Commenting on the efficacy of levers, Archimedes reportedly said "If I had a place to stand, I could move the Earth."
That "place to stand" is a key consideration in the building of bridges and elevated roadways. In modern construction, pylons are created at ground level, each a fixed distance apart from the rest. Pre-fab spans are then trucked in and lifted into place by a crane. But this assumes that the crane has a place to stand, a staging area. When spans must traverse deep valleys, water crossings, unfirm soil or roadways incapable of supporting the load, a crane alternative is needed.
In 1907 the Canadian Pacific Railway solved this problem by devising the Incremental Launching Method: They created a 415-foot steel span on the north embankment of a river crossing in Ontario, and "launched" the entire thing from one side to the other between two massive pulley blocks, using steel cable and a pretty darn powerful hoisting machine. By the 1960s Venezuela had pulled it off with a concrete span, and in the 1970s we Americans developed a way of launching half-spans from both sides at once and getting them to meet up in the middle.
I'd love to see YouTube video of these erections (yeah, I said it), particularly of the 1907 feat, but surprisingly they do not exist. We do have, however, footage of a modern-day Chinese construction company using the 21st-Century version of ILM. It is an incredible feat of engineering:
Here's an animation that lets you more clearly see how the process works, and also reveals the pick-up phase back at the staging yard:
Today the ILM is used around the world, from Scandinavia to Europe, Russia to India, Australia to Asia, with more than 1,000 bridges worldwide constructed via the technique. But it doesn't seem to be prevalent in the U.S., which prompted the National Cooperative Highway Research Program to launch a 2007 study [PDF] to find out why.
Alas, it seems the study's answers are terribly American, related to everything from the obfuscation of financial details, a lack of education on the part of construction firms, and a difficulty with securing required permits for this type of construction. Being an American, I suspect it's more to do with money than anything else. "As is often the case in the highly-competitive construction industry," the report states, "the cost of these specialized bridge construction bid items are not widely published and are not available without considerable research into each specific project. Therefore, the projects presented in the following case studies do not present this information." It's no wonder we don't use the method much when we can't even figure out how much it will cost. Who'd have thought bridge-building would be like healthcare?