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.
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Permit Vehicle Braking Load
- Thread starter BridgeEI
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civilperson
Structural
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.
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
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
- Thread starter
- #5
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
Structural
Change of speed is not "speed". This is acceleration and it is proportional to force,(or friction).
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.
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.
- Thread starter
- #8
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.
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