Maintaining the State of the Art

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Since 1816, the Watertown Arsenal along the Charles River in Massachusetts has served as a hub for innovation, recognized as one of the nation’s earliest engineering testing grounds. Over time, the campus expanded to 125 acres and more than 100 buildings. In 1940, the U. S. Army constructed Building 215—a two-story warehouse at the eastern end of the campus along Arsenal Street, highlighted yellow in Figure 1. Its high-capacity floors with closely spaced columns, tall stories, and a railroad track pit with an overhead crane helped the Army manage and distribute materiel for the war effort. The original design called for a one-story vertical addition, but it was never built.

Beginning in the early 1980s and continuing over the next several decades, portions of the Watertown Arsenal complex gradually transformed from a military and industrial district into a major commercial center. Building 215 came under private ownership and underwent several renovations that removed many of its exterior walls to create a semi-enclosed parking garage and converted the remaining portions to commercial office use (Fig. 2).

In the mid-2010s, Boylston Properties acquired a portion of the site and began redeveloping Arsenal Yards into a mixed-use campus. As part of that project, Building 215, now known as 500 Forge Road, was transformed into a 160,000-square-foot life science laboratory and office facility starting in 2021, developed by Jacobs, the project’s architect, and Simpson Gumpertz & Heger Inc. (SGH), the structural engineer of record (Fig. 3). The adaptive reuse presented unique design challenges, including converting much of the open parking area to new office and laboratory spaces, replacing a long-abandoned parking ramp, repairing deteriorated concrete, implementing accessibility improvements, and constructing a one-story steel-framed vertical addition.

As part of its early 1980s conversion to a parking structure and office building, the building’s owners installed a car ramp in the original track pit and two-story crane bay. But because of the building’s geometry and the need for parking on the roof, the owners created an eye-catching spiral parking ramp on the building’s south side. Over time, the concrete-framed roof over the crane bay deteriorated, and, by 2001, the owners had removed the deteriorated slab, cutting off access between the ramp and the parking areas on the main portion of the roof. The remaining concrete long-span beams over the crane bay were abandoned in place and braced by steel beams.

By the time the latest renovation project got underway in late 2019, what was once the building’s most striking structural feature had become a ramp to nowhere, as seen in Figure 4.
During the early stages of the project, Boylston Properties, the design team, and the construction manager opted to demolish the curved ramp and the remaining portions of the crane bay and construct a new precast concrete parking ramp and structure in their place. This solution was easier and more efficient than repairing and adapting the remains of an original structure into the improved layout. The new parking levels are anchored to the original 1940 ground-floor structure, while the new ramp is supported on conventional spread footings. Because the existing site soils include relatively soft organic deposits and uncontrolled fill, the soil was improved with rigid inclusions, which are constructed by ramming lifts of concrete through the weaker soils to more stable natural alluvial soils below.

While Building 215 was originally fully enclosed, the 1980s renovations that converted the western portion of the building into parking removed many exterior walls and exposed the slabs, beams, and columns to harsh New England weather and deicing salts. Demolishing the building was out of the question due to cost, potential permitting delays, and occupancy deadlines. To accommodate the new program, the building required extensive concrete repair and restoration. Areas in greatest need of attention included the exposed concrete slabs and many of the beams and columns along what had once been the crane bay.

The concrete slab on the ground floor of the parking area drained poorly, causing the original 1 1/2-inch-thick topping slab to crack and de-bond. Over time, water infiltrated and became trapped between the topping and the structural slabs. To address both the poor drainage and the underlying concrete deterioration, the project team elected to remove the original topping and install a new variable-thickness bonded topping. The contractor used hydro-demolition to remove the topping and sections of the slab without damaging the existing reinforcement. The design team limited the new topping’s thickness to 11/2 inches at the drains, but to maintain the 3/16 inch per foot slope, the sloped topping was thickened to 51/4 inches at the high points, and welded wire mesh and anchoring pins were added to restrain curling, as shown in Figure 5. SGH worked alongside Jacobs to optimize drainage while maintaining ADA-required slopes. A similar approach was taken on the second floor, but because the original topping was in better condition, the new sloped topping was applied over the existing one. Additionally, the second floor two-way slab was adequate to support the new topping in combination with the parking live load.

This approach worked well for the lower floors but not for the roof. Previous owners had installed multiple layers of gravel-ballasted asphalt and felt roofing over a steeply sloped, reinforced-concrete topping slab that varied in thickness from 4 inches to 71/2 inches. This topping slab was placed over a 11/2-inch-thick sand layer in some areas and over a rubber membrane and recovery board in others. Since the original roof was to become the new third floor as well as parking, the steeply sloped topping slab needed to be removed. Demolishing the topping slab exposed the original 1940 roofing membrane and, beneath it, the original roof slab, which was severely deteriorated. This left the project team with a dilemma: either remove and replace the original roof slab or place a new structural slab over the existing slab and leave the latter in place. Demolishing the original slab would have delayed the project, so the team decided to repair the original roof slab locally, construct a new two-way slab on top of it, and hang the original roof slab from the new slab. The final solution required temporarily shoring the existing slab from the second floor, making modest repairs, installing through-bolt hanger rods in each structural bay, and placing a new sloped, two-way concrete slab, as shown in Figure 6.

Repairing the deteriorated columns and beams was a much more straightforward process. SGH worked with the construction contractor to identify loose or deteriorated concrete. For the columns, the team first confirmed a minimum cross-sectional concrete area sufficient to support the existing axial loads. The contractor removed the deteriorated concrete, exposed, cleaned, and coated the reinforcing bars, and, where necessary, installed supplemental bars. The contractor formed and pumped new concrete repair material. In some locations, columns and beams were enlarged to accommodate the required cover. Figure 7 shows the repair process.

The need for new elevators and stairs drove an early enabling phase. The goal was to have new elevators and stairs operational before construction of the larger additions began.
At the southeast corner of the building, near what would become the new loading dock, the project team designed a new steel-framed vertical circulation tower with a stair and an elevator. Starting in the basement, the team detailed CMU bearing walls to frame the perimeter of the elevator and stair shafts and support the first floor slab around the new openings. Above, some of the original concrete beams and columns were repurposed to support the new tower and reduce demolition. New steel framing was anchored to the concrete beams and columns using double-angle shear connections and expansion anchors. Figure 8 shows an in-progress photo of the new vertical circulation tower during construction.

Adding a pair of new elevators in the center of the building proved to be a greater challenge. The passenger elevator serves the tenant parking area in the basement, and it requires a 4-feet-deep pit and mat foundation. Unfortunately, the new pit conflicted with the existing spread footings and pile caps, forcing precise detailing to avoid undermining or demolishing the existing foundations. The design solution involved doweling into existing foundations and designing the elevator pit slab to span between pile caps. The right-hand side of Figure 9 shows the new passenger elevator hoistway and pit. The service elevator pit is on the left.

The adjacent service elevator does not access the basement, and it requires a pit 4 feet below the first floor. The challenge then became how to design and construct an elevator pit 11 feet above the basement to accommodate the elevator’s operation and impact loads. The pit could not hang from the first-floor slab, so it bears on a ring of fully-grouted CMU bearing walls extending from the basement to the underside of the pit slab. New concrete beams span from existing pile caps to support the CMU walls, which in turn provide bearing for the service elevator pit. Additional CMU walls extend from the pit slab to the underside of the first-floor slab, providing support for the new opening through first-floor slab. To minimize the loads on the existing foundations, the space below the service elevator pit was enclosed and left unoccupied.

Designing and detailing the new balconies in the northwest and northeast corners of the building created a variety of challenges.

The northwest balconies are the signature architectural features that announce the building to the public along Arsenal Street (Fig. 3). The design intent was to keep the balconies within the building’s footprint and the property line while accommodating the thickness of the new waterproofing, drainage system, and pavers. To keep the new pavers flush with the existing floor levels, the second and third floor slabs and spandrel beams at the northwest corner were removed and replaced with a new, lower steel structure. In addition, the project team decided to demolish the northwesternmost corner column to create an unobstructed view from the balconies.

Anchoring the new steel framing to the existing concrete structure proved another challenge due to the existing circular columns and capitals, as well as drop panels. The design solution involved removing portions of the concrete capitals and nesting new steel framing into the pockets. The team detailed a curved steel plate-and-channel assembly anchored to the circular columns with expansion anchors to support the new beams. To fill the gap between the drop panel and the underside of the concrete slab, WT5 bolsters were added to the tops of the new steel beams. Figure 10 shows both the design detail and the partially erected framing.
After the new framing was in place, the contractor demolished the existing slab and northwest corner column and installed the remaining balcony framing. Figure 11 shows a photo of the new framing with the existing concrete slab and column removed.

Unlike the western balconies, the eastern balconies projected from the main building. To accommodate the new balcony framing, the concrete spandrel beams below the second and third floor slabs were demolished. This approach allowed the transition from the interior space to the exterior balcony to occur slightly within the building footprint, and it resolved issues with head-height clearance caused by the deep exterior beams. To support the new framing, the design included two new steel columns, cantilevered steel framing, and a new detail for the slab step from the existing floors to the recessed balcony structure. Figure 12 shows a detail of the original slab and spandrel beam along with its new counterpart.

With the enabling phase, concrete repair, and minor modifications complete, the building could finally receive the new one-story addition it was originally designed for. The building’s existing lateral load-resisting system consists of ordinary reinforced concrete moment frames, where the building’s flat slabs serve as the beam elements in the moment frames. The vertical additions increase the wind and seismic forces on the existing lateral load-resisting system. A detailed finite element analysis of the existing concrete moment frames showed that they are adequate to resist the increased demands.

The structural system for the vertical additions included steel columns, composite framing, and steel moment frames. (Braced frames, while efficient, would have compromised the adaptability of the floor space for future tenants.) Although the design team considered accounting for rotational restraint at the connections between the new steel columns and existing concrete columns to help control lateral sway, the existing column and slab reinforcement posed potential conflicts with the new anchors for the columns. Every steel column base plate would have needed to be unique. Instead, all new columns were designed with pinned bases and the ability to adjust anchor positions in the field. Consequently, the new column sizes were increased to limit lateral deflections, resulting in a relatively heavy structure.

The vertical additions on both the eastern and western halves of the building include mechanical equipment platforms 5 feet above the main roof level, with steel-framed screen walls cantilevering another 13 feet above the platforms, effectively creating a fourth story in some locations. The platform construction includes galvanized steel framing, cantilevered columns, plan bracing, and metal grating.

The repair, renovation, and additions to the 500 Forge building breathed new life into the eastern end of the Arsenal Yards campus. In addition to creating new usable office, laboratory, parking, and mechanical spaces, the project transformed the building’s exterior with a new fiber-cement panel facade, an entrance canopy, a loading dock, and a solar canopy. Colorful murals now adorn the once-drab foundation walls along Greenough Boulevard. Additional stairs and elevators increased accessibility and serviceability, while new recessed and cantilever balconies enhanced functionality and appeal. Constant communication among the project team helped resolve complex detailing and adapt the building into a space for groundbreaking life science (Figs. 13-14). ■

About the Authors

Robert Evans is a Consulting Engineer who has been with Simpson Gumpertz & Heger Inc. for 11 years, specializing in new building design for projects on occupied urban campuses. Evans’s role includes providing analysis, design, and construction administration services.

Alec Zimmer is an Associate Principal who has been with Simpson Gumpertz & Heger Inc. for 28 years and is a manager and engineer for the structural design of a range of educational, commercial, healthcare, and research facilities. Zimmer is also involved in the analysis and rehabilitation of new and existing structures and has extensive experience in structural investigations.