Footings for overturning discussion

[SouthFloridaPE]

Imagine a 1-story rectangular home with a garage at the front. This is reinf. masonry with wood trusses. The only lateral resisting elements along the front are the two 24" wide masonry piers at ea. side of the garage door. Now the lateral force coming from the diaphragm is about 8000 lbs at 9 ft height (Florida, 140 mph wind, exposure C). There's no doubt the garage door frame (16 ft wide) will have an uplift force on one column and compression on the other (on top of the moments and shears).
Gravity loads are small and counteracted by wind uplift from the roof.
Now here's the discussion: This house has a slab on grade with an 18"x16" monolithic footing around the perimeter. To make it worst, the footing only has 2#5 bottom bars, so it is incapable of receiving any moment from the frame (no top steel). How do they do it without adding a concrete pad to get enough weight for the concentrated overturning forces (0.6D+W)?
My client thinks I'm crazy and overdesigning. Am I?

 

[msquared48]

By putting a continuous wood beam designed for the uplift forces over the two shear walls. The leverage of the additional length will decrease the uplift seen by the foundation. Done it for years...

Mike McCann
MMC Engineering

 

[Ron]

SFPE....think of your structure as a box. If the shear walls are competent, which reinforced masonry walls certainly are, then you can consider OT to be a global condition, not a localized condition as with the garage door. I suspect that if you calculate thusly, you'll find that your structure complies.

I've evaluated a lot of structures, post-hurricane. I have yet to find one that failed in OT. I've seen total building collapse, partial collapse, localized collapse and localized distress....many in your area of the state.

 

[SouthFloridaPE]

Thanks for your opinions. Adding a beam above is what I'm doing (concrete beam, not wood though), which creates a frame capable of transferring moments. And this is one of the reasons why this condition creates concentrated uplift at the end columns.
Now if I use a precast lintel, incapable of transferring any moments to the shearwalls (actually columns due to h/d ratio), then they become cantilevered from the foundation. This kills the concentrated uplift, but then I need to use the footing as a grade beam, and transfer the moments from the columns into the foundation. I used this in a couple of houses, and again I got complaints about having 3#5 top and bottom on a thickened edge slab.

Now with respect to the "whole-house" behavior, this is what I'm doing: The whole frame tries to rotate, so I include the weight of the beam above, (and the uplift coming from trusses above), the weight of the thickened edge footing plus 4 ft of concrete slab on grade, the weight of the columns each end, and 5 ft of masonry wall that runs perpendicular each end. Even with all that, at this location I found the 0.6D+W combination to be insufficient (need more weight). Is everybody else using 0.6D or just use 100% of the weight of the concrete?
I'm using only 5 ft tributary because beyond that, the perpendicular walls receive large uplift from trusses, therefore their weight gets cancelled out already.
Also, the slab on grade can only cantilever about 4ft when lifted, so that's what I'm using as tributary weight.

Thanks for your thoughts.

 

[Ron]

SFPE....OT is a failure condition in the global sense. Your floor slab, if tied together with WWF or rebar, contributes to this resistance whether it fails in bending or not...who cares at that point. If you take the weight of all the materials in the house and apply those at the approximate CG of the house, that is the DL resisting moment as computed from a line along the leeward wall. That has to be 1.5 times the moment resulting from lateral load on the long wall and horizontal component of the roof wind load applied at 1/2 the height + the vertical component of the wind load on the roof, also applied at the approx. CG of the house. I find it hard to believe that the vertical component of the wind load cancels the weight of masonry + floor slab tributary area + truss and roofing material weights.

Stiffness of the box is not a consideration in OT moment resistance...only applied load vs. dead load.

For OT resistance, I do not factor the dead loads.