Shear and Point Load rules of thumb for maximizing window area?

Question

Scenario: One-story ranch house, 25 feet front-to-back, on a concrete perimeter foundation, in California (seismic loads). T-111 plywood exterior.

I would like to maximize the glazing area of the back wall for at least a 20 foot stretch, starting near one corner.

I know how to install headers and windows, but I know that headers move continuous loads on the foundation to be point loads at the sides of the windows... and I know that windows don't have the shear performance that plywood sheathing does.

I'd like to install a series of sliding doors and/or windows with minimal gaps between them, but I don't want to pour additional foundation to pick up the loads created by wide header spans, and I'm concerned about the shear capability of the wall once that much plywood is removed... not sure which becomes the limiting factor here.

So, for starters, is there a rule of thumb for how wide each window in a series of adjacent windows can be before a foundation upgrade to handle the transferred loads is required?

And second, is there a rule of thumb for how much area can be sliding glass doors vs. wall for shear performance? And/or can I use Simpson Strong-Wall the way garage door shear is handled at the ends and/or middle?

(Even if this requires structural engineering, a rough idea of what can/can't be done impacts a whole lot of other design work for a kitchen remodel, so even rough answers are helpful)

Answer 1

Yes, there is a rule-of-thumb for determining shear walls and how wide windows should be to have the posts between the windows transfer the loads adequately to the foundation.

You're in one of the highest rated seismic areas in the U.S. Critical loads are moving the building up-down and side-to-side. We try to tie the roof to the walls and the walls to the foundation for vertical and horizontal loads.

I don't know the width of your house (I'll explain why that's important later), but as a rule-of-thumb, for seismic loads, you need a minimum of 4' of shearwall at each corner (before any windows) and you want no more than 60% of the length of the wall for window / door area. This allows you about 40% of the wall to tie to the roof and foundation.

For your 25' long wall, without structural calculations, I'd use : 25' x 40% = 10' of shear wall (Sorry, I know you want about 20' of window area.)

As a rule-of-thumb, the house length to width needs to be no greater than 1:3. So, if the 25' is the width, then the length should not be greater than 75'. Likewise, if the length is 25', then the width will be no greater than 25' (1:1) and no less than 8'-4"...you don't want it too skinny either.

Now the width of windows: As a rule-of-thumb, the width of each window should not be greater than twice the height of the foundation wall (stem wall plus depth of footing). So, if the stem wall is 18" high and the footing is 6" thick, then the windows should not be any wider than 4'. (As a rule-of-thumb, loads transfer from the wood posts between the windows to the concrete foundation at a 45 degree angle. So you don't want to be further apart than what can be transferred through the foundation to the ground.)

Yes, you are correct, Simpson has some great holddowns and pre-made shearwalls that we often use for garages because the garage doors exceed the 40-60% rule-of-thumb. However, they're expensive and you'll need to calculate the tributary loads. Keeping your house fairly rectangular, installing 4' shearwalls at all corners and keeping window widths about twice the depth of the foundation, you'll be safe.

By the way, use 1/2" minimum plywood (not OSB board for wall and roof sheathing) and nail at all supports with 10d at 4" o.c. at all supports...roof and wall. (If you nail any closer, you'll split the studs and have no resistance.) This all assumes the shearwalls are no less than about 1:2. If you have a vaulted 16' high wall, then the corner shearwall needs to be about 8' wide.

If these rules-of-thumbs don't work for you, you'll need to get it calculated. I'd get a hold of a structural engineer (NOT a civil engineer) in your area. Tell him you just want the calculations, not the plans. It'll cost you about 4 hours of his time (assuming he's got the software) and you can install it as per his design.