# Flexural Stiffness -High speed light craft

Discussion in 'Boat Design' started by Jure Bebic, Sep 1, 2020.

1. Joined: Aug 2020
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### Jure BebicJunior Member

Hi, I'm having a problem calculating flexular stiffness for my midship frame. The boat is MOTH International class Loa=3.2(m). This is an assignment for one of my classes in uni, i know DNV rules are not friendly for a small boat like this but i have to do it using those rules so that i could compare it with FEM results. I chose biaxial E-glass lamination. Youngs modulus of the bottom panel is E=23600 MPa and i intend to use the same panel for transverse stiffeners. But this formula is unclear to me and i just can't get a normall value for stiffness using it. Si=0.0716m^2, I=0.00107m^4, e=0.16 m. Can anyone tell me what I'm doing wrong? Thank you!

2. Joined: Oct 2008
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Location: Japan

Welcome to the forum Jure,

Ok, firstly what do you mean by:
Because
you note that:
Define "normal"...?

Thus, what is your objective, in calculating this flexural stiffness...?
Is it part of the assignment
or
Is it your assumption that you need to calculate it?

Finally... always always always... check your units!

3. Joined: Aug 2020
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Location: Croatia

### Jure BebicJunior Member

Well the primary goal of the assignment is to dimension the stiffeners using DNV and than compare it with FEM results just to see how far off are the rules for such small vessels. The part i think deals with this is DNV-HSLC/Pt.3/Ch.4/Sec.6-Scantlings(4. Latterally loaded beams). I was hoping to get the response of my web frames and than compare it with failiure values( using Tsai-Wu failiure theory, since that is the same theory i used in Siemens FEMAP software) . Am i on the right path? Units i used throughout the whole process are bellow.

4. Joined: Aug 2020
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### Jure BebicJunior Member

Well the value I'm getting using mm and MPa is 6.85E+13 (Nmm^2), and i don't know what to compare this value with since this is my first encounter with composites.

5. Joined: Oct 2008
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If that is the case, then you need define the spans, for the bottom, shown in blue and sides, shown in yellow.

Once you have established the length of each member, and the frame spacing, you then start to apply the rules, such as 5.2.3. This gives you the modulus, based upon your selected frame spacing and the spans of each member, subjected to the applied pressure, from chapter 1.

What you have defined as the structural member, i.e, its dimensions, if it passes the min modulus check, using the material proprieties you have defined, you need to establish that the structural member is not too slender - ostensibly a buckling check, hence the check in 4.2.2. It seems you're doing this back to front?

You have to design the structure to satisfy the rules, i.e what are the dimensions of the frames etc. that must pass the minimum rules of compliance.
That's it.

6. Joined: Aug 2020
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Location: Croatia

### Jure BebicJunior Member

Just to confirm if I understood correctly. Using 7.3.1, for maximum single skin laterall deflection and by plugging it into 5.2.3, i can get the minimum required flexurall stiffness. Then i compare it with the value from 2.6.4 of my preliminary panel.
If the calculated value is smaller then the one from 2.6.4, I can start to;
1. increase the stiffener span, or
2. remove certain stiffeners, or lastly
3. use a weaker panel, calculate its properties anew, until i get as close to the value required by 5.2.3. That would than be an ideally dimensioned stiffener?
It seems i am

7. Joined: Oct 2008
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Location: Japan

Ok... take a step back then.

1...you need to use the main dimensions of you boat..displacement, beam, speed etc etc and obtain the design pressures which are then applied at various locations on the boat, noted in Pt3. Ch1.
2...then looking at your structural layout, you will basically be presented with the following:

You have a frame spacing...and a stiffener spacing if you have longitudinal stiffeners.

3.. you then find the rule that provides you with the determination of the scantlings, lets say the frame. So, looking at 5.2.3,

The Pd = the design pressure you have calculated in Pt.3 Ch.1

The s= span (or frame spacing), and L = length of beam between supports, as per 4.2.2 and the sketch above as an illustration.

The E = you either have this based upon previous designs, or you can use the rules to establish this, based upon your layup.
I = the 2nd moment of area, of the structural member you have 'assumed' for now. Noting in 4.2.4 effective width of plating.
Cdp = boundary conditions, as noted.

You then calculate the minimum required modulus.

But you wont know it is satisfactory, until you do the compliance checks, such as those noted in 7.3.1.

Only once you have performed this process, can you then go back to the beginning.. the parts in 4.2.2.

Design is a very methodical step by step process. You can't do step 4 or 5 until you complete steps 1, 2 and 3!

Does this make sense now?

Jure Bebic likes this.

8. Joined: Aug 2020
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Location: Croatia

### Jure BebicJunior Member

Yes it does. I guess i thought i needed to get the modulus in the first iteration of 5.2.3 formula, that is what confused me. Thank you for your time!!

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