We are used to wonders in Chicago. So much so that often we don't even notice them in front of us. For example, the 32 bridges spanning the Chicago River. They are solidly in place, most of the time, and we hurry across, doing the Point A to Point B two-step, barely registering that we are walking on water, nearly. Walking above water anyway.
But the bridges are not always where they are supposed to be. In summer they go up -- our normality twisting, the street rearing into the sky, light poles tilting at a crazy angle. And even then, we tend not to be thrilled. A routine part of city life; happens all the time. We're used to it. If anything, we're mildly irritated, or else merely wait, docile as cows, swishing our tails, the more romantic among us maybe sneaking a glance over the rail, down at the river to see the sailboats or a gravel barge going by. The rest peck a few "moos" into our phones.
Who stops to wonder, "How do they do that?" Lift an enormous bridge? A motor of some sort, right? Yes, but there's also something more. Something hidden; a secret.
These downtown Chicago bridges are called bascule bridges, "bascule" being French for "seesaw," and they raise with relative ease because the bridge leaf tilting into the air is only half the story, literally. While each half of the bridge lifts up, another half, not of equal length but of equal weight, heavy with concrete and iron, drops down, unseen, into a watertight pit.
Bascule bridges were developed here -- the first Chicago Style Trunnion Bascule Bridge opened March 24, 1902, at Cortland Street, then Clybourn Place. It's still there.
"Trunnion" is another old French word, originally meaning "tree"(we get our word "trunk" from the same root). Originally, it described the stumpy pins that stick out of either side of a cannon so it can pivot on its carriage. With a bridge, the trunnion is the massive steel shaft that the span teeter-totters on. It bears the structure's entire weight.
As with new tires, new bridges must be balanced -- just as lead weights are added inside a tire rim so it spins true, so cast iron cubes, one foot on a side and weighing 445 pounds each, are added to or removed from special "pockets" within the bridge until the span is balanced. The process demands attention to detail. For instance, the fiber reinforced polymer sidewalk decking used in the new Wells Street bridge is lighter than wood, so the counterbalance had to be lightened, while other elements of the bridge were made heavier to compensate -- the pedestrian railings are typically fiberglass, but Wells Street uses cast steel. Fine-tuning the balance is important -- an out-of-whack bridge can abruptly snap up, as the Michigan Avenue Bridge did with near-disastrous results in 1992, toppling a crane onto the riverbank, tossing a wrecking ball into a parked car, injuring six people.
The riverbank below is a reminder why we need bridges that open, because the Chicago River is relatively low banked and narrow -- the giant suspension bridges in New York City wouldn't work here. The East River is 2,000 feet across where the Brooklyn Bridge crosses it; the Chicago River, at Michigan Avenue, only about 250 feet wide. We invented bascule bridges out of necessity.
The bridges are designed to balance at two points -- when closed and when open -- the motors doing the work of nudging them from one state of equilibrium to another. Bridge engineer Dipal P. Vimawala expressed this in a lovely sentence: "Balancing occurs," he wrote, in a report to the city, "when the Resisting Moment equals the Overturning Moment."
Yes, exactly. A similar process occurs in people. Being ourselves out of balance, we try to compensate by making it a habit to hurry. We come upon an open bridge, and even though it doesn't happen too often, our first instinct is annoyance -- damn! -- followed by an urge to scoot a block down and cross there, so as to waste time rushing instead of waste time waiting. This is the resisting moment. But, if you're lucky, you immediately realize that the delay isn't all that long, the thing being rushed toward not all that important, and rather than resist what fate has served you by fleeing, why not just linger and enjoy it? This is the overturning moment, when you upend whatever momentary concern is prodding you forward and just be. Shrug and admit, really, there is no better way to pass a few minutes in the summer in Chicago than to accept this momentary leap of ordinary life. Study the bridge going up. Appreciate it more by remembering the hidden part, the secret that most people don't even know is there. The mystery within. To seize this uplift and welcome it, leaning on the rail, at ease, watching a clutch of sleek white sailboats drift by. For a moment -- maybe the best moment of the day -- equilibrium is restored.
Pictured above: The Lake Street Bridge.