For those who say (paraphrasing) “speed up before you get to the bridge”, and “motion horizontally has no effect vertically”: Easing onto the bridge is preferable to flying onto it with speed. Easing onto it is effectively close to putting a static load on the bridge. Coming onto the bridge at speed allows the mass of the truck to accelerate vertically as the bridge and truck suspension deform under it with dynamic effect. At the extremes, easing onto the bridge is like slowly lowering the truck onto (say) the center of the bridge. Racing onto the bridge is like supporting the truck with the tires just touching the bridge roadway, and then releasing the weight of the truck all at once. In the former case, the dynamic load factor is 1, in the latter more than one, the magnitude depending on the flexibility of the truck suspension and bridge structure.
“Tires just touching”, as I’ve just described it, is unfair, because ideally the truck suspension is already deformed to support the truck weight. You’d have to magically compress the springs to where they were properly deformed, then lower the truck until the tires just touched the roadway, and finally release the weight of the truck and whatever device was compressing the springs. Even then, however, the truck suspension would come into play as the entire system – bridge, truck sprung weight, and suspension – moved dynamically to produce a dynamic load factor.
Bumps encountered at speed just make matters worse. They should be eased over too.
“But I’m also afraid of impact stress as the bridge surface was uneven.”
The uneven surface is far more likely to break up when a heavily laden truck speeds across it. The trucks APPARENT weight can be almost double as it starts bouncing around…
There are many sub-standard rail-road trestles and trains are required to slow down to 10 mph so the weight is stable and not shaking around…
A new bridge was being built between Lompoc and Buellton Ca. A Jersey Bridge was installed temporarily. A flat bed truck-trailer with a 70,000 pound grader (Gradall) on it crossed the bridge at 50 MPH. The truck got across and the bridge collapsed a few seconds later.
The newspaper reported that Caltrans failed to post the weight limit anywhere, so the truck drivers and permit people did not know about the limitations. The newspaper also reported that such a load could cross the bridge under certain conditions.
The conditions were similar to this: Bridge clear of other traffic. Drive down center of bridge at 5 MPH. I can’t remember if there were any other conditions.
I drove across the bridge the day before it went down and I could feel it shaking. It was a bad idea I guess. A Jersey Bridge has rickety structure above that looks like a tall “horse fence” with pickets and four boards widely spaced across it.
The new temporary bridge was three concrete culverts with gravel fill between them and asphalt road surface and it worked well. It would have failed during a wet Winter but we had a dry one that year.
That should help us draw a conclusion about going fast over shaky bridges. We all have to show some guts and avoid panic too.
I ask if you gave dear old Dad a chance to explain his criticisim directed towards you for forgetting his version of “physics”?. Many people make use of other peoples desire not to look dumb and shut them up with comments like “have you forgoten your physics” when they themselves have it all wrong, challenge him.
Sure it is better to go slow but not because of and incorrect application of the theory that mass increases as speed increases. There were so many better ways to explain to you why slow is better in this case than to simply shut you up with a “have you forgotten your physics” comment.
I have seen many discussions on this Forum rely on the “That violates the Third Law of Thermodynamics” type stuff (hoping a good dose of B/S. disorients their oposition). My logic Professor said it another way. He said just throw some Latin phrase in your argument and people will think you are some kind of master at the subject. Owsley may you rest in peace, now that is a man that opened up some minds.
This debate reminds me of something my father told me regarding bridges. While crossing Italy in WW II he said they were told to break out of step when crossing the old stone bridges. It seems that someone thought that marching in step weakened the structure. But they didn’t march very often, I’m sure.
Our favorite “tellers of all that is true” (better known as the Myth Busters) took up the Marching in Step" theory,result? myth busted.
I would have loved to have spent a semester listening to Owsely (with Hunter Thompson tending bar and Ken Keesy handling feeding us all), he died a few days ago at the age of 76, traffic accident in Austraila.
Your assessment would be true except for one thing. The bridge would have to be completely level. If it was not level (and I highly doubt it was considering it’s location in the world) then you are throwing the truck at the bridge. I like your thinking however.
The show was called IRT: Worlds Deadliest Roads. It ran 10 awful episodes and should have been titled India Whine Trail, North American truckers get their butts whupped in India. As luck would have it I traveled many of the same roads on a Royal Enfield Bullet just a few weeks after the show finished taping. The bridge in the show was a Bailey Bridge and are common in India.
The OP is considering the difference between a static failure and a dynamic failure. Load a structure beyond its design limit and in time it will fail due to fatigue. Reducing the time reduces the exposure to failure. Faster is better? Not necessarily. The load applied by the truck to the bridge is dynamic. It isn?t the forward speed of the truck that stresses the bridge but how fast the mass of the truck is applied to the bridge. As the truck rolls onto the bridge the bridge will deflect downward and the mass of the truck will follow. When the force of the bridge applied to the truck balances the mass of the truck the deflection will stop. It is the kinetic energy of both the truck and bridge that determines the load on the bridge. Kinetic energy is determined by the formula E=1/2mv^2. Doubling the velocity quadruples the kinetic energy. The faster the truck rolls onto the bridge the faster the bridge and truck will accelerate downward. The bridge structure will have to resist the acceleration of its own mass plus the mass of the truck. Imagine the bridge as a trampoline. Load it to the point where the springs almost fail. Now walk across. The more you bounce the more likely it will fail.
BTW Myth Busters is a great show, but I wouldn?t bet my life on their engineering. Imagine 20 people on the trampoline. If they are all walking around it?s one thing if they all start bouncing together it?s another.
Mass never ever changes due to speed. EVER! Mass is constant, whether you’re traveling at the speed of light or as slow as a slug. Mass is the same in space and on Jupiter and on Earth. What does change is the weight. Weight will vary depending on different places.
For example: a 10g block of steel will have a different weight on Earth, Jupiter, in space etc. However, its mass remains the same, no matter how fast or slow the block of steel is traveling or were its located.
The answer to this question lies within the inertia something has. The faster something is moving, the more inertia it has. If you push your car along at 5 mph, its a lot harder to stop than if it were only going at 1 mph. This principal also applies to downward force (on the bridge). If the truck were to zoom by on that bridge at 30 mph and hits a bump (which I’m sure there are plenty) the inertia of the downward force on the bridge will be greater than if the truck were only crawling along. Too much force pushing down on the bridge means a not-so-happy trucker and a not so happy (or possibly not so happy bridge as it could collapse).