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Homework Help: Calculation - impact force on a bollard

  1. May 21, 2012 #1
    i need to work some estimations for this case for an assignment

    30mph vehicle hits a bollard with 667kJ
    bollard height = 1m
    vehicle hits the bollard at approx height of 0.7m
    material = mild steel
    ultimate strength = 400mpa

    in one case the bollard is 219mm ext diameter with 10mm wall thickness
    the information given is that the bollard withstood the impact

    The second case i need to work out what the wall thickness should be for a 150mm ext diameter bollard to withstand the same impact

    assume the failure won't be at the floor fixings
    for the time being i will ignore the fact the car deforms as well as the bollard, and there are different levels of impact(front bumper, engine etc.)
  2. jcsd
  3. May 21, 2012 #2
    i've worked out 46mm wall thickness for the second case if i equate second moment of area of the first and second case.
    is that a fair estimation?
  4. May 21, 2012 #3
    If a limiting stress isthe crierion, then use elastic section modulus (I/y), rather than second moment of area I. If collapse is the criterion use plastic section modulus.
  5. May 22, 2012 #4
    Thanks for the advice

    Do you know of a way to calculate the thickness without comparing it to the initial case?
  6. May 24, 2012 #5
    The information on energy given suggests you can obtain an equivalent static force.
  7. May 24, 2012 #6
    what do you mean?
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