Permit Vehicle Braking Load
Permit Vehicle Braking Load
(OP)
When you have a permit vehicle, what/if anything do you use for the braking load? Our state specific permit vehicle is 175k. Using 25% of that, according to AASHTO, will always control your pier/drilled shaft design. I'm wondering if there are exceptions to this in AASHTO that I haven't seen yet or what other people's opiniions are. It's just too much of a force to be applied, especially if you have taller columns.





RE: Permit Vehicle Braking Load
You can specify the maximum velocity at which the vehicle is allowed to cross a bridge, and hence reducing the potential for braking load aplied to the structure.
RE: Permit Vehicle Braking Load
RE: Permit Vehicle Braking Load
I agree that the friction coefficient is a limiting factor, but I still think braking load will be directly related to the change of speed on the vehicle. It has to be.
Quoting the Canadian Bridgem (CSA-S6) code commentary (clause C3.8.6)
Based on energy principles, and assuming uniform deceleration, maximum braking force from one vehicle determined as a fraction of the vehicle weight is:
b=V^2/2ga where a is the stopping distance with uniform deceleration (m), v is the initial speed and b is the braking force fraction. NCHRP provision were developed using a stopping distance of 400 feet at a speed of 55 mph which gives a b=0.253. Thus, braking force exerted by one vehicle would be approximately 25% of the gross vehicle weight.
The 25% seems to corolate nicely with AASHTO
RE: Permit Vehicle Braking Load
RE: Permit Vehicle Braking Load
RE: Permit Vehicle Braking Load
For special vehicles typically 10% is used as horizontal force, which will be similar to AASHTO 5% +5% provision for truck and lane loading.
In the past, when designing bridge for special load (512 metric tons tractors + low bed combo) I used 10% as horizontal loading, which I think is reasonable in your case.
RE: Permit Vehicle Braking Load