Mass in motion

good question.are you suggesting a bit of lift due to the forward thrust of the vehicle?or perhaps some downward drag from same due to poor aerodynamics?certainly one to ponder.

Are you interested in the impact of different speeds on road wear? Fuel economy? What is the motivation for your question?

The question deals with driving a truck across a bridge here in Germany.

If they find a bridge (on the highway) to be damaged in any way they will slow the traffic down while crossing it. A trucks normal speed limit is 90 KmH gets reduced to 60 Kmh. Is there any reason behind this ?

Would it not be wiser to get the weight off the bridge as fast as possible ?

Draw the vector diagram at the tyres and you get the answer.

The truck is working against

a) The wind resistance depends on the velocity (you need flow modelling/ wind tunnel experiment)

b) the tyre friction with road (depends on the weight also)

c) other losses - say due to grip and likelihood of skiddings at various speed.

mathematics is tough for me.

The truck weighs exactly the same at 80km as it does at 60km. However, variations in the road surface will cause the faster truck to exert more force to the road.

Example, picture a truck going at 60km and hitting a speed-bump. The truck will be deflected upwards, then "fall" back to the road, like dropping a weight. Picture then, same truck hitting same speed-bump at 80km. It will be deflected higher, and then fall father, so, dropping the same weight further, exerting more "force" on the road.

So, it makes perfect sense that they do this.

Thank you for all the replys,

So, if there is no apparent damage to the road surface and any defect would be structural then there would be no reason to slow down the traffic, right ?

Im kinda looking for a safety net if i were to get caught above the speed limit in such a situation.

"Im kinda looking for a safety net if i were to get caught above the speed limit in such a situation."

You now have only an illusion of a safety net. When you are standing before a judge, he will care little of what we think here. He will care to remove money from your wallet though.

Your probably right, but if i could dazle him with a formula proving that there is no additional force to the road surface because of the speed, he might not hammer me as hard. allways worth a try.

"he might not hammer me as hard"

Ya, right, actually he will hammer you harder, for waisting his time, and being a smart-ass.

You must be young, (not a bad thing). Tickets are written for "revenue enhancement". Once they get you in court, well, let's just say they turn you upside down, grab you by your ankles, and shake baby shake!

Good points there euro, well thought out. However, I don't remember seeing any potholes in Germany. Maybe there illegal there!

Germans are a strong-willed people, generally. They simply forbid it!

Potholes? Verboten!

Actually the pot holes are still there. They have developed an invisible filler for the holes and now they have mfg. so much of it they can cover the whole darned road with this stuff. But: as always there can be too much of a good thing. They covered a test section of the Autoban with this stuff one afternoon and by the next morning it had disappeared. Shortly thereafter the creators ,mfg's, and installers had all disappeared.

Now I never did get it straight as to whether it was the autoban that disappeared or the disappearing pot hole filler that disappeared. I do know for certain that the pot hole filler that we use here in the good ole U.S. of A. disappeares right before your eyes.

TMF

Well europium, you just convinced me i need to keep to the speed limit, however i might see it.

I wish they were verboten

good answer!

Perhaps you could convince the judge that you had lowered the pressure in your tires so as to soften the pounding the bridge would take.

Please bare with me, I'm typing with 1 less finger (a slight modification with a griner and a slitting disc)..

Oh well, If your curious about the damage heavy vehicles do to the road pavement, check out asphelt roadways that heavy trucks frequent, it's not the pot-holes that you have to be careful of, but the channels that the trucks cause over time as the road pavement is slowly pushed out to the sides of the tyres as the trucks drive along those roads.

Also the "pulling" effect on the roads when trucks take off from traffic intersections (noted by the painted lines on the roads indicating the lanes and crossing lines)

"Please bare with me, ..."

If you remove your clothes, be sure to mark your post "off-topic" and keep an eye out for the CR4 Admins.

I helped develop a bridge testing system. A truck rated at 80,000 # GCW could carry 200,000 # ar 2 mph. Note this was the truck rating, not the road rating, but the principle is the same. Low speed = low impact = less fatigue.

europlum wrote:

"Cracks introduce water which can freeze and expand the crack during the winter months, accelerating deterioration in the deck surface."

Why do cracks also get enlarged when it is continually above freezing like Summer time in wam climates?

"Why do cracks also get enlarged when it is continually above freezing like Summer time in wam climates?"

Perhaps vibration breaks material away and then there's erosion from rain water.

The roadbed expands and contracts over the course of each day because of changing temperatures. In hot, arid climates the temperature difference between day and night can be quite pronounced, aggravating the situation. This movement places additional stress on the road/deck surface which is weakest where it is already cracked.

Furthermore grit, sand, dirt and such can lodge in the crack, keeping it from closing during the expansion phase. This material can function as a fulcrum, with the sides of the crack acting as two levers. As the surface expands (the expansion is differential due to the temperature gradient between the surface and parts farther down) the bottom of the crack is forced open, driving the crack deeper and forcing it to propagate laterally.

Momentum, not weight, is the concern.

Hold a brick over your head and run, then stop suddenly.

Now, same thing, only from a swift walk. Now you see the issue from the perspective of the bridge.

If I were you I wouldn't be so worried about the judge, but about the bridge. I'd hate to be held in contempt by a bridge since the fine could be much steeper, so to speak.

I believe you need to consider Mass X Velocity Squared. 60 squared is 3600, 80 squared is 6400, so at 80 km the force of the truck tires 'crashing into' small bumps in the road surface (and also small bumps on the tires crashing into the road) is roughly twice as great.

The force applied to both the tires and the road surface is considerably greater if the vehicle is braking while traversing the bumps. Best to slow down first, then release the brakes before reaching the damaged area.

And this is for a bridge in a straight line, most have turns and changes in slope. The vector forces are 1.777 times positive or negative to the bridge by the change of the additional 20Kph when you turn or increase your climb/Gee ratio.

F=V²*M/2 put a damper on many of my faster ideas.

Brad

It should be the same, because the velocity travelled is a force tangent to the road. Therefore, the weight would be the only thing that would cause a change in force applied normal to the road. This can be seen by:

sum of the vertical forces = weight*gravity

The equasion isn't all that simple! One must consider that the various shapes of vehicles will cause different down forces upon the surface traveled even with out the natural hammering that occurs as the vehicles suspension system constantly adjusts for a more comfortable ride of vehicle occupants. Further is the lateral forces that occur as the tires wander around in the rutts, as no road is truly flat. Additional stresses occur as said vehicles pass around curves.

Where ever the surfaces change like bridges vs roads the bridges will flex as they are not as stable as the well compacted materials that create the road bed. Temp. changes also effect the movement of the bridges as well as the road surfaces and do this differently.The combination of all of these and more differing conditions is accentuated by speed. Nuff said about this subject!

TMF

I think you must consider time as a factor in your initial question. What may not be a problem on day 1 may be catastrophic on day 1000 on any particular driving surface.

Consider as an example a highway constructed of reinforced concrete slabs of x length. A heavy truck entering the slab pushes down the slab at the forward end. As it reaches the end it "rocks" the slab in the opposite direction, very minutely on a newly constructed road but progressive with use. After many iterations the rocking motion becomes exacerbated as the underlayment breaks down and allows additional vertical motion near the ends.

Extrapolate this to the individual "decks" of a bridge and you can see that the weight of the truck combined with the speed it is traveling can make a huge difference in the stresses involved.

If this is hard to believe, it was easily displayed on Interstate 696 between Detroit and Port Huron, Michigan about 10 years ago when I lived and worked there. The state allowed trucks way too overweight for the design of the road for years. Traveling on the road in a car at a resonant (to the slabs) speed could actually result in a very squeamish, annoying and potentially damaging up and down movement along the slabs.

Hooker