I would say they are welded together piece by piece, but I'm not sure entirely. I did find a website for you to check though, en.wikipedia.org/wiki/Crane_(machine).
The major projects that have made use of bridge cranes or overhead cranes include construction developments. Almost all paper mills in the world utilize bridge cranes. Demag Cranes and component was one of the first corporations to prosper from the use of bridge cranes as a result the company produced the world's first steam powered crane.
24 cranes
The Bridge Crane Specialists is a company that specializes in bridge cranes. You can find out more about them at the following website: http://www.bridgecrane.com
There are several companies that provide Bridge Cranes. You can find information at the following websites. www.washingtoncrane.com www.acecrane.com www.dearborncrane.com www.spanco.com www.bridgecrane.com
You can find some bridge cranes for either rental or to buy by looking in your local yellow pages. Or you can check www.craigslist.com for more great deals
Like the regular joints
bulldozers...cranes...backhoes...
Gemini bridge
How do you constructed bridges?
Tower Bridge opened in 1894 and is constructed of 11000 tons of steel, Cornish granite and Portland stone.
The Skye Bridge is constructed of concrete and steel.
London Bridge is a pre-stressed concrete box girder bridge.