5 storey apartment wood shearwall layout

[Shotzie]

Hello,

I'm looking for some feedback on the proposed shearwall layout (see attached). My main concern for his building stems from the portion of diaphragm spanning between walls 4-1 and 6-1; this portion will have a 2 storey atrium located in the centre of it as well. We are in a relatively low seismic zone in Canada, and it appears that wind is governing the design. The building will be wood shearwalls with Simpson ATS hold downs and a wood diaphragm with 1.5" non structural topping. The plan dimensions are 212 ft x 68 ft, and the floor to floor heights are 10 ft 6 in for the main floor and 9 ft 6 in for the remaining floors. Any feedback is appreciated as we are still early on in the design phase.

Thanks

 

[dik]

Can you introduce anything between B4 and B5? Do you have end stairwells and an elevator? Looks like you have lots of shear walls and other than B4 and B5, the distribution looks good and particularly for low seismic. With good floor diaphragms, the interior walls will take a fair amount of the loading with the outside ones providing torsional resistance. The building's not too high... was in a recent discussion with my wood expert and it appears that Norway is doing 'real wood' to a height of 12 or 14 storeys.

Dik

 

[Shotzie]

Unfortunately between B4 and B5 the top three levels of wall are offset in-plane from the bottom two floors. There may be some out-of-plane offsets in this location as well. We could potentially introduce beams to pick up the offset walls but I'm pretty sure our max beam depth will be 11 7/8" so it may not be possible. I'm reluctant to introduce any steel frames as well as I may take a penalty on the RdRo values for wood.

There are stairwells located at either end of the building (at the ends of the corridors), and an elevator shaft (spanning top to bottom) located to the right of 5-1. These will likely be constructed out of masonry so our intention is to wrap them in wood to separate them from the structure.

I've heard talk of going even higher with CLT, seems like there's a lot going on to push the envelope with wood design. Sounds like North America is trying to follow in Europe's footsteps also.

 

[SJBombero]

It makes a great deal of difference if you design this as a rigid or flexible diaphragm. If you model the diaphragm as flexible, you would add a collector from the end of 5-1 to the opposite side of the building to make up for the missing wall in the layout. However, if you can design it as rigid, the shear wall layout is perfect. You would just need to check the diaphragm deflection to justify the idealization as rigid. If the deflection between 4-1 and 6-1 is too great (> 2 x SW deflection) you can add the collector strap from 5-1 to Line B and then your maximum span would be from 5-1 to 6-1.

 

[mike20793]

If your diaphragm dimensions allow, I would abandon those puny shear walls at the exterior running parallel to the corridors and make every wall in the corridor a shear wall and use rigid diaphragm. Running perpendicular to the corridor, I highly doubt some of those will work or be practical for compression or tension from overturning. I find that almost always controls for my 4+ story wood buildings, rather than shear wall capacity.

Edit: Also watch out for plumbing risers. They will usually want to put them in your shear walls and that is a big no-no.

 

[Shotzie]

@pvchabot We've been running an envelope design (rigid and flexible) for the diaphragm loading to the shearwalls. With regards to the diaphragm design itself, I'll need to take a closer look at the diaphragm deflections to see which assumption (rigid vs flexible) I need to consider. I like the idea of adding the collector strap, I'll see if the design requires it as you've recommended.

 

[Shotzie]

@mike20793 Thanks for the tips. I kept those little wall segments in the initial design for the sake of closing off my flexible diaphragms, given that my corridor to exterior wall span is approximately 30 ft. If I went with the rigid assumption that would help get rid of those, I'll have to take a look at how my diaphragm performs with the rigid assumption. Given the amount of exterior wall we're currently utilizing it seems like the tension/compression forces from overturning will only be approximately 10 kips, although I haven't examined it too closely yet.

 

[mike20793]

That uplift isn't too bad unless your anchoring to a podium slab and then it can get a little tricky but its definitely doable. Check out the 2015 SDPWS. They have a new provision for assuming rigid diaphragm in the direction of the corridor and it's all I ever use since the diaphragm is so deep relative to its width. I've also justified rigid diaphragm perpendicular to the corridor when you actually look at diaphragm versus shear wall deflection. I only use that if I have one little short wall that gets hammered from flexible diaphragm.

 

[Shotzie]

Agreed on the podium slab connection. Luckily this building doesn't have a podium slab and also doesn't even have a basement, so we can get away with quite a bit for foundations below the main floor.

Good reference, I've read that previously but I'll need to take another pass through it to refresh myself, sounds like it might be very applicable for what I'm dealing with.