Wood Shrinkage and hold down straps

[ChasWaughSE]

For those of you wondering about the need for shrinkage compensation devices or careful control of wood moisture content with wood shear walls, consider my recent experience with a multi-story wood building in the Seattle area. The siding on the building had buckled as much as 1 3/8" --- always at the rim joists in upper stories --- in several places. The owner was quite concerned that the rims might be decaying or ???

In fact, the shrinkage at just the top and bottom plates (double plate on lower wall, single sole plate at upper) adds up. With 4 1/2" of wood at 4 percent shrinkage (see Design of Wood Structures, by Breyer, Findley, Pollock, Cobeen) the total is .18" . Shrinkage could be up to 6%, which would be 0.27" of shrinkage. Breyer, Findley, Pollock, and Cobeen note that 6% shrinkage was recommended by Rummelhardt and Fantozzi, "Multistory Wood-Frame Structures: Shrinkage Considerations and Calculations," 1992 ASCE Structures Congress.

So .... even if the joists do not shrink (TJIs can be pretty stable, but not necessarily) with a depth of 19" across the rim, nail to nail, (the building in question had a rim and TJs of 18" depth ) the shrinkage is enough to buckle the strap about 1 3/8". If you don't believe it (closed form solution with chords and segments of circles not available) draw it to scale with a CAD program.

The result is not only siding damage, but a very loose and ineffective strap. More than about 1/8" slop is too much.

This is far different than the conclusions that I saw in thread507-339808 .... but I do see a lot of wood, and it does crazy stuff. It is also in line with some things I learned at a Structural Engineers Assn of WA seminar years ago .... too bad I no longer have the notes to reference them.

Be careful

 

[mike20793]

Shrinkage cannot be eliminated, it can only be mitigated through proper detailing at the exterior. For my multilevel wood buildings, I always let the architect know up front the minimum and maximum expected shrinkage and we are yet to have any issues. Are you using continuous tie rods or conventional holddowns? The continuous tie rods must have shrinkage compensating devices that can take out the slop after shrinkage has occurred. Conventional holddowns usually fair okay since they are only seeing the shrinkage from one floor and it doesn't accumulate.

 

[ChasWaughSE]

My shrinkage, as noted above, is from __one floor__ at a time. Not accumulated. But the multi-story aspect is significant insofar as the hold-downs have to resist a lot of load, and they will have to start with 1/4" slack, and then with nail slip they will exceed about 3/8" or so. That is too much deflection to provide good behavior.

I note that the problems were not at every place on the building (though they were seen at all upper floors). I figure that the variability is due to the variable nature of the wood grain in the plates (some more radial, some more tangential) and how wet they were when installed. Like, were they on top of a pile that got soaked, or under cover.

If you have or can borrow the Breyer, ... Cobeen reference, it is excellent. Again .... it is not a theoretical problem. And it is not confined to lower floors, and does not need to "accumulate." I saw it, and a lot of it.

 

[msquared48]

I can understand the condition you describe, but have not seen that much of a problem with it as you apparently have. I, too, am in the greater Seattle area.

In that take ups are not adaptable to straps as far as I am currently aware, the solution would be to avoid straps, be far more restrictive on the moisture content, and/or baloon framing where the straps would go. Baloon framing would eliminate the need for straps anyway. However, it would not be able to be done in all circumstances.

Mike McCann, PE, SE (WA)

 

[msquared48]

Frankly, over the years, I have had far more structural issues with improperly installed, bent, or missing straps than the shrinkage issue at the floor diaphragm level.

Mike McCann, PE, SE (WA)

 

[ChasWaughSE]

Mike

I would agree ... installation problems are most of the problems in hold downs of all types. But in this case, there is absolutely no other credible explanation. All (of about 12) buckles are in known locations of hold down straps. (No other buckles at all) I cannot believe that 6% shrinkage is typical, but in some cases it could happen ....

Not as bad as a log cabin building that I had to engineer (for a golf course clubhouse) that had lots of openings and needed holddowns. Shrinkage compensating devices there were absolutely essential to the rod tie down system.

Charles Waugh, P.E., S.E. (WA) And if I ever get a chance to get down to Seattle for a SEAW meeting (I live / work in Bellingham) this would be a good one to discuss over a cold beverage. I am going to rejoin SEAW ... even if I hardly have the chance to attend meetings.

 

[msquared48]

I have to update my membership too. That would be good. Always up to a brew.

As for the log homes, I have done many too, and 6" of shrinkage or more is not uncommon.

Mike McCann, PE, SE (WA)

 

[ChasWaughSE]

Mike

My wife is from Montana, and we recently flew into the Bozeman / Belgrade airport for a family visit. There was a big glossy display for log homes .... constructed from precast concrete "logs." Given the fire situation, it would be a great product here .... and eliminate shrinkage problems, too

 

[BUGGAR]

constructed from precast concrete "logs."

We already have that here in SoCal in Frontierland at Disneyland.