The design and construction of the new Nixon Forensic Center represented big changes for the site of the new psychiatric hospital—both in terms of the modern facilities that were planned and the changes that were necessary to prepare the physical site for those facilities. Most dramatically, the site featured a 60 ft drop in elevation from its high end on the northeastern edge of the property to its low end on the southwest corner, explains David Johnson, P.E., M.ASCE, the director of civil engineering at David Mason + Associates, which was responsible for the project’s civil engineering and also the structural engineering of three of the hospital’s new buildings.
Early in the design phase, the hospital staff and the architects—EYP Inc.—decided that the new facility should all be on a single level, “so that meant moving a lot of dirt and rearranging everything on the site,” Johnson says. Primarily, this involved transferring the soil from the high side to the low side—as well as the use of demolition material and new soil from a nearby borrow site—to balance the site as much as possible. The contractor, River City Construction, decided that the site’s existing soil “was inadequate to provide the proper bearing,” Johnson says. So the contractor brought in new material and used some of the site’s original soil to refill the borrow site. Initial concerns that there might be contaminated soil from an old diesel tank proved not to be an obstacle, Johnson notes, because the site had been environmentally remediated before the start of the project. A series of monitoring wells that had been installed earlier were still in place but were decommissioned before the current work, he adds.
Over the roughly 170 years that the site had been used as a state hospital it had been home to multiple buildings that had long since been demolished. But in many cases the original foundations, tunnels, and utilities had been left behind, which meant “a lot of the early work on this project involved cutting off utilities and getting them rerouted,” Johnson says. Stormwater drainage was another concern and resulted in the construction of a retention pond in the southeastern corner of the site to accommodate runoff.
David Mason + Associates designed the structural systems for three of the Nixon Forensic Center’s buildings, notes Kara Marfell, P.E., S.E., the firm’s structural project manager. These include the steel-framed, three-level administrative building and the treatment center and medical and vocational center buildings, which are essentially one-story masonry structures with steel roofs and secure attic spaces for mechanical systems that cannot be accessed from any patient areas, Marfell says.
The treatment center and medical and vocational buildings were founded on shallow spread and strip footings with a bearing pressure of 3,000 psf and an average 2 ft wide trench footing, Marfell notes. For the administration building, however, drilled piers with an average diameter of 4.5 ft were socketed into weathered rock.
The design for these facilities posed certain challenges, including coordinating the ductwork and piping in the attic spaces with the location of shear walls and the need to conceal any structural columns or posts within the wall systems “so the corridors would not have any areas where someone might conceal themselves or [potentially dangerous] objects,” Marfell says.
This article first appeared in the September 2020 issue of Civil Engineering.
