Wood shrinkage and hold-down anchors

[cliff234]

We are designing a 5-story wood framed building and are trying to accommodate wood shrinkage that we are estimating to be about 0.375” per floor. Among the challenges is trying to figure out an economical hold-down detail in the lateral-load resisting shear walls. In the past we have used strap anchors at the hold-down locations and have never had any problems (shorter 3-story buildings), but I am concerned that with 0.375” of shortening per floor (with most occurring within the depth of the floor framing) the straps will buckle and be ineffective. We would like to try to avoid using shrinkage compensating anchors. Is there an easy and inexpensive hold-down detail that we could use that will allow the wood to shrink yet still be effective in resisting tension forces in the chords of shear walls? One solution is to have a sufficient number of walls such that we don’t need any hold-downs. We are looking into that.

Also, at what point does wood shrinkage become a problem? I am asking, because we have designed many 3-story buildings and have never had any problems - and we did not give much thought to shrinkage. I would think that the issues of shrinkage within the depth of the floor framing and it's effect on hold-down anchors would be almost the same for 3-story versus 5-story buildings. Of course the overall amount of shortening in a 5-story building would be greater than in a 3-story building.

Thanks in advance for ideas or suggestions that anyone might have.

 

[msquared48]

If you used TJI's for the floor joists, your shrinkage across the depth of the flooor joists would be next to nothing, leaving the wall top and bottom plates where most of the shrinkage would occur.

If you use the Simpson ATS system - continuous rods at the ends of the shear walls, they make automatic take-up devices for the system, and I think there are others too. But it is expensive...

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[msquared48]

Another method used in the past is to balloon frame the first two stories and hang the joists from ledgers, but TJI's are much easier. (framers just love the balloon framing!)

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[RFreund]

Concur with the TJI or some sort of SCL.

However if you are only strapping across the rim joists (I'm assuming a metal strap from post above to post below?) why would it matter how many stories you have? The amount of shrinkage would be the same for each strap at each floor, no? I can see however other continuous components that extend foundation to roof may be a problem but why is the shrinkage different at each floor than in the other buildings?

EIT

http://www.HowToEngineer.com

 

[msquared48]

RFreund:

It matters on the siding details and any masonry on the outside of the structure, not to mention overall internal plumbing, mechanical, and eletrical runs.

Mike McCann
MMC Engineering

http://mmcengineering.tripod.com

 

[boo1]

Thread had some info:

http://www.eng-tips.com/viewthread.cfm?qid=276910

CSD72 linked

http://www.diva-portal.org/smash/get/diva2:206355/FULLTEXT01

Carair linked

http://www.structuremag.org/Archives/2006-6/D-Fex-Hi-Rise-wood-frame-const-June-06.pdf

 

[boo1]

threads had some info

http://www.eng-tips.com/viewthread.cfm?qid=276910

http://www.eng-tips.com/viewthread.cfm?qid=282403

 

[RFreund]

M^2 - Right, I agree with you on that. That's what I was getting at with my "other components that run foundation to roof" comment. However for strapping at the ends of a shear wall I am not seeing how it makes a difference how tall the building is. I would think it would only be a junction of floor joist depth/material at that floor.

Nice references Boo.

EIT

http://www.HowToEngineer.com

 

[cliff234]

Thanks for all of the responses. Several of you mentioned the same thing I was curious about. The floor-to-floor shrinkage is pretty much the same regardless of whether the building is 3 stories or 5. We have not had any problems with strap hold-downs in exterior walls on 3 story buildings - and now, in retrospect I am wondering why not (not that I'm complaining). If the floor framing shrinks about 3/8" within the depth of the floor on each level, then wouldn't that make our strap bracing buckle? I am now concerned about using strap hold-downs for any height wood-framed structure. The spring-loaded hold-downs seem great, but they are an additional expense. Likewise, engineered lumber is great, but it will also add cost. I think we have a handle on how we are going to handle the issue in our elevator shaft and stairs (expecting 2" of shrinkge on the 5th floor), but we are still working on economical solutions/details to the shrinkage related challenges on the facade.

 

[dhoward26]

Check out this product from Simpson: Good stuff:

http://www.strongtie.com/products/ats/index.html?source=topnav

 

[hokie66]

While the spring loaded devices may cost a bit, surely the cost would be inconsequential in a 5-storey wood framed structure. What type facade is being used?

 

[dhoward26]

I agree with hokie66. In an effort to prevent future issues with the exterior (and some internal components), a more costly bracing alternative would probably be your best choice.

 

[focuseng]

Careful on thing I-joist will reduce shrinkage mentality. A lot of I-joist use sawn wood flanges not LVL. I also notice that in the "Value Engineering" phase of the project (after the contractor gets their hands on it) the really nice I-joist you specified with the LVL flanges gets requested (or no request comes through) to switch a solid sawn flange product to save costs. They're the same -right?! Look at the tables! What's the big deal?

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MAP