Shallow Footing Design Question 1

[kstone]

I am a civil/environmental engineer working for a consulting firm. I was recently asked to determine the approximate size of two footing which will be used to support a 15,000 gallon gasoline AST. I recently took the PE exam and participated in a review class offered by ASCE so I had some reference material pertaining to shallow footing design. There is no information available regarding the soil conditions at the site or depth to bedrock or groundwater, so the calculations I was running was basically to get a footing size that was in the ball park. Using some assumptions, I was able to find the allowable net bearing pressure. With a FOS of 3, the bearing capacity I came up with was 3655 pcf (seem reasonable?). The load with a full tank of gas and structural elements to hold the AST came to 4,100 kips and came up with a footing size of 25x25 for the two footings supporting the load. Considering all of that, I have two questions:
1. Does that footing size or anything else I mentioned seem absurd or ball park reasonable? and...
2. How would I determine a reasonable number for the thickness of the footing.

Thanks in advance for any help with this. These numbers aren't being used for the actual design, but are going to be sent to state agencies as ball park assumptions. Since I just studied this stuff in a limited capacity for the PE exam, I am really curious to figure out this stuff.

Thanks...
-Keith

 

[escrowe]

Have you considered a dart board?

A bearing capacity exceeding 3500 psf would suggest a well-compacted granular (gravelly) soil. Keep in mind that shallow groundwater can significantly reduce bearing capacity.

Why two footings? Is the tank elevated? ASTs are generally mat-supported, and I would expect an elevated tank to have a very thick or depressed mat or pile foundation (generally).

Among other things, the foundation design should consider the maximum bearing pressure under all conditions-- including increased pressures developed under the footing edge as a result of overturning force.

Footing thickness will depend on load distribution, and may be quite thick if supporting a single column or an eccentric load.

Finally, keep in mind that the footing design should limit settlement to the structure design tolerances regardless of foundation bearing capacity.

 

[MiketheEngineer]

Get a soils test!!! Most codes only allow a 1500 psf loading w/o one.

 

[DRC1]

3600 psf does not sound unreasonable for a medium compact sand. You should get a soil test or least a geotech who is familar with the area. Bearing capacity is not that exact a science, so I would stick with 3600, not 3655.
However, 4100 kips is a huge load. If that is a correct,that would be a huge tank. If so that should not be someones first shot at designing a foundation. You may want some more help than what we can offer.
In general though, get a copy of J.E. Bowles " Foundation Analysis and Design" as it goes through both the the geotechnical and structural parts of designing foundations under a variety of conditions.

 

[civilperson]

Your bearing units, pcf, are a mistake. All replies assume you meant psf. The gasoline weighs less than 115 kips, so to get to 4100 kips total is an enormous amount of structure/footing. 25 foot square is 625 ft<2 and a loading of 6.6 ksf. (I am thinking arithemetic error somewhere).

 

[kstone]

Thanks for the replies so far. I should reiterate that the actual footing design will be subbed out to a PE that actually does geotech work. Now that I've said that, I have to admit a very stupid error that I made which DRC1 made me realize. The load is about 130,000lbs, and because I haven't thought about any of this stuff in years, I stupidly multiplied that times G (32.2) to get force in kips. That is pretty embarrassing to admit on this forum, but like I said, I am an environmental guy and haven't even heard the word "kips" since college.
With the new load and using a more modest bearing capacity of 2500 I came up with 5x5 for the footing size.
To answer your question escrowe, the drawing I was handed has the AST sitting on two saddles with piers going from the saddles to the footing approximately 5 feet below ground surface.

Thanks for everyone's input on a question that is vague. I appreciate it. Also, is there some general formula that I can use to calculate an approximate thickness of the footing (based on assumptions I have made and the bearing capacity/footing size calcs)?

 

[kstone]

Thanks civilperson. I was writing my other reply when you posted yours. Also, I did mean psf not pcf, thanks.

 

[SlideRuleEra]

kstone - After a tank is constructed, but before it is filled with product, the tank is often filled with water to check for leaks (hydrostatic testing). If this will happen on your tank, you may need to consider a short-term load for a tank full of water at 8.34 pounds per gallon.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[escrowe]

So, two depressed 5x5 (25sqft) spread footings loaded to about 65 kips each;
about 2600 psf plus live load (wind etc...).

Assuming 'decent' soil conditions, these seem like reasonable assumptions.
I would want test boring data through the foundation zone of influence
(about 15 to 20 feet below ground surface in this case)
to finalize design.

Footing thickness generally depends on soil bearing capacity, column (pier) size and location,
among other assumptions (i.e. type of reinforcement, steel and concrete strength).

For preliminary design, I'd start at a conservative thickness of 24 inches,
and let the structural and geo help hammer out the details.

Here's a spreadsheet. Use cautiously. Notice the large number of blanks...

http://web.eng.fiu.edu/~prieto/geo2/Column Footing Design.xls

 

[kstone]

Thanks alot escrowe! That was very helpful everyone. Thanks.

 

[NVcaster]

Allowable settlement often dictates the allowable bearing pressure rather than soil bearing capcity. Site investigation needed! Without knowing soil conditions I would not use anything higher than 1500 psf for a first cut. On problem with soils they can vary widely within a given area. For shallow foundations I have had recommendations range from 1,000 psf to 3,500 psf. Also if you have poor surface soils you may need to go to piers or piles for support which is in another ballpark altogether. You might check with the local building department and see what others have had to do in the general area. Also if you plan to bring in a geotech you might talk to them now as they might have local knowledge.

A county soil survey may also provide some clues as to soil types, but they tend to be agriculture based. Without even a site visit just to see the surfical soils and general site conditions its hard to really do much.