The tragic collapse of the I-35W bridge in Minneapolis on Aug. 1, 2007—which claimed 13 lives and left 145 injured—provided a stark reminder of the importance of infrastructure in our everyday lives. Remarkably, a replacement structure was designed, built, and opened to traffic in little more than a year. And rather than just a quick utilitarian solution, the new bridge incorporates sophisticated technologies not just for structural integrity, but in its lighting design as well.
Time was of the essence to rebuild the original bridge, an eight-lane steel truss design that opened to traffic in 1967, because this crossing over the Mississippi River is Minnesota's fifth-busiest bridge, typically carrying 140,000 vehicles each day. Tallahassee, Fla.–based Figg Engineering Group was part of the team selected by the Minnesota Department of Transportation (MnDOT) following the selection process for a design-build solution. The new concrete structure provides two five-lane spans of traffic with the option of adding bus transit lanes or light rail in the future. (The northbound span measures 1,222 feet long and the southbound span is 1,214 feet in length.) The bridge's high-performance concrete and post-tensioning in two directions are expected to last for a century, and structural redundancies include two separate spans, multiple box girders, columns, and piers. The so-called Smart Bridge system, which was developed in partnership with the University of Minnesota and MnDOT, includes 323 sensors located throughout the bridge that monitor various conditions, including icing during the winter. Early in the design process, Figg's designers brought Sturtevant, Wis.–based BetaLED into the mix to provide LED technical assistance to meet the specified light level of 0.8 to 1.0 footcandles at the pavement surface as outlined by Illuminating Engineering Society (IES) RP-8 standards. “Their specialists worked closely with MnDOT to provide information and answer all their questions on this new technology for outdoor highway lighting,” says Figg president Linda Figg.
A number of solid-state lighting (SSL) advancements occurred in 2007. “We saw large leaps in lumen output,” BetaLED director of sales Kevin Orth recalls. “The technology and the economics started to make sense, provided you could put it inside a well-designed luminaire.” BetaLED responded to the market by introducing its Edge series of outdoor general illumination products in May 2007. The bridge designers particularly liked the optical design that BetaLED had developed for the new fixture because of its efficient control of the light. “There's no halo effect in the rain or glare,” notes Figg bridge engineer Tom Jenkins. A total of 20 luminaires are used on the new structure: 16 on the main span and two on each of the bridge approaches. The fixtures are spaced 150 feet apart along the center of the bridge's median at a mounting height of 40 feet to ensure travelers an unobstructed view of downtown Minneapolis. Over the main span, the luminaire's rectangular housing with an acrylic lens holds 10 light bars with 20 white LEDs per linear array, and delivers a lumen output of 17,000. The fixtures on the bridge approaches have 12 light bars with 240 LEDs, and a delivered lumen output of 19,200. “This is the first time we've used this technology for a roadway lighting application,” Jenkins says. Likewise for BetaLED, the I-35W bridge is the company's first interstate application of the Edge's advanced NanoOptic refractor technology.
While the bridge designers were convinced that LED technology was the right choice for the new bridge, officials at MnDOT were not so sure. “We had some folks who were skeptical to the new aspects of the technology,” explains MnDOT project manager Jon Chiglo. Color was one concern for the public agency. Like most transportation departments, MnDOT has favored high-pressure sodium's amber light for decades. But the uniform cast that the LED's 6000K white light provides ultimately was accepted by the skeptics. An easier sell for MnDOT's engineers was the life cycle for re-lamping. “Normally we replace our high-pressure sodium on a three-year cycle,” Chiglo says. “We're expecting 10-plus years out of these fixtures.” And even that estimate may be too conservative. BetaLED's Orth notes that Minneapolis' cool climate conditions likely will result in a longer life span for the LEDs—the rosiest prediction is in excess of 150,000 hours of useful life as defined by the IES document L-70, which accounts for 30 percent lumen depreciation over time. With dusk-to-dawn lighting calculated at 4,100 hours per year, it could be a quarter century before the LEDs need to be replaced—a point at which the overall housing of low copper alloy die casts, anodized aluminum extrusions, and powder coat finish likely will be reaching the end of its own life cycle.
The new bridge opened to public traffic on Sept. 18, 2008. It is a remarkable feat that the new structure was designed, built, and in service just 13-plus months after the previous span's collapse left the Twin Cities without one of its major connectors across the Mississippi River. The rapid response speaks not only to significant coordination across design and engineering disciplines, but also the oft-noted civic-mindedness of the area's residents. And their input continues as traffic rolls across the bridge. The U.S. Department of Energy (DOE) has more than 600 community members participating in a survey about the bridge that's sure to inform many of the new infrastructure projects that will be gearing up across the nation this year. Among the findings so far: Nearly 80 percent think the quality of the light enhances their ability to see, while only 6 percent think there is too much glare. “The result overwhelmingly substantiates the advantages that LEDs bring to lighting,” Orth says. He notes that the DOE already was a proponent of solid-state lighting and that further analysis of the bridge's lighting performance as one of the DOE's SSL GATEWAY demonstration projects likely will confirm this position. Research will continue as the DOE monitors energy usage and light levels on the I-35W bridge. BetaLED is relying on the DOE-led monitoring to evaluate the fixture's performance, but they estimate a 15- to 20-percent energy savings over the incumbent high-pressure sodium lamps. “This lighting is a sustainable, environmental friendly solution that sets an example for the future of green bridges and highways,” Figg says.