Bridges are a little like thumbs and electric can openers. You don`t realize how much you depend on them until they`re out of commission.
Just ask the 50,000 drivers a day-and the legions of pedestrians-who use the Michigan Avenue bridge over the Chicago River.
The inconvenience of a two-year, $33 million rehabilitation project that has caused lane and sidewalk closures was made infinitely worse last week when an accident damaged the structure and shut it down completely.
Officials still don`t know when traffic will be able to return to the bridge.
However, the Chicago Park District`s marine department said it was notified by the mayor`s office last week that the city tentatively decided not to allow the bridge to open until spring.
Taken for granted though they usually may be, bridges are a vital part of Chicago`s transportation infrastructure.
The city oversees a network of 52 movable bridges that provide passage over such formidable obstacles as the river`s main channel, its north and south branches, the Chicago Sanitary and Ship Canal and the Calumet River.
And that doesn`t count scores of fixed bridges, viaducts and pedestrian walkways that stretch across everything from expressways to railroad yards.
Of the movable bridges, 49 are what is known as ”trunnion bascules.”
Trunnions are horizontal shafts upon which each leaf of the bridge rotates up and down. ”Bascule,” appropriately enough, comes from the French word for ”seesaw.”
This design has become so identified with the Windy City that it is known in international circles as the ”Chicago style.”
Though some other types of spans can be built more cheaply, the city years ago embraced the trunnion bascule because of what officials considered its pleasing aesthetics and its operational efficiency.
Counterweights sunk into a pit below street level are matched so closely to the weight of the bridge leaf that a small motor can lift thousands of tons of steel. Downtown`s Dearborn Street bridge, for example, has 100-h.p. motors on either side that can raise sections weighing 3.5 million pounds each.
Of the city`s three non-bascules, one is a ”Scherzer rolling-lift”
bridge. On Cermak Road over the south branch of the Chicago River, it opens and closes with a rocking chair-like action.
But the end is in sight for the Scherzer, built in 1906. It`s slated for replacement in five years or so, presumably with a trunnion bascule.
The two other non-bascules, one on Western Avenue the other on Torrence Avenue, are vertical lift models that rise like elevators from their foundations.
The Western bridge, which spans the Sanitary and Ship Canal on the South Side, was converted from a fixed span in World War II to allow passage of military ships built along the city`s waterways, said Raymond Naras, who retired recently as assistant chief bridge engineer in the city`s
Transportation Department.
The Torrence bridge, near the mouth of Calumet Harbor, is at a spot determined to be too wide to be handled by a bascule. It was built in 1936.
Almost three-quarters of the city`s bridges were constructed before 1940. The oldest, built in 1902, is at Cortland Street over the river`s north branch. Age, and the structural decline that comes with it, is a major problem.
An internal federal report in 1989 criticized the city for failing to do enough to inspect, maintain and repair its bridges and viaducts. It also said the city spent too little-about $20 million a year-between 1986 and 1988 to upgrade the structures.
The city plans to spend more than $60 million annually over the next three years, but officials acknowledge that three times that amount could be put to good use.
In an attempt to get a better handle on work that is needed, and to set priorities, the city is in the midst of a 10-week, $770,000 special inspection. Last month, engineers on the project found structural problems serious enough to close the Ashland Avenue bridge near Webster Avenue.
Officials said normal deterioration apparently had been accelerated by abnormally heavy traffic, the result of motorists avoiding the Kennedy Expressway rehabilitation project.
The newest bridge in the city is the Randolph Street bascule, over the Chicago River`s south branch. It was completed in 1984.
Perhaps the most notable of the newer spans is the Columbus Drive bridge over the Chicago River`s main channel. Completed in 1982 at a cost of $33 million, its 269-foot length is exceeded among bascules only by the 295-foot span across the Bay of Cadiz in Spain.
Unlike its peers, the Columbus Drive bridge is set back several feet from the water`s edge to give strollers an unimpeded path along a scenic stretch of river bank.
Bridges may be there to serve the city`s vehicular traffic, but cars and trucks mostly come in second in a conflict with marine traffic. Simply put, bridges open when boats show up; however, mariners must make reservations with the city when they travel the Chicago River so bridge tenders are dispatched to the proper places.
This preferential policy, often questioned by angry drivers made to wait while a lone pleasure craft passes, is tied to history and federal law, Naras said. ”Waterways were the prime carrier of commerce, and everything kind of has to take a back seat to that,” he said.
Even so, landlubbing motorists got a measure of relief in the early 1970s. That`s when the city won approval from the feds to keep downtown bridges down during rush hours.
The frequency of traffic interruptions citywide varies dramatically.
Though some bridges open a few hundred times a year or less, the Cermak Road and Kinzie Street bridges, traditionally the busiest in the system, operate around 3,000 times annually, said Louis Chrzasc, coordinating bridge engineer in the Transportation Department.
That`s because both sit only about 15 feet above the water. They must open even for barges that can scoot under most of the city`s other bridges, Chrzasc said.
Cermak and Kinzie also are among a handful of busy bridges citywide staffed by bridge tenders 24 hours a day. The rest of Chicago`s spans are tended as needed by city crews whose members leap-frog one another to open and close them as boats make their way along a waterway.
If any bridge could be classified as notorious, the Kinzie bridge has made a strong bid in the last few years.
At an unobtrusive spot on the Chicago River`s north branch, just north and west of the Merchandise Mart, the bridge crushed a cab in 1987 when the span closed before the auto had cleared.
The driver was killed.
Last April, the bridge offered a ringside seat to a disaster unprecedented in Chicago history.
Pilings sunk into the river bed to protect the span from being struck by boats punctured a freight tunnel beneath it.
That, in turn, led to the subterranean flood that knocked the Loop for a loop.
