Transformation of the iconic Old Chicago Post Office—from a vacated shell of its former self into one of the Windy City’s prime office buildings—is nearly complete.

The first occupants are planning to move into the building in October.

Once one of the world’s largest buildings, the nine-story, 2.5 million square-foot Old Chicago Post Office served as a mail sorting hub for the Midwest, from its opening in 1921 until postal operations moved to a new facility in 1997. It was notable for handling in-going and out-going mail and packages from two of the world’s largest catalog retail companies, Sears and Montgomery Ward.

After its closure the building, which was listed on the National Register of Historic Places in 2001, gradually fell into a state of disrepair and required a complete core and shell renovation, which included updating all utilities and adding new stairs, elevators and escalators.

“It is a night and day difference,” said Jonathan Sladek, project manager for WSP. “The building had been sitting vacant for more than 20 years and was in poor condition inside and out. The exterior limestone façade has now been renovated along with new windows, so the exterior is looking beautiful once again.”

As the structural engineer, WSP USA served as a key member of the team responsible for delivering the completed project to the client, the real estate firm JLL Inc.; and to the building owner, 601W Companies. The project team also includes Gensler, the architectural firm responsible for core and shell renovations and new design work for amenity spaces, public spaces; and Bear Construction, the Chicago-based general contractor.

Renovations began in 2016. Although the Old Chicago Post Office will soon open for business, construction and fit-outs will continue in the building through 2020.

Reshaping the interior of a historically significant structure proved challenging, but preservation of many of the building’s key elements was a critical component of breathing new life into the old building and maintaining its connection to its storied past.

“The interior was a dark and dingy place, full of all of the old inner workings of the building’s previous life as a mail sorting facility,” Sladek said. “Today it is a bright and clean open space, ready for tenants to build out their future office spaces.”

The National Park Service designated several interior features of the building as historic elements that had to be preserved as part of the restoration, including the postmaster’s suite and the hanging walkways that allowed supervisors to watch over the workers in the mail sorting rooms.

The original north lobby on Van Buren Street has been fully restored to its original condition, and some of the spiral chutes that delivered mail to rail cars below have been preserved.

Although most of the mail sorting equipment was removed and recycled, several pieces remain in the building as a reminder of its role as a 20th Century postal hub.

Most of WSP’s work involved the complex task of determining how to infill thousands of slab openings.

“The slab openings were left in the building after all of the mail sorting equipment was removed,” Sladek said. “This was our first task on the project, and due to the immense size and complexity of the building, we are still infilling slab openings.”

Another technical challenge was retrofitting the freight elevator shafts with new passenger elevators and exit stairs, which were required for vertical transportation. “This was a major undertaking,” Sladek said.

The restoration team made its mark on the project by maintaining a focus on sustainable, environmentally responsible features.

One of the most noticeable improvements will be the conversion of the main roof into a living green roof—a natural public space that can be used by the building’s tenants. It will feature vegetation that will provide a natural cooling barrier for the building as well as other benefits, including rainwater absorption and insulation.

Covering 200,000 square feet—more than four acres—it will be the largest green roof terrace in Chicago.

“It’s gigantic,” Sladek said. “The roof incorporates a combination of super lightweight planting medium to reduce loading and minimize the amount of structural reinforcement required.”

In areas where heavier loads are required for gathering places, a new elevated structure was created above the existing roof.

“The current roof wasn’t built to handle those types of loads,” Sladek said. “It will be one of the great new amenities that this building will be able to provide for the people who work there.”

One factor that had to be accounted for throughout the project involved the active Amtrak railroad tracks running directly underneath the building. When any demolition occurred, the team had to consider its impact on the rails and passenger platforms below.

The contractor was only permitted to work above the tracks from 1 a.m. to 4 a.m. to avoid any disruption of the passenger rail service.

One task that WSP managed was the design of structural framing that supports the new fans that disperse the diesel exhaust fumes generated by the trains. The existing old fans had to remain operational while the structure was strengthened for the installation of the new, more effective fans.

Despite the challenges that came from maintaining the character of an early-20th century building while updating it to 21st century standards—and the unexpected challenges that could emerge anywhere, at any time—the team successfully maintained a tight schedule and managed to hit the client’s completion target.

WSP and the entire design team assisted the client in meeting that timeline by issuing multiple bid packages, which separated the total scope of work into more manageable amounts.

“The scale of the project is such that work needed to begin almost instantly, so packages had to be issued as final design work was yet to be done,” Sladek said. “One of our team’s goals was to meet the client’s tight schedule by providing extremely responsive service – and I believe we’ve done a good job at it.”

Reflecting on the project, he said time needed to meet the client’s time table, to achieve the green goals, and preserve a national landmark, was well worth the effort.

“The fact that this building is able to be reused as occupiable space in the city, and has been redesigned as a super-green facility, is an amazing story of adaptive reuse and its possibilities,” Sladek said. “The leasing team has been extremely productive and the market response has been incredible, as most of the spaces have been leased.”

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