Magical Stone Arches

The stone arch bridge is one of those inventions, like the thermos flask, that just seems to work by magic.  There is no mortar in the bridge pictured above, which carried the Nashua, Acton and Boston Railroad across Stony Brook in Westford, MA.  This particular bridge was built in 1872 and for 53 years this bridge took the weight of trains.  For pictures of other stone arch railroad bridges in Western Mass, see The Arches.

Kim, Kara and I walked along the old line a week ago.  It's not really a rail trail, but it is a good and very pleasant walking path, at least until you reach the now absent bridge over the still active Stonybrook Railroad which runs between Lowell and Worcester.

The oldest known bridge of this type is in Greece and dates from 1300BC.  The design of these bridges owes everything to the fact that stone is extremely strong in compression.  Most modern bridges of course are built on the exact opposite principle: tension.  Stone obviously cannot be used for such a purpose.  But building such bridges had to await the development of steel.  Even the first Iron Bridge (at Ironbridge in England), which was built in 1779 from cast iron, which also lacks good tensile strength, was modeled on the same principles as stone arch bridges, i.e. compression.

Typical modern bridge design, such as the Zakim Bunker Hill bridge in Boston use steel cables which directly connect the roadbed to the towers.  Steel has a tensile strength of about 500 MPa (mega pascals) which is approximately half that of spider silk.  Other man-made materials can have much higher tensile strengths, in particular, carbon nanotubes which can have a strength of 10 GPa (giga pascals) or higher.

In case you're still wondering why the Thermos flask is so magical, consider this.  It can keep hot liquids hot, and cold liquids cold and you don't have to tell it which one you want!