questions about scantling (iso 12215 5-6 :2011)

The picture illustrated is called a "grillage" structure whein the frame and stiffener is of the same height. It can be analyzed in the same way described below. I see some detailing errors/ommission in the picture, a common mistake. The end fixity should have a reinforcing layer, or better yet a "collar" (an edge reinforcing lamination that goes over the top and down again).

A plate and stiffener is analyzed by first transforming the stiffener into an "I beam" as illustrated by LR. This becomes an I beam "welded" to a plate. The effective width of the plate is calculated according to the formula shown or as formulated by the classifying body. If a floor is to be firmly fixed to the stiffener, then a top plate is added to the tabulation as shown by the red outline. Visualize this as a "wide H beam". You can use the same formula to find the effective width as used in the bottom. LR does not use this method but the analysis remains the same.

A word of caution though. Stiffeners are formed with biax layers (+45-45 degree). The calculated stress on the face area is prone to errors as the biax loses about 72-75% of strength/modulus when stress is rotated from its normal axis. No correction necessary if face is reinforced with Unidirectional fiber. Stiffeners are the most difficult to program and design as it is highky directional stress related.

Assign the corresponding tension/compression strength, tension/compression modulus to the elements and find the neutral axis.

Whether you are using the maximum stesss criteria or the maximum strain criteria the initial arrangement of the tabulation remains the same.

 

rxcomposite, thanks for posting the background material.

Perhaps you could help me with a few questions?

In forming the composite structures, do the manufacturers do any physical structural tests to track consistency in manufacture?

It seems to me, that the a single representative Stiffener + Plate assembly could be built and tested (loaded, sacrificed) to verify (fine tune) the calculated results. Is this usually done?

All for now,

Mark Cat

 

MC,

Not exactly. If built to class, the calculations follows the class rules calculations and then a sample (coupon) material corresponding to the laminate schedule is submitted to class to prove that their choice of material meets the minimum standard of calculations. In other words, "you have the calculations, prove it". In some class rules, a coupon is submitted first from which calculations are based.

It would be very expensive to build a duplicate structure only to do some destructive testing afterwards. Those making high performance boats employ composite engineers to optimize the laminate schedule, make it lighter, and narrow down the "fudge dactor" or more commonly known as the factor of safety. Because when in doubt, increase the factor of safety. As a comparison, marine uses a factor of safety of 3 (1/3 of the allowable stress) aerospace uses 2. The smaller the factor of safety assigned, the more test is required.

To ensure consistency of manufacture, class built (or good boatbuilding shops) follows a well defined procedure from purchase of materials, storage, laminating temperature/humidity, attention to details of laminations, sampling technique, barcoll hardness test, ect. All to ensure quality is there.

 

Anyway with the certification, it is the responsibility of the manufacturer which is engaged with the experience we know what is good, or parts that need to be reinforced, the challenge is to optimize a structure (strength / weight) and to reconcile this with the Standards, and it is not always compatible,

nothing prevents to submit for test samples or parts,

in iso 12215 -5, there are 3 module Evaluation, A, B, C,
the A module requires (as said RXcomposite), strength and glass ratio are measured in a laboratory, and you can then use these values ​​in your calculations

B module, only glass ratio is measured by the manufacturer, and you use strength (already) calculated in iso, with own %glass.

C module, iso values with penality coefficient : 0,8 : nothing is measured

(what I say is a synthesis, refer to the ISO 12215 standard, for more informations)

 

rx,
what moment of inertia , and section modulus,
should i use, Because the section is variable,
i can find value of neutral axe in a CAD software,
but how to check with the iso standard,

the picture is not my project, my stiffeners are more complex,
but the principle is the same,

if you want I can send you the stiffeners's picture (private message or mail,
for reasons of confidentiality, I can not post on the forum for the moment).

 

The bending moment is at the center of the stiffener, at the centerline of the cross section of the stiffener. The N.A. and the section modulus can only be arrived at after you have defined the laminate schedule and the materials to be used. Use of a higher modulus material at the top or bottom can shift the N.A.

I will post the tabulation of LR this evening as I'm a bit occupied at the moment. I am not using LR or ISO anymore as I designed my own program to analyse whatever structure I need to. I have added the limiting stress and limiting strain plus the direction (angle) of load to the tabulations. These important (to me) properties are not included in the typical programs available.

Yes, PM me.

 

Before we proceed further, are you using TANSL’s automated ISO design software? He has done an excellent job of programming it and I encourage you to use it. He has also posted a separate ISO calculations in Excel (in this forum) which contains a visible formula. It can be expanded to suit the topic we are discussing here.

Attached is the tabulation of LR based on the formulae discussed in their module Calculation Procedures for Composite Construction. Following this pattern, you can modify the ISO spreadsheet. I have made an Excel version of this but I cannot post it. I am not sure if it still Lloyds property rights (or maybe not as it is my own creation).

My goal is not to question any Class societies’ way of formulating the material properties, just a deeper way of understanding how it works. I think their formula was derived from statistical samples of test laminates. Mine is to make it more versatile, more detailed, as the nature of my work requires it.

None of the tabulations I have seen uses the correction factor when the load angle is changed. The material properties changes considerably when this angle deviates. Note the maximum strain theory tabulation I have posted contains the fiber angle. This came from aerospace composite design where the calculations are more critical. LR shows the angle change modulus formula, poisson’s ratio, and transverse modulus, and others in Part 8 Chapter 3. The software defaults to standard values. It needs a separate spreadsheet to be made and manually correct the default values by entering new ones. That goes the same for ISO too.

Attached are “Engineering Constants” used in composite engineering. As the load angle change the factor deviates. I have derived the formula and programmed it in Excel. Very useful tool.

For example, WR can be tested in the 0/90 degree angle and the result will be identical but if a +45-45 biax is used, the strength/modulus goes down to 35%. Different materials also react differently. A WR follows a sinusoidal pattern. The Kevlar has a much stiffer pattern with the carbon the worst. A seven degree angle and the fiber start splitting. So when the hull laminate uses bi ax, the strength/modulus must be adjusted accordingly. No correction for a 0/90 degree fabric.

In the case of stiffeners, the web act as trusses and behave in shear condition. No correction necessary as it aligns with the load when stressed (more or less). But in the crown area in a +45-45 orientation, it is mostly in tension (or compression) so a property degeneration is required. Not so with a unidirectional fiber as it aligns with the load perfectly. It makes sense to cap the stiffener with Uni as its strength is optimized.

 

IN ANY CASE, BX -45/45 is used when the shear strength is required,

I also have my own excel program based on iso
I also excel software ICOMIA,
my problem is not laminate schedule,
but the dimensioning of the panels and stiffener,
that is the values ​​that I need to take to get the right design pressure, bending moment etc. ..

 

Marlin,

PM me if there is anything you have in mind that cannot be discussed in the open forum.

Rx

 

i've downloaded TANSL iso scantling software, it looks fine...

 

but it lacks a bit of conviviality and ergonomics, and there is some bugs...

for me, the better is I SCANT ISO MONO

you can try demo version here :
http://www.icnn.fr/eng_/DownloadSoft.aspx

license costs a bit expensive, can be profitable, all depends on the use that we make

 

software programs are good, but do not solve my problems
nobody can help me?

 

I guess you need to refresh yourself with scantling rules. Download the LR rules for Special Service Craft. The 2010 version is available in this forum. It contains a lot of information and illustrations.
1. Panel Dimensions/Span points - Part 8.Chapter 1_3.
Covers the determination of plate dimensioning and correction factor for the aspect ratio.Contains information on the Bending Moments to be used in plate and stiffener analysis. Fig 3.1.4 illustrates the midship section and the relevant terminology for the span points. Yours is a hard chine planning craft so this approximates your design.
2. Local Design Loads- Part 5. Chapter 2. Sect 2_5.
Covers the determination of Hydrostatic pressure and Dynamic pressure on plating. Section 5 covers the impact or slamming loads. It also covers the vertical motion of craft, vertical accelerations, and the combination of these forces.
In previous version of LR, the depth of the stiffeners (fig 3.1.4) is illustrated. The deeper it is relative to waterline, the higher the pressure. Along the length and of the sides of the craft, there are zones of impact/slamming zones. These should be taken into consideration.
To be methodical about this;
1. Draw/sketch the midship section and the profile view of the craft to determine stiffener spacing.From the profile view determine the span points and the zones of slamming.
2. Compute the bottom design pressure on the specified plate (center of plate).
3. Compute the bottom design pressure on the stiffener with plate (center of stiffener).
4. For the transverse frame, the bottom pressure is the designed pressure point on the stiffeners. Yours should be taken as continuous beam spanning the breadth of the craft. In the absence of a centerlinegirder, the two innermost longitudinal should not be spaced more than 1.5 meter apart. In classic beam theory, this is a beam with varying concentrated load (it supports the plate+stiffener load). The bending moment varies along the span of the girder. Anyway, just use the formula in LR.
5. You have a grillage structure with varying heights of stiffeners equal to the height of the transverse frame supporting the floor. Most likely, your inner stiffeners/longitudinals will be overbuilt as you have to satisfy the stiffener proportion and the corresponding sheer stress on web. The central part of the transverse will also be overdesigned (in relation to the calculated bottom pressure) as the center of its height is from the deepest part of the keel to the level of the floor.

 

marlín, would be pleased to receive your comments and suggestions to the SCT program. I have long been gathering information from users. Try to improve the program and your opinion will be invaluable.
Thanks for your interest.
Regards
Ignacio López

 

I fear the problem of marlin974 is very simple :

If he wants CE certification, he has no other choice, but to comply with ISO12215-6 & 6. So other scantling rules are of NO purpose.

The manufacturing method choosen leads to stringers NOT perpendicular to panels. (vertical stringers, V bottom).

The question is then "How do you proove formal compliance to ISO 12215 with "slanted" stringers" , when slanted stringers are not explicitely covered in the rule ?

The problem may lie in wrinkling/crippling of the web.