Historic Renovation Demands Delicate Structural Work
Crews have little room for error while adding a new sub-basement to a National Historic Landmark in Washington, D.C.
With the 2013 purchase of 1785 Massachusetts Avenue, NW, in Washington, D.C., the American Enterprise Institute landed an iconic location for its headquarters. But to make the 99-year-old building fit AEI’s needs, a team of designers and contractors had to find ways to expand the available space.
The main challenge was to increase square footage while maintaining the exterior of the five-story Beaux-Arts building—which is a National Historic Landmark—and preserving its historic elements. The National Trust for Historic Preservation, which sold the building to AEI, holds permanent historic preservation easements on the interiors and exteriors.
“It’s a beautiful building that was a little bit too small for [AEI’s] needs,” says Brian Farrell, associate at Hartman-Cox Architects, Washington, D.C. “If we couldn’t add space, then the building would not have worked for them.”
A team led by Hartman-Cox with design-assistance from general contractor Grunley Construction Co., Rockville, Md., devised a plan to expand the 72,000-sq-ft building to nearly 100,000 sq ft by re-configuring existing floorplates; expanding the building into a courtyard; creating outdoor rooftop spaces; and—its most ambitious task—adding a new level below the existing basement.
The new sub-basement required the team to delicately stabilize the steel structure building prior to demolition and construction. The process allowed very little room for error, as too much movement could cause the building’s limestone facade and plaster interiors to crack.
For the interior columns, Grunley excavated to the footings and drilled four micropiles per footing—one at each corner of a footing. The team then erected steel support on top of the micropiles, welded jack-brackets to the columns and burned off the anchor bolts.
Hydraulic jacks were then used to lift each column up until crews saw upward movement of 5/100th of an inch.
“When we saw the upward movement and we knew the load had been transferred from the footing to the piles, we welded everything into place, locked everything off and proceed to demo the footing and excavate below it,” says Shawn Link, project manager at Grunley.
To provide support at the perimeter columns, the team utilized a hybrid approach. Typical concrete underpinning was used, but to ensure that there would not be differential settlement between the interior and perimeter columns, micropiles were drilled in the underpinning piers so all columns would be on the same bearing stratum.
To install column support and excavate the new sub-basement, Link says crews had to work in very low-headroom conditions. “We drilled the piles with a modified tieback rig,” he says. “We had to demo the slab on grade [of the existing basement] to the top of the footing elevation just to have the headroom to drill these piles.”
Initially, mini-excavators and a Bobcat loader were used to get the excavation material out of the basement and into a truck located at grade. Roughly 10,000 cu yd of material was removed from the basement.
Due to the historic finishes above, the team had to monitor each column for movement. “For the protection of the historic fabric of the building—exterior walls and interior plaster—differential settlement was our primary focus,” says Kirk Mettam, executive vice president and principal at Robert Silman Associates, Washington, D.C., which served as the structural engineer.
The team determined that differential settlement of ¼ of an inch could be enough to cause the limestone exterior facade to crack. If differential settlement occurred, the team was prepared to install jacks that could jack down on the piers and up on the footings until they reached proper elevation. However, the team never needed to use that provision.
“We were fortunate that the character of the soils was such that the transfer load was achieved,” Mettam says. “There was very little long-term settlement beyond that point.”
Although there is an historic preservation easement on the interiors and exteriors of the building, the team was allowed to demolish the core—nearly 40% of the interior space. The building originally housed luxury apartments with each floor housing a separate residence. Residential living space was located around the perimeter of the building, while the interior space was for servants’ quarters. The servant areas at the building’s core were on a mezzanine level—at a different elevation than the apartment floors. In order to create a modern office building, the team was allowed to remove the mezzanine level, bringing each floor plate to the same elevation. Within the core, the team built two passenger elevators, a service elevator, an egress stair and restrooms.
While the team worked to retain the existing plaster, some walls were removed during demolition and plaster walls had to be penetrated to install new utilities. Investigations revealed that deep layers of the plaster contained asbestos.
“Any time we needed to get in a wall, we had to abate the plaster under containment and dispose of the plaster as a hazardous waste,” Link says. “We had an industrial hygienist here to run lab samples before we could tear down any containments and do the work that we needed to do in the walls.”
In addition, the building features extensive use of terra cotta, including terra-cotta partitions under the plaster and a terra-cotta flat arch floor system.
Link says those materials complicated structural reinforcement efforts. “The sizing of the columns didn’t match the original construction documents,” he says. “We had to reinforce some undersized columns, which were encased in the asbestos-containing plaster and terra-cotta partitions.”
The building’s design also called for an open event space on the first floor, so Link says four columns had to be removed from that space. “We either reinforced adjacent columns or we imposed the additional load on transfer girders under the removed columns,” he says. “We also installed an 80-ft-long truss where we had removed three columns in a row for the space.”
In addition to the interior work, crews are undertaking a complete restoration of the limestone facade.
When the project is completed, which is anticipated in May 2016, the building will feature office and conference spaces as well as TV and radio broadcast studios, a fitness center, a kitchen and dining spaces. The rooftop will feature outdoor terraces and an indoor kitchen for hosting functions.