# ISO 12215 Bending moment formula

Discussion in 'Class Societies' started by vkstratis, Nov 5, 2022.

1. Joined: Feb 2015
Posts: 75
Likes: 9, Points: 8, Legacy Rep: 14
Location: Athens, Greece

### vkstratisNaval Architect

I know that this issue has been mentioned before in another thread some time ago, but I could not find definite answer.

I am dealing with the ISO 12215 2009 edition for the first time and I have managed to build the enhanced method's stack analysis calculation sheet for single skin laminates. My problem is with the formulas provided for calculating bending moments for l and b directions of the plate.

Both formulas resemble somehow the "standard" bending moment formulas that consist of multiplying the pressure with some factor and the length squared. But the standard's transverse version (according to author's paper stating corrections to the standard) has its pressure element squared instead of the length. Is this validated to be correct or is this also a typo? If one uses the stated version, the result is much lower and of course units are incorrect if we assume that factors are dimensionless which must be the case. Any thoughts from ISO users?

2. Joined: Jul 2021
Posts: 343
Likes: 101, Points: 43
Location: La Rochelle (Fr)

### Alan CattelliotSenior Member

There is a typo in the table ISO12215-5 A.9 Design shear force and bending moment on a rectangular panel :

Md l = 1/6× k2l × P × b² × (EIl/EIb)0,5× 10−3 is correct.
Mdb = 1/6× k2b × P × b × 10−3 should be corrected as : Mdb = 1/6× k2b × P × b² × 10−3

TANSL likes this.
3. Joined: Sep 2011
Posts: 7,200
Likes: 640, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

The correct formula is found in Table H.2.

#### Attached Files:

• ###### Table H.2.jpg
File size:
125.5 KB
Views:
53
4. Joined: Feb 2015
Posts: 75
Likes: 9, Points: 8, Legacy Rep: 14
Location: Athens, Greece

### vkstratisNaval Architect

Thank you. So the author has a typ0 also in his paper which corrects typos in the original published standard.

Another frustrating issue is where the standard computes averages. For example total flexural stress in table H.2 is stated as 165.6 N/mm2 which is neither the average nor the ply thickness weighted average as it probably should. Going forward to the top-hat stiffener example (Table H.6), the author just takes simple averages but he uses values for QX600 without multiplying KAM and KBB factors, resulting to what it seems wrong values. Any thoughts on these?

5. Joined: Sep 2011
Posts: 7,200
Likes: 640, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

The figure of 165.6 N/mm2 from Table H.2 is correct. Calculate the weighted average of the individual stresses by their thickness and multiply the result by kAM and by kBB.
I haven't checked the Tab H.6 but I don't remember hearing anything wrong with it.
Some time ago I consulted Jean Baptiste Soupez, who was proudly running as the author of ISO 12215-5:2019, about some inconsistencies in the standard and he himself told me that he did not agree with what had been published. Amazing, right?

6. Joined: Jul 2021
Posts: 343
Likes: 101, Points: 43
Location: La Rochelle (Fr)

### Alan CattelliotSenior Member

I agree that the average values calculated in table H.2 & H.6 are questionable. At least, should they be weighted by the ply thickness or the product of the ply thickness with the ply modulus. The difference between final results calculated using unweighted or weighted average isn't negligible.

I agree that in table H.6, the values for QX600 are obviously not multiplied by kAM or kBB. There is an error there.

Putting aside this last error, and even with correct calculations of averages, I don't like the way each laminates stacking (table H.6) is used as a single ply in the analysis (table H.7) . I prefer using the ply-by-ply analysis on the full sequence at once, making the calculations on the associated plating together with its stiffener.

#### Attached Files:

File size:
110.9 KB
Views:
45
7. Joined: Sep 2011
Posts: 7,200
Likes: 640, Points: 123, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

Totally agree.

8. Joined: Feb 2015
Posts: 75
Likes: 9, Points: 8, Legacy Rep: 14
Location: Athens, Greece

### vkstratisNaval Architect

Thank you again for your answers. So many inconsistencies in a published and reviewed standard is really strange. By the way TANSL the value of overall (average) stress I think is not correct because individual ply values are already discounted with factors KAM and KBB.

TANSL likes this.
9. Joined: Jan 2005
Posts: 2,656
Likes: 535, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

Wht did ISO increased the constant to 1/6 from 1/12?

1/8 is for a simply supported beam. 1/12 is the standard for a fixed beam primary structure. Does that mean all connections are treated as MORE THAN a primary structure?

10. Joined: Jul 2021
Posts: 343
Likes: 101, Points: 43
Location: La Rochelle (Fr)

### Alan CattelliotSenior Member

There is an additionnal coefficient ( one more ... It seems like it is so fun to spend pages defining coefficients in this standard ... ) K_BM, given in Table A.8 — Design shear force and bending moment in a stiffener according to type of end connections, whose intend is to restore the adequation of beam calculations of the standard with the basic formulas.

What do you think of this wonderfull Table A.8 ? Any remarks, comments ?

11. Joined: Feb 2015
Posts: 75
Likes: 9, Points: 8, Legacy Rep: 14
Location: Athens, Greece

### vkstratisNaval Architect

I think this is copied from DNV rules. They use a similar format.

12. Joined: Jan 2005
Posts: 2,656
Likes: 535, Points: 113, Legacy Rep: 1110
Location: Philippines

### rxcompositeSenior Member

DNV is the same as LR formula. LR uses the same c1, same as standard ME formula.

The only difference is that DNV adds c2 which is point load or concentrated load for machinery or cargo.

#### Attached Files:

• ###### DNV rule- fiber and composite.png
File size:
125.6 KB
Views:
40
Last edited: Nov 10, 2022
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.