shed tank stand footings questions - tie to shed slab?

G'Day,

hoping someone here knows how to work these questions out.....

Background-
I am planning on a tankstand to support a 12KL rainwater tank - the footprint is in the 3mx3m area. It will be located in an internal corner of the shed - Office behind it, main shed beside it.
The tank stand will be paved underneath and the space used for storage.

Questions:
How do I calculate the size of my footings? The soil is clay/gravel, and the water table is 1.5m below NGL. The shed has to be erected before the tank stand, but I can pour in footings/slab as part of the shed pour if it helps.

Should the tank stand footings/slab be connected to the shed slab? - the shed slab will be edge thickened, but then minimum 100mm thick on the office side, and 150mm thick on the workshop side. - the slab under the tank would be min 100mm thick for storage purposes, more if it was needed to form the tank stand footing.

The slab for the tank stand will be about 75mm lower than the shed floor so the shed retains its waterproofing, and the tank slab will have a slight slope to it for drainage.

Thanks,
Des

I may not be able to answer all your queries at once. But I can add my thoughts.
a 12000 litre rainwater tank when full weighs 12 tonne plus tank.
i have a suspended garage floor that is a 200 mm thick slab of high grade concrete, designed to carry two cars weight approx 3 tonne.
rainwater tank stands made of steel have a tank platform made up of 50mm x 50mm RHS spaced at 100mm with corrugated iron on top to spread load evenly. Don't recall the exact thickness of the support frame that went under and this was to hold a 1000 or 1500 gal tank so 7 tonne approx.
if a tank is being placed on a ground level slab it is generally 75mm minimum and 100mm preferred. So not being an expert if you have a 100mm thick slab for general shed floor you can guess it would be ok. But you would need to dig footings to make a |+| under your tank site to support the weight.
cheers

The answer to the first part is relatively simple - keep the two footing/slab designs separate. No matter how well designed, there is bound to be some movement and you don't want one to affect the other. By all means pour the concrete at the same time, but keep them structurally separate.

The main question is much more complex. If the tank was sitting on a slab it would be relatively simple as the load is uniformly spread over a large area, and the worst that is likely to happen is for the slab to settle slightly unevenly, but not causing any real issues, particularly if it is not connected to the rest of the building slab.

But, you are putting it up above the ground, so 1) the load will be concentrated on 4 legs or on 2 or 3 walls depending how you design it and will have much higher loads than on a slab (12 Tonne spread over a small area) and 2) if it fails, it will cause a much greater impact particularly if someone is nearby at the time or even happens to be underneath it.

You can only really determine the load carrying capacity of your soil with some testing and then a proper engineering design of the footings required. I know this is costly, but would be required by Council if you are going through this process.

You may be able to get some idea from similar structures in your area, but at least give it strong consideration before going too far.

Sorry to sound so negative, but felt I needed to flag my concerns as it is more complex than the shed footings and potentially much more dangerous if it is done incorrectly.

Regards

Bauldy

Going to build a new house on our property, and the plans call for a 600mm deep foundation pad underneath the chimney. Class M ground.

I am not sure what the chimney weighs, but would speculate that it would be less than 5 tonnes. Hope this helps.

Contact a structural engineer, if you are going to store stuff underneath it, you dont want it squashing you if it fails.
Unless there is a practicing structural engineer on this forum, you are not going to get the correct answer and hearsay experience is no good in a court of law not to mention the lost dollars.
Whilst those offering advice with good intentions, there are some things best left to the qualified experts.

G'day Des,

If you are building in the Pilbara, as your address implies, there are wind loading criteria that require an engineer to sign off on as rwbuild has pointed out.
As an example, cement silos in that region are required to have a certain amount of product on board in the event of a cyclone, somewhere about 50% from memory. Because this percentage could not always be met most that I am aware of had guy wires attached.

Cheers,
Geoff.