Aluminium Deflection

Discussion in 'Boat Design' started by Adarsh Edakkote, Nov 8, 2016.

  1. Adarsh Edakkote
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    Adarsh Edakkote Junior Member

    Hi All,

    Anybody knows the deflection limit for Aluminium beams?
     
  2. daiquiri
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    daiquiri Engineering and Design

    The deflection limit is given by class rules.
    Hence, it will depend on:
    - the class rules you are designing to;
    - the vessel type;
    - what part, area or mechanical system of the vessel you are considering.

    So your question needs a context, in order to get a reasonable answer.

    Cheers
     
  3. b1ck0
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    b1ck0 Senior Member

    It depends what is the driving factor for the limit. It could be structural, functional and etc. Describe your problem in more details so we can try to advise you.
     
  4. Adarsh Edakkote
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    Adarsh Edakkote Junior Member

    The structure is part of a gangway and it is under ABS class.i didn't find any deflection limit mentioned in ABS rules.
     
  5. Ad Hoc
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    Ad Hoc Naval Architect

    Class do not give deflection limits for aluminium. Despite it being a low modulus material, the Class rules are ostensibly about limiting stress and by inference in the rule fudge factors, deflection too. But this does not always work. (I had to correct a superstructure for a client that 'just' used Class rules for a large RoRO passenger deck because it had excessive defelction).

    So first thing is to ensure it passes the Class rule for stress.

    Then...use some common sense. If the gangway is lets say 10m long and unsupported, would a deflection of 1mm be acceptable, yes of course. Would a deflection of 100mm be acceptable, no. And since a gangway, part of the deflection check would be to check for modes of vibration. Since if the deflection is excessive it also implies the structure is not stiff enough too. Think melenium bridge :)
     
  6. daiquiri
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    daiquiri Engineering and Design

    Yes, in fact Class rules do not treat gangways.
    They are covered by the norm ISO 7061:2015 "Aluminium shore gangways for seagoing vessels". :)

    Cheers
     
  7. Ad Hoc
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    Ad Hoc Naval Architect

    LR has rules for linkspans....and for passenger walkway link spans and they do give a deflection criterion.

    Lloyd's Register Rules and Regulations - Rules and Regulations for the Classification of Linkspans, July 2016 - Part 3 Construction, Design and Test Requirements - Chapter 5 Bridge/Vehicle Ramp Strength - Section 4 Design criteria

    4.6 Deflection criteria
    4.6.1 In Case 1 the deflection of the bridge or ramp between supports under the applied load is to be limited to that given by the following expression:

    L/400 mm, where L = distance between supports, in mm.
     
  8. daiquiri
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    daiquiri Engineering and Design

    AH, it depends on the Class and on the equipment considered.
    Recent cases I have had to consider:

    ABS rules, case of watertight access doors and hatches:
    9.3.1 - max. deflection of steel doors: 0.0056 L
    9.3.2 - max. deflection of aluminium doors: 0.0022 L

    ABS rules, case of deck covers:
    7.1 - max. deflection of steel covers: 0.0056 L
    7.2 - max. deflection of aluminium covers: 0.0028 L

    Then there is also Lloyd's code for SSC:
    Part 6, Ch.7 - Table 7.2.1 - max. deflection ratio of steel structures
    Part 7, Ch.7 - Table 7.2.1 - max. deflection ratio of aluminium structures

    Cheers
     
  9. Ad Hoc
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    Ad Hoc Naval Architect

    Ahh yeah true..my bad, forgot about those. It has been a while since i looked at that side, as i tend to use the SSC software which when using LR rules; & by default, should check the deflections.! I also use my own values too ;)
     
  10. TANSL
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    TANSL Senior Member

    Your own values?. Could you share, please?.:p
    It does not refer to "deflection limit for Aluminum beams" as the OP asked, but it has to do with the comments made : I hope this helps.
     

    Attached Files:

    Last edited: Nov 10, 2016
  11. Barry
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    Barry Senior Member

    So these standards set the deflection but is there also a loading criteria that establishes the load per foot, point load, natural frequency dynamics with resulting forces or does the designer use his own discretionary/anticipated load values?
     
  12. daiquiri
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    daiquiri Engineering and Design

    Of course, loads are also given by Class and ISO rules. In fact, the info about the admissible deflection is incomplete if it is not accompanied by the relative design load.
    For this reason, it is not possible (because not accepted) to use loads given by one Class society with deflections or admissible stress given by another Class society.

    Say that a steel beam of a certain size and cross-section deflects 10 mm under 1000 kg load. It means that it will deflect 5 mm under a 500 kg load (because we design in the elastic, or linear, range of stress-strain curves).
    Then:
    - a Class society "A" could prescribe that the beam should be designed with load 1000 kg and the max. allowable deflection of 10 mm;
    - a Class society "B" could prescribe that the beam should be designed with load 500 kg and the max. allowable deflection of 5 mm.
    Both rules will result in a steel beam of the same size.

    But if you decide to take the load 500 kg given by the Class "B" and size the beam to the allowable deflection of 10 mm of the Class "A", then you will end up with a badly under-sized beam.
    Conversely, if you take the load of 1000 kg of the Class "A" and max. deflection of 5 mm of the Class "B", the beam will be unnecesarily over-sized and heavy.

    It is a situation which is gradually being sorted out with the adoption of unified IACS rules for common commercial ship types, but some differences between various Class rules still remain, and important differences remain for ship types not covered by IACS rules.

    So, to put it in a shorter form - every design is done according to a specified Class, the rules of which prescribe loads acting on various parts of a ship and give the limit values for stress, deflection and, eventually, size of various structural parts.

    Cheers
     
  13. TANSL
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    TANSL Senior Member

    The regulations of the Classification Societies are true treaties of shipbuilding. I would venture to say that all aspects of ship design and construction have been taken into account and studied by technical staff and highly experienced personnel. That is why it is very useful, I would say that it is mandatory, for a responsible designer to reread them from time to time. Simply using the software that many CS make available to the designer can lead to important errors.
     
  14. Barry
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    Barry Senior Member

    So to your comment about "values of stress", do the Class rules then say that the allowable tensile stress for say 6061T6 is X kg/cm2?? (or for zero temper/hardness if welded for a specific series?)
    And if they do, would it include a varying factor of safety for various components of the ship structure. Ie ramp, handrails, carrying a different factor of safety than perhaps an aluminum radar arch?
     

  15. daiquiri
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    daiquiri Engineering and Design

    There are several ways in which the allowable stress, deflection or thickness of an aluminium piece is prescribed by the Class rules.
    For example, Lloyd's Part3, Ch.2 prescribes the thickness of aluminium plating through a correction factor of steel plating thickness:
    1.3.3 Except where otherwise stated, equivalent scantlings (of aluminium stiffeners) are to be derived as follows:
    Plating thickness:
    t_a = t_s sqrt(Ka) C
    Section modulus of stiffeners:
    Za = Zs Ka C
    where:
    c = 0,95 for high corrosion resistant alloy
    c = 1,00 for other alloys
    Ka = 245/ σa
    t_a = thickness of aluminium plating
    t_s = thickness of mild steel plating
    Za = section modulus of aluminium stiffener
    Zs = section modulus of mild steel stiffener
    σa = 0,2 per cent proof stress or 70 per cent of the ultimate strength of the (aluminium) material, whichever is the lesser.
    Same Class, same Part 3, another application (superstructures and deckhouses), gives the thickness of the aluminium plating explicitely:
    3.1.2 The thickness, t, of aluminium alloy members is
    to be not less than:
    t = 2,5 + 0,022 d_w but need not exceed 10 mm
    where
    d_w = depth of the section, in mm.
    And so on.

    Just as another example bit - the HSC code of ABS prescribes scantlings of aluminium (or aluminum, as they call it) parts in case of direct analysis via allowable stress. It is too lengthy to cite here, but is an interesting reading for those interested in the structural issues.

    Regarding your question about the prescribed mechanical properties of an alloy - in the above example of Lloyd's rules, the quantity σa in the equations is tabulated in the Part 2, Ch.8, Sect.1 for each alloy (6061T6 included). See the attachment:
    Aluminium MechProp.png
    As you can see, minimum proof and tensile stress to be used in the calcs are already there. But in general they can be negotiated and higher values are accepted if you have test certificates which demonstrate the better mechanical properties of the alloy batch which will be used for the build.
    In general, yes - as also seen above.
    One of tasks of a designer is to keep up with the rules updates, in order to know where to search for the appropriate design prescriptions.

    Cheers
     
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