Scantling Rules

[Paul Kotzebue]

Originally Posted by MikeJohns

For a small vessel Its a good idea (if you can) to calcutate from first principles using panel sizes pressure head and beam theory. Then run the figures through the scantling rule and compare.

That makes sense and I consider that to be a good approach, but how do you determine design pressure? For example, do you use maximum hydrostatic head?

Using the first principles approach is essentially the same as ISO 12215, Lloyd's Special Service Craft, or ABS ORY. The differences between the above guidelines (they really aren't scantling "rules" like Gerr, Nevins, etc.) are in the design pressures, allowable stresses, and effective width of plating for stiffeners. Once the pressure is defined they all evaluate shell plating using the standard flat plate equation and stiffeners as uniformly loaded beams with fixed ends. Note that there can be significant differences in the results based on differences in design pressures and allowable stresses.

I do like the idea of doing independent calculations from first principles for things like deck deflection and beefing up structure for local loads not covered in the scantling rules.

[MikeJohns]

Yes if you read the rules you quickly see what they consider as the equivalent static head . The class society will juggle that head and calculate, observe and alter accordingly to give a minimum below which they consider unsafe.

It’s been interesting watching the changes over the years.

They all work that way for the panel loads static plus slamming which is assigned as an additional head of water plus a FOS.
When it comes to frames they carry the panel loads and some carry additional loads, rig, keel rudder shaft etc. I’d recommend calculating those loads and adding the additional bending moments before calculating the SM.

Then cross check against the SM of the member that your spreadsheet has given you from the class rule. All you will find is that they use slightly different FOS. It’s a very good cross check and results will be close enough.

Oh yes you need to look at what they consider the maximum stress for the material then that goes into the SM calc.

[conceptia]

Mat-C,

the class societies are more specialized into certain type of vessels, which basically is as per the requirement of that particular country they represent. If you go for ABS or DNV, they are the best in case for Offshore structures,
Germanischer Lloyd is best for Containers, BV n RMS for warcrafts, and Class NK for Bulkers. As the IACS has come into action, now almost all the class societies are implementing the same scantling method. this is to make the class conversions easy.

[LyndonJ]

Has anyone ever posted their spreadsheet of the GL scantling rules for a metal boat ? Or a first principals type approach that aligns with GL rules ?

Anyone interersted in doing a collaborative spreadsheet and making it available on the forum ? How does that go for copyright ? presumably open and free classifiacation design info like GL should be ok.

[fewfish]

Scantling rules as the name says are rules. If you want a boat sailed legally in most countries that means that it has to be built by certain rules.

Classification societies offer you the opportunity to introduce novel features and to so to speak present your case to them. In this case whatever you conceive you can describe either by words or physics and mathematics (FEM results too). If you can prove that your construction is safe, you can build the vessel by your rules. Bare in mind that this costs money and could only be reasonable if you plan to earn money from your design.

There is of course the subject of external loads on the structure. You cannot use loads from GL in ABS. Societies evaluate and apply them slightly different. They are similar and close to reality but different. The hard part for you could be describing those external loads on your structure to them.

But, your approach has to be purpose driven.

What is purpose of your design and question?

Do you want to learn something, or build a boat for your own use or build something you want to sell?

[dskira]

Originally Posted by gonzo

They have to comply with scantling rules so it is not an option.

It is not accurate. No one has to complie, the Lloyd's for example accept you come with your own scantling (backed of course) and ABS also.
The rules can exist only because we research and transmit our result to the body who implement them.
Following blindly will be foolish in certain case.
For the majority of the case following them is safe and make very strong vessel.
Daniel

[conceptia]

dskira,
you are correct.