# Section modulus and Moment of Inertia

Discussion in 'Class Societies' started by ocean_boatyard, Nov 17, 2011.

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### ocean_boatyardJunior Member

Hello to everyone. I would like to calculate the SM and I of GRP top-hat stiffeners. Is any spreadsheet available for those calculations? If not, where can I find the formulas together with examples?

Thank you.

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### PetrosSenior Member

The formulas are complex, and difficult to develop. this web site has formulas for standard shapes:

For irregular shapes you have to add each segment separately, and if it is off set you have to modify it for the radius of gyration (when a simple shape is off set the SM and I gets larger by a function of the square of the radius from the centroid of the cross section).

for a hat section you can likely approximate it pretty good with a rectangular hollow tube of the same dimensions.

Good luck.

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### daiquiriEngineering and Design

Last edited: Nov 19, 2011
1 person likes this.
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### ABoatGuyMember

Here is a link to a USCG Navigation and Vessel Inspection Circular.

http://www.uscg.mil/hq/cg5/nvic/pdf/1987/n8-87.pdf

Check in the back, Enclosure A and there is a simple method that you can quickly make a spreadsheet for. Same can be found in Marine Composites by Eric Greene. Used to be downloadable but I don't know if it still is.3

Al Horseman, now with Gurit, did this many moons ago.

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### conceptiaNaval Architect

you can draw the cross section in autocad and take out the massproperties. This might help you to cross check the self-made spread sheet is giving you the correct values.

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### TANSLSenior Member

I agree with conceptia and I can confirm that this is the best and most accurate method. Just be aware that the drawing should be made full scale, the thickness of each part of the piece be correct and that the profile of reinforcement should be an object "region" of AutoCAD. In this cas the results are super accurate.
In the attached file you will see (pages 2 and 8) how with the help of AutoCAD you can calculate SM, I, Neutral Axis, and more even of the midship section of a vessel.
I hope I have helped.
Best Regards
Ignacio López.

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### rxcompositeSenior Member

Here is something from my early days of understanding section modulus. If you want a clean copy, you can download Marine Composites by Eric Greene and Associates.

I had it in Excell before but have to find it. You need to be able to calculate thickness of a laminate given the fabric weight and resin content.

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