Masonry Cores at Elevator & Stairwells in a Wood Framed Buildings

[Aesur]

NDS section 4.1.5 states: Wood-frame shear walls, wood-frame diaphragms, trusses, and other wood members and systems shall not be used to resist seismic forces contributed by masonry or concrete walls in structures over one story in height.

The exceptions are as follows:
1. Wood floor and roof members shall be permitted to be used in diaphragms and horizontal trusses to resist horizonal seismic forces contributed by masonry or concrete walls provided such forces do not result in torsional force distribution through the diaphragm or truss.
2. *Summarized* - limit to 2 stories provided that floor to floor heights do not exceed 12', diaphragms shall not be considered to transmit forces by torional force distribution or cantilever past the outermost supporting shear wall, diaphragms shall be blocked, 15/32" min sheathing on shear walls & blocked, no out of plan horizontal offsets between stories of shear walls.

I am curious how others view these statements as I have seen many apartment and other buildings buildings that are taller than 3 stories, wood framed and have the masonry cores tied into the lateral system, per these code requirements, I don't see how it's possible to justify dragging the masonry into the wood system when taller than 2 stories and "meet code".

The solutions I keep coming back to are as follows:
1. Design the cores as standalone structures and have a seismic joint all around - architects hate this.
2. Design the cores as standalone structures and "box out" with beams around the structure, but run the sheathing past the members and butt against the core and rely on the floor sheathing being a weak point that may fail/crack during an event.
3. Wood cores - not all contractors have come around to this thinking yet and like that the masonry/concrete cores can be considered OSHA access as well during contruction.

Going one step further, would it be permissible to use a lower R value (of the masonry or concrete system) and design the wood and masonry together, this appears to be a common approach per Structure Magazine and WoodWorks publications, however it doesn't appear that this meets the requirements of the NDS code as technically you are still relying on wood shear walls to offer some resistance to the seismic forces contributed by the Masonry/Concrete cores. What are your thoughts? Are there any other good publications/information on this issue?

 

[chris3eb]

I would think for a low-rise structure, a masonry/concrete core wall would be stiff enough that none of its load would make its way into the flexible wood system and it would just go down through the core to the foundations. If that's the case, then the wood system isn't being used to resist those seismic forces?

It seems like there definitely could be some detailing considerations at the interface as you allude to. I don't really do any wood design, so take this with a huge grain of salt.

 

[XR250]

but run the sheathing past the members and butt against the core and rely on the floor sheathing being a weak point that may fail/crack during an event.

Seems like a stretch to count on 3/4" sheathing failing. Is there any testing that backs this up?

 

[Aesur]

Seems like a stretch to count on 3/4" sheathing failing. Is there any testing that backs this up?

No, I am not aware of any testing to back it up, however if the elements moved at different frequencies during an event (most likely would), would this be a life safety issue or just cause some local cracking/buckling?

My thought is that you could show at what force the decking can buckle based on "unbraced cantilever length" and that the wood building could handle the additional seismic force from the CMU core which technically would be an opposing (lower seismic) force if it was to apply additional loading to the building because it would need to bear against the building sheathing and thereby move in the opposite direction of the building based on stiffness (at least in theory).

How have you seen this done in other buildings?

 

[Aesur]

It just hit me that there may be some sort of testing that could be utilized to show the sheathing is the failure mechanism using the testing and code portions that don't allow sheathing only to brace CMU or concrete wall for seismic separation, I will have to review this further as it's been a while since I dug this deep into it.

 

[KootK]

1) My suspicion is that the seismic gap is the future unless we eventually go to wood shaft walls. Neither is happening with regularity in my markets.

2) The code language suggests to me that the intent is more about things like firewalls where the CMU truly rides along rather than a shaft wall system that has meaningful lateral capacity in its own right. Not that this obviates the need to consider shafts from a code compliance perspective.

3) The code language suggests to me that this is primarily about diaphragm / VLFRS torsional issues. Unfortunately, they don't really quantify that. Some possible interpretations:

a) If it's a rigid diaphragm and the CMU mass doesn't create an ASCE7 torsional irregularity, perhaps that is okay.

b) If one chooses to treat the diaphragms as flexible, perhaps that is okay.

4) I do feel that one could use the wood and cmu shear walls in concert with the lower R-value and still be code compliant. That, because similar to Chris3eb's comments, I feel that a shaft is fundamentally a different animal than is a true ride along wall. I'd be willing to be even more liberal than the structuremag article in that I'd be willing to apply the reduced R-value only for the shear wall lines adjacent to the CMU shaft.

5) For most things, at heart I am still a fan of just tying stuff to other stuff. My preference would be for some kind of prescriptive code provision that just says that you need drag strut coming off of the corners designed to resist such and such force. Wood drag struts are kind of difficult to attach to block walls. In that respect, a box out system as Aesur described combined with drags coming off of that box appeals to me.

 

[masonrygeek]

Aesur, you forgot one other option to consider: 4) masonry loadbearing structure where all the walls work together. Are the rising costs of lumber making this a viable alternative? It seems like we keep trying to make something work when there might be an easier solution....

 

[Aesur]

Masonrygeek, the particular project I'm currently on with this change was driven by "this is how the contractor has always done it and they have never seen wood cores". However I am being told that the price of wood has reached a point where masonry is equal and sometimes cheaper.

I wish the developers would give us the leeway to make these structures all masonry, unfortunately the design was at 50% CD's when the contractor requested/made the change to CMU cores in the otherwise large wood apartment buildings. Being that we are at 50% CD (technically closer to 75%), there is no time nor budget to make such large changes at this stage without considerable delays.

We are hoping this one can be an opportunity for us to provide a teaching experience of all the intricacies of these kinds of changes and designs to the developer and contractor.

We have had further discussions with the team and have helped them understand code requirements and some theory behind this; they are being understanding and the contractor is looking into wood cores for future projects now.

 

[slickdeals]

Is the masonry dragged into the wood? Wouldn't it be self-supporting and designed as a shear wall. I assume the intent of the code is to not have nonload-bearing masonry walls supported on wood shear walls and relying on diaphragm to move those loads.

 

[Aesur]

The masonry will be dragged into the wood, due to height and thickness, it will not be self supporting as it won't meet aspect ratios in many locations and therefore will need to rely on the wood to support it. Because of the stage this is in, there is not enough time to completely redesign the lateral system to use the masonry shear walls, ie lower R value and therefore looking at dragging into the wood structure and utilizing the wood shear walls to support the masonry. Additionally, due to placement of the masonry shafts, we would need to further look at rigid diaphragms as it could induce torsional irregularities, not sure about you, but the last time I went down the rigid wood diaphragm path, it got ugly fast.

 

[phamENG]

I think you have to use a separation joint and make the masonry self supporting. I tried this once with a wind design, and it got ugly fast - can't imagine seismic. The deflection in the wood diaphragm was WAYYY more than the masonry could handle without some severe (failure) cracking.

Or they give you more time to design it properly. Or, they could build it the way you were designing it. (Novel idea, I know.) Or you could compromise and go for a CLT core.