Ballasted bridge decks

[najaB]

I noticed today that the Tay Bridge uses 'regular' sleepers-on-ballast track. Is this common on large metal bridges? Curiousity peaked, I kept a look out and neither the Forth nor King Edward bridge use this method. The masonry bridges like the Royal Borders Bridge do.

A couple of other questions come to mind - the ballast looks quite old, how is it renewed? I'm assuming that standard tampers, etc. cant be used for risk of damaging the deck.

Also, is there any chance it might be changed to a similar fastening method as the other two bridges mentioned? All that ballast must weigh a few tons.
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Anybody?

 

[furnessvale]

I noticed today that the Tay Bridge uses 'regular' sleepers-on-ballast track. Is this common on large metal bridges? Curiousity peaked, I kept a look out and neither the Forth nor King Edward bridge use this method. The masonry bridges like the Royal Borders Bridge do.

A couple of other questions come to mind - the ballast looks quite old, how is it renewed? I'm assuming that standard tampers, etc. cant be used for risk of damaging the deck.

Also, is there any chance it might be changed to a similar fastening method as the other two bridges mentioned? All that ballast must weigh a few tons.
--- old post above --- --- new post below ---
Anybody?

The non ballasted method is called "longitudinal timbers" or "waybeams" and they are a nightmare to maintain with respect to twist.

Many steel or iron bridge decks used to have them but the vast majority have been eliminated.

So to answer your question (and never say never) there is no chance of a ballasted deck being replaced by a longitudinal timber one.

Regarding tamping, providing the ballast depth is reasonable, there is no reason why tampers cannot be used.

 

[DelW]

All that ballast must weigh a few tons.

The weight of ballast is not very significant by comparison with live loads (i.e. trains).

Many years ago I designed a group of six temporary bridges to carry the Brighton line during construction of a second underbridge near Three Bridges station (where the road below was being dualled). Looking back at my calculations, the allowance for ballast was about 1.2 tonnes per m2 while live load works out to about 11.9 tonnes per m2. In my bridge deck calculation, live load was 89% of the total, ballast weight about 8%, and track and deck weight about 3%.

These designs were to an obsolete code and for quite short spans (5m and 6.5m) - the proportions would differ for long spans. But, bearing in mind that eliminating ballast would bring the need for other components (e.g. waybeams, more transverse bracing, separate personnel access) the benefits of doing so in bridge design terms are quite small.

 

[Ploughman]

What you get now is likely to be something like this:- http://davemcalone.zenfolio.com/p228429974/h592F152C#h592f152c

 

[najaB]

These designs were to an obsolete code and for quite short spans (5m and 6.5m) - the proportions would differ for long spans.

Considering that the Tay Bridge is miles rather than meters it must add up!

 

[ScottyStitch]

Considering that the Tay Bridge is miles rather than meters it must add up!

It does add up, but the percentage or proportion of the total weight of the structure would still, I would think, remain pretty small in the grand scheme of things, regardless of the length of the bridge, as DelW alluded to I think....

 

[DelW]

What you get now is likely to be something like this:- http://davemcalone.zenfolio.com/p228429974/h592F152C#h592f152c

Thanks, some great photos at that link - I last visited the area in about 2004 and I hadn't realised how major the Kent viaduct works were.

It does add up, but the percentage or proportion of the total weight of the structure would still, I would think, remain pretty small in the grand scheme of things, regardless of the length of the bridge, as DelW alluded to I think....

Yes just so - the live loads were (and presumably still are) span dependent, and with longer spans the weight of the structure itself will be a higher proportion, but my point was indeed that weight of the ballast will be a fairly small factor in the choice of deck design.