A Stunningly Lit Structure Highlights Boston's Big Dig
Not since they filled in the Back Bay has the city of Boston undergone a structural change as enormous as the Big Dig, which seeks to ease the city's notorious traffic problems. A key component of the Big Dig plan is to replace the elevated six-lane highway, the Central Artery, with an eight-to-10-lane underground expressway that culminates at the north in a 14-lane, two-bridge crossing of the Charles River. One of the latter is the Leonard P. Zakim Bunker Hill Bridge, a stunning new structure that has instantly become a highlight of the city's skyline.
The Zakim Bridge, named after a noted civil rights activist who died in 1999, is the widest cable-stayed bridge in the world, and the first in the United States with an asymmetrical, hybrid design. The concept of Swiss bridge engineer Christian Menn, it is dominated by two inverted Y-shaped (or wishbone-shaped) towers, each of which is 270' (81m) above the deck. These structures are meant to evoke the nearby Bunker Hill Monument, thus providing a visual link with Boston's historic past. According to the Big Dig website, the bridge is 1,457' (437m) long and carries 10 lanes of traffic, eight of which pass through the legs of the towers, with two additional lanes cantilevered on the east side of the structure (These lanes provide the asymmetrical aspect of the design). Girders and two planes of cables support the bridge's 745'-long (224m), 183'-wide (55m) main span. Steel floor beams support the main span and extend out to support the cantilevered lanes. The back spans on the land side of the towers, which are of post-tensioned CIP concrete construction, are supported by a single plane of cables.
According to the principals of The Mintz Lighting Group — David Mintz, Kenneth Douglas, and Faith Baum — this structure offered many interesting opportunities, as well as challenges, for lighting design. (The Mintz Lighting Group worked with the engineering firm HTNB on the project.) According to Mintz, “Traffic passes through the towers, rather than on either side of them. Therefore, we got something rare: an interior surface that's fairly broad. As you pass through the wishbone, you pass through the main-span cable arrays, which extend forward and to the edge of the bridge.” Taking advantage of this arrangement for lighting, he says, “The main-span cable arrays are lighted from off the edge of the bridge so, from a distance, you see the outer side of the cables lighted while the inner side is relatively unlit. You pass by the lighted span of cables and through the wishbone, which is lit in blue and into the open canopy structure of cables.”
Baum says, “The biggest challenges were the size of the bridge and the complicated way in which the cables are arranged relative to the towers and to one another. It was all about finding a way to make these elements work together.” Indeed, the combination of white light on the cables and blue light on the wishbone structures gives the bridge a shimmering, almost iridescent quality. Douglas notes that the light exposes the “basket-weave” of the cables. While all cable bridges tend to have a certain grace built into their designs, the lighting here creates the effect of a glittering apparition that floats above the river. As Douglas says, “Depending on the viewing angle, the shape and the depth of the bridge changes.”
The breakdown of the design is as follows: According to Douglas, “The main-span cables, at the center of the bridge, are illuminated with 250W metal-halide luminaires by Widelite. They are mounted on custom brackets that are attached to the edge girder of the bridge. One unit is placed at the base of each cable to provide uplight. The back-span cables, at the north and south ends of the bridge, are also lit with the Widelite units. In this case, they are mounted in the median between the road beds uplighting the cables.
“The blue uplight,” he continues, “is a 1,000W metal-halide unit from Widelite, with a 1,000W blue-tinted metal-halide lamp by Venture in addition to a blue acrylic filter. Under the bridge, there are some Kim fixtures that also utilize blue color filters to light the wishbone under the bridge, as well as some McPhilben metal-halide downlights to provide safety lighting. Under the roadway, there are Kim LLF luminaires mounted as uplights inside the ‘cells’ formed by the steel structure. These light the cells and create reflections of the bridge in the water.”
There were many practical concerns behind the creation of this magical effect, however. Among them, says Mintz, was achieving the correct light levels and dealing with lighting in an outdoor environment. “Invariably, in a marine environment, you've got salt spray to deal with, along with erosion, wind, vibration.” (Douglas adds that snow is another factor.) As a result, the team went for “sturdy, marine-grade fixtures,” with vibration dampers attached to protect the lamps.
The Mintz Lighting Group handles many kinds of projects, but the firm is something of a specialist in lighting bridges, with projects that include the Chesapeake and Delaware Canal Bridge in St. Georges, DE; the Clark Bridge in Alton, IL; the Maumee River Crossing in Toledo, OH; and the U.S. Grant Bridge in Chillicothe, OH. Mintz notes that the team often works with engineering firms at the presentation stage, helping clients to get a sense of what a proposed bridge will look like when lit. “These presentations are highly technical but, when we are included on the team, we add a bit of excitement and drama by talking about lighting in more romantic and descriptive terms. It's been a successful technique.”
The Leonard P. Zakim Bunker Hill Bridge, which was dedicated in October 2002, has already become a major conversation piece among Bostonians. Aside from its role in easing Boston's crushing traffic problems, it is providing the Big Dig, billed as “the largest, most complex, and technologically challenging highway project in American history,” with a crowning showpiece.
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