# unsupported length from BV NR 500

Discussion in 'Class Societies' started by aldy syahri, Nov 27, 2019.

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1. Joined: Aug 2018
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### aldy syahriJunior Member

Hello guys, I wanna ask some question about BV Rules "NR 500 Rules for the Classification and the
Certification of Yachts".

Here I have some confusing mean between primary structure and secondary structure. in my logic:
1. primary supported by primary
2. Secondary supported by primary
3. platting supported by secondary

is my logic true? or maybe I have some problems with this logic please give me some answer.

if my logic is true, now what is primary and secondary in the construction of a boat?
Can we say that the hull girder is primary and the web transverse is primary?

so when designing the hull girder, the unsupported span is between the web transverse.
when designing the web transverse, the unsupported span is between the hull girder.

is it true? can someone help me? since on this rules the formula for secondary and primary stiffener is the same equation.

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### jehardimanSenior Member

Primary loads are the major loading of the hull girder (NR500 Part B, Chpt 4 Sec 2). Primary structure is the structural material and shape needed to resist these loads; this includes the shell & deck, deep longitudinals (and transverse deep floors in cats) and bulkhead spacing.

Secondary loads are the loads applied to the primary structure to generate the primary loads (hydrostatic, hydrodynamic, and propulsion) as well as large local forces (i.e. cargo/machinery weights, rudder, rigging, or tankage loads)(NR500 Part B, Chpt 4, Sec 3). Secondary structure is the structural material and shape needed to resist these loads; this includes frames/floors between bulkheads, engine beds, crane/mast foundations, bulkhead stiffeners, chain plates, tank ends, etc.

Tertiary loads are localized loads caused by material deflection under primary and secondary loads (which is why I specifically mentioned shape as a function of primary and secondary structure) and the fitting of specific items, such as cleats, winches, hatches, and stern/rudder tubes, etc. Tertiary structure is structure fitted to primary and secondary structure to distribute localized loads/deflection.

NR500 likes to use the terms "primary" and "ordinary", where "ordinary" could be either secondary or tertiary structure. See more in this paper (http://www.shipstructure.org/pdf/418.pdf)

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### aldy syahriJunior Member

Thank you very much, it very helpful and open my mind.

Just try to make it clear,
So when designing the longi hull girder (longitudinal primary strcuture) the span we must include is should be between the tr. bulkhead, or maybe between pillar if it is too long.

When designing the transverse primary such as deep tr. Frame at cross deck in catamaran, the span will be between the connection of one hull and the longi bulkhead or maybe longi girder in middle of the cat.

And when designing the secondary the span will be between the frame.

Also we must have some concern on which loads are applied to the primary or secondary (this one already stated clearly in bv rules).

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It depends what the long.t girder is for. If, for example, it is an engine girder, what is supporting THAT girder?...the WTBs.
So the WTBs become the Primary Supports. So the span is between WTBs, as the WTBs support the girder.

Ahh..this example of the raft structure is a bit tricky since it has several load cases and not all the same conditions.
The span of transverse is between major changes of section...so if a hard chine hull, for example, and if the angle of deadrise is less than 15degrees, the span will be from outboard chine to inboard chine. But, if the deadrise angle is greater than 15 degrees, which means the total angle is less than 150 degrees, the span is from Keelson (Centre line) to the chine. The major change of sectional shape, creates the "primary support".
But for the Catamaran raft (central) structure, the support is technically from one hull to the other, as there is a major change of sectional shape. But for global loads, these structures require a different analysis. Hence a bit tricky, as it is a yes and no answer!

Yes. The span is between each frame, as the frames support the long.t

See below:

The change of section from the bottom plate to the side plate, is a major change of sectional shape. So the span of the transverse frame is from CL to bilge, or bilge to deck.
The long.t stiffener, is supported between frames.
The plate is support by the frame spacing and long.t stiffener spacing.

Exactly..hence my comment on the catamaran raft structure.

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### aldy syahriJunior Member

In here you means that the transverse frame is the primary structure that supported by chine to chine, keelson to chine, or bilge to deck? So we should applied the primary load?

As you said before, if the deadrise <15 degree the span will be chine to chine.
Can we make the span from long. Girder at the middle to the chine? Since I think the long. Girder at mid is support the transvers. Frame too.

This one applys to side structure, when the height of the hull is too height the transverse can be supported by stringer.

Last edited: Nov 28, 2019
6. Joined: Oct 2008
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You need to be careful here.
You will note that whist the definitions given above by JEH are correct, you need to understand the implication of such, and thus how to analyse the structure.

Because this:

Is where it starts to get complicated without a drawing of your arrangement to answer you correctly. Why??..because this is a frame analysis question not a global load analysis. Frames in this sense whilst taking a primary load, in Class rules and for small boats is not strictly speaking a primary load. The primary load is taken by the hull girder, the hull girder supports the primary load and the frames are supported by the hull girder, and frames support the long.ts.

It is all sequential.
What is supporting what?...the nature, or source, of the applied load is thus not so important. So calling this a primary load, can become confusing, especially if English is not your first language.

Does this make sense?

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### aldy syahriJunior Member

The primary load is clearly stated in BV rules and JEH. Like what you said. It is true.
This one is my mistake to state "primary load". Actually, I wanna state about local load for local scantling of the primary stiffener or secondary stiffener.

I am sorry, this one my mistake "primary load". The question should be like this (I will use BV rules definition; they said about primary stiffener and secondary stiffener):

For this one, I will take side structure as an example. From your figure4.45
My question is:
Is the transverse frame primary stiffener or secondary stiffener?

Since, the primary stiffener will have wave load on it.

And secondary stiffener will have wave load and side impact.

8. Joined: Oct 2008
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The transverse frame is a primary stiffener, when you are analysing the long.t stiffener.
Because the transverse frame supports the long.t stiffener between frame the spacing.

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### aldy syahriJunior Member

thank you

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