About first principles

[DUCRUY Jacques]

Hello,

After the reading of the thread "Scantling rules" (february 2010), I have a little (and maybe idiot) question : what are the "first principles" ?

I assume they concern :

the calculation of maximum pressure and design pressure (via longitudinal distribution factor (between 1 and 0.5 for a sailcraft) and pressure reduction factor (between 1 and 0.25)

the design stress determination

the formulas for planking thickness and stiffeners SM.

But I am surprised by some class societies rules : for example, in the LR or GL rules, you dont know really the design pressure or the design stress ...

On an other hand, is ISO rules really more precise that ABS ORY rules ?

Thank you by advance

Best Regards


Jacques

[jehardiman]

1st principles are the basics of engineering, structures and in this case, naval architecture; F=ma, Castigliano's theorem, Poisson's ratio, Archimedes principle,etc.

The classifying societies assume that some one using the Rules understands ship structures and strength at a professional level, and therefor all that needs to be stated are basic requirements for certification, not a detailed list of plate thickness and weld fillets. Remember, at the front of most Class Rules there is a paragraph that usually states that the society is open to considering "modifications or equivalents" to the rules as appropriate. By using the Rules it allows a plan reviewer to quickly calculate if a reasonable hull structure was specified without having to resort to doing it all over by himself.

It is also important to remember that the requirements levied by the Rules are not usually for strength requirements, but for operational realities that the vessel will see over the course of its insurable life. Plate wasteage, mill tollerance, weld inclusion, cargo handling and fendering damage, fatigue....all of this and more is factored into a single line in the Rules about hull plate thickness.