Why does lower speed limit help weak bridge

[TRfan]

I went over a “weak bridge” today, where there was a reduction in speed limit from 60 to 40.
How does a reduced speed reduce the pressure on the bridge ?
I would have thought that if drivers are keeping 2 seconds separation, the faster traffic goes the less vehicles would be on the bridge at any time, and that slowing the traffic down means they’re closer, i e more on bridge.
Any experts out there to explain the rationale ?

[MrAverage]

I have the same train of thought but I think the reality is to reduce the risk of people crashing.

[macushla]

I’d expect something to do with the faster you’re going the more your car is “bouncing” on the bridge when going over any imperfection. Much like how you’d slow down to go over a speed bump to limit the downward forces on your suspension and risk of grounding.

[M4CK 1]

TRfan said:

I went over a “weak bridge” today, where there was a reduction in speed limit from 60 to 40.
How does a reduced speed reduce the pressure on the bridge ?
I would have thought that if drivers are keeping 2 seconds separation, the faster traffic goes the less vehicles would be on the bridge at any time, and that slowing the traffic down means they’re closer, i e more on bridge.
Any experts out there to explain the rationale ?

If you think of the waves in the sea, when the wind speed is low you get a calmer sea when the wind speed picks up, so does the waves in the sea. Similar to a car driving over a bridge, the slower you go, the less vibration, the faster you go the more vibration you'll get.
Hope that makes sense.

[mintybiscuit]

https://en.wikipedia.org/wiki/Sine_wave

[SOL111]

mintybiscuit said:

https://en.wikipedia.org/wiki/Sine_wave

Ha, yes. I was going to suggest it may be a dynamic problem as opposed to static.

The static mass on a bridge is unlikely to be the critical aspect. However, dynamically, loads can be magnified significantly if you're approaching resonance.

The millennium bridge had a similar issue when it was first opened due to footfall. It's normally why soldiers are ordered to break step on bridges but interestingly for the millennium bridge it was the randomness that was the problem.

[TRfan]

Thanks for the replies - i guess that resonance in some form or other is the answer then ....

[jet_noise]

SOL111 said:

Ha, yes. I was going to suggest it may be a dynamic problem as opposed to static.

The static mass on a bridge is unlikely to be the critical aspect. However, dynamically, loads can be magnified significantly if you're approaching resonance.

The millennium bridge had a similar issue when it was first opened due to footfall. It's normally why soldiers are ordered to break step on bridges but interestingly for the millennium bridge it was the randomness that was the problem.

Not randomness.

Every footfall excites the bridge a little at its resonant frequency.
As more people are on the bridge the footfalls cause any sway to become noticeable.
As the sway becomes noticeable people crossing the bridge tend to fall into sync.
Bridge sways more.

It's more an example of +ve feedback combined with insufficient damping.
It was the latter which was fixed.

If people were in step at the resonant frequency the bridge sway would indeed happen sooner and with greater magnitude.

[Eric Mc]

f = ma

[SOL111]

jet_noise said:

Not randomness.

Every footfall excites the bridge a little at its resonant frequency.
As more people are on the bridge the footfalls cause any sway to become noticeable.
As the sway becomes noticeable people crossing the bridge tend to fall into sync.
Bridge sways more.

It's more an example of +ve feedback combined with insufficient damping.
It was the latter which was fixed.

If people were in step at the resonant frequency the bridge sway would indeed happen sooner and with greater magnitude.

Interesting, thanks for the clarification about the footfall.

Yes, I was aware about the damping. I guess it was too late to adjust the stiffness/mass at that point!

[jet_noise]

SOL111 said:

jet_noise said:

Not randomness.

Every footfall excites the bridge a little at its resonant frequency.
As more people are on the bridge the footfalls cause any sway to become noticeable.
As the sway becomes noticeable people crossing the bridge tend to fall into sync.
Bridge sways more.

It's more an example of +ve feedback combined with insufficient damping.
It was the latter which was fixed.

If people were in step at the resonant frequency the bridge sway would indeed happen sooner and with greater magnitude.

Interesting, thanks for the clarification about the footfall.

Yes, I was aware about the damping. I guess it was too late to adjust the stiffness/mass at that point!

Memory+Wiki Millenium bridge
Not so much too late as best fix I'd say.

[kowalski655]

Will this be to target lorries more(but you can't have separate speed limits for them) as their vibrations will be worse.. how many times have we all felt a building "shake" as a lorry goes by,when you don't notice cars.

[Timbo_S2]

speed influences the load envelope used, but its also reducing the traction and braking forces, particularly if the issue is with the abutments etc.

[C&C]

jet_noise said:

SOL111 said:

Ha, yes. I was going to suggest it may be a dynamic problem as opposed to static.

The static mass on a bridge is unlikely to be the critical aspect. However, dynamically, loads can be magnified significantly if you're approaching resonance.

The millennium bridge had a similar issue when it was first opened due to footfall. It's normally why soldiers are ordered to break step on bridges but interestingly for the millennium bridge it was the randomness that was the problem.

Not randomness.

Every footfall excites the bridge a little at its resonant frequency.
As more people are on the bridge the footfalls cause any sway to become noticeable.
As the sway becomes noticeable people crossing the bridge tend to fall into sync.
Bridge sways more.

It's more an example of +ve feedback combined with insufficient damping.
It was the latter which was fixed.

If people were in step at the resonant frequency the bridge sway would indeed happen sooner and with greater magnitude.

.. and as an example of harmonic oscillations in a bridge - this was one that wasn't fixed.

In this case, the oscillations were caused by vortex shedding from the wind and at a particular (quite high) wind speed, the frequency of the vortex shedding matched the resonant frequency of the bridge.

[fast diesel boy]

Eric Mc said:

f = ma

Stick to accounting.

[SOL111]

fast diesel boy said:

Eric Mc said:

f = ma

Stick to accounting.

Ouch!

The equation is correct.

[fast diesel boy]

SOL111 said:

Ouch!

The equation is correct.

The equation's correct, but irrelevant application to scenario.

[SOL111]

fast diesel boy said:

SOL111 said:

Ouch!

The equation is correct.

The equation's correct, but irrelevant scenario application.

If a bridge were to oscillate then any accelerated mass will be subject to f = ma. So not entirely irrelevant but Eric was particularly sparse with his post so don't know how he meant it

Apologies if your 'stick to accounting' was a joke though.

[ludo]

fast diesel boy said:

SOL111 said:

Ouch!

The equation is correct.

The equation's correct, but irrelevant application to scenario.

Not necessarily. If it is a humpback bridge, then as you go over it, your car will be accelerating vertically, as forward speed is converted into upward motion by the action of the ramp formed by the shape of the bridge. The faster you go, the greater that acceleration and hence the greater the force exerted on the bridge. Whether that is the problem, I don't know, but f=ma is certainly an equation that is involved in the motion of a car over a humpback bridge.

[Eric Mc]

That was precisely the scenario I was envisioning - especially as so many weight limits are in respect of older, humpback type bridges over small rivers/streams, railway lines and canals.