Equation 4.4-2

[Bruce McC]

I am trying to work through the equation for the second area of moment of a compound brace, Equ. 4.4-2, at page 4-38 of Design Book first edition. I am using the dimensions given for the example but am unable to arrive at the answer of 6.31 x 10 etc . Attached is my spreadsheet, would appreciate any help in showing the error of my ways.

[Steve]

Bruce, it might be your neutral axis calculation - I'm getting 0.0043333 for that?

[Bruce McC]

Hi Steve

Thanks, yes when I use your value for the neutral axis, 0.00433333 which is the average of the values of the
neutral axes for the triangle and the rectangle I get the same answer as in the book, .
So my problem is either in the formula I used in the spreadsheet for the neutral axis of the compound section
or my assumption as to how it is determined. This was to use the formula shown at the bottom of page 4-37
to determine x the neutral axis of the compound section, not just take the average of the two sections.
I am attaching a screenshot of my spreadsheet showing the formula I used to calculate the neutral axis of the
compound section, it looks ok to me, but if it is meant to give the answer of 0.0043333 then it is obviously incorrect.
Any further comments as to where I am going wrong would be greatly appreciated.

[Steve]

It’s a bit hard to make out all the detail in the screenshot, but I think the difference between yours and mine is that I’ve only got two parentheses following C3/3, and the third one is moved to after C7/2.

[Bruce McC]

Thanks for your assistance Steve, PM was very helpful.