Creating a Concrete Skybridge Connection

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With the advancement of building technology in the past two decades, some high-rise buildings with interconnected towers through skybridges have been developed around the world for their unique iconic architectural profiles. Notable examples include the American Copper Building in New York City, Marina Bay Sands in Singapore, and Raffles City in Chongqing, China. In most of these projects, the skybridges are constructed using steel space truss systems, with members that can be hoisted into place at interconnection levels. One South First also features interconnected towers; however, unlike most similar buildings, it is constructed entirely of reinforced concrete.

Located at the original Domino Sugar refinery site along the Brooklyn side of the East River, just north of Williamsburg Bridge in New York City, One South First is a recently completed 43-story reinforced concrete high-rise building, whose main roof is 432 feet tall. A distinctive architectural feature of the building is the interconnection of the two towers with 16 skybridge floors above the 27th level, which is approximately 270 feet above the ground floor. These interconnected floors span 44 feet between the two towers. The unique architectural configuration was thoughtfully designed to maintain view corridors between the river and the inland neighborhood, while presenting a more porous skyline when seen from the East River. Concurrently, a greater number of residential apartments are positioned at higher elevations within the building (Fig. 1).

Another prominent architectural feature of the building is its contemporary crystalline facade. The building facade consists of angled white precast concrete faceted frames, onto which the glazing is mounted (Fig. 2). These precast frames also serve as sunshades, improving energy performance and reducing cooling demand. As a result of their shading function and architectural expression, the precast facade frames are substantial in size, with depths reaching up to 20 inches and widths extending up to 22 feet over a full story height. The frames are point-supported at the slab edges of the reinforced concrete floor system, where support reactions can be as high as 21 kips. Given the size of the precast facade frames, stringent control of relative deformation is essential to maintain facade alignment and performance.

In typical interconnected-tower skyscrapers, the skybridge sizes are relatively small compared to the primary towers and are often designed to accommodate movement relative to the base structures. For the One South First project, however, a sliding skybridge solution was not feasible due to the substantial number of the interconnected floors and the large size of the precast facade frame. A sliding connection would require big expansion joints along the facade, which could discontinue the architectural feature and pose difficulties to the envelope long-term serviceability maintenance. By structurally locking the towers together through the skybridge floors, the design maintains a consistent precast facade expression across the entire building, preserving architectural continuity.

The popular flat slab floor system was selected for the One South First building due to its simplified formwork, flexibility in column layout, and reduced floor-to-floor story heights. The 16 skybridge flat slab floors work integrally with the two towers as structural diaphragms and contribute to the lateral force-resisting system, which enhances the overall building stiffness. The contribution of the flat slab system to the lateral force-resisting system was explicitly considered in the structural design, resulting in more realistic structural dynamic behavior compared to assuming that all lateral loads were resisted solely by shear walls. By accounting for the lateral participation of the flat slab-column frames, the total number of shear wall piles was reduced, significantly lowering foundation costs.

From a structural standpoint, integrally connected towers can enhance overall lateral stiffness, mitigate cross-wind excitation, and improve occupant comfort at upper levels. The benefits of this integrated configuration were confirmed through wind tunnel testing. Results demonstrated that the interconnected tower system improved perception performance: the estimated 1-year return period peak acceleration at the 42nd floor is approximately 4.9 milli-g, comparing to 6.0 milli-g estimate for a similar standalone reinforced concrete tower.

Due to the 270 feet elevation of the reinforced concrete skybridge floors, erecting falsework from grade to skybridge levels was neither practical nor economical. Supporting the bridge floors and their associated formwork therefore became a primary challenge in the structural system selection. The constructability and cost implications of the skybridge solution also had direct impacts on building zoning, architectural layout, and overall construction sequencing. During the early feasibility study phase, the design team and the client evaluated multiple structural framing options for the interconnected towers.

After assessing construction schedules and cost efficiency, the structural design team proposed an innovative solution: story-high cast-in-place reinforced concrete transfer girders spanning between the two towers to support the 16 skybridge floors; 44-foot long precast prestressed planks were used as the formwork/platform for the story-high reinforced concrete transfer girders at the 27th floor, as well as the shore/reshore bases for the floors above.

To simplify transfer girder framing, the columns of the 16 skybridge flat-slab floors were intentionally aligned into 4 rows without sacrificing architectural apartment layout, so that the columns loads could be transferred through four parallel girders spanned between the two towers (Fig. 3). The parallel girders enabled the design team to utilize the interstitial space as functional mechanical space, where the exhaust and intake louvers were routed through the skybridge soffit. This eliminated the need for facade penetrations and maintains the continuity of the building’s crystalline feature.

Seventeen 44-foot-long solid precast prestressed planks were used to cover the 65-foot width of the skybridge floors. One typical plank was 15 inches deep and about 4 feet wide, with an individual weight of roughly 33 kips. The upward camber of the planks due to the prestressing was controlled to be 2 inches before erection, and mechanical openings in planks were fully coordinated prior to their fabrication. The planks were manufactured in Selkirk, New York, approximately 150 miles from the project site, then transported by truck and hoisted by tower crane to the 27th floor for installation (Fig. 4). Neoprene pads were placed between the planks and the supporting concrete ledges to accommodate relative movement between the two towers during plank installation, and windy conditions were avoided to ensure successful erection.

The planks were designed to span between the exterior column lines of the two towers. Due to the shear wall layout, no columns were located directly beneath one side of the two interior transfer girders, resulting in potentially excessive deflections under the plank ends. Temporary shores were therefore provided at the plank supports. At the time of plank installation, mechanical piping and ductwork routing would already be in progress at the podium levels, making shoring down to the foundation impractical. Instead, the vertical shores were shifted by approximately 3.5 inches at each floor to transfer the temporary plank loads from the tower edge supports back to the shear walls at the 5th floor (Fig. 5).

The precast prestressed planks served as the working platform for the construction of the four transfer girders, which support the 16 flat-slab skybridge floors above. Two rows of dowels, spaced at 5 inches on center, were embedded within each precast plank and cast integrally with the transfer girders and the topping slab, allowing the planks to remain permanently in place. Nevertheless, the structural design conservatively assumed that all gravity and lateral loads were carried solely by the cast-in-place transfer girders, without relying on any strength contribution from the precast planks.

Following installation and stabilization of the prestressed planks at the 27th floor, reinforcement placement and construction of the transfer girders and topping slab proceeded (Fig. 6). The transfer girders linked the 27th floor slab with the 28th floor slab, which effectively formed a 65-foot-wide multi-zoned box girder. The slabs enlarge the compression/tension zones to reduce deflections and minimize reinforcement congestion. Because of their substantial size, the transfer girders were detailed to be cast in four phases to minimize the risk of mass concrete thermal cracking and to reduce temporary construction loads on the precast planks.

The reinforced concrete transfer girders were cast monolithically with the columns and shear walls at the 27th story, establishing the initial integral structural connection between the two towers. Subsequent flat-slab floors acted as structural diaphragms, further integrating the towers into a unified structural system. Although the precast prestressed planks were allowed relative movement during erection, the completed cast-in-place concrete system ultimately achieved full structural integrity between the towers.

Due to the integral structural skybridge floors between the two towers and the large size of the precast facade frames at One South First, careful control of long-term differential column shortening and precast frame support deflections was essential. Several design measures were implemented to mitigate these effects:

Column Section Enlargement
To minimize long-term differential shortening, the column sections supporting the transfer girders were intentionally increased in size. Enlarging the sections reduced compressive stress differentials between vertical elements, thereby limiting time-dependent shortening discrepancies.

Strict Control of Transfer Girder Deflection
To eliminate the need to construct floors above the transfer girder level with cambers, the total long-term deflection estimates of the transfer girders were limited to approximately 1.5 inches, in addition to satisfying the relative deflection requirements in ACI 318 Building Code Requirements for Structural Concrete and Commentary. Controlling the absolute total long-term deflections is critical to prevent differential movement of the skybridge floors, which could otherwise result in facade misalignment and serviceability concerns.

Optimization of Exterior Column Layout
Significant coordination was performed during the schematic design phase to align the precast facade substantial point loads near structural columns. The locations of exterior columns were strategically adjusted so that the support connections for the heavy precast facade frames were placed in close proximity to columns. This adjustment minimizes differential deformation at slab-edge supports and reduces stress concentrations within the precast facade frames. As a result, the facade caulk joint sizes were minimized, maintaining the aesthetic appearance of the architectural facade.

The project incorporates precast prestressed planks, precast facade panels, and cast-in-place concrete structural members. To optimize equipment utilization and minimize schedule delays, the construction sequence was carefully planned to ensure continuous and efficient progress. Owing to the massive size of the skybridge floors, two notable construction considerations warrant special attention.

The first relates to adjustments in the precast facade installation sequence along the inward-facing building elevations below the skybridge. Typically, facade installation at a given floor starts when concrete placement is underway approximately five floors above. Because the precast facade panels in this project were heavy, their installation required the use of tower cranes. To avoid crane conflicts and scheduling constraints underneath the skybridge floors, the panel installation below those levels was expedited. With temporary shoring provided, the inward-facing panels were fully installed before hoisting the precast prestressed planks to the 27th floor. As shown in Figure 4, the precast panels at the inward elevations were completed while the planks were being lifted into position.

The second construction specialty involved installation of the architectural soffit panels beneath the skybridge. Given the approximately 65-foot width of the bridge portion, a dedicated space truss measuring 10 feet by 66 feet was fabricated to serve as a working platform for installing the skybridge soffit facade panels. The space truss was assembled at the podium roof level and subsequently hoisted into position beneath the bridge. To allow the platform to travel between the two towers, a temporary monorail system was constructed at the 29th floor. Figure 7 presents an elevation sketch of the monorail and space truss system.

To the authors’ knowledge, One South First has the highest skybridge floors among fully reinforced concrete interconnected high-rise towers worldwide. Located approximately 270 feet above the ground level, the 16 skybridge floors work integrally with the two towers, functioning as structural diaphragms that enhance the overall lateral stiffness of the building. Precast prestressed planks were utilized as stay-in-place formwork to support the cast-in-place transfer girders at the 27th floor, facilitating an efficient and well-sequenced construction process. Specific design measures were implemented to address the structural and constructability challenges associated with the elevated skybridge configuration. By successfully realizing the fully reinforced-concrete elevated interconnected skybridge floors, One South First demonstrates the seamless integration of architectural intent, structural performance and constructability. ■

About the Authors

Songtao Liao, Ph.D, PE, is a senior associate at Rosenwasser/Grossman Consulting Engineers, P.C., with over 20 years of experience in high-rise structural analysis and design. (stevenl@rgce.com).

Benjamin Pimentel, PE, president of Rosenwasser/Grossman Consulting Engineers, P.C., specializes in high-rise concrete structures. He is a past President of the Concrete Industry Board, the current president of the Concrete Industry Foundation, and co-chair of the concrete panel for the upcoming New York City Building Code cycle.

Matthew Segerman, PE, is a senior associate at Rosenwasser/Grossman Consulting Engineers, P.C., and serves as project manager for several notable high-rise projects, including the new Domino development in Brooklyn, NY.

Yu Huang, Ph.D, PE, is a senior structural engineer at Alan Margolin & Associates Consulting Engineers and Architects, with 15 years of experience.