ISO12215 - 5 Beam Moments and Deflection

Discussion in 'Class Societies' started by willfox, Apr 6, 2017.

  1. willfox
    Joined: Oct 2008
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    willfox Junior Member

    Hi All,

    Any help would be hugely appreciated.

    I have a question about both the bending moments and deflection equations for stiffeners in ISO12215-5.

    Firstly, the moment calculation. I understand that ISO treats all beam as fixed and therefore one must ensure that the beams are designed to ensue the beams can take at least this moment. The question is that if you have a beam which is clearly simply supported, must you ensure that the beam can take the full bending moment at the ends as if it was fixed? I presume not as there will be no moment at the ends of the beam. I suppose you just ensure that the beam can take the "fixed" bending moment at the center of the beam.....

    Secondly, the deflection calculation. The relative deflection factor is 0.05 meaning that the allowable deflection is 5% of the span. This then gives the EI requirement to achieve this deflection. 5% seems a lot, especially as the beams are looked at as being fixed even if they are actually simply supported. If the beam actually is simply supported, the actual deflection would be 5 times the 5% of the span! I suppose this makes most beams strength not deflection critical. It just seems odd.

    Considering both of these observations, would it be wise to design actual simply supported beams with the ISO pressure but using a simply supported bending moment rather than fixed (the ISO requirement). Also, the same could be done with the deflection. Is this too pessimistic as maybe the pressure calculation is pessimistic in its self...

    Thanks for any help in advance
  2. TANSL
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    TANSL Senior Member

    My opinion (arguably like any other) is the following: The ISO standard defines certain assumptions and with the design pressures defined by it, determines the scantles of the panels and reinforcements.
    If you do not want to keep the basic assumptions of ISO, you can not apply any of its formulas. The standards constitute a harmonious whole which can not be used only in parts or in unforeseen cases.
    There may be constructive solutions not covered by the ISO standard. The same applies to any Classification Society. If you want to do something different than the rules, you always have the solution to do a direct calculation of your element.
    A reflection of 5% seems too large. Some formulas of plate theory, applicable to panels for example, are only valid if the deflection is not greater than the thickness of the panel.
  3. Barry
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    Barry Senior Member

    Where on a boat would you find a simply supported beam carrying loads?

    You are suggesting this in your inquiry.

    The 5% deflection would be quite large for fixed end conditions in a static environment. ie a building beam but perhaps the ISO standard is building in an extremely significant factor of safety for dynamic loading in a boat
  4. TANSL
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    TANSL Senior Member

    Let's assume a transverse beam 2000 mm in length. A deflection of 5% is 100 mm. Nor the ISO nor any other norm, for many rare things that we can imagine, can allow it.

  5. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect


    Disclaimer – I don’t use ISO rules, but had a read of them.

    I looked at the definition used in ISO, 12215-6, it states:

    8.2 End connections
    End connections of structural members shall provide adequate end fixity and effective transmission of the bending moment and shear force into the supporting member, and distribution of the load into the surrounding structure.

    So, this is very clear to me. All it is saying is that no matter how you arrange the structure, the ends, must be done in such a way that the load is able to transfer from the member under investigation (the beam) to the structure it is attached to and supported by.

    So if you had a simply supported beam, there is no bending moment at the ends, thus, the end fixity is pure reaction load. Ergo, you need to make sure that there is sufficient shear area and weld strength of whatever you have used that is attached to the ends of the S/S beam so the load can go from the beam, to the structure that supports it, via this shear area of “whatever” you are using.

    Since by implication, is the beam is S/S then the members, (beam) must be stiff enough to take the applied pressure and resulting moment it experiences anyway!

    As for the deflection factor... There is no actual definition of “ y “, where is refers to y/lu in the deflection factor. Thus cannot comment specifically to what they are actually saying. However, it is merely a ‘fudge’ factor, and I would not read too much into it, especially since as noted, 5% is a lot and is also too ambiguous. Deflection with low modulus materials needs a somewhat common sense approach rather than a simple carte blanch absolute. It must also be referred back to the EI calculation and hence checking the “I” rather than the modulus. In their wood calculation example, they do not do a deflection check! All low modulus materials must perform a deflection check, since in general, they pass a stress check first, but are, in general, dictated by their overall allowable deflection!
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