Civic Center Station is one of Denver’s busiest bus transit centers, handling 18 routes that serve an average of 15,000 passengers a day. A familiar fixture of the downtown landscape for more than three decades, the station was showing its age in both form and function.
Many of the problems with the seven-bay facility originated with its design, according to Richard Rost, manager of facilities engineering for the owner, Denver’s Regional Transportation District (RTD). Built as an anchor for the 16th Street pedestrian mall, the two-level station was bunkered into a sloped, landscaped site atop a single-level, 79,000-sq-ft underground parking garage and immediately adjacent to a 20-story office building.
In addition, a medley of 2- to 5-ft-high interstitial spaces scattered throughout the structure’s 30-ft grade change created near-constant moisture leaks, while the original spring-based isolation slab at the station’s entrance no longer effectively dampened bus vibrations.
“The station was, quite simply, in bad shape,” Rost says.
As cost estimates for repairs mounted, so too did the feasibility of expanding the project into a full renovation that also would correct Civic Center Station’s other physical shortcomings. The result, says Rost, would transform “a boxy, clumsy design” into a more efficient, welcoming venue that befits the hub’s prominent role in Denver’s multimodal transit system.
But many of the same issues that contributed to the original station’s woes would likewise complicate the $26-million effort to replace it, encompassing just under 140,000 sq ft of interior and exterior spaces.
Along with the inherent logistical constraints of the station’s urban setting—a zero-lot-line site constrained by major downtown thoroughfares near the state Capitol—the project required the incorporation of structural safeguards for both the adjacent high-rise and the underground garage. It is separated from the station by a 4- to 6-in.-thick waffle slab and beams ranging from 18 to 24 in. thick.
Dave Espinosa, senior project manager with the Denver office of Mortenson Construction, the project’s prime contractor, says that analyzing the intricate structural system of the transit complex and strategizing the demolition process took nearly two months.
“It was like planning major surgery,” Espinosa says.
Despite the meticulous planning, there were surprises once demolition got underway in August 2016. Although wire saws safely severed the station’s upper-level slab and beams from the high-rise, weight restrictions intended to protect the underlying garage limited the size of the heavy equipment used for tearing out reinforced concrete.
“We started with 10-ton excavators but had difficulty making progress early on,” explains Lee Patterson, a project manager with demolition subcontractor ARC Abatement. A follow-up structural analysis found that with additional shoring in the garage, the waffle-slab weight limit could be significantly increased.
“That allowed us to use 40-ton excavators, which worked much better,” Patterson adds.
With improved productivity came another challenge—quickly hauling demolition debris from the site without disrupting traffic and interfering with the movements of the 5,000 people who use the high-rise office building each day. Civic Center Station is still an active transit center, serving passengers from temporary shelters erected near the jobsite perimeter.
Mortenson also had to preserve access to the building’s loading dock and continually monitor nearby structures to ensure that vibrations from construction equipment remained within acceptable limits.
“We kept encountering things that weren’t on the original plans or components that had been built differently than designed,” Espinosa says. When the last dump truck was finally loaded, crews had removed 12,000 cu yd of concrete, along with 9,000 cu yd of dirt to level the southern end of the site with adjacent street grades.
Up From Below
After nearly two and a half months of demolition, the 12,000-sq ft Civic Center Station, designed by a collaboration of Perkins Eastman and SEH, started to take shape in October. The process began in the garage, with 66 micropiles integrated into the foundation to accommodate a new connecting elevator and stair core. The foundation improvements also support the station’s enhanced structural system to incorporate the bus concourse and a cast-in-place connecting ramp up to the street level. The ramp consists of a pan deck with post-tension support.
At the station’s main level, a 29,000-sq-ft plaza supported by 36- to 60-in. joist beams facilitates more efficient bus movements in and out of the station. In addition, the outmoded isolation slab at the station’s entrance adjacent to the high-rise was replaced with an 8-in.-thick sealed concrete slab that “uses neoprene pucks instead of springs to provide a more reliable dampening effect,” says SEH lead architect Jeff Pedersen.
Above ground, the station’s concrete frame is augmented with nearly 220,000 cu ft of rigid geofoam-infill blocks to support the bus loading area and walkways. The infill varies from 18 to 60 in. deep. Many of the more than 2,200 blocks were custom fabricated, based on laser scans provided by Mortenson’s virtual design and construction group.
“That way we didn’t have to cut into them to create the desired shape,” says Espinosa, who adds that the station is probably the largest geofoam-infill installation in downtown Denver.
Topping the station is its signature element—a 500-ft-long, polyester-fabric canopy reminiscent of a similar feature installed at Union Station, another of Denver’s recently renovated transit landmarks. Fabricated with internal illumination, the canopy is supported by 15- to 30-ft-tall structural-steel columns weighing up to 4,000 lb and anchored into structural concrete grade-beam footings.
Along with giving the station a new identity, Pedersen says the canopy complements a redesigned stairway that leads to a multipurpose plaza on the station’s roof.
“It’s a much more open, more interesting and desirable space,” he says, adding that even with the canopy, the station’s lowered profile restores views of the Capitol along the mall. Passengers also will benefit from the addition of two more bus bays, adding capacity as the city’s transit network expands.
Equally important, notes Rost, is the station’s improved sustainability. “Porcelain tile and unfinished concrete finishes inside will be much easier to maintain,” he says, “and the exposed ceilings will give us direct access to utilities—something we weren’t able to do before.”
Aiming for On-Time Arrival
With a scheduled October completion looming, Mortenson is now tackling the project’s last significant challenge—avoiding delays.
“Our goal was to inconvenience our patrons with the temporary shelters for only one winter,” Rost explains. “Mortenson is doing everything they can to make that happen, including working Saturdays.”
That’s hardly surprising, he adds, given the multiple challenges absorbed by the project team over the past year. “They’ve certainly made my professional life better.”
Espinosa confesses that when the project got underway, he feared it would produce even more headaches than ultimately arose. Indeed, he recalls that building inspectors for the high-rise regularly reported they had received no complaints about the bustle of activity close by.
“That means we did it right,” Espinosa says.