# Crossing a weak bridge

1. Nov 12, 2013

### Feynman Bongos

If you need to cross a bridge and you have reason to worry about its load-bearing capacity, do you choose to drive across it fast, or slow, or does it not matter? What are the considerations?

2. Nov 12, 2013

### Simon Bridge

Welcome to PF;
Decent question. How would you go about figuring it out?
How do the different stresses change with speed? How does the vehicle affect the road it is driving on?

3. Nov 12, 2013

### Bandit127

Is the surface flat or real world bumpy? And is the shape also flat or humped?

4. Nov 12, 2013

### Staff: Mentor

Welcome to the PF.

Is this question for schoolwork? You need to try to answer the question yourself, before we can offer much in the way of help. That's in the PF rules (see Site Info at the top of the page).

5. Nov 12, 2013

### Feynman Bongos

Ah, my schooldays were some while ago...
Last weekend I happened to see the old movie Around The World In Eighty Days. There's a scene where a train is about to go over just such a bridge. (Naturally the bridge is flat.) The driver chooses to back up so he can then have a good run at it in order to cross at top speed. But I wondered if that's preferable. On the one hand the structure will be under compression for a reduced time, on the other hand the vibration is likely to be greater... and I've seen signs before bridges requiring a low speed, but I've never seen one advising a high speed (there are of course other safety considerations relating to speed)... it may be that without specific construction details one can't very well get much further...?

PS I say 'compression' as it was a trestle design.

Last edited: Nov 12, 2013
6. Nov 12, 2013

### Simon Bridge

Real-world bumpy would mean the higher speed produces more transverse stresses - you want to go as slow as possible. Railways are quite bumpy - notice the clickity-clack?

If the surface is weak, but not very brittle, it still takes a finite time for a particular part to flex enough to break - so it may be possible to move off one part before it flexes enough to break where sitting on one spot for a while may have you fall through. But you can see that this will depend on materials.

If the supports are weak though - these things have to absorb transverse stresses as well is downwards compression. You can't really do anything about the weight of the train - but you can minimize transverse stresses by going slowly.

If the vehicle accelerates over the bridge - this just adds to the transverse stresses as the wheels push on the bridge. A vehicle will have to apply some force the the bridge surface if it is not to slow down. anyway of course.

And you are right about the vibration.

If you had to walk over eggs, would you be best to go slowly or quickly?
How about if you had to walk over loose bricks standing on end?

Anyway - it reminded me of something but I don't want to hijack so I put it in another thread.

Last edited: Nov 12, 2013
7. Nov 15, 2013

### Feynman Bongos

I have just observed that in the original novel the bridge is, unusually for a railway, a suspension not as in the film a trestle structure. I would think that the characteristics of a suspension bridge would increase relative to those of vertical compression (or tension) the effects of longitudinal and transverse stresses, and give added weight, so to speak, to the argument for slowness.

Personally, I'd be inclined to get out and walk across first.