Mathematical Bridge in Words & Pictures

The bridge is located at Queens' College in Cambridge and spans the River Cam about 12 meters between stone abutments. For me this is a really beautiful structure, so simple and yet fascinating. Its unique mathematical design forms an arch from straight members, what has become known as tangent and radial trussing.

The photo with tangential members highlighted (Image by Cmglee)

Simply put, the arch is formed by setting the long straight timbers at tangent to the circle describing the underside arch of the bridge. Other timbers are positioned radially to hold the tangents together and make the whole structure self-supporting and rigid. The connections between tangent and radial members are bolted together to form a truss. The bridge deck is then attached to the bottom of the radials.  The highlighted timbers (tangents) on the upper photo nicely demonstrate the arch formation.

Image by Bill Reid

About the bridge name

Its official name is simply the Wooden Bridge, but is also known as the Queens' bridge. Its builder James Essex the Younger designed twenty years later a similar structure (to replace Garret Hostel Bridge), which he called a mathematical bridge. After it broke down the name has clung to the bridge at Queens' College.

The construction timeline

The Mathematical Bridge was built in 1749. According some other written sources, the bridge was completed in 1750. The builder was James Essex the Younger and designer was William Etheridge, an English civil engineer and master carpenter who was previously working on several wooden bridges of mathematical design (e.g. Old London Bridge or Westminster Bridge and Walton Bridge). In 1748 Etheridge produced the design and model for the Queens’ bridge. 

The bridge was rebuilt first time to the same design in 1866. Some modifications have also been undertaken. The original stepped bridge deck was replaced with the current sloped timber deck.

In 1905 the bridge was completely rebuilt. The entire timber structure of oak has been replaced with teak and bolted connections replaced the original iron screws and oak pins.

So, the bridge is literally a replica.

The original Etheridge's model of a bridge in 1:16 scale

The Queens' College still possesses an original Etheridge’s model of 1748.

An inspiration for the bridge design

William Etheridge's design for the Mathematical Bridge was based on work by James King, master carpenter and a man with considerable self-taught mechanical knowledge. King used this system of tangent and radial trussing in his 1737 design for a wooden Westminster Bridge, which was not completed. This is the earliest known example of this type of bridge structure.

The construction of a wooden bridge was abandoned, and a few years later the construction of the stone Westminster Bridge has begun.

Arch construction support timbers - The Westminster Bridge centring

James King used the same system of tangent and radial trussing for the timber supporting structure (centring) during the construction of stone arches. This design permitted shipping to pass under the arches while they were being erected. After King’s death in 1744, Etheridge took over the work at Westminster Bridge construction, as well as the whole of King’s system of trussing.

Other bridges of the same design

The Old Walton Bridge across the River Thames was also designed by William Etheridge and was completed in 1750. At that time, the bridge was described as the most beautiful wooden arch bridge in the world. The Walton Bridge’s main span was 40 meters, with two side arches of 13 meters. Unfortunately, it lasted only until 1783 when was dismantled due to the stone replacement.

The bridge was an inspiration for the Italian painter Canaletto. The beautiful white wooden structure has been immortalized in Canaletto's two paintings.

A detail of the painting by Canaletto - A View of Walton Bridge (1754)

As mentioned above, there was a bridge of similar design on the River Cam, which broke down in 1812. It was frequently called the 'Mathematical Bridge'. The bridge at Queens' College owes its popular name to this bridge.

There was also another footbridge at Winchester of the same design which lasted until 1976.

In 1924. was built  a smaller version of the Queens’ bridge, at Iffley Lock in Oxford. The Iffley Mathematical Bridge was a tribute to the bridge over the Cam River.

Mathematical Bridge at Iffley Lock, Oxford (Image by Matt Fascione)

Despite the fact that present Mathematical Bridge is a replica of 18th century original, it became a Grade II listed building for its special architectural and historic interest.

The President's Lodge and the Mathematical Bridge, 
Queens' College Cambridge

Sources:

1.  Engineering Timelines

2.  Queens' College Cambridge

3.  Dulwich Picture Gallery, London

4.  Ruddock Ted (1979), Arch Bridges and their Builders 1735-1835

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The Writer in Me Just Woke Up!

I am not a writer.

I don’t know how many times I've repeated these words to myself, although I am writing dozens of emails and memos every day at work. True, it is related to my profession (I am a civil engineer), but still, I am mainly writing memos of several pages.

Well, then, why am I afraid to write a blog post?

The reason, however, is the different type of audience. I write emails and memos to people I know, or at least know their name. This new audience is a mystery to me and I don’t know what to expect.

Still, the decision fell!

Now I need to find a topic. Once again I am facing a dilemma. I could write about the things that interest me, but I don’t know will it be interesting to readers. I like maths, use of new technology in construction, green tech, traditional building methods, but also other things related to nature and cultural heritage.

So many ideas go through my mind. I’ve come to the conclusion that all these topics have already been written. How to write about familiar topics in a new way? I remember the title of a documentary I recently watched: The Unknown Known. Should I write something like that? On the other way, there is always some sort of new “lands” to explore and I see myself as an explorer.

For now, I don’t want to use any writing tips. In fact, I don’t like headlines with number of tips for “how to do something better”. There are so many posts of such content that, frankly, annoy me.

And then, after some hesitation, I decided.

My first ever blog post will be exactly about what I was thinking before I started writing, the doubt whether to write for the public.

And here it is; the ice has broken!

Regardless of any future comments, I’m relieved that I’ve managed to write a few meaningful sentences (at least I think they are). Then it occurred to me that maybe nobody will read my post, but I think disappointment is also a part of the writing process. After all, there is a first time for everything. I must wait to see if my first post is going well.

I hope next time I will write about something concrete and linked to my profession. Many new ideas are already emerging.

The writer in me just woke up!

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