huangyhg

foundation wall
i have a project that has a 20 foot tall cast in place concrete foundation wall that will be back filled full height with sand. the wall is 14" thick, i am using 30 pcf for my active pressure on the wall. i want to design the wall as pin-pin. the top will be pinned by the main floor diaphragm. how do i achieve a pinned condition at the base? i have a 4000 lb/ft horizontal reaction at the top of footing that i dont know how resolve. can i use the basment slab as 4"thick x 12" wide column to support the reaction? the slab is 8" above the top of footing so i am ignoring passive pressure. this seems like a very simple issue i think i am just looking at it the wrong way. please help
basement slabs are used to resist passive pressure but if the pressures are large enough you need to check for potential upward buckling of the floor slab.
what sometimes happens in larger structures is that the footings settle a bit, causing a downward movement of the edge of the slab at the wall, thus, an upward bow in the slab occurs (very slight) that then creates pδ forces and upward buckling.
usually slabs aren't all that thick and have very little resistance to this upward buckling. of course the dead weight of the slab itself counters this buckling to an extent.
you should consider thickening the slab, thickening the footing, etc. to account for this lateral thrust.
if you want to pin the base of the basement-wall, then you will have to design the pin to be a pin. with a wall 20' high and 14" thin, the wall is not very stiff, hence rotation will occur at the base4 which could cause the concrete (and possibly) waterproofing to crack. a more stocky-wall would reduce the rotation leaving axial shear (if only)at the base. the base joint could be designed to resist shear and not moment(shear studs)and, if the slab is stiff enough to resist the axial forces, then you will have a pin joint that can be waterproofed.
because you're wall is particularly high, i think you're right to give the lateral resistance at the base thorough consideration. you may need to thicken the slab for additional friction or thicken the footing for additional passive resistance, in my opinion. this isn't like a house basement. special conditions call for special measures.
other issues you might consider:
1) the sequence of construction may be such that the backfill is placed before the basement slab. you'll need to be clear in your communication about what's required.
2) i only use at-rest pressure for a restrained wall. in my thinking, the active pressure implies a deformation that relieves the pressure and this deformation is generally not acceptable in a basement wall.
3) since you're assuming loading due to sand, make sure the sand occupies the entire zone which will influence the lateral pressure.
how about a key-way in the top of footing?
add a few dowels and just 'drop' that wall right in...
i think one thing that is being overlooked is the idea that if you want to design the wall as pinned-pinned you need to use the as rest pressure, not the active pressure.
answers your questions:
1. yes, you can design the wall as pinned both on top and bottom. the support mechanism is the same for both - reinforcing according to your analysis to allow relative deformation (rotation) at the joints. you may need to add water stop at the bottom slab-wall joint, because the concrete will crack as intended (preferabily the slab shall be casted after the wall).
2. yes, the base slab acting like a beam-column. you need to account for both the compression and subgrade reaction.
3. i will ignore earth pressure below the level of base slab, and, as structuraleit pointed out correctly, using "at-rest" earth pressure, not active pressure since no movement will occur on the upper support, which is the single requirement for soil to be in the active state.

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