July 13 – Under Stress

Today’s factismal: Eugène Freyssinet, father of the modern highway, was born 134 years ago today.

If you’ve ever looked at a highway overpass being built, then you’ve probably wondered “what are those wires in the concrete and why are they there?” The wires sticking out of the concrete are called tension cables and are there to make the concrete stronger by squeezing it. And the engineer who first figured out why they should be there was named Eugène Freyssinet.

Without prestressed concrete, this is what our highways would look like (Image courtesy Pont du Gard)

Without prestressed concrete, this is what our highways would look like
(Image courtesy Pont du Gard)

Large bridges are nothing new. The Romans had aqueducts (large canals to carry water) that stood 160 ft high and stretched more than 1,000 ft. But their bridges were built from brick and mortar and literally required tons of material in order to keep from breaking apart from the sheer weight of the top sections. If you look at the Brooklyn Bridge, you can see how little had changed in the world of construction between the time of the Romans and the 1800s.

Tension wires sticking out of a prestressed concrete arch; without them, the arch would fall apart. (My camera)

Tension wires sticking out of a prestressed concrete arch; without them, the arch would fall apart.
(My camera)

The reason that large bridges (and buildings and monuments, for that matter) needed so much stone was because most things are very weak when they are being pulled apart (are in tension) but are much stronger when they are being squeezed together (in compression). To understand this, consider the difference between a small pile of sand that you can easily deform with a finger and the same pile of sand stuffed into a rubber balloon where it takes a lot more strength to change its shape.

Though a lot of people knew about this effect, it took Eugène Freyssinet to develop a way of applying it to the real world. He discovered that by stretching wires out and casting the concrete around them, he could prestress the concrete and make it much stronger. As a result, he was able to build bridges that were longer and stronger than anything that was ever done before.

The TWA Flight Center at JFK (Image courtesy Ton Stam)

The TWA Flight Center at JFK
(Image courtesy Ton Stam)

And his discovery wasn’t limited to bridges. If you’ve ever seen the TWA Flight Center at JFK airport, or the LAX airport, or the Sydney Opera House, then you have seen prestressed concrete at work.

If you’d like to explore more, or search through thousands of other science topics, then why not look over the National Science Data Library? It is a free website with education plans for teachers and fun facts for everyone else!
http://nsdl.org/

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