# LR-section modulus

Discussion in 'Class Societies' started by Walid, Aug 28, 2020.

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1. Joined: Aug 2020
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### WalidJunior Member

I have simple question about the application of the section modulus formula of LR rules.

for a primary members we have a different section modulus coefficient (1/12) adjacnt to the ends and (1/24) at the mid span.this value can make a reduction of about 50% of the SM if we chose the mid span value (1/24).thinks are not clear for me here...i guess it's relevant to the stress transformation if we suppose that the both ends of this element are fixed to another elements so the stress which comes from this latest will transformed to this member at the posistions (1&3) in the attached picture .

regards

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2. Joined: Oct 2008
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Welcome to the forum Walid

How much of structural theory do you know/understand?

For a built-in beam, with a uniformly disturbed load, q (udl), what is the bending moment?
M = q.L^2/12

So at the ends there is a constant of 1/12.

What does it show in table 3.1.1...at position 1 and 3 (the ends)... it shows... 1/12

What is the bending moment, for the same beam, same conditions, but at the mid-span location:
M = q.L^2/24

What does it show in table 3.1.1...at position 2 (the centre)... it shows... 1/24

All the table is showing, is what any text book will show you, or, if you wish to calculate it from theory.

The bending moment is 1/2 of what it is at the ends, that is all it means.

So, if you elect to use the SM at the centre, that's great...but what is supporting the centre??.... the ends.... so if the member is supported at the ends....what factor must you use... 1/12.

To understand this better, let's see this in action in a similar example, look at a long suspension bridge... like this one, a classic cantilever bridge design, that takes advantage of knowing where the maximum bending moment is located and the bending moment at the centre:

You can see the central section is not as stiff, structurally, as the ends.... this design works because the central section is supported by the ends, which are "built-in", much like the beam.

So, if you design your structure to the 1/24 at the centre, all you're doing is the part that is noted as "suspended span" part of the bridge above.
But what is holding it up??... you can't ignore this part..

Make sense now?

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### WalidJunior Member

Enough,to say it's completely clear Ad Hoc;

yes it make a sense and linked directly with the requirement of the openings in the webs .

Last edited: Aug 29, 2020
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### WalidJunior Member

hello again everyone,

according to the LR rules :

we have to use one longitudinal bulkhead at the cross-deck area for craft with length less than 24m and two for craft with length greater than 24"to prevent cross flooding and the spread of flame and smoke".
so we can use a partial longitudinal bulkhead attached to the transverse web frames to achieve their purposes and guarantee the transverse strength provided by the primary members,

am I right!!!

5. Joined: Oct 2008
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That depends.

If you can prove that cross-flooding between the 2 hulls does not exist, as I assume you're referring to a multihull, it is not a requirement to satisfy.
You just need to prove it.

As for the smoke and flame...one assumes your only source of such, would be in the ER. And the ER has SFP anyway...which satisfies that part!

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### WalidJunior Member

I guess there is no debate about the damage scenario,

my question is about the structural point of view
for boats with length around 24m the longitudinal hull girder strength is not required,but in the transversal direction we must make the cross-deck area stiff to absorb the stress anduced by the transversal bending & shear and torsional moment.so let's roturn to the main question;can I design the the longitudinal BHD as a broken members fixed on the web of the transversal primary memebers"cross-deck web frames/transv BHD" and make it watertight & gastight above the ER <<pic-01>>or should I design it as one single element,which I think it's not the proper arrangement in this case<<pic-02>>

pic-01

pic-02

Last edited: Sep 1, 2020
7. Joined: Oct 2008
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Aaaahh, ok.. that wasn't clear to me.

So, if you have identified that the dominant load path is transverse, which members need to be continuous?
The transverse members.

Thus:
Like that

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### WalidJunior Member

it's all about the design decisions,I understand now

it was a usefull conversation Ad Hoc and thanks again for your time

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### WalidJunior Member

it's me again

as I mentioned before for a twin hulled craft around 24m the long strength is not required,so I want to put a continuous girder in the bottom centerline which extend to the bow where it will become a stem bar,but I dont want to make it a full depth as CLG "see the attached pic"
as I know,the web depth of CLG according to LR or another CS is the floors depth,so it will be a keelson,in addition we have the web thickness and the flange section area,the result is a major member which serve the hull section modulus .
I'm confused,shall I considered this member as a bottom longitudinal stiffener,who carry the keel plate loads and transferred to the supporting members ,can I calculate the section properties by the local pressure like the other long stiffeners,except for the stem bar which have his own sectional area !!!

Last edited: Sep 6, 2020
10. Joined: Oct 2008
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It is up to YOU, as the DESIGNER.

You have merely elect to have the keelson as a member to help support the build, or you have have it a s structural member to help support the transverse frames, or, you can have it as a docking keel.
Depending which you want the keelson to be, influences how you design it, and thus - the final scantlings.

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### WalidJunior Member

I understand that,it is possible to make a combination of roles to achieve all this in a single element:
build process
local strength
dry-docking

if it designed properly

12. Joined: Oct 2008
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Yes of course - that is design.
You decide which is the most important load case, one, or some, or all of the conditions - that is where being the designer comes into play. You design the structure for the purpose at hand.

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### WalidJunior Member

This is what I wanted to hear,the role and the purpose

thanks

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### WalidJunior Member

hello again gentleman's
can I post something about the propulsion systems here!!!or I must create another thread!!!

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### Boat Design Net ModeratorModerator

If it's somehow related to this "LR section modulus" thread and the thread discussion here, then that's fine to expand in this thread.

If it's more a separate topic of discussion, it would be best to create a new thread in the appropriate subforum with a thread title describing the new topic at hand.

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