Six Story Wood Frame Construction: Tips & Resources 7

[KootK]

I've got a number of projects in the works where six story wood construction, which is new to me, is being considered. They are all to be six stories of conventional wood (stick not timber) constructed above ground level transfer slabs. I'm seeking advice regarding this kind of construction as well as any published resources that may be available. Certainly, I'll be speaking with trade organizations such as the Canadian Wood Council. I'm hoping to jump start my knowledge quest, however, by requesting help from the crew here.

So far, I've got this from British Columbia's licensing body: Link

And for clever ideas, I have the following:

1) The dominant issues seem to be shrinkage, lateral load resistance, and wall construction at the heavily loaded lower floors.

2) Potentially, once could rotate the unit framing direction part way up the building with the goal of spreading the load, and some of the shrinkage around.

3) The height which brick is used ought to be limited to mitigate differential movement between brick and the doors and windows behind the joints.

4) Special detailing can be used for mechanical and electrical risers to mitigate the impacts of shrinkage on those systems.

Any other great ideas, articles, or manuals out there?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[wannabeSE]

Woodworks has a lot of information on midrise wood design. But, some (most) of the information is geared toward US codes.

http://www.woodworks.org/

 

[hokie66]

My tip is not to do it, but realize that is not helpful advice.

 

[KootK]

My tip is not to do it, but realize that is not helpful advice.

It's helpful if you elaborate upon why you feel that it should not be done.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Canuck67]

I have attended a couple of the Woodworks seminars on tall wood buildings. There are lots of details to consider, many of which you have mentioned. The presentation by Thomas Leung, structural engineer for many in the B.C. region certainly reinforced the complexity of the lateral analysis (i.e. there is no commercially available analysis program that will accurately model the lateral analysis of a tall wood structure).

fp innovations seem to be the ones that are publishing a tall wood building structural design guide, I believe it was published in the last few months. Hopefully it will save reinventing the wheel on new details specific to this type of construction.

As an aside, I am not quite yet convinced that at this point in time it will result in cheaper construction. Down the road, possibly.

 

[Brad805]

I am not convinced they are cost effective if one considers the costs carefully. I left the woodworks seminar unconvinced when I attended in 08.

 

[hokie66]

KootK,
My test is whether I would choose to live in it or not. A six storey wooden building is not for me.

 

[KootK]

So where is the weakness when it comes to cost competitiveness? I'll be using LSL plates, double/triple studs at the lower floors, and tie downs on the shear walls. At first blush, that would seem pretty economical relative to, say, six floors of concrete. Many folks in the real world express this view as well though so there must clearly be something to it.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[Ron]

KootK...I'm not a fan of wood construction for such buildings for the concerns you have mentioned. Considering that it might not be your decision, think about a couple of things....

You could use a combination of platform and balloon framing. That would make your transfer slab conditions easier to handle as you would have fewer such conditions.

Warn of but disclaim ANY responsibility for waterproofing. Water will be the absolute enemy of this construction and if waterproofing is not done correctly, the structure can fail miserably. That includes things like the selection of the exterior cladding, designing for serviceability deflection for that cladding (be very conservative!!), and detailing the waterproofing for constructability and serviceability.

Good luck if you must do it.

 

[Brad805]

I agree it will be far more cost effective than CIP options, but if you compare it to some of the steel options I do not see it being significantly less using Canadian labor rates. There are too many pieces and owners always have crazy ideas that lead to non-repetitive framing. I believe if you add in the course of construction insurance, all the simpson parts, and carefully consider the framing/material costs it would not be as great a savings as one might think. I still believe there is a savings, but my personal belief is the real savings is in the range of 10 - 15% of the structural budget.

I agree there are many out there, but many times I see the decision to use wood is not always carefully considered.

 

[Guastavino]

KookK,

I recently turned down some multi-family wood (~3 stories) because I was concerned my client would not like my design (other reasons too). My client was used to getting multi-family wood designs from an Architect with no engineered stamp. Frankly, I don't understand how architects get away with that, but that's another story...Anyways, I turned it down because I knew I would be doing it right, with diaphragm chords, collectors, etc. etc. Simpson and or USP would have loved me. But, I had worked with this client a few other times and he proved to me that cost reigned supreme. I worried that I would upset him by designing it right. Meanwhile, he is used to it being basically done using prescriptive methods which, IMHO don't have a complete load path.

With all that said, I think that it's still a worthwhile project to take on, as long as the owner/architect aren't concerned solely about price. Also, RISA has some great software that you could potentially tackle the project with.

One other thing to consider. Have you ever heard of

http://www.infinitystructures.com/?

I'm sure there are other comparables too. These folks specialize in cold-formed stud construction for multi-story. They compress the walls as they build them in a shop. These go up very fast and don't have nearly the issues that wood does. They do a lot of the engineering in house too.

Also, check out this article by woodworks

http://www.woodworks.org/wp-content/uploads/5-over-1-Design-Example.pdf

 

[TehMightyEngineer]

Having designed the gravity framing system for a 5 story wood apartment building a while back and recently done a 4 story elevator shaft out of wood I can offer a few general things.

The biggest piece of advice is make sure the contractor, architect, and all else involved know what they're getting into. As you're aware the lower floors will be much larger and heavier than is used in typical construction. Make sure the initial drawings, estimates, etc. are based on an accurate rough design so that everyone can make sure they aren't caught off guard when you start specifying 2x8 studs at 6" o.c.

Make sure to budget and provide contract language so that you can put extra time into this if/when the revisions come in. Small changes can easily ripple through the building causing massive headaches with rechecking all your details. Large wood structures are the worst for revisions and "small" changes IMO.

I'd prepare everyone to expect some steel/concrete in the first floor framing at a minimum. Interior bearing walls are also a good idea.

Floor trusses will be your friend here, they're a pretty good method to economically span a long way.

Make sure that the architect gives you enough wall length to have proper shearwalls. Don't go to the steel or expensive wood shearwall details unless you have to. Make sure holddowns have a path to the foundation as they are going to be critical.

As you mentioned shrinkage, bearing, and deflection can be an issue. I'd say you're correct with limiting the use of masonry or other items that can't shrink with the wood. The large building I did were 100% wood framing (minus a few steel beams) so we avoided that issue.

Maine Professional and Structural Engineer.

http://www.fepc.us

 

[bobby83]

Hi,
I attended a seminar in Quebec City on 5-6 story lightweight wood structures. I recall 3 major challenges specific to these structures:
1. Fire safety requirements (non combustible cladding, fire separations everywhere, etc);
2. Vertical deformations from shrinkage and creep in sill plates and top plates (that can exceed 30 mm!). Special plumbing fittings and shrinkage compensating anchors must be used);
3. Minimal length of shearwall to respect the lateral drift criterion. If I remember correctly, you need a shearwall length of H/3.5 minimum. That might be problematic with the architect!

 

[appot]

a 4 story elevator shaft out of wood

How did you control deflections? Defections at the center of the hoist beam can be handled by using a steel beam (or a very large LVL, would that even be allowed?) Did you ignore the deflection of the supporting members?

Or did this elevator use a telescoping cylinder?

 

[BUGGAR]

You mentioned brick. Check out the prefabricated panelized brick veneer sections they used up in Seattle. They were essentially free standing like cladding and were used on wood framed structures. They were shop fabbed, shipped to the site, and crane erected. I don't know how they were tied together though.

 

[ChiefInspectorJeff]

Would not the OM at the first floor shear wall be tremendous?The chords would have to be almost solid wood.
Would braced frames work better?

Inspector Jeff

 

[rlflower]

I agree, ChiefInspectorJeff, that the OM at the first floor shear wall would be tremendous. Assuming that this is a "stacked" system in which the walls providing lateral restraint are aligned from one floor level to the next, the structural designer will almost certainly have to use structural steel in lieu of wood where the load becomes excessive. Steel braced frames may be necessary.

Simpson Strong-Tie has developed a lateral restraint system in recent years involving continuous threaded rods with devices that would take up the slack as the wood framing in the walls shrink over time. This system appears to have advantages in such "stacked" framing designs. I an intrigued if anyone has had experience with such a system; furthermore, would it be a feasible system for a tall structure as proposed on this thread?

Richard L. Flower, P. E., LEED Green Associate
Senior Structural Engineer
Complere Engineering Group, Inc.

 

[TehMightyEngineer]

Swiver: not to pull this off-topic but I forgot it was actually 5 stories. It was a newer elevator without a machine room, this greatly simplified the loads. The rail loads had very tight deflection limits and were supported on heavy multi-ply LVLs. The LVLs were braced against heavy timber floors and a masonry wall. It was definitely pushing wood design and we tried to convince the contractor that using masonry walls would be cheaper and easier but they were determined to make it out of wood.

More concerning than the deflection was fire protection issues. That was much more problematic.

Maine Professional and Structural Engineer.

http://www.fepc.us

 

[mike20793]

You are right; the major areas of concern are lateral resistance, shrinkage, and brick differential movement (related to lateral load and shrinkage). I've done several 5 story wood buildings in the last year and I've found that my shear wall locations are often determined by holddown capacity or boundary stud capacity, rather than sheathing capacity. Shrinkage happens (mostly) in the plates, so there's no way to avoid it; you can only mitigate it. We've been convincing architects to place a stud wall around the concrete shafts so shrinkage isn't an issue. They've been more than willing once we explain the anticipated shrinkage and how it may affect the upper floors. The elevator sills can be adjusted and you get a slight drop at the stairs, but even that can be mitigated to less than a half inch. We've had good success with lipped brick and shelf angles at each floor supported by LSL rim boards. Turning the framing up the building is a great idea. I've tried pushing it before, but it didn't take. I'll try pitching it again, though.

 

[manstrom]

I've done it and will continue to do it. A few tips / items.

*Woodworks is a good resource. Also, Terry Malone's book.

*Shrinkage is the big issue. Relative shrinkage between CMU, brick, steel and pipes are an issue. Don't fight it, allow for it. Check the coefficient of thermal expansion for PVC.

*Do not run brick over 4 stories. Use a relieving angle if you go taller; even that is a bad idea.

*I've never had to use LSL plates for loads. It may help in shrinkage, but keep in mind, you can't have engineered wood be FRT. I assume this is 3A construction, so all wood in the exterior walls needs to be FRT and therefore can't be engineered lumber

* 2x6 exterior walls. 2x6 corridor walls are nice, but not necessary. 2x4 interior bearing walls will work, but expect triple studs at the bottom. 18" trusses and

* Lateral is perfectly doable (unless you are in a crazy wind or seismic zone). Use a shrinkage compensating system like Simpson, CLP or Quick tie. Use all of the unit separation walls and all of the Corridor walls. You may need OSB or thermoply at the bottom few floors. Exterior walls will be fairly useless in shear.

*If the owner intends for these to be high end condos, use another system. Condo owners don't want to see settlement cracks in drywall.

A few lessons learned.

*FRT studs do not dry out the same way as non FRT studs. Expect differential settlement.

*Exterior sheathing can restrain the shrinkage of the wood. The outside wall may not shrink like the inside walls.

*PVC pipes need allowance for movement vertically.

When I am working on a problem, I never think about beauty but when I have finished, if the solution is not beautiful, I know it is wrong.

-R. Buckminster Fuller