Question regarding how to populate the P&M arrays for interaction diagrams.

[wbell]

I am trying to develop an application that allows the user to see if a given value of axial load and moment can be will be safely supported on a spread footing.  Civil engineers are familiar with using interaction diagrams for columns and I am trying to implement the same principle for a spread footing.  The attached sheet works for the first two regions of the analysis where the load-moment curve is defined by decrementing the soil pressure from one side to the point where the pressure is maximum at one side and zero on the other.  If you look at the P array created by the sheet, it is correctly populated using a while loop which terminates when the pressure becomes less than zero.  After exiting the loop, I created a second while loop which starts calculating the axial load by moving the pressure triangle across the footing surface in specified increments.

The problem is that although the second while loop executes correctly, I am not able to append the calculated axial load to the P array created in the first while loop.  This can be verified by tracing the output of each while loop.

I will try creating separate arrays within the definition of P, but I would appreciate any comments/suggestions on how to populate the entire array within the object definition.

Thank you for your assistance.

[RichardJ · Accepted solution]

You have three errors:

1) On the LHS of the program assignment you have P[i. There should be no index (so just P).

2) Get rid of the i<--i-1 between the loops. It causes the last value of the first loop to be overwritten by the first value of the second loop.

3) In the second loop you have a literal subscript (typed with "."), not a vector subscript (typed with "[") on P. This is also true in the trace statement.

[wbell]

Hello Richard,

Thank you for the prompt, correct responses.  I revised the statements per your comments and now the routine works as intended.  The calculation is based on an article in Concrete International, March 2003, by Sam Eskildsen (pp. 104-109), where he presents a method of calculating a footing interaction diagram using Excel.

I intend to share the routine with Mr. Eskildsen and discuss some differences I have noticed in his approach.  If you are interested I will share the results of our discussion.

Sincerely,

Wayne J. Bell, PE

[RichardJ]

Glad to hear it now works. I'm not a civil engineer though, so I don't have a great deal of interest in the subject matter.