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Bridge structure and mysterious arches

Average: 5 (3 votes)

Friend of the blog, Smelter, sent me this picture of a bridge from his work commute saying that he thought I'd find it interesting. He was right. I was intrigued. From what I could see, the bridge was a standard box section railway bridge. The steel, box shaped sides made for a very strong rigid structure perfectly capable of holding the weight of a train. 

But what about those arches? What were they for? They looked like they too were made from steel. The triangular braces would give the arch rigidity, yet keep the structure light in weight. They just didn't seem to be doing anything. Why have arches on the top of a box section? Smelter kindly sent through another picture. I admit that I was none the wiser. If anything, I was a little more puzzled. The arches seemed to have some kind of twist in them. Weird. 

Finally, Smelter let me off the hook sending through this link to the same bridge on Google maps. All became clear. The arches ran across the bridge, not along it. They were there to join the box sections together. The arch would provide clearance for the trains to fit under. 

If you visit the site on Google maps you can have a virtual wander round with street view and see the arches from a few different angle. Thanks Smelter! I really enjoyed that.

Find out more about bridges here.

Saturday Structure - railway bridge

Average: 5 (1 vote)

We went walking near Keswick this morning. On the return leg of the journey we came across this rather attractive railway bridge, now converted to a foot bridge. It was the structure that caught my eye.


To span a gap this wide the bridge needs to be more that a simple flat beam, an arched bridge being the most obvious choice. Unfortunately, a train wouldn't be able to cross a traditional hump-backed arch bridge. Too humpy! So in this bridge the sides are arched but the deck is suspended below the arch. This makes it an arched bridge with a flat surface.


The weight of the bridge decking and the vehicles crossing the bridge is taken mainly by the two girder arches on either side of the structure. The arch shape is very strong in the vertical plane but there is a danger that it could twist sideways and collapse so it needs to be supported. Hence; under the bridge deck there are a series of large girders running across the width of the bridge and extending beyond the bridge sides. From the ends of these cross beams, support girders are attached to the top of the arch. These make strong triangular shapes which give the bridge rigidity. Neat huh.


To make the strongest structure using the minimum amount of material the bridge is make almost entirely from girders. There are two type (that I could see) on this bridge. 'I' beams and 'U' beams. (Named for their shape.) The principle is the same for both and you can try this out for yourself with a strip of card. A flat sheet of material may be flexible but add a couple of right angled creases and the strength increases dramatically. Girders are simply creased steel.
Anyway, it was at about this point in my thought process that my True Love was beginnng to look impatient so, onward with the walk!



Average: 5 (2 votes)

 BridgePagesInstant lesson activity!

The downloadable file has 18 pages of parts, information and instruction ideal for home or school use.

This pack comes in three parts:

A river for you to build a bridge over.

All the parts you need to make a bridge demonstrating how arches work, the importance of keystones and how surprisingly stable the arch shape is even without cement! Finally the pack contains the parts you need to investigate strength in simple beam bridges; which shapes are strong? Which make the best bridges? All you will need is cardboard to print the parts onto, scissors and glue for assembly.