsure if it is the best path for multi-storey.

This paper briefly discusses general details. Anybody have any experience or advice to share?

 

[Aesur]

When I came across this I typically used engineered lumber plates and balloon framing to reduce shrinkage. Additionally using Simpson ATS holdowns are recommended because of the take-up devices.

 

[jayrod12]

Many times locally we have additional slope built in to the balconies to attempt to ensure drainage away after the shrinkage. My concern with the leveling nut is much of the shrinkage doesn't occur until the building is enclosed and conditioned. Therefore it's too late from a finish standpoint.

And if we were to use engineered lumber plates and additional hangers to balloon frame walls, we'd never get the work around here.

 

[skeletron]

@jayrod12: I agree on both fronts. The leveling plate seems like a detail with really precise intent that can't be attained in real practice (re: no real control over a contractor's sequencing). Microllam plates would take care of the issue, but we'd be out of business in a flash.

 

[Aesur]

@skeletron & jayrod12, I understand where you are coming from, however I would like to know what you do at cmu/concrete elevator or stairwell cores to deal with the building shrinkage?

 

[Aesur]

Additionally, I have found that plates sometimes are controlled not by shrinkage, but by studs bearing on them... do you just use 3x or 4x plates when this occurs in lieu of engineered lumber?

 

[phamENG]

A thicker plate wouldn't increase its allowable compression stress perpendicular to the grain. You either need to use larger studs (3x), tighter 2x stud spacing, or engineered lumber plates. If less frequent, steel bearing plates can get you below the perp allowable stress.

If you're getting above 3 stories, sometimes it's best to forego the CMU/concrete shafts. With advances in gypsum products and code acceptance for fire ratings, you can do wood framed shaft walls which more or less removes the issue of differential shrinkage there.

[URL unfurl="true"]https://www.woodworks.org/wp-content/uploads/wood_solution_paper-Accomodating-Shrinkage.pdf[/url]

 

[jayrod12]

At concrete and CMU walls, I intentionally frame shorter joists into the wall, with some backslope, to accommodate the shrinkage as well. But I only do it about half of the expected per floor. i.e. if you're expecting 1/4" of shrinkage per floor, I'll have them put the 2nd floor 1/8" low at the elevator/stairs, 1/4" low at the 3rd.

Now, whether it gets built as I detailed, I have no idea. But that's my intent. I've never yet had a call back for an issue regarding shrinkage of wood framing adjacent to CMU/Concrete. I feel as though that it may have happened in the odd extreme case, and now engineer's everywhere are paranoid.

I can't imagine with the number of 3 and 4 story wood framed buildings that many people are doing anything special.

 

[phamENG]

You also have to consider the kind of buildings this applies to. If you're building a 5 story building and have to resort to using wood in lieu of structural steel or even light gauge, you're probably not building the nicest place in town. Often times these are apartment buildings, and once their built and leased the management typically won't care about some slight sloping around the elevator/stair. And, as long as you keep the joists fairly long that frame into the shaft, the slope will be really gradual and may not be noticeable. Just have to pay attention to how the floor will deform in the area.

They will care about the cracks in the drywall, the leaking pipes, and the back sloping balconies letting water in. That's why details around finishes and MEP and the balcony details skeletron initially brought up are the most important.

 

[XR250]

You either need to use larger studs (3x), tighter 2x stud spacing, or engineered lumber plates

I know on paper this seems accurate. However, on a number of occasions I have been reviewing a crawlspace where LVL's were used for beams and SYP was used for the pier caps. The LVL's seemed to be crushing and the SYP was not. So either the LVL is not as strong as they advertise it to be or the SYP is a lot stouter than it's supposed to be.

 

[phamENG]

Interesting. Was there water/humidity involved? If the pier cap was SYP, I would guess it was treated. LVLs can't be treated, and so they would lose strength a lot faster than treated pine in a crawl.

 

[XR250]

Was there water/humidity involved?

Always humidity in a crawl, but no signs it was excessive. Seems moisture and LVL's are not a good combo - even if rot is not involved.