# BUOYANCY ANALYSIS FOR CONCRETE PIT OR TANK

Rating:
40

### Description

Program Description:
"BUOYANCY" is a spreadsheet program written in MS-Excel for the purpose of performing a buoyancy analysis, checking the resistance to flotation, for either an open or buried/enclosed rectangular concrete pit or tank.
This program is a workbook consisting of three (3) worksheets, described as follows:
Doc
Open Pit or Tank
Enclosed Pit or Tank
Program Assumptions and Limitations:
1.  This program is based on the Archimedes Principal on buoyancy which is stated as follows:
"Any object, wholly or partially immersed in a fluid, is buoyed up by a force equal to the weight of the fluid displaced by the object".  Thus, the volume of water displaced by the object multiplied times the unit weight of water is equal to the buoyant force.
2.  Use of "overhangs" (extensions) on the pit/tank base allows the user to account for a "block or wedge" of soil around the periphery of the concrete pit/tank.
3.  For soil above the water table, the dry unit weight is used for determining the weight considered to contribute in resisting flotation.  For soil below the water table, the submerged weight (gs - gw) is used for determining the weight considered to contribute in resisting flotation.
4.  The full concrete unit weight is used for the walls, base, top slab (if applicable), and tributary grade slab (if applicable) as weight considered to contribute in resisting flotation.
5.  The total weight resisting flotation is the sum of the pit/tank concrete weight, the tributary grade slab weight (if applicable), the unsubmerged soil weight, and the submerged soil weight.
6.  The total "buoyed volume" is determined as the total volume of water which is displaced by the pit/tank.
7.  The total "buoyant force" is then determined by multiplying the "buoyed volume" times the unit weight of water.
8.  The factor of safety against flotation is the total weight resisting flotation divided by the total "buoyant force".
9.  This program assumes a Factor of Safety >= 1.2 to be satisfactory, taken against the highest anticipated water table level.  This also allows for some uncertainty in the contributing factors and variables.  Where the weights calculated are well established, this Factor of Safety could be reduced to 1.1.  In determining the Factor of Safety, allowance should be made for future removal of weight, for example the removal of soil.
10.  This program contains “comment boxes” which contain a wide variety of information including explanations of input or output items, equations used, data tables, etc.  (Note:  presence of a “comment box” is denoted by a “red triangle” in the upper right-hand corner of a cell.  Merely move the mouse pointer to the desired cell to view the contents of that particular "comment box".)

### Calculation Preview

Submitted On:
12 Oct 2016
File Size:
224.00 Kb
655
File Version:
1.3
Rating:
40

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Heinzig 4 years ago
Alex, Thanks! Lovely analysis! Enjoy your retirement!
ATomanovich 4 years ago

Flynn,
Sorry I took over a year to respond. I've been retired now for a couple of years, so I don't check on this website as often as I used to.
Anyway, I do not have an SI units version of this spreadsheet. However, all of my spreadsheets, including this one, are protected, but not with a password. So you can unprotect it and "massage" it to your liking.
Regards,
Alex
Flynn 5 years ago
Is there an SI version available for this? Standard units are only used in the US and the sheet is locked making it a bit useless for the majority of non US users.
ATomanovich 6 years ago
I made some minor appearance/cosmetic changes to the "WALLPRES.xls" and "BUOYANCY.xls" workbooks. What I did was, where same variables existed in both workbooks, I changed the variable names and nomenclature so that they would match up for consistency.
ATomanovich 11 years ago
It was brought to my attention in the "BUOYANCY.xls" workbook in the "Enclosed Pit or Tank" worksheet that there was a problem encountered when the water table was located above the top of the pit or tank. The calculation, while in error the way it was, turned out to be conservative for the total buoyed volume when the water table is above the top of the tank.
I have added the necessary program logic to cell C60 in calculating the total buoyed volume to address the condition where the water table is above the top of the tank.
The version of this workbook is now 1.2.
crystalinv 12 years ago
Keep up the good work...
ATomanovich 12 years ago
It was brought to my attention that in the "Enclosed Pit or Tank" worksheet of the "BUOYANCY.xls" workbook, there were some errors/typos in the input data validation for the input of the Water Table Depth, Dwt, at cell C21. These errors occurred in the "Settings - Validation Criteria", "Input Message", and "Error Alert". None of this had any effect on the resulting calculations. However, this would have incorrectly limited the range of acceptable user input values.
These same errors did not exist in the "Open Pit or Tank Worksheet".
victormchavez39 13 years ago
Excellent, Alex. Thanks.