Steel Elements Connect Slim Towers
Hanking Center Tower in Shenzhen, China
A 350 m tall tower, located in a typhoon region, has been structurally divided into two along its vertical axis. One side contains office space while the other-10.5 m away-contains the tower's circulation core.
Introduction
Hanking Center Tower rethinks the traditional commercial office building through an innovative approach to circulation, social, and work spaces. Offering flexible tower office space anchored by high-end retail and dining in the podium, the Tower serves Shenzhen's growing body of global professionals and brings density to the suburb of Nanshan.
Occupying a place of prominence on Shennan Boulevard, Hanking Tower's slender profile redefines the local skyline. The Center utilizes folded angles to elegantly merge public components in the podium with private commercial space in the tower - a departure from conventional towers, where differing program is often relegated to separate and disjointed volumes. Surrounding the tower's podium, a grand plaza and dimensional hardscape create a new neighborhood landmark and enhance public activity at the street level.
Unique Feature
The only thing that differentiates a typhoon, a cyclone, and a hurricane is the location of its formation; each term describes the same powerful weather phenomenon. Buildings along the coast of Shenzhen, China, must therefore be designed for what the United States would term hurricane-force winds, which can reach upward of 160 km/h. One such building, the 350 m tall Hanking Center Tower in Shenzhen, China, takes a unique approach to such design; it is structurally divided into two parts along its vertical axis to create two visually separate, yet connected, slim towers. A steel structural system will support the two slender towers-which behave monolithically, despite appearing as visually distinct-against typhoon-strength winds.
Upon its completion, the 65-story structure will be the tallest steel braced-frame tower in China, according to Eric Fenske, S.E., a senior project engineer in the Chicago office of the global engineering firm Halvorson and Partners, and the project engineer for the Hanking Center Tower. Fenske wrote in response to questions posed by Civil Engineering online.
"An all steel-braced lateral system is uncommon in China, where concrete or composite construction is the norm for high-rise structures," said Fenske. Steel was chosen for this project, however, in part because the engineers devised an economical steel design and in part to meet the needs of the architectural form. The design of the 110,169 m 2 tower was created by Thom Mayne, FAIA, the founder and design director of Morphosis, which has offices in Los Angeles and New York City.
The south tower will boast a multiplane sloping facade and will house office spaces, while the north tower-located 10.5 m away-will contain the building's main circulation and service core. Two secondary cores in the south office tower will house private elevators, freight elevators, and mechanical services, according to material distributed by the architects. Sky bridges at each level and horizontal steel braces located at every fourth floor will connect the two towers.
By "freeing" the services core from the center of the building, daylight can be admitted and views can be afforded from the public circulation spaces, according to the architects. Every fifth floor will contain glazed lobbies and so-called "sky gardens" that will expand on these day-lit public spaces, creating a communal hub for tenants, according to the architects.
For stability, however, the two towers had to be linked so that they behave as one, according to Fenske. Each was too slender to be stable alone, and relative movements between the two-if they acted independently from one another-would have been unmanageable. "Typhoon gusts generate very large wind loads on the tower-nearly twice the wind pressure when compared to towers in non typhoon regions," Fenske said. "In fact, the factored overturning forces due to 100-year wind loads were almost identical to those due to a rare (2,500-year) earthquake event."
The tower rests on a mat foundation that is supported by bored reinforced-concrete piles.
For the Hanking Center Tower, the design team selected a steel mega braced tube system with multistory diagonal bracing that repeats every four stories to resist lateral loads. "In order for the tower to efficiently carry the overturning compression and tension forces created by the typhoon-level wind loads and rare seismic events, vertical columns were located to create a box in plan," Fenske explained. "To supplement the main bracing located on [each] four-story module, additional bracing was located on a one-story module around the elevator and stair/mechanical cores." This mega bracing system is distinct from the horizontal bracing that transfers the diaphragm forces between the two towers, he added. "Further linkage was achieved by providing outrigger and belt trusses at three of the five mechanical levels, which also provide additional load paths, creating a redundant system," Fenske noted.
Structural design
The building is designed to withstand large wind loads during typhoon and seismic events that are common to the Shenzhen region. The lateral load resisting system chosen to strengthen the structure is a braced steel frame with concrete-filled box columns. The use of concrete fill reduces the cost of the columns as well as making them heavier, therefore minimizing uplift tensions caused by wind or seismic load combinations.
The intermittent bracing will tie the core elements together and outriggers will join the core to the opposite face of the tower at mechanical levels. The horizontal diaphragm forces will be transferred between the two towers by placing two horizontal 'X' braces every four stories.
Vertical columns will be placed far from the centre of the tower to carry the overturning compressive and tensile forces created by the large wind loads. The columns will be connected by mega bracing consisting of steel box shapes to create a closed tube to resist the lateral shear forces.
Structurally, the towers' floor framing comprises a concrete-on-metal-deck system supported by composite steel beams and girders. The tower rests on a mat foundation that is supported by bored reinforced-concrete piles.
Connecting the separate tower sections so that they behaved monolithically under the extreme wind loads of a large typhoon proved tricky. "The open floor space between the two towers presented a unique challenge for a tall building," Fenske noted. "A typical high-rise building has a continuous floor plate that acts as a diaphragm and is able to distribute the lateral loads to its resisting system," he explained. With the two separate towers, however, this was not possible.
To transfer the lateral loads in the Hanking Center Tower, two horizontal X braces were instead placed between the north and south towers at every fourth story to "match the module of the megabraced tube," Fenske explained. "We completed many special analyses to both follow the horizontal loads through the diaphragm and framing system and check that this bracing met the required performance for both wind and seismic events."
The facade
The facade of the building will comprise a transparent glass shell, which will provide a glimpse of the interior mechanics and structure. The glazed facade will offer panoramic views of city, as well as allow abundant natural light and airflow from operable windows into the building. The south face of the building will have a sloping facade with slope reversals in three locations, thus creating four different planes on the facade. The podium structure will have a facade made of folded angles, which will merge seamlessly with the tower component of the building.
Aesthetics vs. Economy
Cost and appearance also had to be balanced. "The architects wanted to keep the gap between the two as open as possible-both vertically and horizontally-while also expressing the structure," Fenske said. "The current Chinese codes allow for an inter story drift limit of height/250 for a steel-framed tower and height/400 for a tower using reinforced concrete," Fenske explained. Because the allowable inter story drift in steel braced-frame towers was higher than that allowed for reinforced-concrete towers, the cost premium for using steel was reduced, according to Fenske. "To further reduce costs, concrete was substituted for some of the steel for all of the vertical columns, creating rectangular concrete-filled tubes (RCFTs)," he explained.
Because the Chinese codes treat rectangular concrete-filled tubes in similar fashion to reinforced concrete for assessing their allowable drift, the design team conducted additional analyses with respect to the anticipated drift. They found that that because the concrete caused the column to become heavier, it minimizing uplift tensions. "After completing analyses of the tower with both all-steel columns and RCFTs, we were able to show that the behavior of the structure …was essentially identical, and received approval from the authorities to use a maximum height/250 drift limit," Fenske explained.
The engineering team also had to accommodate the angular face of the south tower, which will contain three slope reversals. This will create four different planes in the massing of the tower, according to Fenske. "The slope reversals in the massing cause additional horizontal forces that needed to be resisted by the tower's lateral system," he noted. "Depending on the geometry of slope changes, these forces can be significant and even greater than the wind or seismic forces." The engineers studied a variety of slope angles and reversals to find a solution that both met the architectural requirements and kept the horizontal forces to a manageable magnitude.
In addition to the two main towers, a six-story folded-plane structure will be located adjacent to the south tower's base and will contain retail and restaurant spaces. Construction of the tower is expected to be complete by 2016.
Quotes:
Upon its completion, the 65-story dual-tower structure will be the tallest steel braced-frame tower in China. Sky bridges and horizontal steel braces will connect the office tower with the circulation tower so that they behave as one structure under lateral loading.
The 350 m tall Hanking Center Tower that is being built in Shenzhen, China, has been structurally divided into two along its vertical axis. While one side houses office space, the other contains the tower's circulation core. Courtesy of Morphosis
The folded planes of the main tower's south facade and of the six-story podium at its base hint at the structural complexities contained within the building. © Luxigon
References:
1. www.morphopedia.com
2. American society of Civil engineers. To read more: www.asce.org