Safety factor applied to structural design

Discussion in 'Boat Design' started by makobuilders, Jan 18, 2017.

  1. makobuilders
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    makobuilders Member

    Typically what sort of safety factor do NA's work to when designing different parts of a vessel. Like the deckhouse, pilothouse, hull, etc.?
     
  2. Richard Woods
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    Richard Woods Woods Designs

    A "safety factor" is also a "fiddle factor". The more accurately you design the lower the SF can be. So you are, I am afraid, asking a question impossible to answer.

    RW
     
  3. upchurchmr
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    upchurchmr Senior Member

    I don't know about NAs, but in military aircraft we used 1.5 x the expected breaking strength.
    That is with hugely expensive material property testing, Finite element analysis prediction, detail part analysis, a fairly strict quality control program, a great deal of historical data, and small scale and full scale testing.

    No boat builder could afford most of that (except possibly America's Cup design).
    So the safety factor would have to be higher - IMHO.
     
  4. TANSL
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    TANSL Senior Member

    Normally, when calculating scantlings, it is not considered a certain safety factor, but the designer uses the maximum stress allowable by the regulation that his boat must comply. Each Classification Society indicates, for each material, the maximum permissible stress. If a direct calculation is performed, larger permissible stresses can be used.
    ISO 12215-5 (for small vessel scantling) allows a design stress for E36-AH36 mild steel plates of 294 N/mm2. The breaking load of this material is 490 N/mm2. That is, the safety factor is 1.67.
    However, all the rules also take into account other factors, such as loss of thickness due to oxidation, or others (which I do not know) that cause the scantlings to increase in various proportions.
     
  5. messabout
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    messabout Senior Member

    In building design, for bridges and other structures it is common to use a 300% SF. Some of the structures still manage to fail because of unforeseen events.... the Tacoma Narrows bridge and a few others. A boat is not immune to failure despite good design engineering and generous safety factors.

    A current trend is to make boats as light as possible in order to enhance performance on the one hand, and to mitigate the cost of material used on the other. The latter is probably the less noble of the reasons.
     
  6. JosephT
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    JosephT Senior Member

    Speaking of unforeseen events. When in doubt, build it out of steel. You never know when a giant squid will attack your boat.

     
  7. rxcomposite
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    rxcomposite Senior Member

    It is expressed in many forms. In safety factors, it is usually a whole number and a fraction greater than 1 (used in aerospace and civil design). The other term is the limiting stress, expressed as a fraction (used in Naval Architecture).

    The limiting stress for alloys varies from 0.60 to 1.0 depending on the location. For superstructures and deckhouses, it could be from 0.60 to 0.75 of the welded proof stress, not ultimate or break strength.

    For composites, the Lloyd's limiting stress is 0.25 to 0.33 from the Ultimate tensile/compression stress. ISO uses 0.5. limiting stress. When materials are mixed in composites (glass+kevlar+carbon)or (uni and biax), the limiting strain (the plot of the modulus vs the elongation/strain) is used. Other class societies uses the limiting strain in lieu of the limiting stress.

    As the deflection and shear must also be controlled, it varies also. For deflection, it varies from 400 to 775 (alloy) depending on the location. For composite, limiting span/deflection ratio is 100 to 200 with 150 being the norm.

    limiting shear for stiffener webs is normally 0.425 for alloy and composite is 0.33.

    All three must be satisfied.
     
  8. Ad Hoc
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    Ad Hoc Naval Architect

    One does not use UTS as a measure of FoS, nor indeed any design values. Thus, this is misleading and not correct.

    Any FoS is a measure against the yield/proof stress alone, not the UTS. Once an applied load has subjected the material to go beyond the yield/proof stress it is degrading. The material has been subjected to strains that no longer return the material to its original state, as such, the material changes shape, necking (less XSA means less load carrying ability too), and also reduces its capacity for further deformation.

    As an example highly strain hardened aluminium alloys have very very little capacity for further deformation from their proof stress to the UTS, i.e very little FoS. Hence one never uses UTS for a design value or as a base for a FoS.

    ally overload.jpg

    The UK MoD uses a FoS of 9 on any item that is to hoist or carry personnel. Thus any FoS is not just material related but its duty, its frequency of usage and location on a vessel too.
     
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  9. Alik
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    Alik Senior Member

    2AdHoc:
    In ISO12215-5 it does.
     
  10. Ad Hoc
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    Ad Hoc Naval Architect

    Alik

    Indeed it does, but I would never advocate using UTS as a measure of metric for design values, ever!

    In ISO it si also pointless, since the minimum value they cite to use (of the two 0.6 v 0.9) always defaults to the yield values anyway, thus, why even put the option in!
     
  11. rxcomposite
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    rxcomposite Senior Member

    Alik- There is a mention of the aluminum Ultimate Strength in ISO 12215-5 but in the design stress, it says use minimum tensile yield. Did I miss a page?
     

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  12. makobuilders
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    makobuilders Member

    I don't see how the SF can be considered a "fiddle factor" at all. I work in rail and building construction and in general all components are designed/modeled and tested to a SF of 3.0, except for those that support workers where we increase to 5.0.

    However, design of a small boat is different. I understand that classification societies will dictate what loads to design to depending on the class of vessel. Then the NA will work within the working stress limits of the material. But that would only provide a SF of 1.0 which is not logical.

    Hence my question. Thank you.
     
  13. daiquiri
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    daiquiri Engineering and Design

    Actually, it is not so. A Class Society (or a technical rule) will dictate BOTH loads and the allowable stress (or allowable deflection) for various components/areas of the vessel. The combination of these quantities (loads and allowable stress/deflection) makes the calculation framework (Class or rule-specific) which implicitly includes the SF too.

    Other times, the minimum value of thickness or modulus of the structural member will be given by the Rule. In that case too the SF has already been accounted for (again, specific to the Class rule used).

    There was a similar discussion in this recent thread: http://www.boatdesign.net/forums/boat-design/aluminium-deflection-56746.html#post790360

    Cheers
     
  14. TANSL
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    TANSL Senior Member

    The factor 0.6 is applied to the ultimate tensile strength while the factor 0.9 is applied to the yield stress. So, at least ISO 12215-5 does use the breaking load for some of its calculations.
     

  15. FAST FRED
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    FAST FRED Senior Member

    With GRP a simple rule of thumb is if it doesn't flex , you are good "forever".

    On inspected USCG vessels the coasties used to spec 300 or 400% (I forget) but it will be in their Subchapeter T rules.
     
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