# Iso 12215 - H/30 Rule

Discussion in 'Class Societies' started by willfox, Mar 16, 2012.

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### willfoxJunior Member

In ISO 12215-5:2008 11.7.2, the ISO rules say that you can use the square root of the kAS factor to determine a new denominator instead of th usual 30 to size the maximum height to thickness ratio.

I have a stiffener with varying height. The smallest height is half that of the largest height in the beam. Using the laminate stack analysis I have detarmined the scantlings of the beam. I have made sure this height passes the H/30 rule. The rules say that kAS is Actual shear force / FD determined from the pressure calcs which I must meet for ISO.

As I am designing this beam to ISO, I must meet FD which is the iso requirement. How do I determine the actual shear force?

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### fcfcSenior Member

I have neither understood it.

I have taken as a dummy thing.

Kas > 1 because we have also Kas = actual web area/required web area.

The rule limits stifferner h/t because of web crippling.

So table 21 allows you thinner web or higher web, depending on Kas (> 1).

IMHO, actual shear force is simply the force you used to compute your beam. So 99.9% of the time, actual shear force = Fd and Kas = 1.

The only I would see is when the stringer is part the the boat layout, and you have strange looks, well suited for human beings, but less evident from a structural point.

Now, if someone can put more light on this, I would be interested, too.

Thanks.

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### willfoxJunior Member

I was thinking that if your beam just passes in bending and in shear using the minimum height of the beam, you must then use this minimum height of you beam for the H/30 as this part of the beam is fully working. Where the beam is deeper, it is under less shear stress as the CSA is greater than you have specified for the smaller part of the beam. I then thought the amount of shear stress would be inversely directly proportional to the depth of the beam as the thickness is a constant. I.e. You double the depth, you half the shear stress, therefore it will be less susceptible to buckling. I thought the shear FORCE was a constant you took from ISO pressure calcs....................

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### fcfcSenior Member

Buckling/crippling is not linear. Buckling depends on lengh squared. Double the length, half the load. You double buckling.

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### willfoxJunior Member

Ok, well I suppose to be safe you should alway use H/30 for FRP structure. I am still a little confused over this rule tho.......

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### fcfcSenior Member

My understanding: (may be wrong).

You are computing a bottom stringer.

For practical and manufacturing reasons, your bottom stringer is also a bunk side.

So mandatory heigh = bunk heigh, say 30 cm for ease of computations.

H/30 rule mandates you minimum web thickness = 1 cm.

But when computing your stringer with the actual Fd from bottom pressure, it has a compliance factor of say 5. (bottom column 25). So it can bear Fd 5 times bigger.

So you can reduce web thickness by square root(5) = 2.2

I repeat it is my own understanding, and I may be wrong.

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### willfoxJunior Member

I think that sounds spot on. Is there anyone out there who agrees with this or knows this to be the case? It makes sense to me this way....

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### mizkuziJunior Member

Iso 12215-5:2008

anybody who has copy of ISO 12215-5:2008 in pdf file that is willing to share it with me? I cant find a free download... Also, is ISO 12215-7 already available and finally released? Can i have a free copy please?.. Thanks!

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