Bridges on the Move
Many of you are probably unaware of the number of different types of bridges. Just the basic list includes beam bridges, cantilever, arch, truss, suspension, cable-stayed, movable, floating and combination bridges. Within these basic categories there are a number of subcategories as well. Movable bridges, for instance, include the bascule (drawbridge), vertical lift and the swing bridge to name just the most common.
The basic characteristic of movable bridges is that they can be moved to allow other traffic to pass. The drawbridges of medieval times were the original movable bridges. These were raised by ropes and pulleys. In the 16th century, Leonardo da Vinci designed a movable bridge for military purposes. The most famous bascule bridge in the word is probably Tower Bridge across the Thames in London (below). Tower Bridge was originally hydraulically operated using steam power from coal-burning boilers. Today, the hydraulic mechanism is driven by oil, and it is powered by electricity.
Drawbridges in which one or two opposing leafs are raised to allow marine traffic to pass underneath are the most familiar type of bascule bridge. The “modern” era of the bascule bridge was christened with the Van Buren Street Bridge in Chicago in 1893. Today, Chicago has 35 movable bridges – more than any city in the world! A website dedicated to movable bridges in the British Isles currently documents almost 1,400.
Working much like a child’s seesaw, a counterweight balances the span (or leaf) during its entire upward swing. In fact, bascule is a French word for seesaw and balance. The counterweights may be located above the bridge or below the deck. One type of bascule bridge is the trunnion design in which the bridge rotates around a large axle called a trunnion. Many of Chicago’s movable bridges are this type. The other is the Scherzer rolling lift, which raises by rolling or rocking like a rocking chair on a track. Joliet, Ill., has a number of this type of bascule bridge.
In 2008, the American Society of Civil Engineers (ASCE) awarded the Outstanding Civil Engineering Achievement award to the new Woodrow Wilson Bridge spanning the Potomac River linking Maryland and Virginia. Each leaf of this structure weighs 4 million pounds, and the total load to move is over 32 million pounds.
Thermal processing is at work in many of the details of bridge construction, particularly construction of a bascule bridge. A 1918 schematic of a Chicago-area bridge shows specific types of steel and processes such as forging being required for specific parts. Steel continues to be a huge development area in bridge construction. High-performance steel has been used in bridge construction for the last decade. Research at Lehigh is using this steel to re-engineer I-beams with corrugated webs making them thinner but stronger than previous beams.
In bascule bridges, which may require a range of gears and motors, state-of-the-art material and processes such as induction hardening or vacuum carbonitriding might be specified for gears. Some bearings will be surface hardened and others will be through hardened. The largest of these bearings and gears will be also be forged for optimum integrity and longevity.
Although it is not a bascule bridge, one of the more interesting bridges constructed in the past decade is the Magdeburg Water Bridge. Completed in late 2003, this water bridge is actually a canal through which boats can float over the Elbe River. Using 24,000 metric tons of steel and 68,000 metric tons of concrete, the water bridge connects Berlin’s inner harbor with ports along the Rhine River.
Now you know that whether a bridge raises to allow ships to safely pass underneath or a water bridge transports ships over other ships, thermal processing and metallurgical advancements are making bridges safer for years to come. IH