Viaduct or Bridge?

[Fushi_cho]

What is the difference between a Viaduct and a Bridge?
Some say it's arches - But not all bridges with arches are Viaducts. Some say it's built by Romans but again not all bridges built by Romans are Viaducts. (And in case anyone mis-understands I do not my Aquaduct which carries water)

[Rock'n'Roll]

I think a bridge spans railways and rivers,whilst a viaduct may span gorges,valleys. Can't really see any difference

[skylight]

Hi fushi, A viaduct is a bridge made up of several small spans. A bridge is one structure to span across road ,water, gorge etc cheers skylight

[Catso]

I second skylight's opinion - multiple span => viaduct

[Quizmonster]

Chambers Dictionary...
A. Bridge - a structure spanning a river, road etc giving communication across it...
B. Viaduct - a structure carrying a road or railway over a valley...
Chambers Thesaurus lists bridge and viaduct as synonyms.
It's true that viaducts have several spans, but then so have many bridges. The Encyclop�dia Britannica describes the Pontchartrain Causeway in Louisiana as (quote) "the world's longest concrete bridge". It goes on for almost 24 miles, so it has quite a few 'spans'!

[Fushi_cho]

Thank you quizmonster. I think your answer is most complete. There isn't anything that clearly sets a viaduct apart from a bridge. The words are synonyms.
It's wierd how you know when you see one though!

Cheers to everyone for their answers too.

[gary baldy]

The large structure that carries the M6 over the Manchester Ship canal is called the Thirwell Viaduct.
The almost identical structure that takes the M60 over the Manchester Ship canal is called Barton Bridge.
So that clears that up then - doesnt it?

[Octavius]

The Romans did not build viaducts, they built bridges and aqueducts.

A viaduct is a bridge composed of several small spans. The term viaduct is derived from the latin via for road and ductus to conduct something. However the Ancient Romans did not use that term per se, it is a modern derivation from an analogy with aqueduct. The Roman bridge at Serta, which has all the characteristics of a 'viaduct' is a .... bridge.

[SteveD]

Imagine you are passing UNDER the railway line, road etc. When does a bridge become a tunnel?

[Octavius]

A bridge over a road will still be a bridge!
A tunnel is usually underground, even when carved into a mountain, it is technically underground.