During the Qin dynasty of the 3rd century BCE, the first Chinese emperor Qin Shi Huangdi ordered the construction of canals to transport the resources of the agricultural hinterlands to the burgeoning cities under his control. Unfortunately, these canals cut existing roads in half, making them impassable. In order to overcome this problem, Qin Shi Huangdi hired engineers to construct bridges over the canals and rivers. Depending upon the materials at hand, bridges were made either of stone or wood.
The Roman empire was built upon roads, but sometimes those roads came upon rivers and streams. While it was possible to wade across, or "ford," a river not all parts of a river were viable fording points. In order to keep the empire intact, Roman engineers constructed a vast network of bridges over the many rivers and valleys of Europe and the Mediterranean. Invariably, Romans chose stone for the construction of bridges because of its durability. Classical engineers during these times perfected the art of arches which distributed weight in such a way as to keep the bridges stable for hundreds of years.
Perhaps one of the most famous bridges in Europe is the "London Bridge." London Bridge was the only bridge across the River Thames from the 10th century until the 18th century. It is believed that in the 11th century, Anglo-Saxons pulled the bridge down in an effort to divide Viking troops which had taken the town. Regardless, the solid stone construction lasted until the 18th century when its size and general state of disrepair prompted engineers to tear it down and rebuild it, again in stone. The 18th century bridge was again rebuilt in 1973, but this time it was constructed of steel and concrete.
Bridge construction remained essentially unchanged until the 19th century when mass-produced steel became available. Strong and inexpensive, steel allowed John A. Roebling to pioneer the use of "steel rope" bridges; thick cables of steel were intertwined like ropes into a vastly stronger "rope" which was used to suspend the roadways of bridges between support pylons. Roebling is credited with building such suspension bridges in Cincinnati, Ohio; Pittsburgh, Pennsylvania; and Brooklyn, New York.
Bridge building seems simple enough, but the slightest miscalculation can mean a a disaster of epic proportions. In 1940, the Tacoma Narrows Bridge across Puget Sound earned the unflattering nickname of "galloping Gerdie" when a combination of flawed design and high winds caused the bridge to sway and buck wildly before finally tearing itself apart and falling into the water below.
In 2007, the I-35W bridge outside Minneapolis, Minnesota collapsed suddenly, causing several deaths and a general public outcry over shoddy roadwork and inspections. The state sued an engineering firm for not spotting the fatal flaws that led to the bridge's collapse.
