huangyhg

slah on grade
my reinforced concrete slab on grade is approx.
30 meters by 8 meters. thickness is 150 mm.
design bearing pressure is 120 kpa. the only
loads on my slab or spread foundation is the
own self weight (reinforced concrete) and some
fluids loads, as this foundation is being used
for containment of fluids. what do i need to
consider to design my slab/foundation? i know
i need to make sure that my actual bearing
pressure is less that the allowable, i need to
provide minimum reinforcement (temperature or
shrinkage) steel.
do i need to design for moment? i would think
that my main mode of failure is settlement, or
more appropriately, uneven settlement? what
about failure in bending, though? how do i
check this? top of concrete is approx. 150 mm above the top of grade. i am designing to ac 318.
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how high is the fluid your supporting? is your slab supporting walls that contain the fluid?
if there is a high water table, you will have to design for uplift of the tank. and this would be for the maximum seasonal water table elevation seen.
mike mccann
mccann engineering
the max. height of the fluid is only 150 mm, or 6 inches. mine is a containment slab to take any spill from drums that have their own designed foundations. what do i need to design my containment slab for? i have checked bearing capacity, that is ok. i have provided minimum steel reinforcement, per shrinkage or temperature steel. the ground on which my slab sits is reasonably good at 120 kpa, or 2.5 ksf. do i need to design for failure of the slab by flexure? i do not believe i have too much uplift on my slab -- how can i verify that?
i don't see why the type slab you describe needs to be designed for any other loading condition than restraint by the subgrade. the restraint will cause cracking of the slab when the concrete shrinks, thus the requirement for shrinkage crack control reinforcement. jointing of the slab should limit the amount of cracking, but you then have to seal the joints to ensure the integrity of your containment. if it is required to have no joints, the slab could be posttensioned to hopefully eliminate cracking. theoretically, if the slab could be cast on a frictionless substrate, the slab could shrink without stress, thus no cracks. that is not feasible, but casting the slab on a flat, smooth surface with plastic sheets as a slip layer will help.
the 30m x 8m is problematic with out some joints across the 8m direction. where are you located and what delta t do you expect the slab to be subjected to? what structural elements are within your containment slab? presumably they, the tanks, are within your containmaent slab. how are you isolating them from your slab on grade? tank foundations within your containmemt structure will behave quite differently than your containment structure.
i believe for your situation, the slab needs to be jointless! you can design a jointless slab on grade, if you provide reinforcing of about .005ag. but, this will not eliminate the cracking. the slab will still crack, but these cracks will remain tightly closed.
since, it is a contaiment slab, consider applying some type of penetrating sealer on the surface after completion. this will fill the cracks and will effectively seal the slab.

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