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Working stress vs allowable stress

  1. Oct 10, 2016 #1
    1. The problem statement, all variables and given/known data
    In my book , the author stated that working stress is the maximum stress premitted under design load . Whereas the allowable stress is given by below ( refer to the notes attached )
    http://imgur.com/a/YnimQ

    2. Relevant equations


    3. The attempt at a solution
    So , the working stress is the maximum stress before the object will fail ?
    Allowable stress is the stress that we determine ourselves ? If the working stress is 4000N , by safety factor of 2 , so the allowable stress is 2000N ?
    here , the maximum stress that we can apply is 2000N due to safety reason ?
    Correct me if i am wrong .
     
  2. jcsd
  3. Oct 10, 2016 #2
    image here
     

    Attached Files:

  4. Oct 10, 2016 #3

    PhanthomJay

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    The terminology can be confusing.

    Typically, the working load, or sometimes called the design load , is the maximum actual load that will be applied without any overload factor. Under this working load, the stress shall not exceed the maximum allowable working stress, where the max allowable working stress is the failure stress divided by a safety factor.

    The other way to do it is to apply an overload factor to the max actual load, then the stress under this factored load shall not exceed the failure stress. The failure stress is usually the yield stress of the material, or the stress at buckling.

    There are Code provisions also that modify this somewhat.

    For the problem at hand, the stresses shall not exceed the given allowable stresses when the system is subject to the given load.

    Failure stresses at yield are a property of the material, and must be given or tabulated somewhere.




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  5. Oct 11, 2016 #4
    So
    OK, what you mean is working load < allowable load < failure load ?
     
  6. Oct 11, 2016 #5

    PhanthomJay

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    working load = allowable load < failure load.
    Suppose you want to fasten a rope to a ceiling to support a load of 4000 N. The 4000 N is the working load; that is the max load you ever expect to hang on the rope. Now in choosing a rope to use, and fasteners, you don't want to use a rope that will fail at 4000 N, because there would be no safety factor. So instead, you choose a rope and fasteners that can support 8000 N (factor of safety = 2) before it breaks. In this example, the working load is 4000 N. The allowable load is 4000 N. The failure load (yielding or breaking load) is 8000 N.
     
  7. Oct 11, 2016 #6
    So, after

    So, after adding fasteners , the stress is is failure load ? How if no fasteners is added ?if so, the failure load = max load = working load = 4000N?
     
  8. Oct 11, 2016 #7

    PhanthomJay

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    No. I mentioned the fasteners only because it's no good if the rope is strong enough but the bolted connection to the ceiling is weak. Anyway, the working load is the allowable load of say 4000 N so using a safety factor of 2 you want to be sure that the rope is strong enough to handle 8000 N before it fails. It would be unsafe if the rope failure or breaking strength was only 4000 N. This is why safety factors are used in order to provide a margin of safety against failure.
     
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