Daylight basemetn Foundation wall sliding

[jnichol]

Wondering what other people do for foundation wall sliding when you have a daylight basement (supported at top and bottom of wall and able to transfer top reaction to the shearwalls). In our company, we look at the base reaction from the wall caused by at rest pressures, and subtract out any resistance from friction of the wall/fdn/soil weight, friction from the wall dead lodings (ie floors and roofs), possibley the floor slab friction, and passive resistance. If we can not get a FS of 1.0 for a fdn wall (dont believe the retaining wall FS of 1.5 from chapter 1610.2 applies)we increase foundation size to achieve 1.0.

To me that seems logicial, and if anything Im not really sure I believe we are liberial by not keeping with the 1.5FS, but lately we have been competing aginst a full service firm that has a 2'-0" wide footing under the foundation wall for a commercial building with a daylight basement with 12'-0" of backfill on one side, when I would have needed an 8'-0" wide footing. The building is way to long to say that the wall is capable of spanning side to side.

Because of the daylight basement issue, I dont think you can simply blow off sliding or say that you have a slab present which resists the sliding, but maybe I am missing something.

Does anyone have any ideas as to why my foundation is so large in comparison, or is the competing engineer probably just undersizing their footings. Also, what do you do when you compition is obviously missing something in their design. Thanks

 

[jdmm]

This isn't an easy question to answer because there are so many varibles. You suggest that the horizontal force is due to an undifferentiated force equal to 12 feet of soil. Is this really true or is there some passive resistance.

If there is a 12 foot horizontal force there is also a component vertical on the footing that is resisting movement equal to the frictional force on the wall times the horizontal force.

Are you using a friction resistance at the foundation level that is consistent with that used to calculate the horizontal force or are you just using some concervative structural friction factor.

You should discuss your approach with an experienced geotechnical engineer. In my experience, structural engineers underutilize the capacity of the soil. Your competition, sorry to say, probably has already developed an approach based on geotechnical principals.

 

[msquared48]

Remember that it is not just the slab dead load you are talking about to resist sliding, it is the whold building dead load. If the wall is going to slide with the slab in place, the entire building, more than likely, will have to slide as a unit. I have taken their approach for years with no problems. There will be special circumstances though, like the building longwall resisting the soil load being very long with respect to the endwalls. The floor diaphragm above must be designed to transfer the load to the endwalls too.

Takde a look at the total dead load of the structure, apply the slidingcoefficient, and compare it to the base force. The result might give you the FS you need.

Mike McCann
McCann Engineering

 

[msquared48]

Additionally, don't forget the passive pressure on the stemwalls, front and rear.

personally, I see this scenario failing at the foundation in one of at least three modes...

1. Shear through the stemwall (assuming it is short), or
2. Shearing of the soil at the base of the stemwall from the back to the front of the structure, or
3. A vertically diagonal slippage of the underlying soil, actually lifting the slab, possibly buckling it. This would imply that the failure was inside of the endwalls - kind of like crushing a pop can from the side. It implies that the structure would have to be lifted, at least in part, in order to fail.

#1 is easily calculable as is #2. #3 is a much harder to model.

Mike McCann
McCann Engineering

 

[civilperson]

What is the depth of the footing at the walkout side? Use this area as a passive pressure to resist sliding as well as all the friction forces listed by others. The width of a footing under a wall for vertical loads only can vary as to what is the allowable bearing pressure beneath. If the footing is resisting moments at the base by cantilever action, the width will grow.

 

[COEngineeer]

I agree with the responses. 8' footing is just too crazy! If I were that worried, I would have made the wall even deeper than the slab instead of making 8' footing. But I think you would need to do that. I

Never, but never question engineer's judgement