Slab-on-grade design for supported tank 3

[CleanWater]

This question is also posted on the Structural forum.

I am dsigning a concrete slab for two granular activated carbon tanks for water treatment. I am using the method described in "Concrete Floors on Ground" published by PCA. Each tank has four supports with base plates that will be located near the edge of the slab. My questions are:

1)Should the operating load (unfactored) or factored dead and live loads be used to determine the post load for use in the design charts?
2)Can anyone recommend a value of "Stress per 1,000 lb. of Post Load" where it may be better to design a reinforced concrete slab instead of an unreinforced slab. The weight of each tank when full of carbon and water will approach 100 kips. The conditions (safety factors, concrete flexural strength, etc.) are giving me values that barely allow the charts to be used. If I use factored loads and a safety factor of 2, I can't use the charts. The design charts start at 10 psi on the "Stress per 1,000 lb. of Post Load" axis.
3)If the loads are high enough that I can't use the charts, could I use a reinforced footing design approach?

Any suggestions would be greatly appreciated.

Thanks,
Jack.

 

[steve1]

CleanWater

Funny how some posts in this forum receive lots of reply's, and others receive none. It would make for an interesting study to try and figure out what causes this.

But I digress.

I have engineered many a foundation for various kinds of mechanical equipment. There are not alot of references for this type of project. Generally the design procedure that I use is as follows.

First set up a design criteria. This includes a determination of the appropriate design loads, including live, dead, snow, seismic, wind, etc. These loads (other than dead) should be taken from either the appropriate building code or ASCE 7, Minimum Design Loads for Buildings and Other Structures. Dead and operating loads are a function of the equipment.

Second, set up an Acceptance criteria. These are the values that you design to: Cmpressive strength of concrete, yield stress of rebar, allowable bearing stress on soil, etc. Note that often you won't have access to a soils report, so you must visit the site to determine the existing conditions. Generally speaking I use an allowable bearing capacity of 2000 psf, unless the site is a swamp or something like that. If you are not comfortable with this, then by all means require a geotechnical evaluation.

Start the design by assuming a size of the footing. I generally use plan dimensions of the equipment plus one foot on all sides, and start with a one foot thick slab. Generally speaking I would not go less than one foot thick since you will most likely need both top and bottom reinforcing steel.

Next check for bearing stress and stability. Bearing stress is based on P/A +/- Mc/I. You should use unfactored loads for this and be sure that your maximum is less than the allowable, and that the minimum is greater than zero (ie. no net tension). Also you may need to check several combinations of loads. You shouldn't add wind and seismic together as an example, and also be sure to check for the unloaded condition of the equipment with no other loads (this will provide the minimum bearing stress condition).

After one pass with the assumed dimensions you may need to resize your slab (note: adding dead weight by increasing the slab thickness will solve net tension problems).

Next comes the structural design. Use factored loads for this, and recalculate the bearing stress. For equipment installations I general use only the maximum value calculated this way. I take this maximum value and apply it as a uniform upward load on the base of the footing. Next determine your maximum positive and negative bending moments (any consistant sign convention will work here). I check for bottom steel by calculating a cantilever moment from the edge of the pad to the centerline of the equipment feet. I check for top steel by considering a simple beam span between the equipment feet. What you are looking for here is the maximum moment (positive or negatine) that you can use to size your rebar. You want only one size (say No. 4's at 12" o.c. each way, top and bottom) for the rebar.

Lastly check for punching shear at the equipment feet.

I hope this helps and good luck.

Steve1

 

[CleanWater]

Steve1-

Thanks for your very helpful reply. I assume that the check for punching shear is at the distance d/2 from the edge of the base plate?

Thanks.

Jack.

 

[steve1]

CleanWater,

Yes that's correct.

Steve

 

[jcm1212]

Jack,

When it comes to support an "all flat-bottom tanks" American Water Works Association (AWWA) D100-84 recommends that the foundation mau be designa as a:

-tanks supported on ring walls (that is a concrete ring with stabilized sand inside the ring),
- tanks supported in concrete slabs,
- tanks within ringwalls,
- tanks supported on granual berms, and
- tnaks supported on granular bemrs with steel retainer rings.

So there are several methods to desing tanks foundations.

Juan McLane