New foundation on existing slab

[hamza41]

Hi everyone,

We want to provide a slab foundation for a new steel tank (22.5' diam, 12' H) inside a building which already has a general slab of almost 2 feet over concrete piles as shown in the sketch below.

My question is: can we constrcut the new foundation over the existing slab with no mechanical connections (dowels) by just pouring the concrete (reinforced) over it and considering the existing slab as a hard soil. I don't think we need to connect them with bars or bounding. What do you think ? Is there something I might have missed ?

Thank you for your help.

 

[Greenalleycat]

You could do that, though I’d be concerned about the seemingly sloped surface as this creates a sliding tendency
Why do you not want to dowel in? Just drill and epoxy bars at a metre grid or whatever
Seems not worth the risk given the scale of what you’re putting on it

 

[AskTooMuch]

We pour mud slab a lot of times below foundation in muddy areas.
So this is just a reinforced mud slab, if it's on soil, I guess.

This is on piles though so will need to check the pile.

 

[Tomfh]

It’ll probably work without dowels however I agree a few dowels should be added for good measure.

 

[Aesur]

I would add the dowels due to the sloped slab. Is this an elevated slab or slab on soil with piles? Wouldn't you need to check the slab to span between the piles for the new loading?

 

[hamza41]

Thank you all for your answers.

Yes Aesur, it's a slab on soil with piles.

I forgot to precise that we are actually replacing the old tank by a new one with almost the same specifications, so there will be no significant load change. The old tank was supported on steel beams wich are supported on concrete beams as shown in the following drawings. I don't know why they chose this complicated design, especially the concrete beams are sloped and there is no pipes to pass through them.

 

[SlideRuleEra]

1) The old tank was supported on steel beams which are supported on concrete beams as shown in the following drawings. I don't know why they chose this complicated design...

2) Is there something I might have missed?

1) The floor slopes about 3.7% (about 10" fall over approximately 22'6" horizontal). This is very steep, we typically used 2% floor slope in critical areas of our electric generating stations. They wanted positive floor drainage underneath the tank to the trench; this can be quite important for some industrial applications.

2) Check with your Client to see if loss of under-tank drainage is important. If not, put in the dowels as others have suggested. If seismic loading is required, dowel details become more important.

 

[JStephen]

The steel grillage is sometimes used to allow access to the bottom, usually for leak detection. And used for some elevated tanks in the olden days.
On the design shown, it looks like they were trying to transfer loads to specific locations/piles rather than to the slab. Maybe they deduced or assumed the slab was inadequate for that purpose?

 

[hamza41]

On the design shown, it looks like they were trying to transfer loads to specific locations/piles rather than to the slab. Maybe they deduced or assumed the slab was inadequate for that purpose?

The slab should be more than enough to support the tank. As per shown below the load is not transfered directly to the piles. I think it's most probably used for leak detection and drainage below the tank.