Tay Bridge's condition

[d9009alycidon]

It is interesting to consider these apparent restrictions on the Tay (Railway) Bridge while the parallel road bridge will be carrying much higher axle loads than was expected when the bridge was designed!

 

[furnessvale]

It is interesting to consider these apparent restrictions on the Tay (Railway) Bridge while the parallel road bridge will be carrying much higher axle loads than was expected when the bridge was designed!

To be fair, I think the same applies to most rail bridges, including the Tay Bridge.

ps. It is also untrue to say road axle loads have risen greatly. Overall weight and size of HGVs has risen but more axles per HGV means axle load is static.

 

[Spartacus]

Do you mean a Tornado F3 or GR4, though?

I'd originally put 'a Peppercorn A1', but then thought some clever bugger would say there was some difference between Tornado and the old ones!

 

[Lucan]

Overall weight and size of HGVs has risen but more axles per HGV means axle load is static.

Axle loads in themselves are not important to the bridge structure as a whole; they are important to the localised area under the wheel. For railways that is the sleeper, rail chair, rail, and rail joints. For roads that is the surface tarmac/concrete and any shallow drainpipes and ducting under the surface. It is the total weight of the vehicle(s) that matters to a bridge structure as a whole.

Dynamic loads also need to be taken into account for both the localised stresses and the whole-structure stresses. Not only do steam locos have dynamic "hammer" at speed, they have a higher unsprung mass in the wheelsets (larger wheels plus con rods) and I suspect that their suspension is a lot harder than in a modern loco; all that adds dynamic load.

 

[Wilts Wanderer]

Axle loads in themselves are not important to the bridge structure as a whole; they are important to the localised area under the wheel. For railways that is the sleeper, rail chair, rail, and rail joints. For roads that is the surface tarmac/concrete and any shallow drainpipes and ducting under the surface. It is the total weight of the vehicle(s) that matters to a bridge structure as a whole.

Dynamic loads also need to be taken into account for both the localised stresses and the whole-structure stresses. Not only do steam locos have dynamic "hammer" at speed, they have a higher unsprung mass in the wheelsets (larger wheels plus con rods) and I suspect that their suspension is a lot harder than in a modern loco; all that adds dynamic load.

Historical sources suggest that hammer blow was greatly reduced, if not eliminated, in the best of the BR Standard series of steam locomotives.

 

[johnr57]

platform ender wibble?

Sir
What a wonderful expression !

 

[Chris125]

Happy to be corrected, but I believe the Tay Bridge has a RA8 weight restriction - large pacifics like Tornado are RA9. That would explain the issue.

 

[AM9]

Historical sources suggest that hammer blow was greatly reduced, if not eliminated, in the best of the BR Standard series of steam locomotives.

So how has the linear moment of the motion been matched by the rotary moment of the wheel counterweights? Every steam loco up to Evening Star balanced the fore and aft forces with weights on the rotating parts, (usually at rims of the wheels as they required less overall mass than locations closer to the rotation axis for the csame effect), and the BR standard class designs were with the exception of 71000, all two cylinder ISTR so the forces would be even less balanced.

 

[Lucan]

So how has the linear moment of the motion been matched by the rotary moment of the wheel counterweights?

It cannot be balanced perfectly (except with some very fancy and rather impractical designs), and two cylinder locos must impose significant yawing forces on the loco and track as the two cylinders do not go back and forth in unison, and are about 2m apart. I believe they are at 90 degree phase with each other to prevent a dead spot, making things worse. The counterweights on the wheels are primarily to offset the mass of the crankpin and big end of the con rod, not the piston.

In internal combustion engines, two-cylinder in-line layouts (I2) are pretty rough even though at 180 degree phase. I4 is better, as all the primary dynamic forces and moments are balanced, but secondary forces remain because the cylinder movement is not truly sinusoidal. (I don't believe that 4-cyl steam locos were configured the same way as an IC I4, again to avoid dead spots and making things worse.) I6 is better still as more of the secondary dynamic forces can be balanced. I8 even better but makes a long engine. Other configs like horizontally opposed ("Boxer" engines"), V, W, Deltic, and rotary engines can be very well balanced, but it gets complex and nothing is perfect.

It may be that the standard locos, while only 2-cyl, were just more scientifically designed than earlier locos, so hammered less despite the basically poor balance.

 

[Macwomble]

Happy to be corrected, but I believe the Tay Bridge has a RA8 weight restriction - large pacifics like Tornado are RA9. That would explain the issue.

Well, Tornado appears to have traversed the Tay Bridge yesterday.

 

[Highlandspring]

Well, Tornado appears to have traversed the Tay Bridge yesterday.

No it didn’t, it went via Perth out and back.

 

[Macwomble]

No it didn’t, it went via Perth out and back.

OK, thanks....I stand corrected.

 

[bella]

I've now noticed that Tornado safely crossed the bridge from north to south on 22 May 2012.