Structural design - some brief on approaches

[Eric ruttan]

Agreed.
But...

If you take an I-beam, apply a load for a given span, it is easy enough to calculate the deflection and stress.

If you now increase the span, but the same load is applied, what happens? Well the the bending moment increases.

Firstly, thanks for recognizing this as an actual question, and not a non question.
Secondly, yes, but, let us be reasonable.
If you take a 20' fast cat like a tornado and only stretch the hulls make it a 40' cat, yes your example shows a problem.
But no one does that. Well, maybe some people think about taking a cat and cutting the hulls and adding some number of feet. and so in this case your example would apply to a limited extent.

But, if one does the reasonable thing and also increases the web depth of the stretched I beam, (hull depth and width) than the ability of the "I beam" to resist deflective load can increase more or less than the load.

But, again, where is the magic point? As I see it there is not one. yes it seems as if it is a rhetorical question friend Alik, but it is a question.

I was just reading about a designer who needed less reinforcement for a build of larger hull than was needed for a smaller hull on a smaller boat. Because the cross section increased more than loads.

 

[gonzo]

Stiffness increases to the cube of the increase in depth for a beam. It is not a linear relationship. However, shear and tensile strength of the material remain constant.

 

[Eric ruttan]

Perhaps someone can deduce from all this (I say perhaps) that when the global loads of the beam-ship have to be considered, the local loads are no longer considered, but it is not correct, in addition to the global loads, when they are taken into account. , local charges must be taken into account.
On the other hand, it should be clarified that there are boats less than 24 m in length in which the global loads on the hull must be taken into account.

this is my point! this is all just confusion! rebuke magical numbers!
Just say the loads need calculated. local and global. It is simpler to think and takes less words to write.

 

[DCockey]

these are not standards, or rules, in many jurisdictions. They may be guidelines.
Not to quibble over words.

They do not have to be legal requirements to be called "standards" or "rules".
ABYC calls what they publish "standards" even though they are voluntary in many jurisdictions. What are ABYC Standards - American Boat and Yacht Council https://abycinc.org/page/standards
ABS calls what they publish "rules & guides" even though they are voluntary in many jurisdictions: ABS | The American Bureau of Shipping https://ww2.eagle.org/en/rules-and-resources/rules-and-guides.html
LLoyds calls what they publish "rules and regulations" and use the shortened term "rules" even though they are voluntary in many jurisdictions. https://www.lr.org/en-us/rules-regulations/

 

[DCockey]

this is my point! this is all just confusion! rebuke magical numbers!
Just say the loads need calculated. local and global. It is simpler to think and takes less words to write.

Regulations, requirements, rules, standards, etc commonly contain seemingly arbitrary numbers to denote the range of applicability. These numbers are not "magical". They represent a choice which was made in an attempt to ensure that the range of applicability is broad enough but not too broad.

 

[Ad Hoc]

Firstly, thanks for recognizing this as an actual question, and not a non question....

Thanks for the confirmation

But, if one does the reasonable thing and also increases the web depth of the stretched I beam, (hull depth and width) than the ability of the "I beam" to resist deflective load can increase more or less than the load.

That would be correct.

..But, again, where is the magic point? ...

The “magic” point, as you all it, is the value of the stress of that “I-beam” when subjected to a load.

For example, an I-beam of 80x8 riders with a web of 200x5mm if the span is say 3m and you apply say 2tonne centrally, the stress, if aluminium will be 92.5MPa, which is fine as it is below the design allowable stress for the material.
Let us assume if we double the span to 6m, the stress would also double and be 185MPa. This now fails, as it is over the design allowable stress for that material.
So the “magic” point has been exceed, this being the design allowable stress for that material.

But, if the web depth is increased, as you rightly note, from say 200mm to 400mm, the stress in the beam reduces from 185MPa to 76.5MPa.
Thus clearly demonstrating that as you noted, increasing the stiffness reduces the stress.

..Just say the loads need calculated. local and global. It is simpler to think and takes less words to write.

I read your question as requiring more of an explanation than a simple one-liner which I noted in my reply. Sorry about that, I’ll just keep it brief then.

 

[gonzo]

this is my point! this is all just confusion! rebuke magical numbers!
Just say the loads need calculated. local and global. It is simpler to think and takes less words to write.

What is your point? What magical numbers are you rebuking? Your posts have been rather obnoxious, criticizing methods and rules that have been historically proven to generate better and safer vessels. Can you provide any data to back your negative comments?

 

[Eric ruttan]

What is your point? What magical numbers are you rebuking? Your posts have been rather obnoxious, criticizing methods and rules that have been historically proven to generate better and safer vessels. Can you provide any data to back your negative comments?

What engineering need be done on a 23m boat that should not be done on a 25m boat? Or vice versa?
What engineering need not be done on a 99 T boat that must be on a 101T boat? Or vice versa?
It seems the obvious answer is none.
Then why, if the topic is
Structural design,
would one suggest otherwise. Or indeed confuse the topic with any such meaningless distinctions as 24m or 100T.

These numbers are not magic and nothing is special about them.

 

[gonzo]

The "magical number" claim is yours and only yours. You have posted plenty of obnoxious comments on this forum without showing any basis for your opinions. Laws, regulations and class requirements have no need to be "magical". They are what they are. If you don't want to engineer a 550T vessel, simply design it to be 449T; it is a very common choice. Engineering must comply with laws, rules and regulations, otherwise the design is useless. Further, the numbers are not magic, but are special because they make the design applicable and that fulfills its purpose.

 

[Alik]

<24m is a 'magic length', from where simplified requirements apply. The reason is purely financial, i.e. small craft=small money. This is about cost of engineering, cost of certification/classification, cost of equipment, manning costs, etc. In some countries this cutoff is 20m, not 24m (say, in Russia).

 

[CDBarry]

ISO 12215-6 requires a global load check for some boats.