Bridge 185 on the GSW is here (I think). The truth is, I have never been there, so it was a bit of a hunt. I came across the photos while searching for something else. These photos were given to me many years ago by Lewis Brown.
As usual, there is a pdf version of this post.
.This view through the arch is diagnostic of position showing the row of houses running away to the right of the bend.
This is a typical John Miller accommodation overbridge, so minimal as to be almost sketchy. The span to rise ratio was originally more than it looks. Scaling from the pic says more than 7:1 but 6:1 seems more likely. There are wide cesses under the bridge so lets say 30ft span 5ft rise. But look at the dip in that string course.
There is something of a dip in the parapet too and a hollow in the metalling near the crown.
And looking closer at the spandrel wall those courses dip away right back from the springings.
Indeed, there is a substantial makeup under the string course.
That looks as though it was intended to create a slight hump. The other side doesn’t look quite such a mess.
But look at those Vs in the silhouette. They look way too deep.
Back to the bad side and there is surely a cusp in the curve under the middle of that brick patch.
Let’s push that picture just a little further.
That bleaches out the arch but we can now see a big wet patch near the middle of the span. And look how loose those stones are under the string course.
So what is the story here?
Miller was a brave engineer. He built the enormous Ballochmyle viaduct, but he was also rather tight with his clients money. There are skew bridges on the line built on the same formwork as this and they absolutely give the lie to the idea that skew bridges are stronger.
To be fair. When this was built, the heaviest thing it was likely to carry was a farm cart and a horse. Maybe a ton on an axle. Modern cars are nearly as heavy as that. So with 5 axle 44 tonne vehicles going in and out of a farm the poor bridge doesn’t stand a chance.
Here we can see that the spandrel has lost its support as the arch sagged and has been free to move out at a step.
So, a few lessons here. When the arch is obviously bouncing under load, filling the gap with hard cement mortar is asking for trouble. Notice the spalling voussoir. And I wonder what the wedges were supposed to do? And here is the same thing happening in the diagonally opposite corner.
Notice how, to the right of the crack, the spandrel is sitting on the arch, while to the left it is corbelling and leaving a gap. The hard mortar is older here but has caused the same damage.
That dogleg probably pushes the loads into the corners but if I were to get access here I would want to be checking levels to see whether the arch was twisted.
This loses a bit of sharpness when pushed to reveal the detail, but I think some of those stones have worn away all their mortar and are beginning to drop a little.
Finally, let’s have a look at the environment, as far as we can.
This pic over the parapet shows that the railway is on a low embankment here.
Very slightly higher and he would have gone under. There are bridges under this track where I can only just stand up.
As it is, the abutments are surely built on original ground and much taller than they look. The dogleg in the embankment to the south means there is less support than you would hope and it is not unlikely that the abutment has tilted back.
Having said that, Miller’s abutments were not sketchy at all. He clearly understood that you could get away with a thinner arch if the abutments were stiff enough and here the structure will be hollow and at least 6ft thick but probably more.
Here is the abutment of a bridge in Castle Douglas designed by Miller’s son. Notice the arch at the far side and the filled void in the forground. This is a 45o skew bridge over a two-track railway. And is fully 10feet front to back (3m if you prefer).
The arch is similarly slender and flat. Below you can see the rubble concrete backing from that bridge which surely mimics what is present on Bridge 185.
The nature of that concrete haunching is seen better here below.
And that was rather hastily put together but life is remarkably busy just now. Off to New Zealand and Australia in a week and much to do before then.