Luis Benitez spends a lot of time thinking about bridges. Benitez is the chief engineer for the City of Chicago and it’s not uncommon for him to receive a late night phone call about one of the city’s 377 bridges.
It usually isn’t good news.
Sometimes, a truck driver will misjudge clearance width or height. It does neither the truck nor the bridge much good when that happens. That’s the cue for Benitez to get to the scene and make sure the bridge is still structurally sound.
Come next June, he’ll have one fewer bridge to fret much about. The 606 Trail (formerly called the Bloomingdale Trail) bridge at North Milwaukee Avenue and North Leavitt Street will be a full foot taller, and the lanes will be wider. It will also be a very cool-looking pedestrian expansion bridge.
During a community education event at the Bucktown-Wicker Park Library on October 11, Benitez explained some of the nuances of caring for—and rehabbing—Chicago’s bridges for The 606. The “Trail Mix” event also offered kids a chance to build their own model bridges, using cardboard and tubes, or marshmallows and toothpicks.
The focus of this event was the Milwaukee/Leavitt Bridge, where workers are currently dismantling and rehabbing the old trusses.
“This was built in 1915 when the rail line was elevated,” Benitez said. “Most bridges are designed for 50 years life. These have been up for 100 years and are still kicking. These were designed for railroad loads; we’re using them for pedestrian loads.”
The weight of an occasional bicyclist, walker or jogger obviously doesn’t need the same level of structural support as a 120,000-pound locomotive. However, adapting a rail bridge for pedestrian use still requires a bit of finesse, Benitez said.
“You have to get the fine-tuning of the sculpture just right,” he said. “Also, the frequency. People synchronize their steps and that causes vibration, so we have to keep the frequency at 3 hertz or higher so it doesn’t vibrate. Too strong isn’t necessarily better—you have to find the right balance.”
To accomplish this, each cable connected to the spans will need to be adjusted with the same precision a violinist would use preparing an instrument for a symphony performance.
“We’re going to do as much rehab as possible to save costs,” Benitez said. “We had to do clearance, too—some biological, some cultural. Every bridge over 50 years old is considered historic, and that affects of most of the bridges in Chicago. You don’t want the history and architecture of these bridges to just go away.”
He said working with architects on the Milwaukee/Leavitt bridge had its challenges, too.
“This bridge, I had to redesign it 10 times. I wanted to kill the architect,” he said, laughing. “Architects and structural engineers don’t always get along, but you need both. We make sure things are strong, and they make sure they’re aesthetically pleasing.”