A guest at the Gatlinburg SkyBridge cracked one of the bridge’s glass panels, USA Today reported. The guest was attempting a baseball-style slide along the bridge when a metal piece of his clothing caught on the panel and chipped the panel. The 700-foot suspension bridge is a marvel.
According to USA Today, the Gatlinburg SkyBridge is a modern wonder with the numbers to prove it. “The SkyBridge is nearly 700 feet long, making it the longest pedestrian suspension bridge in North America,” the article said. “The structure features three five-by-five panels at its center. The glass has three layers with the upper layer serving to protect the other layers.”
Building a suspension bridge takes precision, care, safety, and intensive labor, but the result is a marvel of engineering. Here’s how it’s done.
Building Bridges, Part One
Suspension bridges are functional, time-saving, and can be works of architectural beauty, but most of us haven’t overseen their construction before. So how would a suspension bridge over a river or creek, for example, be built once it’s been designed?
“Construction begins with two towers, placed on foundation blocks on opposite banks of the stream,” said Dr. Stephen Ressler, Professor Emeritus from the United States Military Academy at West Point. “At a yet-to-be-determined distance behind each tower, we’ll place the anchorages, so named because they’ll anchor the ends of our main suspension cables, which run across the tops of both towers.”
Dr. Ressler said that in order for the anchorages—which resemble massive couches—to work, they must be put underground and they must be very heavy, having what he called “sustainable weight.” In order to make them heavy enough, the “empty” portion of the anchorage’s couch shape is usually filled or stacked with something like concrete.
“At this stage, with the [main] cables supporting only their own weight, they naturally assume a curved shape called a catenary, which closely resembles a parabola,” Dr. Ressler said. “But the cable shape changes substantially when we add the deck, supported on transverse beams, each suspended from the main cables by a pair of vertical wires called suspenders.”
The deck makes up the main platform that we travel along, while the transverse beams, which are under it, run across it at regular intervals. As Dr. Ressler said, the transverse beams are suspended from the main cables by the thinner vertical cables known as suspenders, which are what we see along suspension bridges.
Building Bridges, Part Two
Before building a suspension bridge, each component must be designed first so it can accommodate the loads it will bear. Like the construction itself, the design has to happen in a specific sequence, mostly for safety concerns.
“Our design process will entail purposeful consideration of six different structural elements,” Dr. Ressler said. “We’ll start by designing the deck, because it directly supports our 90-lb. per square foot pedestrian loading. Next, we’ll design the members that directly support the deck—the transverse beams. Then the suspenders, which support the beams; then the main cables, which support the suspenders; and finally, the towers and anchorages, which support the cables.”
To look at it another way, a suspension bridge is basically designed from the inside out, or from the least load-bearing component to the most load-bearing. Dr. Ressler said this sequence, which is called “following the load-path,” is defined by “the transmission of internal forces through the structural system.”
“If we don’t design by following the load-path, we’re quite likely to overlook some of this accumulating load,” he said.
A delicate balance applies to the design and construction of a suspension bridge. Mistakes in either of those processes could lead to fatalities—as misuse of the bridge by a baseball enthusiast could, also.
Dr. Stephen Ressler contributed to this article. Dr. Ressler is Professor Emeritus from the United States Military Academy at West Point and a Distinguished Member of the American Society of Civil Engineers (ASCE). A registered Professional Engineer in Virginia, he earned a BS from West Point and an MS and a PhD in Civil Engineering from Lehigh University, as well as a Master of Strategic Studies from the U.S. Army War College.