CLT Shafts 2

[phamENG]

Working on a 4 story wood design. The initial plan was to use CMU shafts for LFRS. This is a wind dominated design. It's unlikely that a building designed to meet our wind requirements would not meet our (nearly) laughable seismic requirements, but I'll pay them due homage when the time comes. (I'm in Coastal/Southeast Virginia.)

The framer reached out to the design team today and suggested CLT in lieu of the CMU. At first glance, I love the idea. As far as the architect and I are aware, only one project has used CLT in this region - and that was a roof. So having an opportunity to work on what is likely to be the first lateral CLT project in the region is exciting.

Here's the rub - CLT lateral design info seems to be a bit lacking. I've found a few white papers, and read the lateral chapter in the CLT handbook, but I'm not coming away with the level of confidence I'd like to have. Most of them are limited to story height shear walls in platform construction - the idea here is to build the shaft full height (50' +/-) and the come back and build up around it like you would with a typical CMU/CIP shaft.

Does anyone have any insights into this? The benefits of CLT are clear, and I'm sure it can work in this situation, it's just a matter of figuring out how. Any resources or experiences would be welcome. Thanks.

 

[r13]

Sorry for my ignorance, just out of curiosity, what is CLT, concrete lathed timber?

 

[Agent666]

Cross Laminated timber. I'm surprised it didn't come up in a Google.

 

[KootK]

Exciting. I'll help if I can. I've not done one myself, however:

1) I've got a lot of CLT docs so if you're interested in whether or not a particular one has worthwhile info, let me know and I'll dig.

2) My wife is a local mass timber expert and I believe that she's done at least one three story shear wall system. I'll try to procure the details as I feel that this exercise will be mostly about the connections.

3) My wife tends to be a bit terse in discussing her expertise but her first response was "all he needs is the FP innovations manual". [link Link]https://web.fpinnovations.ca/clt/[/url]. That's what you've already got, right? If so, please elaborate on what you feel is lacking and I'll see what I can do to help.

 

[KootK]

What's our floor construction going to be?

 

[phamENG]

Thanks, KootK. Floor system is floor trusses with concrete fiber structural panels. (First time I've mixed them with wood, but I've used them in light gauge successfully and USG says they'll work with wood framing.)

I have the US version of that handbook...which is 6 years older. Looks like there's some additional info in the Canadian version that may be of use. I'll dig a bit and let you know.

I'm a document junkie - if you're sitting around with nothing better to do, I won't complain about you posting some CLT lateral design/connection papers. I do agree with you - the connections will be the key piece of the puzzle.

I'll be reaching out to the manufacturer proposed by the framer today, but for now need to figure out just how much effort this is going to take to change the design.

 

[jayrod12]

I've currently got a project on the go with CLT floors on glulam columns. Unfortunately (fortunately?) the lateral system is CMU shafts instead of CLT. But I likely shear walls to diaphragms, and based on my discussions with the supplier the diaphragm capacity is whatever you can get out of the connections. They have multiple different options for the splices between panels depending on the required shear transfer between adjacent panels. Otherwise it's just rigid body motion of each individual panel.

I feel for shear walls, it would be extremely similar, except you'd need to pay much closer attention to panel buckling. If it were me, I'd be assuming a elastic compressive stress distribution with a discrete tension hold-down. Then looking at a portion of the end of the wall as a column for the buckling type checks. I feel the actual shear capacity through the panel will be far in excess of the other potential failure modes.

 

[KootK]

I won't complain about you posting some CLT lateral design/connection papers. I do agree with you

I'm afraid that you'd need to identify something you were interested in and then let me dig for it. I haven't read most of what I've got. It might also be worth contacting the Canadian Wood Council. They're interested in promoting this kind of construction and may be able to offer some meaningful help. Hopefully we here from some of our Euro-peeps here too.

 

[Brad805]

What was the reason for the proposed change? If for enviro reasons, ok, if not, I would let the manufacturer propose a solution with a budget for the design team to consider. Delivery can be brutal with CLT, and right now wood price in our locale has went from $300 per thousand BF to almost $2000 per thousand BF.

 

[phamENG]

KootK - understood. Once I'm done digesting the differences between the Canadian and US manuals, I'll let you know.

Brad - primary reason is schedule. The framer is saying he can complete all of the shafts in days to a week, where CMU could take several weeks to reach the same height. At this point, it's mostly a question of feasibility since nobody local has done it before. We want to make sure it is (which I'm pretty sure it can be, just need to enough convince the rest of the team) and get an idea of the design side costs. The framer is working up the construction side of it and we'll decide to stay the course or switch later this week.

 

[Brad805]

Ok, that makes sense. We have done quite a few precast cores for elevator shafts when the forces are reasonable. I have nothing against CLT's, but paying the same for wood or concrete always makes me pause.

 

[skeletron]

Definitely go for the FPInnovations manual as it is the most recent.
Check where your nearest CLT supplier is. I think there are 3 in the eastern USA, and a couple in Canada on that side of the country.
Find what your supplier's panel size restrictions.
Determine the fire ratings for your building code too.

From my understanding (which is a 5 on a 100 point scale) so much of mass timber is based on standardized grids and connections that are defined by the supplier's production and the contractor's ability.

 

[phamENG]

Thanks for the support, everyone. Looks like we're not bringing in CLTs this time around. Next project with this developer, however, is likely to use CLTs. So I'll keep reading up on it and studying.

KootK - if you can get your hands on those details, I'd still love to see them. Thanks.

 

[XR250]

Pham,

Might want to consider the current lumber shortage and corresponding sky-high prices. CMU may be more attractive.
(Just noticed Brad805 made a similar commment)

 

[bones206]

The folks from Woodworks are very helpful and a good resource for education and project support.

 

[phamENG]

XR - yes, that was a factor we considered. While not a DB, the contractor is already on board. So my focus was primarily on design feasibility and costs. The contractor was determining material costs.

 

[KootK]

KootK - if you can get your hands on those details, I'd still love to see them.

So... as is sometimes the case, my wife doesn't want me splattering her intellectual property all over the interwebs. That leaves us with two choices:

1) I very sloppily hand reproduce the details of interest and post them here. I'll be honest, there's at least a 50% chance that I'll never get around to following through on this.

2) We connect burner to burner and and I get you what you seek that way. And if you screw me over and get me divorced I'll make it my life's mission to ensure that you never utter another word online without it being more hotly, and uselessly, contested than the 2020 election. If you wish it, you can sort out my contact info here: Link

 

[molibden]

I've done a couple CLT shafts and a ton of CLT buildings in my short career. Ask me anything

 

[phamENG]

KootK - option 2 sounds good. Unless you have a professional clone wandering around, we connected on LinkedIn 2 months ago. I'll give your riddle a go, though. Seems easy enough.

molibden - great. Thanks. Since it was pulled out of this project, I can't devote as much time to it, but as I dig into it I'll come back with questions. In the mean time, how do you feel about some of the other comments? Particularly what jayrod had to say about his analysis approach, etc.?

 

[molibden]

I'm from Europe so keep in mind things are different.
Tools I use:
MAIN SOFTWARE: Timbertech. Italian software with built in stiffnesses for walls, connections.. I do seismic and wind loads analyses. It designs CLT walls, floors, connections. Almost everything in a common building.
FEM: I use Dlubal RFEM with additional module Laminate which designs also CLT. Great software, I highly recommend it. Especially for complex stuff.
APPS: Bigger manufacturers provide good software, online or offline, to design different setups of CLT. Look into Calculatis by Stora Enso. You can see some theory behind different cases, but it's for Eurocodes.
HAND CHECKS: A few years ago we had span tables etc., now I use hand checks mainly for connections.

I design CLT floors using sofware I described above. Then you design walls for vertical loads (together with buckling and also fire).
Check compression perp. to grain.
For lateral loads almost always thicknesses of walls stay the same. CLT panels are really rigid and strong. Even buckling of walls is almost never critical for lateral. Fire design is a lot more demanding for the walls.
Then you come to the core: connections.
Diaphragm is almost always considered rigid in Europe. So you need to check connections between panels. Design of diaphragms is really lacking in our area, so we really don't check deformations but only resistance. We tend to overdesign connections between panels with full thread screws to keep it rigid as possible.
Design of hold-downs is the same as for timber frame (a force couple - tension/compression on each wall end).
Then there are connections wall/floor, wall/wall etc. You need to figure shear flows and design the screws, brackets... accordingly.