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wood shearwall aspect ratio

wood shearwall aspect ratio

(OP)
I have a wood framed elevator tower that is 3 stories tall and is only connected to a floor diaphragm on 1 of the 4 sides.

The tower walls are taking load from the floor it is connected to.

I have provided beams at each level in the 3 walls not connected to the floor so that the studs aren't spanning all the way to the roof. They transfer the out of plane loads to the perpendicular walls.

The question I have is about the aspect (height/width) ratio of the 3 walls not "braced" by the floor. I'm of the opinion that the height of each panel is from beam to beam (level to level) but I'm being questioned on this by the Building Official. He states that the panel height should be measured from ground to roof (from diaphragm to diaphragm) .... this is in regards to the allowable aspect ratio of 3.5:1 for wood SW's, not overturning moments....

It is my opinion that the horizontal beams we have framed at each level act similarly to a diaphragm in that they transfer out of plane loading to the perpendicular walls just as a diaphragm would. I don’t see how having a “diaphragm” at each level (supposing there is no elevator) would make the exterior SW’s behave any differently than the beams we have provided….

I would like other opinions on the “substitution” of the diaphragm with the horizontal beams at each level…

RE: wood shearwall aspect ratio

I would agree with your approach per that written explanation. The beams will act the same as a small floor system would, possibly better than other situations that may pass by inspection. I would say that your approach seems logical. I do think that some detailing is required at the beam-to-wall connections to ensure all works and transfers forces. I am a little concerned with how the overall system will handle your loads since it is only connected on one side (1 of 4).

If you have an elevator tower, is it exterior? The 3.5 ratio is for wind and 2(?) for seismic. If that tower is inside than those walls don't have to get any load... or wont get load if not attached to the floor systems.

RE: wood shearwall aspect ratio

2:1 is to have non-reduced load values. Max is still 3.5:1 for seismic. At that point you are getting a sizeable reduction in capacity.

This is an interesting post. My first thought was you could justify the block being connected to the adjacent diaphragm and resisted in rotation, like a typical deck/low roof with no lateral at the outer edge. However, thinking about it more (especially how the elevator will be connected to the walls) I'm not so sure.

Rails will be tied to the side walls if I'm visualizing correct. With lateral load in that direction (in line with the walls that tie to the floor) I agree there is no problem. In general I would not try to resist floor load with these walls, but I would shear them to span beam to beam and deliver load to the floor.

However, in the other direction (parallel to the elevator entrance) the rails will push on posts in the walls which will deliver load to beams above and below. These beams then deliver load to the "connected" side, which is fine assuming there's lateral somewhere there. But they also send load to the outer wall, which I think will act like a tall wall.

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