Vehicular Centifugal force on bridge
Vehicular Centifugal force on bridge
(OP)
For a bridge structure on a horizontal curve does the centrifugal force result in an additional horizontal force and/or vertical force on the structure. I understand that the force is a horizontal force @ 6' above the deck, but am a little unsure if it results in an additional downward vertical load on the deck or if it is a horizontal force transmitted through the deck to the bearings, or both.
Any thoughts would be greatly appreciated.
Thank you.
Any thoughts would be greatly appreciated.
Thank you.






RE: Vehicular Centifugal force on bridge
I imagine that additional downward force is neglected in the deck and most superstructure members by the majority of designers. But I could see it being significant at bearings.
RE: Vehicular Centifugal force on bridge
Centrifugal Force = (mass of vehicle)(velocity^2)/radius of curve
Now, with this knowledge, I also know that this vector is applied at the center of mass. Which causes a moment on the vehicle. Perhaps not that significant when a small car is driving at 60 miles/hr versus 18,000 lbs ESAL truck loads are applied it would cause an eccentricity on the road, and therefore cause the truck to increase force on one side of the bridge decking which would infact increase your vertical load on one side and reduce it on the other. There is also a shear lateral force that occurs in the connections causing horizontal forces. I would assume that roadway engineers take this into account.
How would I approach this? I do know that the centrifugal force CANNOT exceed the coefficient of friction between tires and roadway. Now this is something I am not versed in so, I would assume that the dynamic friction and static friction should be a parameter that should be observed and studied. Also, dynamic friction could possibly be neglected in that situation I'm not sure. But applying that centrifugal force formula should give you your horizontal shear force conservatively (not to mention you should be adding appropriate safety factors in accordance with your local codes ((ours is aashto))).
Now the moment causing a differential distribution of forces, you should be able to account for a standard truck mass, compare it to your centrifugal force, find the center of mass, then apply the load to figure out how much eccentricity is actually occurring. Apply it to your reactions.
RE: Vehicular Centifugal force on bridge
Correct. You don't need to worry about dynamic friction, if you assume static friction is 1.0 you'll be good to go. In practice trucks drive at about 0.3g max. In order to resolve the forces directly you need the cg height of the vehicle and the track. There will be no net change in vertical load, but you will have a horizontal force.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
Leftwow, that should be "the centrifugal force CANNOT exceed the coefficient of friction between tires and roadway plus the superelevation of the roadway." The Green Book recommends using side friction factor of 0.15 above 50 mph. Combine that with an emax of 10%, and you could get lateral forces considerably higher than from friction alone.
GregLocock, would the vertical component of the superelevation force be enough to need consideration?
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
The point of the tire in contact with the road is not moving relative to the road at that point in time, unless there is some slip.
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
Car wheels on pavement have static friction, unless the wheels are not gripping the pavement and are slipping. (Even when rolling.)
RE: Vehicular Centifugal force on bridge
In theory, yes, but in the real world, no. When the tire meets the road it deflects, and the concrete-rubber interface is subject to creep without slip. This effect is large enough to be measureable on a normal street car, and visually evident in slow motion video of something like a drag racing vehicle with soft tires and a lot of power.
In short, tires are really complicated.
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
Also, weight is the only vertical component of force.
RE: Vehicular Centifugal force on bridge
yup.
I would say it's definitely not on the bridge engineer to calculate cornering forces.
If the type of traffic is known, this is pretty light work- you could use Greg's number of .3g for an average truck. For passenger vehicles, 1g would be going really really fast. .5g or something similar would be more realistic as a worst case I would think.
RE: Vehicular Centifugal force on bridge
Some standard road vehicles can corner at 1g. You can engineer your bridge to withstand this. If you are going to protect the car, you will have to re-think your guard rails for when the car rolls over.
--
JHG
RE: Vehicular Centifugal force on bridge
On topic - the tire is nominally at zero velocity where it is in contact with the road when it is rolling. But that is not really true, as the local velocities in the contact patch actually create the in-plane forces. As such the high school friction model is a gross simplification, and 1.0 is good enough.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
How did "lbs" get tied up in this?
RE: Vehicular Centifugal force on bridge
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vehicular Centifugal force on bridge
RE: Vehicular Centifugal force on bridge
Regarding the vertical effects, as stated by others, there will be no increase in the total vertical load but there will be an additional moment, so the distribution of loads will change. One difference between the Australian code and the requirements stated in the OP is that in Australia the centrifugal force is applied at deck level, so the effect of the vehicle centre of mass being some height above the road surface is ignored.
Another interesting difference in how friction forces are applied is that the factor for braking loads is 0.45 for a single vehicle, or 0.15 for multiple vehicles. This also seems quite low for an Ultimate load, especially the multi-lane load, since events that would cause all vehicles to brake simultaneously do happen.
Doug Jenkins
Interactive Design Services
http://newtonexcelbach.wordpress.com/
RE: Vehicular Centifugal force on bridge
http://www.nhtsa.gov/DOT/NHTSA/NRD/Multimedia/PDFs...
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Vehicular Centifugal force on bridge