interior pad footing

[AlpineEngineer]

I have recently noticed that in order for an interior pad footing, in a crawl space, to resist lateral loads it must be below grade. While this makes sense I have never seen them built this way in my area. They are always poured on top of the crawl space grade. Is this how they are being built in your areas? In addition I have noticed they aren't trenching in the perimeter spread footer, thus on the interior the spread footer is not under grade. Are these things of concern?
Thanks.

 

[csd72]

Depends how much lateral load. If the pad is not completely under uplift then it will probably have enough friction against the soil surface.

Can the floor diaphragm be used to transfer lateral loads out to the external columns?

csd

 

[msquared48]

The reason that the footings are supposed to be buried into the ground (7" I believe the detail shows) is to prevent the footing from literally walking away from its intended location during a seismic event. If it walks too far, suppport for the column may be lost, or the footing loaded eccentrically. If there is a good positive comnnection between the post and the footing and the post and the beam, walking may should not be an issue.

Regarding the exterior wall - same idea - to have some earth for passive pressure to prevent sliding from lateral soil pressures. This would have to be into undisturbed native soils, otherwise, with fill, I would have to neglect the top two feet of the soil. For two foot high stemwalls, this probably not an issue. Four foot, maybe. Eight foot, definitely.

Mike McCann
McCann Engineering

 

[whyun]

To effectively resist interior lateral loads, concrete stem walls are provided below the shear walls with continuous footing embeded below grade.

If the footprint of the building is not too large and you do have interior shear walls, design the first floor diaphragm to deliver the lateral force to the perimeter stem walls. You dont need embeded footings along the shear line now, only need to take care of end uplift/down force at the shear walls.

 

[sdz]

Normal practice where I come from is for footings to be embedded. I would be particularly concerned about edge footings in case there is erosion or scour. Are footings close to outside ground level?
Technically if all loads are designed for there should be no reason that footings can't be on grade.