Perforated Shearwalls per IBC section 2305.3.7.2

[tdawgui]

Can someone please help clear up some of the grey areas I have about this section???

1. "The maximum opening height shall be taken as the maximum opening clear height."

Say I have a 5' window that is 3' off the sill plate. I first took this to mean that the opening height is from the top of the window down to the sill plate (totaling 8'). However, I then begin to wonder when I would ever have a maximum opening height that would fall under the H/3 category. I think having the H/3 category and showing sample opening heights of 2'-8" and 3'-4" tends to suggest that the opening clear height is actually just the height dimension of the opening (5'). This does make some sense too since as long as you have sheathing with the appropriate nailing pattern in the areas above and below the window, they will add stiffness and should be accounted for. Am I right in my thoughts?

2. Uplift anchorage at perforated shearwall ends

The equation for calculating the uplift doesn't incorporate the vertical resistance to uplift that you will get from applied dead loads. My solution to this is to calculate Vh, subtract 2/3 of the dead load multiplied by 1/2 the total perforated shearwall length (its moment arm), and then divide that value by the adjustment factor multiplied by its sum of perforated shearwall segment widths. Any reason why I shouldn't incorporate my dead loads?

3. Anchorage for in-plane shear

Issue 1
The equation divides the Shear Force (V) by the adjustment factor applied to the sum of perforated shear wall segment widths. First of all, isn't this very conservative as the sections of sheathing under and above the window will help stiffen the shearwall? I think that starting with just a portion of the wall, not even the whole wall, and then reducing that even more with an adjustment factor is simply way too conservative. I imagine that you should be able to use the whole length multiplied by the adjustment factor.

Issue 2
Ok, assume the method is correct for Issue 1... Now, when it comes down to the sill plate and its fasteners, I would think that you should be able to use the full length of the wall instead of just its segments. My reasoning here is that, unless the opening is a doorway, the sill plate would be continuous and would spread the load through the entire length. Therefore, I think that for sill plate fasteners I should be able to take V and divide it by the adjustment factor applied to the entire length of the perforated wall. What are your thoughts on this?

I know this is quite a bit and I hope I haven't thrown too much confusion into my cry for knowledge. If you have questions or think I may be heading off on the wrong track, please let me know... I've thought about this way too much today and my brain is a little fried.

tdawgui

 

[DaveAtkins]

The maximum opening height applies when you have more than one opening--you must use the largest opening height when entering the Table.

You can use 0.6*DL to resist overturning. I don't have the Code in front of me, but I recall you still must design the uplift for the wall shear times the height of the wall.

I talked to Ed Keith of the APA about this once--he said in seismic zones when a perforated shear wall is "rocking and rolling" the sill plates under full height segments of the wall see more shear. His comment applies to shear walls above the first floor of a building. It does not apply to shear walls attached to a foundation wall, but the Code does not differentiate.

DaveAtkins

 

[tdawgui]

Dave,

I understand that the maximum designates the largest opening, but what dimension is used as the clear height? Is it the dimension from the top of the window to the bottom of the window or is it the dimension from the top of the window to the sill plate?

Using 0.6*DL makes sense to me. For some reason I thought it was 2/3, but i can't find that anywhere so I'll use 0.6.

Any comment on #3-Issue 1?

 

[KBVT]

The 0.6 DL factor is to be used, check your IBC load combinations for opposing wind/dead load effects. Clear dimension is top of window/door to bottom of window/door. If you have no doors in your wall, it is just the height of the window itself.

 

[DaveAtkins]

I commented on # 3 in my first response. You must follow the Code. If you have further questions, I would call APA--they have been very helpful to me in the past.

DaveAtkins