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.

 

[Jerehmy]

Sounds fun. I don't have anything to add besides the largest wood building in North America is the Wood Innovation and Design Center at the University of Northern British Columbia. It's 90 feet tall. You live in Cananda kootk, no?

And that in Vienna they're planning a 275 foot tall wood skyscraper .

 

[ChiefInspectorJeff]

Here's my tip: memories are short,sprinklers fail, fire dept.s have short ladders. A death trap in the making and pushed by the wood industries.
Fire rated wood might help but that stuff is problematic.

Inspector Jeff

 

[boo1]

here is one

 

[boo1]

one more

 

[ChiefInspectorJeff]

Scary.Too many places to make mistakes and obviously ill suited for the use.Complex details that won't get done.

Inspector Jeff

 

[manstrom]

Jeff, how do you really feel?

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

 

[ChiefInspectorJeff]

I love wood! In the fireplace or in a tree

Inspector Jeff

 

[MikeHalloran]

Closing remarks from the second document provided by boo1:
{
A LOT MORE WORK THEREFORE YOU WILL NEED MORE FEE.
REMEMBER APEGBC’S POSITION IS THAT AN INADEQUATE FEE IS
NOT AN EXCUSE FOR INADEQUATE WORK

GOOD LUCK TO THOSE WHO ARE PIONEERS IN THESE BUILDINGS,
PAY ATTENTION THERE WILL BE MANY MORE PITFALLS THAN WE
HAVE IDENTIFIED HERE TO DATE, MORE TO COME
}



Mike Halloran
Pembroke Pines, FL, USA

 

[boo1]

NFPA
Fire Safety Challenges of Tall Wood Buildings

http://www.nfpa.org/research/fire-p...fire-safety-challenges-of-tall-wood-buildings

 

[mike20793]

The 2015 SPDWS have updated info regarding use of rigid diaphragms. It may be useful for loading parallel to your corridors. I also avoid transfer diaphragms whenever possible and use full depth collectors or take out the resulting couple from offset shear walls through the shear walls in the corridor.

 

[KootK]

Thanks, everyone, for the excellent recommendations.

Regarding Fire Protection Issues

While I understand and share the concerns, I don't believe that this is my purview as the structural engineer. Fire protection engineers and scientists have influenced the fire protection regulations that apply to tall wood structures. I can only assume that they have taken the necessary steps to create levels of fire safety in tall wood buildings that are compatible with the levels of fire safety provided in other buildings.

This is just as we would do for, say, the seismic hazard level for different types of construction. Let us also not forget that there was a time when many of our ilk objected to the use of structural steel on account of the perceived fire safety risks. These are technological issues that often find resolution over time.

On With the Fun Stuff

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.

I'm not clear on how combining platform and balloon framing would eliminate transfer slab conditions. Can you elaborate?

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

Thanks for the advice. Part of this is a costing exercise being performed by a dominant (perhaps the dominant) player in the residential development market. It will be interesting to see what they come up with. We've had difficulty in the past with load bearing steel stud projects in our market (also your market I think). There seem to be relatively few CFM contractors skilled at doing anything other than infill wall. We get relatively few bids on the work and have endless CA issues. Is your experience with CFM significantly better?

Is construction insurance really more expensive for wood frame projects? Do you know the premium? Is it a fire risk issue as well?

I recently turned down some multi-family wood (~3 stories) because I was concerned my client would not like my design (other reasons too)...I knew I would be doing it right, with diaphragm chords, collectors, etc. etc. Simpson and or USP would have loved me.

In my latest incarnation, part of the team that I'm responsible for does nothing but multi-family wood frame buildings all day every day. It's about 30% of our portfolio and all of it rolls out the door with my stamp on it. I no longer have the luxury of avoiding light frame wood altogether. Time will tell if I can survive being... the way that I am.

Check out the prefabricated panelized brick veneer sections they used up in Seattle.

I must know more. Can you give me a project name or a link of some sort? How does prefabricated, free-standing brick mitigate differential shrinkage issues?

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?

It's something that I'll be keeping a close eye on for sure. If steel braces were required, that might exacerbate the differential shrinkage issue.

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.

Great idea. We already do this on our four story projects. Can you tell me any more about how the elevator sills can be adjusted and why that's less possible for the stairs? I'm afraid that I don't have the hands on construction experience to understand how that works.

We've had good success with lipped brick and shelf angles at each floor supported by LSL rim boards.

I've considered this. Any issues getting the numbers to work where the LSL is parallel to the floor framing?

Turning the framing up the building is a great idea.

Can anyone confirm whether or not this is really a great idea? It would spread the load around better but, on the other hand, would make the framing less repetitive. I've heard plenty of talk of turning the framing part way up but have yet to see a structure done this way out in the wild.

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

Wow, thank goodness you brought this up Manstrom. This is likely the case in my jurisdiction (Alberta) as well. I would have felt pretty foolish messing this up.

Exterior walls will be fairly useless in shear.

This is exactly how it's panning out, even with reasonably spaced windows. It's good to hear this confirmed.

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.

Whoa!! Six story stick = drywall cracking?? That could be a problem. This is shrinkage related cracking predominantly?

I don't have anything to add besides the largest wood building in North America is the Wood Innovation and Design Center at the University of Northern British Columbia. It's 90 feet tall. You live in Cananda kootk, no?

Yesir. Check this one out at 13 stories: Link. Canadian bad asses I tell you!

REMEMBER APEGBC’S POSITION IS THAT AN INADEQUATE FEE IS
NOT AN EXCUSE FOR INADEQUATE WORK

Yeah, I got a kick out of that. As though inadequate fee wasn't the root cause of inadequate engineering in light frame wood since the invention of the 2x4.

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.

 

[KootK]

. there is no commercially available analysis program that will accurately model the lateral analysis of a tall wood structure).

Why not? Can you elaborate?

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.

 

[manstrom]

I'll comment on a few questions:

I have seen drywall cracking on a few 5/6 story wood buildings. All due to differential settlement. Most renters and building owners will accept this, but not condo owners. The biggest place that I see it is between exterior walls and interior walls. On the top 1 or 2 floors there can be a vertical crack where the two walls join. If there is a door perpendicular to and just adjacent to the exterior wall, the outside part of the door will want to push up since the outside wall doesn't shrink like the inside wall.

Balloon framing in this sense refers to running the wall up from the top of the sheathing to the bottom of the sheathing and hanging the trusses off the side (where it is bearing). Instead of having 5 plates in a platform framed wall (2 top, 1 bottom, 2 in the truss), you have 3 plates (2 top, 1 bottom) to limit shrinkage. If you are going to do this, you should do it at all bearing walls. Otherwise you risk too relative settlement issues between walls framed different ways.



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

 

[mike20793]

The shaft openings can be left short and grouted to the appropriate height after shrinkage occurs. The elevator stops are infinitely adjustable, so they get tuned after shrinkage has occurred. The stair shaft landings can't be adjusted so you will often experience a step down coming out of the shaft. This can be mitigated by coordinating the landing height ahead of time to reduce the step down. There is a Woodworks Webinar coming soon about detailing for shrinkage; I'm sure there will be some good information in there, too.

My thought with the framing is that I usually have (not always) 2x6 exterior and corridor walls and 2x4 party walls. I get stuck requiring triple or quad 2x4's on my bottom level for the party walls (trusses spanning between party walls) but a singe 2x6 for the corridors and exterior walls. Why should we be taking all the load to 2x4's instead of 2x6's? Switching directions would relieve the load on the 2x4's in this case, but I'm not sure how it affects the mechanical systems, particularly the mechanical chases.

When I have the floor trusses parallel to the LSL, I use a 2x4 knee wall next to a 1 3/4" LSL rim board and spec 1/4" plywood filler to match the thickness of the 2x6 walls above and below. Then I kick off the floor trusses every so often to brace the knee wall. We've used a similar condition with lapped rim boards to act as headers over windows that have no room for conventional headers with trusses framing into. Contractors haven't complained yet, though some have asked if they can use OSB. The 1 3/4" LSL rim board is certainly more expensive than a 1" or 1 1/4" LSL rim board, but you get the properties of an LSL beam going to the wider thickness. The only problem is the throughbolt spacing for the shelf angle to the rim board gets a pretty close spacing, usually 16-18 inches.

 

[Canuck67]

Kootk,

I unfortunately cannot elaborate, that comment was attributed to the presenter in the Woodworks seminar (Thomas Leung), a structural engineer from B.C. who is part of the committee on tall wood structures and also structural engineer for quite a few of the initial tall framed wood structures. I found the comment interesting .... but have no further information to elaborate.

Other interesting items from the seminars:
- Sound transmission mitigation must be dealt with by architect, but can have an impact on how structural details may need to be detailed.
- Fire safety during construction is very important (Contractor's responsibility). See link to article (

http://news.nationalpost.com/news/c...-edge-of-crane-as-massive-inferno-rages-below

)
- Lots of Simpson's type hold downs required to compensate for shrinkage
- Panelized construction techniques were emphasized over and over and over. One of the ways they justify cost savings is by speed of construction using panelized floor and wall systems, i.e. build 1 storey per week, therefore you save costs by reducing overhead items such as A) time for site trailer on site, B) reduced site supervision time during shortened construction period, etc.... Construction insurance is more expensive, so also need to offset those costs as well. Stick framing was not being promoted as cost effective.
- Engineering and architecture time is higher at both the design stage and construction phase. The devil is in the details.

I think the tall wood structure is in it's infancy in Canada, but will likely become popular as engineers and architects and specialty contractors become familiar with the in's and out's of the system.

 

[KootK]

Shortly after initiating this discussion, I attended a Woodworks seminar on six story buildings. They addressed a number of the issues that we bandied about here so I thought that I'd report back and pass some of that information on. Please keep in mind that none of the information below originated with me. I'm just parroting what I heard at the seminar which, naturally, has a propaganda dimension to it.

Economics

Apparently, in many markets, it is parking requirements that render six story wood buildings unfeasible. In my market, a four story wood building can be developed on a tight site with one level of parking below grade but a six story building must have two stories of parking below grade. Apparently, it is that extra below grade parking level that really hurts.

One of the presentations was by a structural engineer practicing in British Columbia. Apparently, there, there's no such thing as a light framed wood building currently being built in the metropolitan areas. Virtually everything that he's done in the last few years has been five or six story. That being said, BC is wood country and they have legislated several initiatives that tilt the scales in favor of tall wood.

Fire

Apparently, modern fire codes for mid-story wood establish parity wrt fire hazard between wood and other systems, including non-combustible systems.

1) Building officials seek to establish comparable heat release curves. An example is shown below. Apparently, if the other rules of the road are followed, you can have a light framed wood apartment with a heat release curve comparable to that of a non-combustible system.

2) For now, while we're waiting for research to disseminate, Canadian codes are keeping the volume of wood the same in six story buildings as it is in four story buildings. i.e. If your building is taller then the footprint must be smaller, keeping the fire load similar. Really, in a building that is taller but smaller in plan, your travel time to a fire rated shaft would be shorter in a mid-rise building.

3) I guess they've been doing these kinds of buildings for a while now in the pacific northwest (US). The wood industry has queried fire officials in that region and, apparently, they don't feel that mid-rise wood buildings pose any more of a threat to occupants or firefighters than do similar sized non-combustible buildings.

4) In my opinion, what follows was the most intriguing bit of all. It seems that, with modern fire safety technology, fire hazard has become decoupled from construction material. Statistically, fire safety is almost 100% about having functioning sprinklers and alarms now. If you have those things in play, mortality is extremely low and it's unlikely that a fire will even get beyond the room that it originated in. The main issue lately in sprinklered buildings has been balcony to balcony, BBQ initiated fires on balconies without sprinklers. Now that they're adding sprinklers to balconies, this is also ceasing to be an issue. In summary, if you've got sprinklers and alarms, combustible/non-combustible isn't really relevant, despite the emphasis still placed on it by the code.

Construction Insurance Premiums

Increased premiums are, not surprisingly, related to the propensity for enormous wood buildings to burst into flame during construction. And, as far as I heard at the seminar, there isn't a great answer to this. Most mid-rise buildings that catch on fire during construction burn to the ground in spectacular fashion and the fire load is such that not much can be done. Some mitigating strategies include:

1) Having watchmen on site 24/7
2) Constructing firewalls in advance of the wood framing in between.
3) Constructing units, complete with drywall, at intervals ahead of other, unsheathed units. The drywalled units act as fire barriers.

Solid Wood Elevator Shafts?

In BC, they're building their elevator shafts out of solid, laminated 2x6's. See the photo below. This baffled me and continues to. I asked several presenters about this and never received a satisfactory answer. Some of the suggestions provided include:

1) Provides fire rating!
2) Dampens elevator vibration!
3) Required for shaft stiffness in seismic country!
4) Makes it easy to install elevator rails!
5) Shafts must stand while surrounding building burns down!
6) It seems like a reasonable way to replace a CMU shaft!

I'm skeptical of many of these reasons and believe that many of them could be dealt with more economically. If anybody knows the definitive reason for using solid wood elevator shafts, I'd love to hear about 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.

 

[msquared48]

Four other possible reasons:

1. The use of wood may eliminate the coordination of another trade, and

2. In that elevator shafts are you ally shear walls too, they would integrate better laterally and vertically with wood shear walls, and

3. They may take less time to erect and load up than CMU.

4. They a be cheaper.




Mike McCann, PE, SE (WA)

 

[KootK]

Thanks for chiming in Mike. Are you doing that in your area? It's not the use of wood that surprises me, it's the use of solid wood. Stateside, I did a lot of wood elevator shafts that had studs at 12-16" o/c.

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.

 

[msquared48]

Haven't seen it in newer structures yet, but have seen it in very old structures. Did a remodel on a Sorority house at the UW three years ago, and some of the fire walls and stairwells were if this construction - probably at the turn of the last century.

Another reason for using it presently, at least in high seismic regions, would be to reduce the dead load of the stricture. Compared to fully grouted CMU, it is 1/3rd of the weight. Considering the weight of an entity structure it is probably not much, but every little bit helps - ant you do not have to use strut and tie!

Mike McCann, PE, SE (WA)

 

[mike20793]

I've heard solid shafts be mentioned in my area for the past couple years, but I haven't seen it yet. I got similar responses as to why solid and not studs and, similarly, I was not satisfied. It seems like it was primarily a fire rating and noise transmission issue for the architect that brought it up. I've done wood stair shafts recently and they were just studs at 12". I remember having trouble keeping the integrity of the fire rated assembly, but that was for the landing framing and how it framed into the sides of the shafts.