Unequal Grade Basement Foundation Wall

[bbookz]

I'm designing a two-story storage building (Like Extra Space Storage) about 320ft x 135ft. It's a walkout basement condition with grade differential between the long sides being 11'-4". I'm the foundation designer. The floor, roof, and above grade walls are a pre-engineered system; but essentially light gauge bearing walls 10ft o/c with composite deck for the floors and Z-purlins at 5ft o/c bearing on the same light gauge walls for the roof. I have not spoken to the pre-engineered system designer yet, but is is in there scope to design the lateral system which will likely consist a regularly spaced light framed shear walls or flat strap walls. We'll have to coordinate foundations for these items.

These things are built on the cheap. Typical interior bearing walls are supported on thickened slab foundations poured integral with the slab on grade (I don't really have a problem with this, but may have to do something different at hold lateral system downs). In an example project, they also stepped the side walls with grade (Properly designed as cantilevered retaining walls) so that the basement retaining wall stops below the floor level and has a light gauge knee wall between the top of concrete wall and upper level floor.

My question is if I consider surcharge (Passenger vehicles - I was going to use 100 psf, but maybe fire truck in an emergency), drainage is adequate and I properly design the wall as a cantilever are there any potential problems (maybe long term deflection or something) with saving wall concrete by stopping the wall short of the floor above on the sloped grade sides. I wasn't intending on designing the walls as a purely pin-pin basement walls anyway since there is unequal grade, its easy enough for the contractor to miss the note about not back filling the wall until the floor above is in place, and I would have to coordinate the top of wall connection with another engineer who is hard to get a hold of.

 

[bbookz]

I was planning on designing the wall stem for at-rest pressure and will check the wall which goes full height as fix-pin (for inside face bars) since there will be a wall to floor connection at these locations. For footings, overturning and sliding I'm going to stick with the active earth pressure cantilever wall design; otherwise the footing will get very large. Also, I am designing the footing for 1.2 SF sliding (includes surcharge pressure) since in the final built condition there is more than enough slab on grade to bring the safety factor above 1.5.

 

[oldestguy]

Maybe a dumb question, but does your upper floor have any expansion joints? Otherwise I assume it acts as a horizontal beam with the shorter end walls as the support for the higher long wall pressures and tied in there or just sitting. Comment?

 

[KootK]

My question is if I consider surcharge (Passenger vehicles - I was going to use 100 psf, but maybe fire truck in an emergency), drainage is adequate and I properly design the wall as a cantilever are there any potential problems (maybe long term deflection or something) with saving wall concrete by stopping the wall short of the floor above on the sloped grade sides.

Works for me. When a retaining wall is used to support axial loads this way, I think that it's prudent to give some credence to the P-delta effect of the axial load acting on a potentially out of plumb / displaced wall. I'd think it pretty rare that would add up to much compared with direct lateral load from the soil/wind though. This system might actually be advantageous in the sense that you no longer have to worry about the tops of the side walls pushing against the diaphragm against your wishes.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[bbookz]

Thanks KootK.

oldestguy, if you are asking about the lateral system for the overall building, I'm not certain yet. I'll be discussing this with the pre-engineered system engineer soon. If the question is about basement wall design, I'm going to have cantilevered wall footings so will not be designing for floor diaphragm earth pressures. I think at 320ft we are just small enough to not need an interior expansion joint. Since we will only have the three concrete walls down low, I was anticipating that there would be multiple interior light gauge shear walls at regular spacing in each direction. Since there is a grid of storage spaces there are a lot of interior walls to utilize.