safety factor for wooden design

Discussion in 'Class Societies' started by Walid, Mar 7, 2021.

  1. Walid
    Joined: Aug 2020
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    Location: algeria

    Walid Junior Member

    hello everyone
    I usually find some answers in this forum without asking,obviously not today.
    so,I've passed through some CS rules in order to find something about wooden structural design,I guess only the ISO 12215 had mentioned this topic with 0.45 limiting stress factor for stiffening members.
    my question:
    1/:in cas of an FRP hull renforced by a wooden stiffeners below and above the WL,how can I deal with the design stress value.
    2/:knowing that I m using LR rules the next problem I will facing is the span/deflection ratios especially for the deck compenents.any advices!!!

    regards
     
    Last edited: Mar 10, 2021
  2. jehardiman
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    jehardiman Senior Member

    The real problem with wood is knowing exactly what properties you have in ....that piece of wood right there.... While there are published minimum expected strength (tensile, compression, with/across grain, etc.) and modulus, the selection, fastening, and fitting of the piece becomes crucial, much like off-axis CF errors and fastener zippering affect that piece's strength. This is why wooden most vessels are generally built to scantling and framing rules vice calculated stresses. In cold molding, the relatively thin laminate layers can be treated in typical non-isotropic layer fashion based upon minimum expected strength. Bulk stiffeners present a special problem, especially if not encapsulated. You need to carefully select your Neutral Axis and provide sufficient trans-laminar tensile and shear strength. So...
    1) Just figure out the FRP "top-hat" stiffener you need to put the Neutral Axis at the inner skin, make your wooden stringer the interior size, glass them in with proper fillets and tabbing and then forget about them for many, many years. As long as they are undamaged, they are good for the flexural life of the vessel. (Otherwise, just calculate it directly using anisotropic flexural analysis, accounting for all loads and failure modes).
    2) Meet the rules and properly treat any holes in/through the deck beams, paying particular attention to the fastener squeeze to prevent damage to the most important part, the outermost wood fibers.
     
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  3. Ad Hoc
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    Ad Hoc Naval Architect

    Welcome to the Forum Walid,

    In addition to JEH's comments above,

    If you are:
    Then use whatever values LR offer for design values, of the wood you are selecting, to input into their rules. If they do not show any, then you must approach the plan approval office for guidance. I suspect they wont have much..as I did the same a few months backs, for a catamaran design. Thus I suspect you may have to opt for older ABS or GL rules which do note values of wood to use, as per CS rules, or as I did, obtain technical data from the mills supplying the wood and getting LR to confirm said mechanical properties. for a design allowable stress.
     
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  4. Walid
    Joined: Aug 2020
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    Walid Junior Member

    thanks for both of you jehardiman & ADHOC

    but I need to clarify some things here,
    for the propreties there is no probleme,I work in parallel with ISO_Annex_E wich provide the mechanical properties of some common woods.

    as ADHOC said,I must use onother CS regarding this matter,

    for me I must give the wooden design the priority,here in algeria the FOAM core&top_hat stiffeners doesn't exist unfortunately.
    based on the experience of some active members here,I decide to treat the wooden stiffeners as a structural members not as core,covered and encapsulated by FRP only to prevent moisture effect and bonded them to the hull,the cross section calculated to carry the bending moment induced by the applied loads using the 0.45*compressive strength as a design stress,but due to the lack of informations regarding the attached plating in case of wood as stiffening members,I neglected the contribution of the lower flange which can be considered as a strength reserve in case of an overloaded situation as the picture below shows.
    now,I need to validate this assemption !!
     

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  5. TANSL
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    TANSL Senior Member

    If you follow ISO 12215-5: 2019, in addition to the factor 0.45 you must apply a coefficient kAM = 0.9 or 0.95
    Only in the case that the reinforcement, made of wood or edge plywood, is associated to a strip planking panel, you cannot include the attached plate when calculating the I or SM values of the reinforcement. In other cases, the value given by the standard can be taken.
    Coating the reinforcement with fiber, in my opinion, does not make much sense, unless you also coat the shell panel with the same fiber.
     
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  6. Walid
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    Walid Junior Member

    hello Mr ignacio and thanks for your contribution
    as I mention above,the shell is an FRP_single skin renforced by a wooden members,so can I include the effective plating and treat it as a single unit after the idealization or I must keep it as it is!!
     
  7. TANSL
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    TANSL Senior Member

    Frankly, I don't know, but in the case of two such different materials, I don't think the union between them allows you to consider them as a beam with web and flange. I would not consider the attached plate. You should check that whatever you use to join panel and reinforcement, it supports the shear stresses that will occur between them. You should do, in my opinion, a similar calculation to the one proposed in Appendix "H" for the reinforcements to determine the tension / compression in the areas furthest from the neutral axis and the shear stress between the reinforcement and the panel.
     
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  8. Ad Hoc
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    Ad Hoc Naval Architect

    Now I think I understand your question.

    Basically you are saying you have a structural member which is made of 2 different materials, and hence two different material proprieties. How to analyse it?
    Is that correct?

    If so, then it is about the ratio of the 2 material properties.
    So you have the higher E(h) divided by the lower E(L). So you a constant C = E(h) /E(L).

    When calculating the sectional areas, to calculate your I, the material with the higher E is multiplied by the the ratio C.

    So, in your example the 180x100 = 180cm^2.
    If, for example, this has a higher E than the Glass you are using, and let's say the value of C= 2 then this area becomes 180 x 2 = 360cm2 ... But you don't gain any "thickness", just 'width'.
    So instead of being 180mm high and 100mm wide, it becomes 180x 1 = 180mm high and 100x2 = 200mm wide.

    The proceed with the calculations as normal.
     
  9. Walid
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    Walid Junior Member

    good evening Mr ignacio
    the tensile and compression induced by the bending moment applied on the stiffener will result a maximum shear stress at the NA plane,so Id like to say yes I must check it to ensure a smooth transmission of the shear stress through the laminate"no structural core principle",but I m not sure about it in cas of wooden reinforcement due to the weight and mechanical properties of solid wood.
    according to LR,the required web thickness to meet the shear stress is almost 4mm in each side,with 0.46 fiber content and 74.8N/mm2 ultimate shear stress using CSM&WR.
    for a 0.85m span of pinewood and a 1200g/m2 fiber,there is an increase of 8% in weight without taking in consideration the resine and the lower flange weigth,in addition to 0.33 limiting design stress I guess we will ended with a heavy structure.
     
    Last edited: Mar 10, 2021
  10. TANSL
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    TANSL Senior Member

    Hi Walid,
    The wooden structure is never the lightest possible but, since you cannot use another construction procedure, try to achieve the minimum weight (wood) structure, playing with the spacing between reinforcements or with the shape of the cross section. of the reinforcements. A solid, rectangular stiffener has a lot of material that does not contribute to the total strength in an appreciable proportion. Perhaps a "T" -shaped stiffener will achieve the same strength, in its critical pints, with less weight, even though the labor to build it is greater. It depends on how you want to optimize your structure. The attached plate cannot be taken into account, but only if the panel is of the strip planking type and perpendicular to the stiffener. Try another type of construction that allows you to use the attached plate. Investigate if plywood on edge reinforcements are more favorable than solid wood, or vice versa. That is, you have some option, although not many, to try to reduce weight.
     
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  11. Walid
    Joined: Aug 2020
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    Walid Junior Member

    good evening adhoc
    first of all,thanks for your cleare explication.
    the selection of stiffener cross section for a homogeneous materials based on the attached plating,my question is about the selection of a wooden cross section in case of a FRP hull!!!
    I guess if one can insure a strong bond between the structural compenent and thier attached plating "by the equivalence method"as u mention above, the resulting section will behave as a single unite,in that case the attached plating will form the lower flange of a tee section
     
    Last edited: Mar 10, 2021
  12. Walid
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    Walid Junior Member

    completely clear TANSL,I will try my best to achiecve a suitable arrangement

    thanks
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    The wood... shown in your section...is not really providing much 'strength. It is just carrying the shear load..and very heavy at that.
    If you removed the wood and replaced it with a light weight foam, the structure still does the same job.
    This is of course assuming the FRP is sized to pass the min rule requirements on its own.
     
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  14. Walid
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    Walid Junior Member

    I get the picture adhoc
    even it's not easy to shape a wooden beams in a complex areas in order to respect the structural continuity,but as I said earlier I have to pass through this.

    after all,we returned to:

    "A vessel is nothing but a bunch of opinions and compromises held together by the faith of the builders and engineers that they did it correctly. Therefor the only thing a Naval Architect has to sell is his experiences."

    thanks for all
     
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  15. jehardiman
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    jehardiman Senior Member

    Sniffff....Makes me cry...
     
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