huangyhg

slab on grade with tank slab
i was asked to do a slab and footing design for a building addition that will house a 4300 gallon equalization tank. the addition is 14'x20' and the tank takes up part of the building with a footprint of 8'x12'. the architect proposed a 4" concrete pad atop the building's slab. my supervisor thinks it should be a floating pad separate from the building's slab. is this common procedure? i don't see a small slab like this helping anything if it isn't composite with the floor slab.
also, what is typically done when a tank is on part of a building's slab as far as bending? i'm more of a bridge guy so slab on grade design isn't my forte. i've found the tf 705-r-03 slab on grade design by wri. it mostly covers design on clay soil, whereas mine is strictly on sandy soil. i have the crack control down but worried about the large load from the tank. looking for a little guidance
i would probably put in on an separate foundation, isolated from the slab with expansion joint material all around. then analyze the foundation for bending.
i'm with your supervisor in seperating the slab under the tank from the building slab, as the part under the tank will likely try and settle some.
once you seperate the two, then you can design the portion under the tank as a regular concrete footing, and the slab as regular floor slab.
not knowing the situation exactly, you might consider seperating the tank footing from the slab with 3/8" or 1/2" expansion joint material caulked at the top.
for the tank footing i would place the top 4" above the top of the adjacent slab to make a 'house keeping' pad look. it also will somewhat disguise a bit of settlement by the tank footing in the future.
is your building heated? if frost heave might be a problem in an unheated building, then i would sink my footing down to frost depth. if the building is heated (or are you in a place where frost heave is not a problem), then i might think about say a 16" thick footing. 4" above the floor slab elevation, 12" below.
just my thoughts, not being fully aware of the exact situation.
thanks for all of your help. this tank is shoved in the corner of the building. i planned on having a perimeter footing below the prefabricated building walls, typical of all slab on grade designs. if i were to have a floating slab with footings, there would be two footings side by side on two sides of the tank. the building is not heated and the purpose of the building is to remove groundwater contamination and this tank is basically a holding tank for the air strippers.
so based on your guys recommendation, the tank's slab should be separated from the building's slab and the tank's slab will have footings on the perimeter as well. the slab will be elevated the 4" so any settlement will be not as noticeable. my calculated pressure on the slab from the tank is 400psf when full. again, thanks for helping this young engineer
a lightly loaded slab like the portion under the tank will require minimum thickness if the soil is capable of supporting the 400 psf from the tank and the weight of the slab. either tank footing connected to adjacent slab or separated by expansion material will work. if connected, use reinforcement at 6 inch spacing both ways .

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