Wood Podium Bldg - CMU Walls/Urban Infill Issues

[bookowski]

We've only recently started dabbling in wood podium buildings and have an atypical one that's bringing up some questions.

This is an urban infill lot, about 100ft wide x 175ft deep. It'll be 5 wood over a 1 story (+basement) concrete podium. It will go lot line to lot line, with only a seismic gap to the adjacent buildings. The adjacent buildings are similar height, around 4/5 stories masonry buildings.

The lot line walls will be CMU, this is for waterproofing/durability/fire/etc since they will be built inches away from existing buildings. So along the lot lines we'll have about 175ft of cmu wall. A concern is shrinkage and how we deal with this. Based on a quick run with the simpson shrinkage calculator the shrinkage is expected to be on the order of 3/4". There are a few different framing conditions, often the joists frame perp to the cmu but there is a corridor condition and one area where they are parallel. So the impact of that differential shrinkage may be expressed in a few ways in the framing/floors but seems problematic in general.

I've seen examples of and suggestions to build a secondary wood wall inboard to alleviate this issue. In the grand scheme of things this probably isn't a lot of wasted wood. However, when I look at all this cmu wall it seems like I should be using it for lateral. I could eliminate all wood shearwalls in the cmu direction if I use the CMU lfrs. It does seem wasteful to not use the cmu for lateral. Can I have a secondary wall and make a reasonable flexible connection (to allow for shrinkage) that still competes a lateral load path? Or do I forget the secondary wall, use the cmu for lateral and make some kind of other provisions for shrinkage? In another post on here about general shrinkage issues I see that someone suggested that they always frame into the cmu walls and specify an initial slope that will undo itself as the shrinkage occurs. Anyone else doing this? I had thought about this option but it didn't seem practical to me (are the framers going to do that?) and unless you decouple/detail the finishes it seems like you'd still have the same problems with cracking etc.

 

[driftLimiter]

What is the floor system at the podium level? you mention joists framing to and parallel to the cmu. Perhaps you could use a portion of the CMU wall near the point of no movement, then release the rest of the CMU from the floor system and just use it for bearing. I have used a combination of building felt, and foam covered dowels to provide a slip between non-shear heavy walls and podium slabs.

 

[Aesur]

Wood shrinkage is usually highest perpendicular to grain, one thing to consider is using engineered plates in your walls. Another trick is to use balloon framing instead of platform framing. The downside of balloon framing is that your walls are taller which can result in stud design issues.

 

[masonrygeek]

Being a masonry guy, I don't see why you don't just switch over to an all masonry solution. With wood price increases and the often divergent issues of stiff CMU elements and flexible wood elements, it makes sense just to go one route and try not to build a double frame to make it work. Aesur has some good suggestions to minimize shrinkage of wood. Let me know if you need further information on masonry and where the project is located.

 

[phamENG]

To masonrygeek's point, here's an ongoing study that looks at comparative costs for this kind of structure: Initial Cost Study Report and Addendum Cities

 

[jayrod12]

Eliminating or significantly reducing the amount of dimension lumber being used perp to grain will provide fairly significant reductions in shrinkage as alluded to by Aesur. I've also seen some solutions where the interior bearing lines of wood are incrementally framed a bit taller. Generally speaking for standard wall construction with engineered lumber floor systems (i.e. minimal shrinkage in the actual floor) we usually expect about 1/4" per floor of shrinkage. So I've seen it framed such that all of the wood bearing is taller than the masonry/concrete bearing surfaces, but not usually the full amount because you end up with wood bearing plates on masonry often. I'm not sure how that would work at 5 floors, maybe it's a matter of no increase at the second floor, 1/8" per floor after that. By the top level, you've gained back almost half of the expected shrinkage.

After reading the study posted by Pham, masonry and hollow-core sure seems like an attractive option. In the addendum for my area, that's only a 3% cost increase, and that was pre-covid. With the lumber pricing now, I can only imagine that it's a wash, or masonry and hollow-core may be cheaper than a wood building.

 

[phamENG]

5 stories of light frame wood is a special case. Mixed materials on a 3 story building can be handled easily. 4 stories is manageable with careful detailing, but 5 stories is not fun. The best bet is usually to go with an all wood system using wood shear walls and wood shaft walls. When that's not an option, you either have to abandon wood or accept that somethings will have to be done very differently and/or you'll have some sloping floors and cracks somewhere. If you can keep bearing points a good distance from the CMU and keep the line consistent, you can frame the floors perpendicular to the CMU and let the floor slope. The further the interior bearing is away from the CMU, the shallower the angle and the less of a problem it is. If you have a wall 10ft from the CMU, this probably won't work.

 

[XR250]

My girlfriend lives in a four-story with CMU elevator cores. The floors slope pretty significantly at those locations. Even the slab on the first story slopes LOL.

 

[bookowski]

Pham - Thanks for that study. I didn't see the appendix b that they refer to which is drawings of the test structure but from the description it does not sound like it's a podium type. I wonder if that would even further lessen the savings of wood. I would think that there's some premium to mobilize a concrete sub only to do a ground floor and transfer slab and then get going in wood vs other systems which would be consistent all the way up. This particular one does not seem like it wants to be wood but it'll likely stay that way (some decision makers on this one have a strong preference for wood, I think it's mostly due to a good job at marketing by the wood trade).

I've also seen some solutions where the interior bearing lines of wood are incrementally framed a bit taller

I was wondering about this but it seemed too fussy, can you really get framers to do this? It seems that the cracking will be the same in any case but you could at least shoot to cut that final floor slope down. Rather than frame the walls high I'd think that I'm just specifying the ledger to masonry as low (semantics I guess), which isn't too bad. We've managed to get all framing perpendicular but there is still a long corridor condition so it's a short span. Maybe shooting for 1/2 the expected is worth a shot here along with a flag to the client of what to expect.

 

[phamENG]

Correct - not a podium. But then above the podium, you just have a multistory apartment building so you're still comparing apples to apples. Might be granny smith and golden delicious...but they're still apples.

If there's no turning them back, make sure you spell out the risks, how to plan to mitigate them, and emphasize that mitigation is not prevention - cracking will occur.

Here's another good document if you don't already have it: Link

 

[masonrygeek]

If you haven't given up on a masonry solution, here is an NCMA TEK that talks about the design:

https://ncma.org/resource/construction-of-high-rise-concrete-masonry-buildings/.

TMS's Masonry Designers Guide also has some examples of building designs. Of course, you don't need a podium for a concrete masonry building since it does what a concrete podium would do and, it is more economical than having a couple of different trades on the jobsite. My 2 cents.