ire impact loading on pos

huangyhg

tire impact loading on post
i have been designing the barrier system for v8 a race in australia.
for the concrete barriers the calculations were pretty straight forward. we assumed that 4 - 3 barriers would act together through catenary action to resist the load. i assumed an inelastic collision between the car and the wall and determined system velocity. from  there i used the weight of 4-3 walls and calculated the stopping distance of the wall due to the friction. i did the calculations for several situations and confirmed my results with the fia standards which uses the same logic.
the problem occurred when designing the debris fences.
to calculate the load of the tire on the mesh i calculated an equivalent spring constant of the mesh, treating it as simply supported between the posts. from there i calculated how far the fence would need to deflect to take out the kinetic energy of the tire in an inelastic collision - (this combines the fences weight and the tires and gives a systems velocity which is resisted by its elasticity) . it came to be about 8 kn which is close to the fia standards.
for the post i realized there is no way i can assume an inelastic collision and it would be much closer to perfectly elastic.
the question is how can i calculate my impulse loads on the posts?
using conventional mechanics will give a ridiculous force because the deflection of such a stiff post is so small.
say for a tire weighing 9 kg with a velocity of 27 m/s.
fence post will be chs 114.3 * 3 spaced at 4 m.
does anyone no of any standards or have experience in this area?
i have only been out of uni for 12 weeks so tread lightly lol.


well, why not assume that the post will bend plastically?
if a wheel and tire assembly hit the catch fence over my head i am not going to complain if the posts bend permanently.
cheers
greg locock
if you could give me any tips on how to anaylise it plastically it would be much appreciated.
i have sort of come to a mental road block.
ive been reading some complex impulse theory and the math is beyond my ability to follow and probally most engineers and is not practical out side of the lab.

bump.
i think most of us are reluctant to comment for fear that any advice we give will get you in trouble.  designing race courses is a very high risk area, best left to specialists who have done it before.  if you are in an office which has taken on this work without prior experience, tread carefully.
research the energy absorbed in a fully plastic hinge. it should involve the first moment of area and the yield stress.
have you contacted cams?
cheers
greg locock
bender, i don't have the credentials to formally help you, but i did touch sides with a problem that had some similarities some years back. it was in the form of a eurocode that required the structure of busses to have a minimum lateral strength.  the test was that the structure had to deflect less than a set distance when the bus was allowed to roll & fall over certain height.  
based on this work i did, i believe that you will have to approach the problem by allowing the posts to contribute to the point where a plastic hinge develops.  there were some specialists in the field who ran sophisticated software that could "marry" the changing load diagram with the energy absorbed by plastic deformation.  i don't have the name any more, so you'd have to trawl the 'net.  
thanks.
i have been reading into plastic hinges in seems the way to go for this type of analysis.
but i think the direction were heading is the post will fail for all intesive purposes in the worst case scenario. we will have to design the debris fences to act as system which will give the required deflection to be on the safe side.

the book "design of welded structures" by blodgett has a section on absorbtion of impact loads by beams in bending. its a relatively inexpensive book published by the lincoln arc welding institute.
thanks everyone this information has been really useful.