Eli5 how brick bridges are built.

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I’ve been watching a narrowboat channel from England and there are very many beautiful brick bridges with a nice arch over the canal. I understand how the arch holds the bridge up.

I don’t understand how they built the arch in the first place without the bricks falling. They were built a couple hundred years ago.

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5 Answers

Anonymous 0 Comments

Wood frames are built to lay the initial stones on, until the key stone can be set in place. Once the key stone is in place, the arch is self supporting.

Anonymous 0 Comments

There are different techniques. For example you can first build a wooden frame and place the brick arcs on them so they don’t fall down before the entire thing is finished.

Anonymous 0 Comments

The most common technique is to first build a wooden frame in the shape of the arch. Of course no traffic can move under the bridge as it would hit this wooden frame. But when the brick arch have been fully constructed on this wooden frame and the mortar have dried the wooden frame can be slid out and the brick arch will remain standing. A lot of canal bridges are built this way. The wood frame can be reused for multiple bridges. The canal is exactly the same width under each bridge so the same frame fits all of them.

Anonymous 0 Comments

Shuttering, temporary wooden supporting structure.

Look up Brunel’s GWR Bridge at Maidstone, wide flat arches, opened 1839. All the pundits said it would fall down when the shuttering was removed. The shuttering was washed away in a storm, the bridge is still in use today.

https://en.m.wikipedia.org/wiki/Maidenhead_Railway_Bridge#

Anonymous 0 Comments

Generally speaking bridges were built starting from the bottom and going up. As such the first consideration was how many foundations it would have in the water across the span. Small spans might not even require any foundations in the water, while longer spans may require several arches. With longer arch spans supporting scaffolding below the deck is crucial until the bridge is complete whereas with multiple small arches you may get away with minimal supporting scaffolding as the two halves of the arch can mostly support themselves even before being connected because they don’t stretch over a long gap.

Building bridges is hard, and it was especially hard in the older times. The Romans were able to build surprisingly long and complex bridges with single or multiple arching spans, many of which survive today. However that did not mean that it was an easy feat. My personal favorite story is the one of the Arta bridge in Greece, which is a fairly long and tall multiple arch span stone bridge that was built around 400 years ago. The bridge has some folklore surrounding it, according to which supposedly the bridge kept collapsing and the master builder had to entomb his own wife in the foundations for the bridge to remain stable. Of course that’s just floklore, and we don’t actually know how the construction went, but there’s always a hint of truth in myths and folklore which is that probably collapsing during construction or shortly after it and needing multiple revisions to get it right was probably not uncommon. This was also an issue with domes for churches and temples.