If you have been able to read ISO 12215-5 to the end and have understood the formulas, let me show you my admiration because few technicians have been able to do that feat. If you continue on that path I have no doubt that you will become a good calculator of scantlings.
ISO 12215-5 is very difficult to read and almost impossible to understand because it gives no explanations of what it does or why it does. In many occasions you must have great experience in calculating scantlings to know, more or less, what you are doing and, therefore, know how to give the appropriate solution when there is a problem.
On the other hand, ISO 12215-5 has large gaps or inconsistencies. For example :
- there are no floors for ISO or, at least, does not specifically say how to calculate them.
- If you have a tank bulkhead, in a 0.8 m depth boat, you must apply a loading height of at least 2 m, which is totally absurd.
- the minimum thickness of the deck required by the standard is ridiculously small in small boats.
Answering your questions now:
- the ideal structure depends on many things but you always have to calculate the scantlings of a boat thinking on the shipyard that will build it, in the human and material means that it possesses and in the customs of that shipyard. You can try to get the structure of minimum weight, structure of lower cost of materials or structure with lower cost of labor. Each of these criteria will lead to a structure with different elements, distributed in different ways and with different laminates schedule.
- When something does not comply, there are several solutions: changing the composition of the laminates or changing the spacing between reinforcements are the most common. For example, the same layers, otherwise distributed, can solve a problem.
If you need to consult anything else, out of the public domain, I will be happy to assist you in my email address.
Good luck with your work and encouragement.