Hangzhou Century Center: Engineering a Landmark

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The Hangzhou Century Center, also known as "The Gate of Hangzhou," is a prominent urban development situated in the Xiaoshan District of Hangzhou, China. Positioned between the Hangzhou International Expo Center to the east and the Olympic Sports Center Training Hall to the west, the project is envisioned as a striking gateway to the city. This iconic development comprises two high-rise towers, each reaching a height of 310 meters, connected at their base by a 60-meter-span steel arch bridge. The surrounding commercial blocks form a dynamic mixed-use complex, offering office spaces, luxury hotels, and retail facilities.

With a total construction area of approximately 526,000 square meters—370,000 square meters above ground and 160,000 square meters below—the twin 63-story towers are designed to resemble the letter "H," a symbol of Hangzhou's identity. A suspended steel grid structure connects the two towers starting at the 21st floor, creating an expansive public space above the bridge, enhancing both the functional and architectural scope of the complex.

Hangzhou is located in a region of moderate seismic activity and medium wind pressure. The lateral design of the towers’ structural system is primarily governed by seismic effects. The structural system itself is a highly efficient and robust integration of distinct yet complementary components. It is designed to leverage the stiffness, mass, and damping characteristics of a rigid central core and a ductile perimeter moment frame, optimizing resistance to dynamic wind forces while efficiently dissipating seismic energy.

Each tower features a centrally located reinforced concrete core tube, which transitions from an elongated octagonal shape at the base to a rectangular form by the 44th floor. The core resists the majority of lateral seismic and wind loads. The perimeter frame is a composed of reinforced concrete beams and composite SRC columns which could provide the excellent ductile for seismic and minimize the sizes. The perimeter moment frame is proportioned to ensure effective load sharing, maximizing efficiency as the second defense system for seismic.

The towers’ oval-shaped floor plans taper along their height, resulting in a highly dynamic, aerodynamically optimized massing with a rounded corners and tapered shape, highly integrated with architectural layouts. This shape helps to disrupt wind vortex shedding, thereby reducing wind-induced vibrations and loads, leading to significant material savings.

The floor system consists of one-way reinforced concrete beams and slabs within and beyond the core. Although cast-in-place concrete floors typically extend construction timelines, the contractor was able to implement a three-shift work schedule per day for concrete construction, whereas steel construction was limited to one shift per day due to local regulations. As a result, the concrete system did not delay the schedule while simultaneously achieving structural material cost savings.

The steel arch bridge spans 62 meters between the two towers, elevating 34 meters above the ground. Comprising six parallel arch trusses aligned with the tower columns, the bridge is supported by rigid connections at the ground level, where it integrates with the steel-reinforced concrete columns of the towers. The horizontal thrust of the bridge is transferred to the core tubes of the towers via the continuous floor slab system at the ground level, ensuring stability and load distribution.

The bridge serves dual functions: a pedestrian walkway at the 6th floor and a banquet hall at the 3rd floor. The arches of the bridge are defined by the Funicular form-finding to minimize the bending moment under gravity. The diagonal braces are incorporated to enhance lateral stiffness since this building-type bridge still needs to meet with building structural control ratios. The bridge’s natural frequencies are 1.23 seconds in the X-direction, 1.18 seconds in torsion, and 0.99 seconds in the Y-direction, ensuring its stability under dynamic loads and providing a resilient structure for varied use.

The suspended roof structure spans approximately 60 meters between the two towers, hanging from the 21st floor. It is composed of bidirectional steel grid members with vertical curvature optimized through the catenary geometry concept; each segment of the grid is straight with the H section. The horizontal curvature is carefully designed to simplify node connections and enhance the architectural aesthetics.

Horizontal restraint systems located on the 18th, 14th, and 11th floors provide lateral stability for the suspended roof and incorporate tension cables and compression struts. Vertical steel members support the side walls of the suspended structure, hanging from the edge beams of the roof. These vertical members are laterally restrained at the 6th and 3rd floors of the bridge but are free to move vertically, accommodating differential displacements.

The drape roof is one of the project’s most innovative features, drawing inspiration from the natural shape of a hanging chain, which forms a catenary curve under its own weight. This geometry ensures the roof predominantly carries axial forces to minimize bending moments and optimize material efficiency, which contributes to both structural performance and sustainability.
The initial geometry of the longitudinal drape grid was derived from the classic catenary equation. However, this equation assumes uniform segment lengths and constant gravity loads per segment. Given the varying segment lengths and loads in the drape grid, iterative calculations and adjustments were required to account for the actual gravity loads at each node.

This process ensured the final catenary geometry efficiently supports its self-weight
The integration of the steel grid roof and glass panels was a key design challenge, demanding precise coordination to balance structural integrity with aesthetic refinement. The objective was to maintain the majority of the glass panels in a flat configuration while accommodating the doubly curved geometry of the draped roof.

To achieve this, the roof's surface was designed using a scaled-translational approach, beginning with defining a central catenary curve, which is then subdivided into multiple straight-line segments based on the target glass panel dimensions. Due to the tapered form of the tower and different hanging ends, adjacent catenary curves span slightly longer distances. These adjacent curves are generated by scaling the central catenary—preserving the segmentation—and translating it to its designated location. This procedure is repeated to generate the series of catenary curves across the surface. Finally, corresponding division points between adjacent curves are connected with straight lines, forming a quadrilateral grid, which defines the layout for the glazing panels. This method ensured that the glazing panels remained perfectly flat while conforming to the overall curvature of the structure. Each glass panel was supported by a quadrilateral steel grid, with panel edges precisely aligned to the grid lines. The flatness of the panels was meticulously controlled through careful geometric calibration of the grid and strategic orientation of the steel members.

The optimized geometry allows 97% of the grid members to use standard H-section steel (HN300x150x6.5x9), with localized reinforcement (H300x150x10x25) at stress concentration areas. The total steel weight of the roof, including nodes, is approximately 700 tons.
The sidewall of the drape roof connects the edge of the drape, which is straight in plan, to the edge of the bridge structure, which is curved in plan. It also meets the towers along a line angled in elevation. The hanging mullions of the sidewall are designed to be as evenly spaced as possible while maintaining a funicular shape under gravity to minimize bending.

To achieve these design constraints, 3D graphic statics were employed. In this approach, force magnitudes within the structure are represented by the lengths of lines in a force diagram, while equilibrium at each node is ensured by the closure of force polygons. By imposing geometric constraints on these diagrams, a 3D funicular structure in equilibrium was developed.
Although the structural logic of the sidewall precludes the use of entirely flat glass panels, the relatively shallow curvature ensures that cold-bent glass panel warping remains within acceptable limits, even under wind-induced deflections.

The grid nodes of the suspended roof are designed to simplify construction and ensure structural efficiency. Each node consists of a steel tube with four H-section members connected to it. The flanges of the H-sections are welded to circular plates at the top and bottom of the tube, ensuring continuity of the load path. The orientation of the I-section member axes is carefully specified to ensure that the webs of every I-section intersecting at a node align along a common line, known as the "node axis." This eliminates geometric torsion and simplifies detailing and documentation. The sidewall connections were designed as hinged joints, allowing for construction tolerances and accommodating differential displacements while maintaining structural integrity.

Wind tunnel testing was conducted to determine the wind loads on the suspended roof and side walls. The tests provided eight wind load cases for the structural design, with wind pressures ranging from -2.27 kPa to 1.73 kPa. The natural frequency of the suspended roof was controlled to be around 1 Hz to ensure accurate wind load predictions.

The maximum deformation of the suspended roof under wind loads is 132 millimeters, which is acceptable for a 60-meter span. The lateral deformation of the side walls at the connection points with the towers is 47 millimeters inward and 68 millimeters outward, within the allowable limits for the curtain wall system.

The deformation of the twin towers under wind and seismic loads has a minimal impact on the suspended roof due to the high stiffness of the towers relative to the roof structure. In the integrated structural model, the lateral deformation of the towers at the connection points with the roof is 30 to 50 millimeters, which has a negligible effect on the overall behavior of the grid roof. The porous grid roof reduces wind loads while promoting natural ventilation at the concourse level.

The deformation of the suspended roof is controlled to ensure the safety and functionality of the glass panels. The out-of-plane deformation of the panels is limited to 1/50 of the diagonal length, while the in-plane shear deformation is controlled by designing the panel connections to allow for relative displacements.

The structural performance of the suspended roof was evaluated using finite element analysis. The analysis considered gravity loads, wind loads, and seismic loads, as well as the interaction between the suspended roof and the twin towers. The results confirmed that the roof meets all design requirements, with sufficient stiffness and strength to withstand extreme loading conditions.

Following the completion of the foundation and basement, construction of the two towers commenced simultaneously from ground level. A climbing formwork system was utilized for the reinforced concrete shear wall core, which progressed approximately two floors ahead of the column, beam, and slab floor system. The typical construction pace averaged six days per floor. Upon reaching level 24, construction of the steel bridge began. The two cores were engineered to resist the thrust forces from the steel bridge. Each bridge arch was prefabricated in two segments at the shop, with the six steel arches erected first, followed by the installation of steel braces, columns, and floor members. The superstructure construction for both towers was completed in 13 months.

The construction of the drape roof presented several challenges, including the need for precise positioning of the grid members and the installation of the glass panels. The construction of the suspended roof involves the following steps:

The Hangzhou Century Center project demonstrates the feasibility of using catenary geometry and advanced structural analysis techniques to design long-span suspended structures. The project’s success provides valuable insights for future projects, particularly in the design of lightweight, efficient, and aesthetically pleasing structures.

The successful realization of the Hangzhou Century Center project would not have been possible without the dedication and expertise of the entire project team. Special acknowledgment is due to structural team members William Baker, Dane Rankin, Toby Mitchell, Han Ding, Max Cooper, and Ben Johnson for their innovative approach to architectural integration and structural aesthetics. Their collective efforts and seamless collaboration were pivotal in transforming this ambitious vision into a tangible landmark.

The suspended grid steel structure of the Hangzhou Century Center stands as a remarkable achievement in contemporary structural engineering. Through the innovative application of catenary geometry, advanced wind tunnel testing, and meticulous construction planning, the project has produced a structure that is not only highly efficient but also visually striking. Its success offers valuable insights for the design and construction of future long-span suspended structures and super-tall buildings, pushing the boundaries of engineering excellence. ■