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Denver Designs an Intermodal Powerhouse

Exterior rendering of the new train hall at Denver's Union Station, which displays its polytetrafluoroethylene roof
The roof of the new train hall at Denver’s Union Station is made of polytetrafluoroethylene, coincidentally echoing the material used on the iconic roof at Denver International Airport. © Ryan Dravtiz

A new train hall beneath an undulating canopy and an underground bus terminal transport Denver’s Union Station and its neighborhood into the 21st century.

May 13, 2014—Denver’s original Union Station, at the northwestern edge of downtown, was built in 1881. Burned down in 1894 and rebuilt later that year, the historic station has seen its share of changes. The clock tower on the second incarnation of the station was eventually demolished, and the center section of the station rebuilt. During the 1920s and 1930s, up to 80 trains a day arrived and departed, and the station was an important transportation node for soldiers heading off to fight in both world wars.

In more recent years, though, those numbers dwindled, with Amtrak making only two stops per day and a ski train heading to the mountains making regular runs. Mostly, the station’s orange “Travel by Train” neon sign has stood silent watch as nearby warehouses blossomed in the 1990s into a residential and nightlife district known as Lower Downtown. Over the years the city has begun to weave a new urban fabric into the dead zone behind the station, building pedestrian bridges over Interstate 25, the South Platte River and nearby freight lines, and fronting the river with a park and some new development.

A light-rail station was eventually added near the rear of the station, but the station itself remained thoroughly underutilized. (Read “Denver Opens Long-Awaited Extension to Transit System” on Civil Engineering online.) But after years of planning, the train station and the surrounding blocks are in the midst of their most profound change yet. Union Station is roaring back to life as an intermodal transportation hub for trains and buses, the centerpiece of a generation-long infrastructure overhaul taking place across metropolitan Denver. The station’s new bus terminal opens this week.

In 2001 the Regional Transportation District (RTD)—Denver’s mass-transit operator—acquired the station and a 19.5-acre parcel of adjacent land. In 2004 a plan for redevelopment of the station and land was approved, but the scale was modest: only a light-rail station and an underground parking garage were planned. 

Then everything changed. “By coincidence this happened to be the same time that the voters of Colorado passed this giant bond program, called FasTracks,” says Kristopher Takacs, AIA, an associate director of the New York office of Skidmore Owings and Merrill, the firm designing the project. Funded through a sales-tax increase, the $6.5-billion FasTracks project is a major commuter rail and bus rapid transit (BRT) program—122 mi of new light-rail and commuter rail, plus 18 mi of BRT—that will knit together the city and surrounding suburbs.

The oval-shaped opening in the train hall canopy both keeps costs down and opens the platforms to Denver’s abundant sunshine

The oval-shaped opening in the train hall canopy both keeps costs
down and opens the platforms to Denver’s abundant sunshine.
© Robert Polidori

Union Station was now reimagined as a commuter rail hub. “That changed the pressure on how to use the land around Union Station,” Takacs explains. “FasTracks really was a game changer for the use of this public asset.”

In 2006, a consortium of two developers, East West Partners and Continuum Partners, operating as the Union Station Neighborhood Company, won the right to redevelop the station. These developers brought another 20 acres contiguous to the project, and the scale of the project morphed from a limited transportation initiative into the creation of an entire neighborhood.

For starters, designers found that there was too much pressure to jam everything together on the site, so the light-rail was pushed back from the station and now terminates in the right-of-way of the central freight-rail corridor. A long bus concourse will replace an aging facility a few blocks away. Downtown’s free mall shuttle was extended to operate at the station, and a new downtown circulator is being added. Lastly, the station’s eight tracks will handle Amtrak trains and, in a few years, FasTracks commuter lines as well—including a line to the airport that is being planned. The station itself is being renovated to include new shops and a boutique hotel, and surrounding blocks are primed to be developed as office and retail spaces. 

The engineering planning on the project was overseen by Los Angeles-based AECOM. During construction, a temporary train station was built a few blocks north, and the tracks in the station were demolished to facilitate the underground bus terminal. The 22-bay bus terminal, more than 1,000 ft long, runs perpendicular to the train station. It was built atop a 4 ft floating slab. Richard Romig, P.E., M.ASCE, an associate vice president of AECOM and the firm’s Denver area operations manager, is serving as the design project manager. He notes that the slab floats on top of “extremely dense sandy soils,” so it made more sense than putting in what he refers to as “a whole forest of piles” down to bedrock. The floating slab is also much each easier to waterproof because it doesn’t involve penetrations through the soil, he says.

The walls at the edge of the terminal were built from the top down using soldier piles and lagging with tiebacks. “In essence,” Romig says, “as the wall extends deeper, the earth pressure on the wall increases, and you compensate by providing tensioned tie-backs. Tie-backs resist the lateral earth pressures and in turn minimize movement.”

Engineers were concerned that excavating a 35 ft hole in close proximity to the station would lead to some cracking of the plaster in the historic station—especially given that the station itself is built on shallow foundations. They predicted less than half an inch of movement, but the work went so well that there was no cracking and movement was less than 1/8 in.

Because the station is located near the river at the lower end of downtown, the surrounding topography drains toward the site. AECOM used a two-dimensional flow analysis, which can map anticipated water movement across a defined area, to develop strategies for minimizing storm-water runoff at the historic station during a 100-year event. While planners originally intended to build a 66 in. diameter pipe to intercept runoff along Market Street, a few blocks away, that proved too expensive. Instead the flow analysis led them to build a 72in. pipe beneath the tracks near Coors Field, a less-expensive option. To minimize excavation and the impacts of groundwater, the engineers provided for a very gentle grade of roughly 2.7 percent on the streets approaching and above the terminal.

Interior view rendering of Denver's Union Station

Amtrak service returned to Union Station earlier this year; in 2016
several new commuter rail lines will also call the station home.
© Robert Polidori

If stood end to end, the underground bus terminal would be, as SOM’s Takacs puts it, the tallest building in town. “Traditionally, in America at least, bus stations usually don’t get the royal treatment. We wanted this to be a noble space that was uplifting,” he says. SOM wanted an experience that was akin to strolling an airport concourse, not walking along a thousand-foot-long basement. So the concourse utilizes yellow glass tiles on its upper walls to reflect sunlight from skylights. It’s finished with terrazzo flooring, a metal panel-wall system, stainless-steel railings, and glass storefronts near the bus gates. “It’s a surprisingly pleasant experience,” says Takacs.

The same should prove true for the project’s above-grade spaces. Directly above the concourse is a short continuation of 17th Street, downtown’s main artery. It will run a few short blocks from Union Station to the light-rail station. The street is wide enough to protect the view corridor of the train station from the Highland neighborhood that rises above Interstate 25 a mile or so away. Designers crafted a series of pedestrian-friendly “garden rooms” that transform the street into a kind of linear plaza. Originally a pedestrian median split the road, but landscape architects with Hargreaves Associates, of San Francisco, shifted the plaza to the north side of the street, where it will enable pedestrians to enjoy more sunlight.

Seven sets of skylights will channel sunlight into the middle section of the concourse. They are concrete boxes that puncture the box beams that support the roof and the roof itself. The boxes rise up about 4 ft and are capped with insulated glass panels. (The front of the station, formerly two large parking lots, is also being redeveloped into pedestrian plazas.)

Belowground the designers also built a large fan plant to make sure the concourse is properly ventilated. At the light-rail station three conical tubes rise up, forming the intake and exhaust vents for the plant below. “We took great care to treat the massive precast tubes as architectural elements that could be appreciated as public sculpture, aside from their important functional purpose,” says Takacs.

But the visual heart of the project is a new train hall in front of Union Station, a soaring canopy that covers new train tracks. Originally the tracks were planned to go underground, and the space above was to be developed. Then RTD opted for diesel trains instead of electrified track, and diesel trains wouldn’t work well underground. So the tracks had to come back to grade, but this meant that the planned real estate development directly behind the station would no longer be viable. This put more pressure on the entire project to work as an asset to the community. “The rail terminal as a piece of architecture needed to be spectacular so that it was a value-generator for the neighborhood, a focal point, something iconic,” says Takacs. “A place that was so unique that the land value around it would really respond.”

The firm’s original impulse was to create a grand shed worthy of a new train station, similar to those being developed throughout Europe. But out of a $350-million budget, only $10 million was allocated for the train hall itself. “We needed an act of magic,” Takacs continues. “It needed to be a building that wasn’t a building.” The solution is the canopy, he explains. “It creates an environment and a space that’s much more impactful than its actual physical presence.”

The result is a grand but light, curvy covering that runs 500 ft long and nearly180 ft wide, with a large oval-shaped opening at the center. The structure is held up by 11 trusses that span 176 ft and are raised about 18 ft above the pedestrian level. The trusses are supported by pin-supported A frames, which cantilever out of the ground and carry the loads down to the pile foundations.

The roof is made of Polytetrafluoroethylene (PTFE), a Teflon-like material similar to that used for the iconic roof at Denver International Airport, which resembles a series of teepees or mountain peaks, and though some may see that move as deliberate—part of an infrastructure “language” unique to the Mile High City—Tackas says it’s pure coincidence. “We would have loved to [have built] a big glass-and-steel structure, but of course that’s about 20 times more expensive,” he says. “So we needed to find something that would make a long span, that would feel like something incredibly special, but would be about as cost effective as you could imagine.”

The open canopy will still provide 50 percent coverage along the platforms from rain and snow (a secondary canopy will complete protect Amtrak trains), while opening up the station to Denver’s reported 300 days of sunshine.

“We wanted this to be a grand space, composed from a grand infrastructure,” says Charles Besjak, P.E., S.E., AIA, M.ASCE, the director of structural engineering for SOM. “And it had to be cost effective. We wanted it to be repetitive as much as possible.”

The column spacing, 15 ft at the edges, grows wider toward the center to accentuate a sense of porousness. To meet the view corridor requirements the center of the canopy descends to 22 ft in height from 70 ft at the ends.

“To make the whole structure seamless with the architecture, we designed all the connections,” Besjak says. “Typically we’d let the fabricators do all the connections. We did every single bolt on this job, because we were concerned about the way it looked, and keeping the consistency in the detailing, which really makes the structure much more relevant.”

The Union Station project wraps up what may be one of the more successful runs of urban planning any American city has seen. Starting with the opening of Denver International Airport in 1995—which itself is being expanded—the city has embarked on a generation-long civic infrastructure binge, building light-rail, a new convention center, new museums and sports arenas, and new public parks.

Amtrak service was restored to Union Station earlier this year. The bus terminal opens May 11. Many of the parcels surrounding the station are already being built on. “This is what the architects and urban planners always preach,” says Takacs. “Use a public investment to catalyze private investment. Here is a national example of how that may be true.”



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