Vehicle Barrier Design
Vehicle Barrier Design
(OP)
I need a physics refresher. Why is 1/2mv^2 used to determine the force on a barrier and not F=ma?
When was the last time you drove down the highway without seeing a commercial truck hauling goods?
Download nowINTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS Come Join Us!Are you an
Engineering professional? Join Eng-Tips Forums!
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail. Posting GuidelinesJobs |
|
RE: Vehicle Barrier Design
mv^2/2 is the kinetic energy of the moving body (vehicle in the case of a barrier).
The difficulties with F=ma, as far as barrier design is concerned, are that the deceleration 'a' is difficult to estimate, and will probably vary during the impact anyway.
For a conservative barrier design, the kinetic energy before impact is equated to the strain energy of the deformed barrier structure after impact (including plastic deformation if relevant) to calculate the deflections in the barrier. From there it is relatively simple to calculate the final stresses.
If you want to get very 'fancy' you could possibly include an estimate of the amount of strain energy that would go into damaging the vehicle, and only design the barrier to take up the balance of the energy input.
RE: Vehicle Barrier Design
a = (v1-v2)/t, where v2=0 and t=1 second
Then you would have F in lbf and the problem could be a statics situation. Am I oversimplifying?
RE: Vehicle Barrier Design
Take a car crashing into a barrier at 54 kph, 15 m/s.
Say it will crush by 0.5 m.
So roughly v^2=2as, a=225 m/s/s
s=ut+1/2 at^2
t=1/15 second
The advantage of an energy method is that if you can account for all the energy it will be accurate, and you only need to know the initial and final states, what happens in between makes no difference.
The disadvantage is that you need to know the energy of each state, for instance in the example I gave you need to be able to work out the energy of deformation of teh car's structure.
Cheers
Greg Locock
RE: Vehicle Barrier Design
If you are happy to base your design on a pure guess of the deceleration time (where Greg has shown just how wrong your guess could be), then why bother with the dynamic formulae at all? You could equally well guess the design impact load.
Just don't pretend that such a procedure belongs in the realms of professional engineering.
RE: Vehicle Barrier Design