North Yorkshire Moors Railway General Discussion

We had a similar thing last year, an email suggesting we had been "selected" for a small business award for South Yorkshire.

Got as far as speaking to them on the phone, realised it was a pay and get some recognition type event, as opposed to actually being based on merit- wasn't cheap either!

 

Although in fairness I was stewarding a DCE, where when we arrived at Basingstoke NR suddenly said a bridge just East of Wool was not able to stand the weight No 9 crossing it. A fine time to decide, especially as the same loco had gone over twice either the week before or the previous week (forget which). Anyway after many phone calls and about a 30 minute delay a 5 or 10 mph pass was agreed, and stayed in place for the rest of the DCE programme that year.

 

;
I'm not a bridge engineer but can understand why RA9 locos were/are permitted on what was an RA5 line. It is more than simple weight; dynamics have a large effect, especially hammer blow, and that goes up by the square. Thus, an A4 at 30 mph (the old line speed) as compared with the present 10mph would exert a load of 900/100 =9 times more. There's more to it than this simple calc but it illustrates the point.

 

I'm not a mechanical engineer, but I am an engineer with some training in mechanical engineering; so, with that background, let me offer a few thoughts.

Stone bridges are a generally a whole different kettle of fish from metal bridges, because when they fail, it's usually for very different causes. Not always; both kinds can fail when the footings of the supports are washed away by the actions of flowing water (like the recent Garmouth Viaduct failure on the River Spey in Moray). A 'failing' bridge that's failing for this reason, it generally just incrementally gets closer and closer to the point of collapse. The collapse can be triggered just by the weight of the bridge itself (the so-called 'dead load' - as happened with the Spey bridge); or, a passing load on the bridge may push it past the point of collapse. Note that these failures a not a failure of the body (the usual jargon among mechanical engineers for that is 'structure') of the bridge itself, but a failure of the footings holding the bridge up.

Both kinds of bridges can also collapse as a result of impact damage (the most famous recent example being the Baltimore harbour bridge), but those obviously aren't really 'failing' until the collision - and those failures generally happen shortly after the collision.

As to failures of stone and metal bridges for other reasons, they are usually related to a failure in the bridge structure itself - and that's usually related to the properties of the material. Stone is strong in compression, but not in tension - so stone bridges are usually arched bridges, where the material is loaded in compression. Unless the stone decays (which it might - e.g. limestone, from acid rain), those should be pretty resilient (and have been - 9 of the 10 oldest bridges in the world are stone arched bridges) .

Metal bridges often include elements loaded in both tension and compression. (The common truss has both kinds.) Those usually fail by a tear in an element loaded in tension. The tear may result from corrosion, fatigue (the growth of cracks in metal from fatigue would need a lecture to even begin to scratch, alas), etc (possibly aided by overload). The famous interstate bridge collapse in Minneapolis was a combination of all those.

There is no general rule as to how safe a 'failing' bridge is; it all depends on the failure mechanism: e.g. a metal tear can propagate slowly or quickly.

What exactly is the issue with Bridge 42, does anyone know? Given that it's a stone arch, which does not seem to have supporting pillars in the river, the abutments must be moving?

Noel

 

https://northyorkmoors-publicportal...Next/publicportal/planningapplications/820983

Has a document listing the issues with the bridge

Sent from my LGN-NX1 using Tapatalk

 

Be quicker to list what isn't wrong with it.

 

Ah, thanks. The "Defects Table" was not very detailed (e.g. "Missing brickwork - Location - Varies"), but the "Essential Maintenance Work Document" had a great deal of detail, which enabled me to mostly figure out what the issues are.

To give that, one need to know that Bridge 42 is essentially 3 arched bridges, side by side. (This is not obvious from the usual photographs!) The outer ones, one on each side, are about .25m wide (I didn't see an exact number given), formed out of large stone blocks ("masonry", in the Work Document). Between them is an arch composed of 7 layers of brick (a little less than 1m thick), about 18.5m wide.

This latter span has a number of issues - primarily many bricks missing, mostly from the bottom layer (one missing patch, probably the largest, looks to be about 8 bricks by 2), but also some other issues, such as some cracks. There are a few photos from below, in which one can see some of the issues (they have enough resolution to be blown up for a better look).

How far it is from failure I have no idea - one would need to be an expert in brick bridges, and see it closely, to tell, I reckon. I'm quite happy it has been closed for repair, though!

In looking up a specialized term, I ran across this site: Long Span Arches: Design and Limitations (and associated pages such as A Stone Arch Bridge’s Weight-Handling Abilities); it's about arched bridges made out of stone, not brick, but they have a great deal in common.

Noel