GL: transverse, longitudinals and mixed framing

Discussion in 'Class Societies' started by Arvy, Jan 10, 2008.

  1. Arvy
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    Arvy Senior Member

    Hi all,

    I am still puzzled with the germanische lloyd rules. I am designing a steel 40 ft sailing yacht, and right now I am working on the framing.

    I have decided to use transverse frames with a spacing of 400 mm, and longitudinal stringers with a spacing between 250 and 350 mm (max is on maximum beam, becoming closer to the aft and fore.

    Using the formulas in the rules (GL for vessels <24m) I get the following (for the first frame)
    Longitudinals: a section modulus of 2.39mm^3 wich would lead to a flatbar 40x8 or 60x5
    Transverse webframe: a section modulus of 3.1 mm^3 which could be achieved with a L of 60 x 40 x 4.

    I find the stringers extremely large sized compared to the webframes. When I would make the slots for the stringers in the webframes, I would nearly have to cut them through.. bending the stringers in a fair way will become rather difficult I think.

    Can anybody tell me if these scantlings are realistic? I don't want to use an L or T for the stringers.

    Or is it with GL that you can either use transverse of longitudinal framing?

    Or did I misinterpret the unsupported lenght? I used lmin in the formula in table 1.43 (page 1-80). In my case the real unsupported length = 0,4 m and lmin = 0,815m more than twice the real length. The rules don't make clear if I should use lmin or the real unsupported length, so I used lmin to be safe.

    For the webframes I used the formula in table 1.44 (page 1-80). Beside this formula there is also a formula in table 1.42 to calculate the section modulus of transverse frames.

    I just can't figure out if I use the right formula or not. Can anyone help me with this?

    Grtz,
    Arvy
     
  2. MikeJohns
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    MikeJohns Senior Member

    Hello Arvy
    I'm an engineer and I design steel vessels.

    I think perhaps you are getting confused. I don't have the rules you are trying to apply , do you have them in an electronic format? If so and in English send them to me and I'll have a quick look. Some of the guides are very confusing as there are Girders and stringers both of which are longitudinals as well as transverses.

    Just transverses alone at around 400 is common but this will depend on the skin thickness, Also I cannot be sure without looking at those rules but the MOI SM calcs may include the plating itself. Some rules allow a strip of plating some allow the full width.

    I would consider spacing the transverses at 1m or more if you are going to use longitudinals.

    If you want you can email me at engden@aapt.net.au I may have some other rules that you might find easier to interpret or to help as a guide.

    Cheers
     
  3. Arvy
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    Arvy Senior Member

    Hi Mike,

    You are right about me getting confused.

    I do have the rules in electronic form and in english, so I will send them to you. I will also send you together with a drawing in which you can see what I want to do and will add some extra info.

    Over here in the netherlands we mostly space frames at 400, so I want to stick to that (as yards are used to it, and it is easier to create a fair hull for the people rolling the shell plates as they have more templates).

    I would like to use stringers to prevent the hungry horse look when you only use transverse frames, so maybe they don't have to be so heavy as the rules say.

    Grtz,
    Arvy
     
  4. Olav
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    Olav naval architect

    Arvy,

    just a few remarks:

    I'm a bit confused about your numbers. First, I think you meant cm^3 instead of mm^3 (which the GL rules use for section moduli). Second, given this and provided your numbers for Wmin are right, there seems to be something wrong with one of your selected FBs. According to my calculations the section modulus of a FB40*8 is 2.13cm^3 (not enough to satisfy the need of Wmin = 2.39cm^3), while FB60*5 is ok (3.0cm^3). Also, purely based on the requirement of 3.1cm^3 for the transverse frames, an L60*40*4 is too large (my calcs give W = 40.48cm^3), but of course there are other factors to be considered too (like the need to make them big enough to pass the stringers through), so this may be ok.

    Using lmin in table 1.43 was perfectly right as it is intended not to use values smaller than this at all. The real length is only to be used if it is > lmin.

    Just my $0.02...
     
  5. Arvy
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    Arvy Senior Member

    Hi Olav,

    I understand your confusement... First of all it is indeed cm^3, the software I use to calculate the section modulus of flatbars etc is using mm^3 so I would get figures like 2.13*10^3 mm^3 which of course is 2.13cm^3. I left out the 10^3 as this is more readable, but forgot to write it down in cm^3.

    I just rechecked my calculations, and they are from the time I was playing with the stringer spacing, by varying them between 350, 300 en 250 mm. The 40x8 is from a different calculation (300mm) than the 60x5 (350mm).. that wasn't too clever. In my calculation a L60x40x4 has a W=3,98cm^3 which is indeed larger than the required 3.1 but the stringers do have to pass.

    At least I interpreted the lmin correctly.

    Looking back at the time when I did construction drawing (not the designing itself but working out the design so the yachts could actually be built some 12 years ago) for a lot of yachts I remember that the frames were L-shaped. The stringers were rather small (nothing compared to the numbers I came up with) and the maximum height of the stringers was never more than half of the longest leg of the L shape. Too bad it was all a long time ago for me and memory gets blurry over time. I did find a part of a drawing in an old cupboard for a 48foot yacht in which the frames were a mere L60x40x5 unfortunately there was nothing on the drawing about the stringers.

    So now I am thinking are the numbers I came up with for the stringers and webframes for truly longitudinal framed yachts, and not for transverse framed yachts with stringers? I think this might be the case if you consider the Wmin for the stringers = 2.39 and for the webframes only 3.1 so only slightly larger.. I find this confusing.

    Or maybe I am just confused by the terminology or my inability to interpret them, I looked up if there was a german edition as well (hoping it would help me interpret them better) but unfortunately there is no translation for this chapter. The rules speak of transverse frames, webframes (which are also transverse) but they don't say anywhere which one should be used when and if longitudinal frames are the same as stringers.

    Grtz,
    Arvy
     
  6. JohnHeart
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    JohnHeart Junior Member

    Hi, Arvy

    I've been using ABS, BV and DNV rules for section modulus calculation but I don't have the copy of GL Rules. I might be able to help you if you send me the electronic copy of the rules as well as some data you use in the calculation.
     
  7. Arvy
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    Arvy Senior Member

    Hi John,

    Do you have an e-mail address I can sent the rules too (they are available freely on the internet) and the numbers I have used for the calculations.

    Grtz,
    Arvy
     
  8. JohnHeart
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    JohnHeart Junior Member

    I have pm you my email address.
     
  9. JohnHeart
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    JohnHeart Junior Member

    Hi Arvy,

    I received your e-mail and I did a quick look at it. I'm not impressed with the GL Rules. It's my first time to look into it and this Rule was written year 2003. I guess GL's latest rule is more comprehensive than this.

    I think the confusion arises on the unsupported span of the frame. So, it's better to understand this basic that all Class Rules are using.

    For GL, transverse frame here means, a stringer arranged transversely. And the longitudinal frame is the stringer arranged longitudinally. Choose between the two framing system. Either transversely or longitudinally.

    The web frame simply put is bigger than the transverse frame.

    You can forget about the transverse frame formula if you have decided to go for longitudinal framing.

    A simple explanation for the unsupported span of each frame.

    -For longitudinal frame, it's the transverse spacing of web frames.

    -For web frames, it's the longitudinal spacing of girders. Without girders, it's from chine to chine or bilge to bilge distance (in short detail)

    -For girders, it's the transverse spacing of bulkheads.

    800mm web frame spacing and 250mm longitudinal frame spacing is the common spacing we use in the design.

    If you're going for 400mm web frame spacing, use it consistently in all your calculations. But, I'm not 100% sure if GL's formula supports it, since 750mm is their minimum.

    To come up with better the solution, check your frame spacing, unsupported span and formula. I did not check your formula one by one but to point one typo error: For instance,

    Shell side Pdss
    >= 0,4 L + fore =2.96*$B$7-2.94

    it should be 2.06 not 2.96.

    And also, for your frame section modulus, you have to add the shell plate in the area considered.
     
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  10. Arvy
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    Arvy Senior Member

    Hi John,

    Typo's are very easy to make, I will recheck my formula.

    A transverse framespacing of 800 mm also sounds acceptable to me when using longitudinal framing. It is old habbit to hang onto 400mm as all designs I have seen (from 16 to 12 years ago) had 400mm spacing. So I guessed they were transversely framed, with stringers for ease of getting a fair hull??

    You mentioned that there was a minimum framespacing of 750mm in the rules, I must have missed that apparently. I know that there is a minimum of 750mm that one should use if lmin is less than 750 with lmin being the minimum unsupported span being used (in my case around 1,04m iirc don't have the exact numbers around right now).

    About adding the shell plate, I searched for it through the rules as I expected that I should do this, and couldn't find it anywhere (except in the part for grp hulls), Olav however said that for metal hulls one didn't need to add any shell plating.

    But in the end you say: use longitudinal framing with 250 mm framespace and use webframes at 800mm?

    One more question: the unsupported span of webframes, this isn't completely clear to me, I have a roundbilge design with a centergirder (except the enginebedding, boxkeel etc but in these locations I would still use the higher), would the unsupported span be from the girder to the sheer (as this is the first "chine")?

    Thanks this far
     
  11. Arvy
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    Arvy Senior Member

    Hi John,

    Forget the question about the unsupported span of webframes, this is clearly stated in the rules (just looked them up), it is like I thought, from the centergirder to the sheerline (deck).

    Finally things are falling rapidly into place thanks to you, it was indeed the terminology for the transverse and longitudinal framing and webframes.

    I will go for the 800mm webframes with 250 longitudinals and do the number crunching this evening (after thorouhgly rechecking the formula's in my excel sheet).

    The only question now remaining is the added shellplating to the area that I am working on.
     
  12. JohnHeart
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    JohnHeart Junior Member

    You're right, that is where I found the minimum.

    Once you have calculated the required section modulus, you can proceed looking for the right section to use with a higher section modulus than the required.

    If you have a 60x5 flat bar, 250mm bottom longitudinal frame spacing and 3mm bottom plate, you need to add that 250mm wide bottom plate in the calculation of the section modulus. Thus, forming an inverted T. Flat bar standing upright the bottom plate.

    Same thing with the web frame. Consider the plate below.

    Also consider that you cannot always use the whole frame space as the effective width of the plate. There are limitations to that.

    Effective width of the plate for longitudinal frame is the minimum of the following:

    - long'l. frame spacing
    - 60*plate thickness

    Effective width of the plate for web frame is the minimum of the following:

    - web frame spacing
    - 0.33*span
    - 750mm
     
  13. Arvy
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    Arvy Senior Member

    Thanks John,

    I have researched the GL rules, but I cannot find anything regarding the effective plate width in those rules for metal hulls. They only mention the effective plate width in FRP and wooden hulls (although, for wooden hulls they explicitly mention the section modulus withouth the effective platewidth).

    Where did you find those formula? If they are from another ruleset, I wouldn't just mix them.

    I will sent the GL-Group an e-mail with this question and hope they will answer.

    Grtz,
    Arvy
     
  14. Arvy
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    Arvy Senior Member

    Hi all,

    I very kindly received an answer from GL-Group regarding the effective platewidth. The answer is below:
    ===============
    We could confirm that no special effective plate width is defined in our pleasure craft rules (yachts up to 24m). Therefore we would refer to our seagoing ship rules which were developed much earlier. Attached please find this special chapter about design principles in which you will also find a paragraph about effective plate width.

    Regarding the defined effective plate width for FRP structures in our “small craft” rules, the reason will be found in the relative new material of FRP and therefore “new” defined.
    ================

    The attaced part of the rules is chapter I- Part 1 Section 3 Design Principles. There the effective platewidth in general is the framespacing: "Generally, the spacing of frames and stiffeners may be taken as effective width of plating."

    Further in the chapter there is a little differentation for different loadtypes.
     
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  15. Arvy
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    Arvy Senior Member

    Hi John,

    You suggested that I should use 250 mm for the spacing of the stringers and 800 mm for the webframespacing.

    I tried to do this, but had some problems with having to reposition the seams for the shell plating. In my original structural setup I have used 250mm stringer spacing for the bottom plating, and used between 300 and 360 mm amidships down to 200 mm fore and aft stringer spacing for the side plating. Do you think this would cause a problem? I will be using 800mm framespacing except amidships where I will add some extra frames (at the position of the mast and keel).

    Myself I don't think it will cause mucht trouble (after all it is at its maximum only 110 mm extra).

    Thanks for you thoughts about this.

    Arvy
     
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