Floor to Floor straps buckling.... tolerance ? 1

[NFExp]

I have a 3 story wood frame apartment building where the floor to floor straps are bowing at the floor system. I am aware that the bowing is caused from shrinkage of the building. The straps are nailed top and bottom and not at the floor system as specified by Simpson.

We expected about a 1/4" shrinkage at each floor level and instructed the GC to install the straps after the roof was installed. I guess my question is what is the amount of bowing that is acceptable before re-install is required.

FYI, the buildings are currently in construction and all the straps are installed on the interior face of the walls.

 

[jayrod12]

To me it's odd that the straps are on the inside. Wouldn't that complicate the load path since the sheathing is on the outside face?

Either way, I don't think any bowing is acceptable. That would mean the building would have some give before the straps engage. Not sure how that would perform in reality.

 

[gte447f]

I would be curious what Simpson has to say about this situation. Surely they have encountered it many times with their products. I am also sort of surprised that this condition has already presented during construction. I would have thought it would take longer for the shrinkage to occur. Was the wood kept reasonably dry before construction?

 

[phamENG]

For what it's worth, as a junior engineer I had to do an analysis of these straps in a buckled state and help write a brief for a lawsuit filed against the EOR for a project where these buckled after the finish was installed and some hardipanels landed on a car in the parking lot. The EOR lost.

I agree that inside is strange. Can you provide a picture of the installation?

I also agree that any buckling of the strap means it's no good. Needs to be removed and reinstalled. make sure they are fastened through the floor band as it will help to prevent that buckling failure.

 

[NFExp]

PhamEng.. What was your analysis of the straps in a buckled state.

FYI Simpson has a footnote on the CS16 straps
"Wood shrinkage after strap installation across horizontal wood members may cause strap to buckle outward."

If the census among engineers is that zero bowing is acceptable, I dont see how floor to floor straps can ever be specified.

 

[gte447f]

"If the census among engineers is that zero bowing is acceptable, I dont see how floor to floor straps can ever be specified."

I agree, that's why I think Simpson must have an opinion on this condition. It must happen all the time. I assume they (Simpson) must have analyzed it and determined that some degree of buckling does not invalidate the intended function of the straps.

 

[phamENG]

No, they have a sheet in the catalog that explains you have to wait until shrinkage is done to fasten the bottom end, fasten in the floor band to restrain the buckling, or both.

My analysis showed that the buckling was caused by shrinkage.

 

[XR250]

For what it's worth, as a junior engineer I had to do an analysis of these straps in a buckled state and help write a brief for a lawsuit filed against the EOR for a project where these buckled after the finish was installed and some hardipanels landed on a car in the parking lot. The EOR lost.

Wow!
No multi-family for me! I do run into plywood buckling occasionally.

 

[Celt83]

"The straps are nailed top and bottom and not at the floor system as specified by Simpson.", be careful with this statement Simpson's catalog specifically notes that attachment along the rim is not required so it falls to the design professional to specify that. Edit: I read your statement as saying Simpson specifies nailing at the floor system but reading it again could be interpreted as intended by the snipped detail, so just a word of caution either way.

The catalog has recommendations to "limit" / "reduce" bowing by either nailing along the Rim or attaching the bottom of the straps after the roof is on:

 

[jerseyshore]

The graphic that Celt posted is exactly what should be done. Use all of the nail holes or install half the strap first then come back and do the bottom half next (which is what I see most often). Shouldn't be any bowing in the straps.

I do see a lot of straps being installed on the inside these days. Everyone complains that it messes up their siding (probably because of the straps bowing haha).

 

[NFExp]

Thanks all..

I was hoping there was published tolerance with the straps but it seems that any bowing is not acceptable. Im gonna tell the contractor that all the straps need to be re-installed.... which is a big undertaking for this project..

For future projects, I will have the above install instructions on my plans.

 

[XR250]

So they recommend waiting until the roof is installed? Is that for weight or keeping things dry? What happens when you have full LL and DL on the floors and roof (not likely but for arguments sake let's say so). That load would trump any roof DL. Will that cause buckling too?
Seems like this is a liability for any EOR (as pham's story indicates)
Maybe we should be using the floor to floor rods with take up devices instead?

 

[phamENG]

XR - I don't specify them at all for this reason. I use the plywood itself or floor-to-floor screws. I've spec'd full height a couple times, but invariably I show up to the site and the contractor decided to make up his own system rather than buy the Simpson stuff, which means it doesn't work. Because that same shrinkage happens and you have big gaps between the plates and the nuts. You need the expensive shrinkage compensators to make it work.

 

[gte447f]

I disagree that "any bowing is not acceptable". I don't think that is what Simpson is saying. They are making recommendations to reduce the likelihood and severity of buckling of the straps, but it may still occur, especially if you just wait to nail the bottom of the straps and don't also nail the strap at the floor cavity. Ultimately I think this is "just" a serviceability issue, otherwise I think Simpson would take it more seriously. In my opinion, it probably does not matter whether the straps buckle or not, except that the straps buckling might cause localized damage to non-structural finish materials, which, granted, could result in a fairly serious problem like the one phamENG described.

If you wait a few weeks to nail the straps, that might help, but I would expect the majority of shrinkage to occur over a longer period of time after the building is completed and put into service. I think nailing the straps across the floor cavity would help by forcing a different pattern of deformation of the straps, but they still must deform because the shrinkage will still occur, so there still will be slack in the system, which must not be a problem for the strength load case.

 

[NFExp]

All

Simpson responded back and provided the diagram in the attachment.

I am not aware of any guidance for a buckled strap beyond the warning that buckling can happen after wood shrinkage in the installation notes found on page 283 of the C-C-2024 catalog and guidance to follow to avoid the buckling due to wood shrinkage located on page 51 in the High Wind Guide.

I did reach out to the Simpson product engineer assigned to the CS product lines and he provided this information for you to consider.

We don’t have guidance on how much bowing is too much. The strap bowing would not create a strength limitation on the connection. But the load wouldn’t get transferred into the strap until the slack is taken up, so there may be some additional deflection. Designer would need to evaluate that based on the size of their bowing and floor geometry. As long as it’s less than ¾” of bowing, it doesn’t seem to create much additional deflection.

Maybe I was overthinking this situation... It seems that the bowing does not create movement that I thought it did..

 

[driftLimiter]

If the straps are for shear walls, you would want to add the amount of vertical slack that needs to be taken up into the hold down deflection term of the shear wall deflection equation.

This problem is exactly why the NDS requires designers to account for shrinkage in mid-rise wood framed construction. It is also why shrinkage take up systems were developed in the first place.

 

[jerseyshore]

Bowing of one strap might not seem like a lot, but add it up over 4 floors. Then as DL mentioned include that with the holdown deflection and suddenly it becomes a not so negligible number.

I don't know what other people are seeing across the US or Canada, but most of the lumber we see around the northeast, especially for this type of mid-rise construction is super duper green. Wet as hell and ready to shrink as soon as there's a dry day.

The last two majors problems we had on a 3 story and 5 story building due to shrinkage happened before they even completed the interior finishes. Waiting to nail the bottom of the straps for a few weeks even would help a lot to this bowing problem. Short team shrinkage in wood construction is a big problem these days (and even more so for PT lumber).

 

[Flotsam7018]

Why not just use floor to floor hold-downs (HDU or sim) - you could specify the nuts be backed off to account for the shrinkage. I use straps less and less as most of the framers we work with complain about them getting in the way of just about everything. Sometimes they're inevitable of course.

 

[jerseyshore]

I get a lot of requests to swap out hold downs on single family houses a lot of times, but that's because it's only 4-10 extra hold downs. For big 500 unit buildings it's hundreds and hundreds so using CS16's instead of HDU4's adds up big time.

 

[Flotsam7018]

Fair enough jerseyshore - however, lawsuits also add up pretty quick.