Bridges in the sky

Sky bridges are a growing trend in contemporary tall building design. More than mere pedestrian links, these often-immense structures provide formal distinction, expanded means of egress, increased floor area, and access to spectacular views atop the buildings they adjoin.

Every sky bridge must be designed to meet the demands of its particular form, function, and location. Engineers must develop appropriate solutions to ensure the safe performance of the linked towers and sky bridge, even under the most extreme conditions.

Building connections

At the very outset of the project, the team must make a design decision that will have a greater impact on structural and architectural performance, construction, and overall cost than any other: how to connect the sky bridge to the towers.

Having led the structural engineering for several projects requiring exploration of multiple attachment scenarios, Arup is at the forefront of this technology.

CCTV Headquarters, Beijing

For Beijing’s CCTV Headquarters building, which houses television studios in a 13-story cantilever connected to two leaning 230-meter-tall towers, we elected to integrate the bridge into the towers’ overall support structure, creating a single monolithic structural system capable of enabling the tremendous overhang.

For Singapore’s Marina Bay Sands Integrated Resort, where critical hotel amenities are positioned within and along the massive Sands SkyPark that spans three inclined 55-story hotel towers, we articulated the sky bridge with movement joints.

Credit: Arup

 

This allows the towers to sway independently of each other as the sky bridge glides along sliding bearings in response to these movements.

Credit: Timothy Hursley

Marina Bay Sands, Singapore

Dynamic links

When designing the striking sky bridge Conservatory atop the Raffles City Chongqing development (for which foundations work is scheduled to begin this fall), Arup determined that neither of the above strategies was practical.

The relatively large wind and seismic demands imposed on the slender towers would have induced tremendous force within the buildings and through the sky bridge if these were made monolithic as in the CCTV, resulting in very high steelwork costs.

Credit: Arup

Time history analysis showing the behavior of two design scenarios in an earthquake

Conversely, impractically large sliding bearings would have been required if the deck were allowed to slide freely atop the buildings, as was done for the Sands SkyPark. Another solution was required.

The team devised a creative third option, which employs specialty seismic bearings located at the interface between the towers and sky bridge.

In this dynamically linked system, the bearings are designed to dissipate a significant amount of the energy generated at the interface between bridge and tower as the structures slide past each other, reducing the magnitude of movement and improving the structural performance of both the sky bridge and the towers that support it.

Moreover, tapping the energy generated at this critical interface reduces building damage during an extreme earthquake event, enhancing the project’s overall seismic robustness.

Next steps

In China, proposals for tall and complex buildings that fall outside of standard code must be reviewed by a seismic review committee made up of academics, researchers, and designers. (Buro Happold posted an interesting piece about the process on its blog.) Raffles City Chongqing is currently undergoing this review, which to date has consisted of non-linear seismic time history analyses of different kinds of bearings, wind time history analyses, and demonstrations of sufficient robustness and redundancy beyond code-defined loading events.

We are optimistic that the design will be approved and the building will be completed on schedule in 2018.

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