Renovating Existing Buildings for Modern Research

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One of America’s leading research centers has dramatically reshaped its existing campuses through a combination of major new construction, renovation, and expansion. Part of this transformation included the modernization of the existing 23-story research building known as the “Avenue.” Originally constructed in the 1990s, the Avenue building contains public spaces, laboratories, and staff residences. The renovations of the Avenue included 120,000 square feet of existing space on the second through sixth floors of a 23-story building to create modern, flexible labs that support critical research. In order to achieve a modern and efficient lab space, existing shafts and mechanical cores had to be relocated, which created some structural challenges and the need for innovative design solutions.

The Avenue building superstructure consists of one-way and two-way reinforced concrete flat slabs. The existing floor plans located the main mechanical shafts within the middle of the structural bays of the building with plumbing risers located on a select number of columns. The existing slab design only included drop caps at columns with the plumbing risers to account for the reduction in the punching shear capacity. While the current layout of shafts and risers may have been ideal for research 25 years ago, this layout limited the design team’s ability to optimize and modernize the space for current research consistent with other buildings on campus. For example, the Avenue building did not have the same flexibility as new steel construction in terms of layout of columns and mechanical openings to fit within lab spaces. The solution was to infill the existing shafts and risers with reinforced concrete and relocate the shaft openings to areas that maximize the layout of modern labs. This required penetrating the existing slab with large new mechanical shafts and adding plumbing risers adjacent to columns, greatly reducing the punching shear capacity of the slab and column joint.

Dealing with any existing structure comes with unique challenges and requires careful coordination between all design team disciplines taking into consideration the current conditions and limitations. During the modernization of the Avenue, consideration of occupants above and below the stories under renovation needed to be considered. This meant limiting the hours of noisy work such as drilling and jack hammering. Another challenge was that operations and utilities that pass through the building to other parts of the building complex needed to be operable throughout construction, which meant phasing of mechanical, electrical, and plumbing (MEP) demolition and installation. Some ducts and pipes had to remain in place and the structural modifications needed to be coordinated around them. As the building had undergone several prior minor modifications, as-built MEP drawings were not available and as such many of these obstacles could not have been known during the project’s initial design phase. Given the existing conditions and constraints, the design team reviewed multiple building materials and methods, with the goal of providing a solution that would allow for flexibility based on the actual in-situ conditions.

The first step the design team took was to create two different finite element models using CSI SAFE. The first one was to recreate the existing structure with all the current openings and penetrations, and the second model included the existing structure modified with the updated mechanical openings. This approach allowed the team to set a baseline model for tracking the redistributed forces that come with creating openings in a two-way concrete slab. This made it easier to identify locations where the slab was overstressed and required strengthening. It also allowed the team to verify that the existing conditions were still valid for the current building code loading. Based off the analysis, it was determined that if the new mechanical shafts were created without additional reinforcing, the existing slab would be overstressed, and certain areas would see large deflections.

The design team reviewed several different building materials to reinforce the existing slab which included reinforced concrete, steel plating, FRP, and steel framing. Utilizing fiber-reinforced polymer (FRP) to strengthen most of the slab was one option the team reviewed, as it provided a lightweight and non-intrusive solution that wouldn’t impact above-ceiling space MEP. After a careful review of the analysis models, in many instances the size and number of openings created a large change in moment demands that went over the FRP’s capacity to increase the slab strength for flexure and punching shear.

Another option the design team considered was the use of additional concrete and reinforcing to improve the concrete superstructure’s load-carrying capabilities. Strengthening of existing slabs would require new slab and beam sections to be poured and bonded to the existing. This would increase flexural capacity but would also require very close coordination between the existing rebar to remain and the large amount of rebar being drilled and epoxied into the existing sections. Additional challenges with this approach included the selective demolition of existing rebar, working around many existing plumbing utilities that had to remain operational, and working around existing electrical conduits in the slab that were to remain. With so many unknowns, there was the risk of major design changes during construction. In addition, the large number of reinforcing bars that would require drilling and epoxying into existing concrete would create much noise, impacting occupants above and below, and forcing the contractor to work during off-hours. For similar reasons to the concrete approach, the design team elected not to pursue reinforcing the slabs with steel plates and post-installed concrete anchors.

While exploring the steel plating and FRP approaches, another option presented itself. To reduce the amount of drilling and provide maximum flexibility for the MEP team, the structural team explored using steel beams to frame out the openings. In this approach, the steel beams would support the concrete slab around openings and bring the loads directly back to the columns. This approach reduced the demand on the slabs, also reducing the amount of FRP required. FRP was still used in isolated locations away from the new openings to meet the slab strength design for the newly distributed forces where steel beams were not nearby.

To reduce drilling for anchors and help with the construction schedule, the steel beams were designed for the full unbraced length for lateral torsional buckling. Steel jackets were installed around columns to transfer the beam forces into the columns safely without the anchors breaking out and having to rely on the column reinforcement for concrete breakout. The face of the jacket that the steel beam framed into utilized anchor rods with an adhesive that were sized to transfer the beam reaction into the steel jacket and column; while through bolts with adhesive were used on the opposite column face to help with confinement and provide reinforcement for the concrete breakout. All oversized holes drilled into the concrete columns were infilled with epoxy or grout to minimize reduction to the column’s axial capacity. The column sizes at the lower levels of the 23-story high rise measured 2 feet by 3 feet, which made it difficult to drill holes for through bolts. Holes of this length have the possibility of not aligning and would need to miss rebar on all faces of the concrete columns. The steel contractor’s solution was to scan the columns and drill the holes on all four column faces, then survey the hole locations and create a custom-plated jacket for each column location. This ensured that each steel jacket fit in the field and had minimal adjustments.

The use of steel beams was found to be very useful as the contractor started to demolish the existing labs spaces. Demolition activities uncovered previously unknown plumbing mains that served the whole building and could not be interrupted or shut off. Steel framing was quickly redesigned and resized as the demolition progressed, allowing the contractor to stay on schedule. The use of steel jackets also limited the need to redesign for field conditions since the jackets are very flexible in their ability to support any steel framing members. Steel beams were able to be offset and skewed to avoid existing mechanical ducts and pipes.

In addition to the new mechanical shafts, the plumbing risers were relocated to different columns than original. Since several columns were already steel jacketed, steel drop caps with stiffener plates were used to reinforce the beam-column joints for punching shear. The addition of the drop cap not only helped with punching shear capacity but also reduced the negative and positive bending in the slab since the slab design spans were shorter. The use of steel framing and steel drop caps reduced the demand on the slab’s top reinforcing at the column locations, which reduced the amount of FRP strips and did allow for a few existing rebar to be cut when locating the new plumbing cores.

To maintain the existing slab’s negative bending capacity, all concrete slabs needed to be scanned and marked out so the plumbing contractor could locate their cores to avoid the existing rebar. The steel contractor utilized plexiglass templates that traced the plumbing cores to accurately fabricate the steel drop caps. A laser cutting machine produced very accurate steel members, saving significant time and labor to install the new plumbing risers.

In addition to modernizing the main lab spaces, the central knuckle which provides circulation between adjacent buildings was to be renovated and included a redesign of the monumental stairs between floors. To provide a sense of openness, the design of the new steel stair did not include any supports at the mid-level landing, instead cantilevering out around the stair plinth. This allowed the architects to maximize circulation and use of space and provided optimal views of the two-story art wall.

Support of the new stair required the existing floor opening to be modified, including infill and cutting of the existing concrete slab. Steel beams were added around the opening that connect back to existing columns and beams. The C10x30 stair stingers were supported by a steel HSS16x12x3/8 header that connected to two existing concrete beams, originally not designed for such forces. To minimize impact and cost, the existing concrete beams were reinforced with FRP for both flexure and shear.

Upgrading any existing building requires careful consideration and analysis and outside-the-box solutions. The Avenue building modernization makes no exception, and the project showcases the design team’s ability to perform major structural modifications to an existing concrete structure by utilizing steel framing and FRP while maintaining building functionality and minimizing impacts to users during construction. The use of structural steel framing allowed for flexibility for unknown field conditions and created an elegant solution to a very challenging project. ■

About the Author

Timothy Schuster PE, is a structural engineer at HDR in the Princeton, New Jersey, office. (Timothy.Schuster@hdrinc.com).