report tables - moment of inertia

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report tables - moment of inertia

While checking some output, I was having trouble verifying the moment of
intertia reported. I may be wrong here, but it appears that the Ad^2 term is
reduced by half, that is shown in the WSDOT summary report tables. At any
rate, my calc I = bh^3 + Ad^2 for rectangular shapes is giving higher values.

Binary Data PGSuper1_051111.pgs77.63 KB
Rick Brice
Steve, I'd be happy to take a

I'd be happy to take a look at the section properties. Please post your file and I'll look into it.
I'm not sure if this is a typo, but the moment of intertia of a rectangle is (bh^3)/12 + Ad^2. Perhaps this is the difference?
(BTW, I'm off to the dentist soon so I wont get to post an answer until first thing tomorrow)
Richard Brice, PE
Software Applications Engineer
WSDOT Bridge and Structures Office

Rick Brice
Steve, I've reviewed your


I've reviewed your spreadsheet and it has some issues with regards to computing section properties. The issues are:

  1. You have measured "d_i" from several different datum for the various elements. d_i for Element 1 is measured from the top of bulb, d_i for elements 2, 3, and 4 are measured from the top of the overall section, d_i for element 5 is measured from the top of element 1, etc.
  2. You are not applying the parallel axis theorem correctly. Your "d^2" term should be (Yb - Ybcg)^2 where Yb is the CG of each element measured from the bottom of the section and Ybcg is the CG of the section measured from the bottom of the section.
  3. The moment of inertia of a triangle is (bh^3)/36 not (bh^3)/12

Attached is your spreadsheet and my hand calculations for the section property. The value I compute is about half of yours. This is consistent with the difference between your spreadsheet and PGSuper.

Please send me your PGSuper project file so I can further confirm the computed section properties.



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