Drawbacks to reducing number of exterior plywood shearwalls

[skimboard20]

Hi all,

I am working on a two-story residential project in the Pacific Northwest. The project is typical wood framing spanning to stud walls and plywood shearwalls. In an attempt to save on construction costs, the contractor for the project is looking to reduce the number of shearwalls. His idea is to limit each side of the structure to one stiffer shearwall with larger endposts and holdowns instead of the several shearwalls on each side of the structure that I designed. In theory I don't see why this would be an issue, as long as one segment of shearwall on each side of the building can resist the wind and seismic demand (and there is a lateral load path from the diaphragm to the single shearwall). But I suppose I've always maximized the number of shearwalls for a structure (i.e. if a wall segment meets the NDS requirements for a shearwall, then it gets designed as such). So significantly reducing the number of shearwalls goes against my typical design intuition.

Does anybody have any thoughts on this? I'm looking for any potential drawbacks to redesigning the lateral system with fewer shearwalls.

Further food for thought: All of the shearwalls are currently designed as our minimum shearwall ('SW1' in our nomenclature). We specify that all walls not designated as a shearwall get sheathed and nailed per our SW1 designation. Essentially, the whole structure is sheathed and nailed as an SW1, unless a particular shearwall has a different designation (SW2, SW3, SW4 etc. with increasingly stringent sheathing and nailing requirements). If I reduce the number of shearwalls, but the rest of the wall segments are sheathed and nailed as an SW1, aren't we kind of back to what I currently have designed? Except we've presumably added more nailing since one wall on each side of the structure will presumably increase its shearwall designation (and potentially increased the construction cost).

Thanks for the help!

 

[driftLimiter]

To your first point. There is nothing inherently wrong with doing this but the code and the flow of forces will make you pay a bit.

First off you need to re-evaluate the redundancy factor on those walls for seismic. If you get down to too few shear walls you get a 30% bump in load. Could still be worth it to the contractor.

Second you have drags and collector forces. Using your normal strategy you might not pay them much attention (maybe you do) having just 1 pier a very few piers will greatly increase the drag forces. The connection of these forces to the shear wall may become prohibitive.

Third the foundation this is a big one. You gotta get those overturning forces down and the original way of having them spread over the length of a building can make a conventional wall footing work decently well. As you concentrate the overturning forces you end up needing a pretty big footing to resist them. Also will need a fair amount of work to actually anchor the hold downs into the footing. But if you can get away with using off the shelf hold downs then you will probably be okay in this regard. Just think about overturning stability of that footing as you size it. Even on two story buildings with few shear walls I have designed some pretty big footings for this reason. They often extend around 5-6 feet beyond the extents of the shearwall. Granted I often neglect contributions of adjacent typical wall footings if I need to engage long segments of wall footing to resist overturning then I model it as beam on elastic foundation.

The minimum SW1 on all the walls not designated as shear walls don't have hold downs and they don't need inspections. This is always done in my experience and I wouldn't even really think about it much if posed with this problem.

Contractor said he wants less shear walls for (whatever) reason. As long as your being adequately compensated for the time, I say give him less shear walls and let him decide if its worth the trade offs.

 

[ryaneng]

Not really any drawbacks to this, except for the fact that it may not actually decrease the cost. Typically what I would do on a design like this is determine the total load along a resisting line, then see if this load can be resisted by the total strength of the unblocked wall segments (this is essentially the default exterior wall). If the capacity is not high enough, determine a length of shearwall that is needed with edges blocked and nailed at 3" o/c. Allocate that shearwall length as advantageously as possible to minimize the number of hold downs required.

 

[Aesur]

My thoughts are that I only grab the shear walls I need and don't care what they sheath the rest with. Is there a reason you require SW1 on all "non-shear walls"?

If you think about the construction effort it takes to install multiple shear walls versus a single shear wall, it's significantly more effort for multiple ones, not to mention the increased number of holdowns and potential for construction mistakes thereby increasing the headaches you may have to deal with down the road.

Occasionally we have requests to sheath the walls in one plane that aren't shear walls to create a flush finished surface, but this is rare and typically only for corridor walls in apartments. We make it clear that this is an owner decision so they know the cost implications.

 

[skimboard20]

Thanks for the input everyone. It's sounding like there's not really an issue performance-wise with reducing the number of shearwalls. For this particular project, it does seem like it's stepping over dollars to pick up pennies. But at the very least I can go through the design exercise and inform the contractor.

 

[driftLimiter]

@Aesur, I don't use the strength of stiffness of these walls for LFRS so I don't require holddowns or increased anchorage on all of the exterior wall not designated as shear walls, only the sheathing of matching thickness with minimum shearwall nailing.

I've always seen it done that way without really thinking much about it, because I've never heard anyone complain. I suppose it serves as the structural envelope, could serve as stud bracing, so it is nice to have a specified spacing for that. It allows the finishes plane out, probably increases the shear stiffness and perhaps strength too.

 

[skimboard20]

@Aesur, I'm not exactly sure our firm's justification for sheathing the entire structure as SW1. But I'm not sure I've ever seen a construction site in our area where the entire building isn't clad with plywood before the WRB, etc. goes on.

To @driftLimiter 's point, I also ignore the additional segments of SW1 in the overall lateral design. I feel better knowing that they might provide some stiffness/lateral stability that is not explicitly accounted for in the lateral design.

 

[Aesur]

For this particular project, it does seem like it's stepping over dollars to pick up pennies.

I don't know the size or scope of your project so I cannot comment directly to this, however considering how many walls there are in multi-family residential projects, sheathing all walls can get quite expensive. In fact we try to use corridor walls and party walls for most multi-family to keep from sheathing any exterior walls as this is a huge cost savings for the owner. Again, it depends on the project to see any potential savings.

I don't use the strength of stiffness of these walls for LFRS so I don't require holddowns or increased anchorage on all of the exterior wall not designated as shear walls

Correct - I was commenting to the fact that per the original post skimboard20 was using all the walls that can be shear walls as shear walls which could be significantly more holdowns than one or two walls in a shear line with heavier holdowns. Again, I'm not sure if perforated or segmented is being used however. I agree there is no reason to have HD's on non-shearwalls. I personally only use segmented and block and strap in my designs.

I believe we are seeing two scenarios currently:
1. Some contractors are locking into GMP's and with costs going up, they need to find places to cut anything they can. I have received RFI's with substitution requests for things as simple as H-clips to save a few pennies each which on a large project can add up.
2. With interest rates increasing money is harder and harder for developers to come by which will result in a slow down and value engineering of projects, so the cheaper the project is, the higher the change it will go to completion.

 

[jerseyshore]

I have done a ton of multi-family recently and these are common comments I get on most of the projects. I know for me it's not so much the nailing, but the reduction of hold downs that owners are always trying to eliminate. And with the crazy window to wall ratio it gets harder and harder to accomplish on each project.

Typically it's 7/16" plywood for the exterior walls and 5/8" gypsum (sometimes two layers) on interior walls. I try to avoid changing sheathing at all costs. I start similar to what ryaneng mentioned about figuring out the total load and seeing how much wall I need along that line. For exterior walls it's all segmented designs since you won't see 8' of solid wall anywhere anymore.

I know some engineering firms ignore the exterior walls and just use corridor/ unit demising walls as SW's, but I don't like to mess around with all those windows and sliders. I want the exterior walls to be as stiff as can be. Using segmented designs also helps to eliminate a ton of those pesky hold downs.

I specify my shear wall(s) and their nailing patterns and any non shear walls are just nailed per code.

I don't think reducing the number of shear walls will impact the performance that much as long as you're smart about it. I feel confident with fewer shear walls as long as I know they are spread out and well balanced throughout my structure.