huangyhg

designing concrete slab/pad
i am looking into designing a 10' by 10' concrete slab/pad. this concrete slab/pad is going to be used for settlement monitoring. it will be covered by 70ft of soil (135pcf).
appreciate your advice on how i can design this or any reference i can use as a guide.
thanks
check bearing capacity of soil beneath slab. assume some soft spots, (maybe 10-15" span) for flexure check. reinforce if desired or design as structural plain concrete.
a crack will not render this slab unusable for its purpose.
agree with civilperson that the soil beneath the slab is most important. assuming that the soils are "resonable" it is really almost impossible to design projecs like this - not because it can't be done, but the true loading conditions are "not knowable".
imho, "design it backwards": look at the scope of the job, a 10' x 10' slab. the minimum practical slab thickness to have useful top and bottom mats of rebar is about 12" - (the combination of top and bottom mats will handle just about any loading condition that comes along). because of the small size, no joints are needed. at 12" thick, less that 4 cubic yards of concrete are required - (not much concrete to be saved by good engineering). put in a resonable quantity of rebar - we have used #4 @ 12", each way, top and bottom (as a minimum design ) with success for all sorts of industrial applications.
...and be done with it (saving quite a bit of engineering time).
i would recommend a 300mm thick pad footing with minimal reinforcement for shrinkage & cracking. for this purpose i would just use d12 u-bars @ 350 cts eachway top and bottom. "black reinforcement fsy=300mpa" should be adequate. save you some money. cover say 30mm.
as im not very familiar with the imperial system, is your overburden soil about 4m deep or less?if yes, my recommendation for the above should be ok. preparation of foundation base is critical here. you want a nice flat and consistent homogenous surface. as we all know soils are almost always never homogenous.
sre's right....the scope is too small to do any brain damage over. make it stiff for several reasons:
1. you do not want much if any flexure since you're using the slab for settlement monitoring. if your datum is in the center and you get a movement, how will you know whether it is flexure, settlement or both?
2. you do not want the slab to tilt, so i assume that's one of the reasons for the slab size.
3. the engineering time required to "fine tune" the slab will cost more than the slab.
4. you might consider putting some strain gages on your rebar so you can tell what the slab is doing relative to what the soil is doing.
5. your unit weight of soil is quite high! are you sure it is 135 pcf?
thanks for the advice, it has been very helpful.
say, i am interested in designing a concrete slab in a parking lot, in a channel, to be used as a sidewalk, etc.
do you guys recommend any reference/guidance/tables i can use to design this slab/pad? just curious, so that i can be prepared for future projects.
thanks

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