# Preliminary Still Water Bending Moment

Discussion in 'Class Societies' started by marufuddin0, May 7, 2021.

Tags:
1. Joined: Feb 2016
Posts: 25
Likes: 2, Points: 3, Legacy Rep: 10

### marufuddin0Naval Architect

Hi,

I need help with the class approval of a MIDSHIP. I am a little confused between design still water bending moment (SWBM) and maximum still water bending moment (SWBM).

Is the design SWBM different from the max preliminary maximum SWBM in the sense of calculation?

I calculated the design SWBM from the rule book both for hogging and sagging conditions. Should I calculate preliminary maximum SWBM from different loading conditions, both for hogging and sagging? What I understand design SWBM should be higher than the preliminary maximum SWBM if the two are different.

Could you please highlight how to calculate the preliminary maximum SWBM?

I have calculated considering different loading conditions using Maxsurf. However, I need to be assured the maximum SWBM from this calculation is the preliminary maximum SWBM.

2. Joined: Sep 2011
Posts: 6,626
Likes: 446, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

The regulations of the Classification Society will give the maximum value of the bending moment. You have to use the greater of the two values obtained, in sagging or hogging condition. On the other hand, the standard will tell you the maximum allowable stress for the material you are using. With these values, you calculate the resistance modulus (first moment of inertia) that the main section of your boat should have and check if your frame complies with that value.

3. Joined: Feb 2016
Posts: 25
Likes: 2, Points: 3, Legacy Rep: 10

### marufuddin0Naval Architect

Hi, I am still confused about your answer. Please explain more to understand better. I am going to explain more to let you know what exact problems I am facing.

1. Bending moment calculated from section modulus and material allowable stress --- is this bending moment design swbm?
2. Bending moment calculated from the class rule books — isn't it preliminary maximum swbm?
3. Bending moment calculated from the weight distribution — isn't it the actual still water bending moment at each station?

Actually, I submitted the midship section drawing for class approval without mentioning bending moment values. The class said it assumed the max preliminary still water bending moment 70000knm and 15000knm for hogging and sagging conditions, respectively, and the class asked for confirmation. So, when I calculated using the rule book, I found 50000knm (hogging) and 25000knm (sagging). So, I labeled these values as design still water bending moment and submitted the midship drawing again. However, the class is still asking for the max preliminary still water bending moment and it is also saying the vessel should be equipped with loading manuals. I have not submitted the station-wise bending moment values that I calculated using maxsurf considering the weight distribution. I would like to submit it with an intact stability booklet.

4. Joined: Sep 2011
Posts: 6,626
Likes: 446, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

If the says that the bending moment in still water has a certain value, try to find it with your calculations, or ask the class to explain how you get that value. They are usually right.
There are two different themes that should not be mixed:
• The maximum bending moment in calm waters, which will serve to define the minimum resistant modulus of the main section. Here the displacement corresponding to the scantling draft that you have decided will be used.
• The curve of bending moments in a given load condition. With it, you must create the loading instrument that will allow you to check that under no load conditions, in any section, the admissible stresses are exceeded.
They are different things.

5. Joined: Mar 2021
Posts: 10
Likes: 0, Points: 1
Location: France

### ScarfJunior Member

Design Still Water Bending Moment - calculate this using the lightweight distribution of the hull, buoyancy, and deadweight items for your intended loading condition(s).

Maximum Still Water Bending Moment - this is the maximum SWBM that the structure is capable of, at a chosen transverse section. It will be greater or equal to the design SWBM. You can calculate it using the allowable hull girder bending stress and section modulus to first determine the total allowable bending moment. The maximum SWBM is the total bending moment minus the wave bending moment but may also be limited by buckling considerations.

6. Joined: Sep 2011
Posts: 6,626
Likes: 446, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

7. Joined: Mar 2021
Posts: 10
Likes: 0, Points: 1
Location: France

### ScarfJunior Member

Apologies for any previous confusion caused. The OP does not say which set of Rules are being referred but here are some further thoughts:

Class Rules do not usually include design still water bending moment (SWBM) approximations, these are to be determined by the designer and usually submitted as part of the midship section approval.

Total design bending moment (DBM) = SWBM (based on still water loading) + Wave Bending Moment (from Class Rules). This total DBM determines the required modulus for the midship section and will be used for the preliminary approval of the midship section.

In terms of a maximum SWBM there may be two possibilities:

1. Class may want to be confident that design SWBM values obtained from preliminary loading conditions have sufficient margin to allow for any changes in weight and buoyancy distribution during the build period of the ship. They may therefore anticipate a margin to be applied to design SWBM to give a maximum envelope that is unlikely to be exceeded and consequently require a re-examination of the midship section.
2. If permissible SWBMs are going to be approved and stated in a loading manual, these are based on the maximum capacity of the midship section i.e. total DBM - wave bending moment. Are these permissible SWBMs the 'maximums' that are being requested by Class?

8. Joined: Sep 2011
Posts: 6,626
Likes: 446, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

@Scarf , do not complicate your life more than necessary, do not mix things that cannot be mixed, continue studying the regulations of the CS and read again, if you don't mind, my post # 4.

9. Joined: Mar 2021
Posts: 10
Likes: 0, Points: 1
Location: France

### ScarfJunior Member

I'm not 'mixing', just trying to offer some thoughts on what Class may be asking for as part of the approval process. Which particular Rule and clause should I be reading again ? ABS, BV, DNV, LR, RINA ?

10. Joined: Sep 2011
Posts: 6,626
Likes: 446, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

The one that you like the most, because, as you know, or should know, the concept of bending moment does not change according to the rule that is read.

11. Joined: Mar 2021
Posts: 10
Likes: 0, Points: 1
Location: France

### ScarfJunior Member

I can assure you that I understand very well the concept of longitudinal bending moment and its relationship to hull girder strength as I have applied it many times in practice. Also that actual still water bending moment is determined from a balance of hull buoyancy and weight. Loading manuals sometimes require permissible still water bending moments to be included, so I added that to the discussion for interest.

12. Joined: Sep 2011
Posts: 6,626
Likes: 446, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

You can assure many things, but the evidence is the evidence.
I thought we were talking about a bending moment around the transverse axis of the ship, not the longitudinal axis.

13. Joined: Mar 2021
Posts: 10
Likes: 0, Points: 1
Location: France

### ScarfJunior Member

We are, you introduced the word 'axis', I was referring to distribution. For clarity then, how about 'vertical' bending moment to include both hogging and sagging conditions.

14. Joined: Sep 2011
Posts: 6,626
Likes: 446, Points: 93, Legacy Rep: 300
Location: Spain

### TANSLSenior Member

That is a totally impossible situation

15. Joined: Mar 2021
Posts: 10
Likes: 0, Points: 1
Location: France

### ScarfJunior Member

Perhaps you should contact DNV and tell them they have it wrong as well.

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.