thickened slab or separate foundation?

[delagina]

What is the rule of thumb when to thicken a slab or just use a separate foundation?

If I have a slab supporting say a water tank/drum with four legs on the slab. Should I thicken the slab or just design a separate foundation with joints between thin and thick slab. Say water tank is 30 kips full of water + wind load.

 

[larsacious]

30 kips seems like a substantial load. I would provide a separate foundation for the tank.

 

[pwht1]

If anything it'd depend on the builders schedule. Some will not want to take the time in digging a foundation and doing separate pours and some will. My advice, do whatever you're gut tells you and coordinate with the builder before they start excavating.

 

[delagina]

how do i design the thick slab.

can i just design thick slab as mat foundation with 4 pedestals ignoring the effects of the thin slab.

but the rebar detail would still be continuous from thin to thick slab without joints.

is that a good assumption?

 

[pwht1]

Design it just like a pad footing. Find out what area you need to support each post, check punching and flexure. Extend the thickened slab beyond each post by a half the 'footing width' + 10% and specify the reo the pads need over the whole lot.

Stick a joint around the thickened section to separate it from the adjacent slab or add nominal top reo to fix any negative moment.

 

[JedClampett]

I'd check punching shear of the pedestals through the thin slab. Then I would check the effective punching shear area vs. the soil allowable. If you don't have enough soil capacity then you're going to need to activate some of the thinner slab. Check the thinner slab for bending.

 

[msquared48]

I do not know where your project is, but I would certainly worry about uplift from overturning due to seismic and wind events. I cannot see just a thin slab resisting that. I really stongly suggest providing a new integrated foundation.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[a2mfk]

I agree with pwht1 and others about designing this as a mat/pad. I would provide isolation joints around the mat from the rest of your slab so it can settle and move independently, and the slab around the mat can shrink during curing..

 

[delagina]

thanks,

i guess my rule of thumb will be no portion of the thickened slab should be in tension/uplift. if it gets too big, i'll use a separate foundation with isolation joints.