Stress Imposed in Soil?

[caleb]

I have been assigned the task of deciding whether or not a predetermined landing area will be adequate to support a transformer that my client has on order. The transformer weighs 94000lb and has a landing area of 5x8'. Assuming that I can obtain the appropriate soil classifications I am wondering how I can calculate the stress or distributed load that will be imposed on the soil. Is it as simple as calculating the distributed load produced by the transformer and applying this to the soil? Or is my ignorance in regards to soil science causing me to miss a crucial point in this calculation? If there is a readily accessible code or standard that I can reference in my analysis that would also prove very useful. In short my questions are:

- How can I determine the load affected area as a result of the transformer base?

- Where could I find the published information for the above calculation for reference purposes?

Any help in regards to this will be greatly appreciated.

Respectfully,

Caleb Bell

 

[jheidt2543]

If the transformer is fully supported on the 5'x8' area then the soil loading is 94,000 lbs./ 5'x8' = 2,350 psf. If there is a concrete pad you have to add the weight of the pad. Next, you have find out if the soil will support the load, which should be covered in your soils report. The 2,350 psf is not a very big load so, unless your soil is very poor, it should not be a problem.

A concrete base should be set on a 6"-8" thick compacted stone base course and the pad should be 6"-8" thick with reinforcing steel selected on the basis of the pad thickness and the transformer load.

 

[MRM]

Caleb,
I think one of your questions was how much soil (depth) is affected by this new loading. You can usually assume that the the significant portion of the new pressure will be carried a depth about equal to twice the footing width (for a mat like yours) into the ground. For your situation, this would be approximately 14 feet or so. At this depth, the pressure increase may be about 10% of the new pressure due to the transformer. The stress distribution with depth due to the new footing pressure depends on a few things such as the soil density or consistency, layering, and other items, but this is usually a good estimate. The next question is... are there any soft soils down to those depths that would be affected even at the diminished new pressure levels imposed by the transformer mat? Soil borings will give you the info you need to answer that.

 

[caleb]

Thanks jheidt2543 and MRM for the information. I'm in the process of finding the appropriate soil information so I can get this thing on the ground.

Thanks again,

caleb

 

[Grouser]

Caleb

Is the transformer related to a switch? Electrical equipment can have large horizontal forces applied when current in applied/removed. The foundation design needs to address this contingency. Find out from the client what this equipment is for and if it will be subjected to switching loads. Also find out what amount of settlement is acceptable so that you can insure the foundation design limits settlement/consolidation appropriately. Good luck.

Grouser