Footings for overturning discussion
Footings for overturning discussion
(OP)
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?
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?





RE: Footings for overturning discussion
Mike McCann
MMC Engineering
RE: Footings for overturning discussion
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.
RE: Footings for overturning discussion
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.
RE: Footings for overturning discussion
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.