Equation 4.4-2

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Bruce McC
Blackwood
Posts: 374
Joined: Thu Apr 07, 2011 1:29 pm
Location: Canberra ACT Australia

Equation 4.4-2

Post by Bruce McC » Sat Jul 11, 2020 1:23 pm

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.
Screenshot (11).png
Bruce Mc.

Steve
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Joined: Sat Oct 18, 2008 10:18 pm
Location: Coffs Harbour, NSW

Re: Equation 4.4-2

Post by Steve » Sat Jul 11, 2020 4:11 pm

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

Bruce McC
Blackwood
Posts: 374
Joined: Thu Apr 07, 2011 1:29 pm
Location: Canberra ACT Australia

Re: Equation 4.4-2

Post by Bruce McC » Sat Jul 11, 2020 7:42 pm

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, :D .
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.
Screenshot (12).png
Bruce Mc.

Steve
Blackwood
Posts: 165
Joined: Sat Oct 18, 2008 10:18 pm
Location: Coffs Harbour, NSW

Re: Equation 4.4-2

Post by Steve » Sat Jul 11, 2020 8:37 pm

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
Blackwood
Posts: 374
Joined: Thu Apr 07, 2011 1:29 pm
Location: Canberra ACT Australia

Re: Equation 4.4-2

Post by Bruce McC » Sun Jul 12, 2020 11:05 am

Thanks for your assistance Steve, PM was very helpful.
Bruce Mc.

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