# Required Section Modulus for Side Elements

Discussion in 'Class Societies' started by comandante, Dec 3, 2012.

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### comandanteNew Member

Hello gentlemen,

I am working on structural design of a high speed supply vessel based on Rina High Speed Craft rules.

I have a basic question which confused me, if you help me I will be pleased.

Rules have one formula for calculation of section properties for deck, bottom and side. For instance, I found required section modulus Z = 18 cm3 for deck ordinary stiffeners. And I used an element (19cm3) which satisfies this by taking into account X axis.

The problem is for side longitudinals. If I consider general X axis and make calculation for side element which has same dimensions, it has 47cm3 section modulus instead of 19cm3. Therefore I am confused which SM should be considered for side elements.

Rules do not mention which axis I should take into account.

Murat Tosun

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### Eric SponbergSenior Member

Murat,

Welcome to the forum. The axes to use are always local to the stiffener, and the vertical axis should be normal to the plating where the wave pressure is applied. So, let's say that your hull side is not quite vertical, but maybe 85 degrees to the waterline. The stiffener is normal (perpendicular) to the plating. You have to orient your local axes for the stiffener to correspond with the plating: The horizontal axis through the neutral axis of the stiffener is parallel to the plating, and the vertical axis is perpendicular to the plating. You calculate your moment of inertia and section modulus for the stiffener with these local axes, not the axes parallel to the ship and the earth's horizon.

I hope that helps.

Eric

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### comandanteNew Member

Thank you very much Eric,