Calculation - impact force on a bollard

[Ziv7]

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.)

 

[Ziv7]

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?

 

[pongo38]

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.

 

[Ziv7]

Thanks for the advice

Do you know of a way to calculate the thickness without comparing it to the initial case?

 

[pongo38]

The information on energy given suggests you can obtain an equivalent static force.

 

[Ziv7]

what do you mean?