Permit Vehicle Braking Load

[BridgeEI]

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.

 

[kelowna]

If it is a permit vehicle, you can specify the conditions of that permit.

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.

 

[civilperson]

Speed will NOT effect the "braking load", the coefficient of friction between the tires /pavement is the limiting case for locking the brakes and causing horizontal load.

 

[kelowna]

Civilperson,
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

 

[BridgeEI]

Agree w/ kelowna, AASHTO assumes the same philosophy as the canadian code. Just an fyi though, CALTRANS has exceptions to the AASHTO code that they do not include a braking force w/ a permit vehicle. It would be nice if our state spec would be clearer but what can you do.......

 

[civilperson]

Change of speed is not "speed". This is acceleration and it is proportional to force,(or friction).

 

[wiktor]

AASHTO is not specyfying 25% for the axial load of permit vehicle - it's for the design tandem or truck.

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.

 

[BridgeEI]

Thanks all for the input. I decided that depending on the height of the pier/columns I would eliminate the braking force associated w/ the permit vehicle. Looking at it from a practical perspective, the probability of having a permit vehicle traveling across the bridge with another HL-93 design truck to either side of it is pretty low.