Bridges and Speed restrictions

[richieb1971]

Please help me understand speed restrictions on bridges. I understand weight restrictions, but I don't understand speed restrictions.

If the train is going fast it will cross the bridge quicker, going over it slowly will mean the weight distributed on the bridge will be there longer.

I assume it has something to do with vibration.

I did a quick google but nothing comes up in relation to why bridges have speed limits.

Thanks.

 

[Llama]

Ballast depth? Inspection frequency?

 

[GB]

I'd say its more likely to be due to the suspension forces. Higher the speed the more the suspension has to work and the more force placed onto the track and structure.

 

[stuu]

It's called dynamic load. The faster an object is moving, the more forces it imparts to the structure it passes over. It's not my area of expertise at all, but if you google that term then you will find plenty of detail (although not necessarily very layman-friendly)

 

[4F89]

If you hit a wall gently with a sledgehammer 50 times, nothing much will happen. If you smack it hard just once, you will likely do more damage.

 

[SteelWeelApeal]

On a curve the sideways force will be greater at higher speeds as the momentum of the train is changed faster (the same force that you feel pushing you, and things on tables etc, sideways). I am not sure how vulnerable bridges are to this, but it must be quite a strong force for an entire train.

 

[Teaboy1]

Kinetic Energy = 1/2 M V squared.

Therefore if you were to double the speed, KE increases by the square ie 4 times!

But if you were to halve the speed, KE increases by the square ie 1/4 times !

 

[Llama]

Surely 'decreases'?

 

[edwin_m]

If there is a minor bump in the track, then (putting it crudely) a train passing over at a higher speed smacks it that much harder. If the track was perfectly smooth then that wouldn't happen (except for the "hammer blow" from steam locomotive rotating parts), but that can never be guaranteed. But then there is the similar issue if the wheel has a minor imperfection, such as if it's had a slip and has a flat spot.

 

[najaB]

As others have pointed out, a dynamic load and a static load of the same magnitude can have massively different effects on a structure. As a crude analogy, would you rather stand still wearing a 25kg backpack, or have a 25kg child jumping on and off your back? It would be easier to hold the static load than the live load.

The slower the train moves, the more it is like a static load.

 

[dm1]

Some possible reasons for such restrictions:

• Limited loading guage clearance (See the most recent episode of Gareth Dennis's RailNatter for much more detail on this).
• There is usually a difference in stiffness between ballasted track and track on a bridge structure, which can act as a bump when entering or leaving the bridge (think walking from a grass lawn onto a concrete path). Higher speeds make this effect more pronounced, due to dynamic loads as mentioned above.
• On very long bridges or viaducts, expansion and contraction of the bridge can be significant, so a so-called bridge expansion joint device is needed in order to connect to continuously welded rail on either side. This device can lead to speed restrictions being required.
• Maintenance and wear considerations

 

[MoleStation]

That's interesting. Durham viaduct's speed has been increased over the years, mainly by easing the curve on the upline. That bit of the track heading south is where the power is mega applied it seems.
And then there's the trains heading north downhill, braking, on a curve over the bridge especially those that call at Durham.
That viaduct wasn't originally a mainline bridge.
Hardly any vibrations felt while a train passes overhead

 

[edwin_m]

That's interesting. Durham viaduct's speed has been increased over the years, mainly by easing the curve on the upline. That bit of the track heading south is where the power is mega applied it seems.
And then there's the trains heading north downhill, braking, on a curve over the bridge especially those that call at Durham.
That viaduct wasn't originally a mainline bridge.
Hardly any vibrations felt while a train passes overhead

Masonry can bear an impressive amount of compressive loading but can't be put into tension, and the dynamic loads are much less than the deadweight of the structure itself. This leads to designs which, if they are to stand up at all, are able to withstand far more loading than was expected at the time. Hence, as long as obvious problems are repaired and the foundations don't shift, a masonry structure will last for ever despite bearing loads being far more than when it was built. Think, for example, of river bridges from the Middle Ages that now carry modern road traffic.

Steel or concrete structures (more recent ones in particular) have been designed based on analysis of the expected loads, and only as much material used as is necessary to bear those loads plus a factor of safety. So they are much more likely to need strengthening or replacement if loads increase, or if some unforeseen problem reduces their ability to carry the load originally intended.

 

[MoleStation]

Masonry can bear an impressive amount of compressive loading but can't be put into tension, and the dynamic loads are much less than the deadweight of the structure itself. This leads to designs which, if they are to stand up at all, are able to withstand far more loading than was expected at the time. Hence, as long as obvious problems are repaired and the foundations don't shift, a masonry structure will last for ever despite bearing loads being far more than when it was built. Think, for example, of river bridges from the Middle Ages that now carry modern road traffic.

Steel or concrete structures (more recent ones in particular) have been designed based on analysis of the expected loads, and only as much material used as is necessary to bear those loads plus a factor of safety. So they are much more likely to need strengthening or replacement if loads increase, or if some unforeseen problem reduces their ability to carry the load originally intended.

Thanks for that reply. Really interesting.
It's all timber on the inside with stone built all around it and the structure with the piers is on marshy ground.
How does that work for 120 years without altering approaching gradients and etcs?

edited for reasons