huangyhg

basement slab on grade design
i have a basement for a hospital facility with 16' tall walls. assuming a restrained retaining wall with pinned supports at the first floor slab and basement slab i am getting about 5000 lb of load per foot in the basement slab.
the opposite walls is over 150' away to provide the balancing load. i am currently showing a 6" slab reinforced with #5 @ 12" oc ew. i am not sure how to analyze this slab. i am sure in the past i have somewhat blindly assumed the slab can tranfer the load as an axial member but would like to hear some oppinions. the soil is not great so the slab is a slab on grade but the remainder of the building is pile supported. so passive pressure at the base is not an option.
any opinions would be appreciated.
thanks
perhaps a grade-beam system could be used to provide designed load paths for force collection w/ the slab acting as a diaphragm.
or if your 16'wakll is on piling - how about adding batter piles to ake the lateral soil kick?
mike mccann
mmc engineering
are there perpendicular walls to the one you are trying to restrain? if so how far are they apart?
how about using a lopsided footing to engage some of the 16'-0" backfill on your footing. maybe this in combination with a key below the footing to engage passive pressure will work.
don't forget to add in any floor dead load into your calculation.

mrkei:
just out of curosity, what equivalent fluid pressure are you using? the base reaction seems pretty high for a 16' tall baement wall.
jmho: unless there is something unusual about the situation, your slab should be fine. i've done basements this wide with 5" slabs reinforced with wwf or fiber mesh, and have not had any trouble with the slabs not being able to take the thrust from the bottom of the walls.
the stress in the slab only ammounts to 70 psi. not really that much.
the equivalent fluid pressure is 60 psf per the geotech. i think that is high also but.......
also in response to the other posts. no perpendicular walls.
the batter piles might work but are costly and dont engage unless vertical load is applied to the pile.
thanks so much for all the advice. i still am not sure about if the slab will have a buckling problem even with the low axial load of 70 psi factored.
another issue i have is a portion of the opposite wall is a walkout so i need to tie the first floor diaphragm into the return walls for lateral stability. with 170' of wall and only a 45' return life in the world of the first floor slab could get interesting for that kind of diaphragm shear. any help with this one???
it's hard to make large walkout basements work. i generally end up using a cantilever retaining wall.
daveatkins
i would agree with ljkh345 about the basement slab. i can't see it displacing upward unless subject to hydrostatic pressure. i would omit any isolation joint material and just turn the polyethylene up against the wall face.
as to the diaphragm, if the first floor is a concrete floor, there should be no problem. anything else, i would agree with dave, use a cantilevered retaining wall.
it is being tied into a concrete composite slab 6 1/2" thick normal weight concrete. from my experience the 5 kip/ft i need in the 45' return wall will be working that slab hard. i may need to reinforce that zone. i don't think designing as a cantilever is an option because at some point it is going to think the top of the wall is supported etc. plus adds a bunch more piles to the footings.
the slab as a compression member is obviously braced against buckling downward, so the question is how do you treat buckling upwards, right? so in several standards it recommends bracing be capable of taking 2% of the axial load in the member (i like 5% but thats just the cya in me). so if you have a slab that has 5000# compression, the buckling restraint has to account for 100# you have a 6" slab and therefore your slab weight is 75 plf. this leads me to say that your slab is unbraced from upward buckling at 16" o.c. (100/75*12"/'). so now you have a concrete column 1' wide 6" thick and 1'-4" tall with fixed fixed ends. easy to analyze. re

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