Suspending the Limits

A cast steel rocker mechanism acts as a pulley, allowing the relative lateral drift of the upper half of the main building to be distributed through the upper portion of the cable net wall. A full-scale model of the rocker mechanism was constructed to confirm its efficacy. photo credit: Tim Gritth, all

A new corporate headquarters building in Beijing takes the concept of cable net glass walls to its limit. But the real surprise is the building within the building, suspended in midair. By Mark Sarkisian, P.E., S.E., M.ASCE, Neville Mathias, P.E., S.E., and Aaron Mazeika, P.E.

Prominently located in Beijing northeast of the Forbidden City at a major intersection along the capital’s busiest ring road, the 110 m tall New Beijing Poly Plaza is a unique structure that includes 24 stories of office space, a 90 m tall atrium enclosed by what is believed to be the world’s largest cable-net-supported glass wall, and an 8-story museum—referred to as, in translation, the lantern—that literally hangs from the main structure.

In commissioning a new headquarters building, the structure’s owner—China Poly Group Corporation, which is involved in importing and exporting, real estate, and cultural and theatrical productions—expressed a desire for a structure that would represent its disparate subsidiaries as a unified whole. The instructions for the building called for a wide range of spaces, including offices, retail establishments, restaurants, and a museum (the Poly Museum). The museum, established by one of the company’s subsidiaries, has as its goal the repatriation of China’s cultural antiquities through purchases at international auctions.

Based on its previous work in China as well as on client referrals, the San Francisco–based architecture and engineering firm Skidmore, Owings & Merrill LLP (SOM) was chosen to design the $180-million structure.

The project is located on the southwest corner of a major intersection along one of the city’s ring roads, in this case a major highway constructed in the 1980s and expanded in the 1990s that forms a rectangular loop around central Beijing. The site’s primary orientation is northeast, facing the intersection and the client’s existing headquarters building. The structure assumes the shape of a triangle in plan; the main building is shaped like an L, and the glass-enclosed main atrium forms the triangle’s third side. (See the plans on pages 40 and 41.) This design reduces the extent to which the perimeter is exposed to the elements. A series of smaller interior atria maximize access to daylight from the office spaces.

The Poly Museum is located in the lantern, which is suspended from the structure within the main atrium space. Its crystalline surface of laminated, patterned glass is pleated to increase its light reflection and refraction characteristics. Within the lantern, exhibition and leased spaces are enclosed by wooden walls that control the admission of daylight, while common circulation areas occupy the voids between these wood-paneled areas and the glazed perimeter walls.

Secondary atria cut through the legs of the L, or wings, which form the south and west sides of the headquarters. These smaller atria admit sunlight into the main atrium at various times throughout the day. The exterior walls of these atria are composed of minimal membranes supported by two-way cable nets in order to maximize their visual and solar transparency. The main atrium’s cable net is stiffened by two V-shaped cables that in turn are counterweighted and kept in tension by the weight of the suspended lantern.

Each leg is 76.5 m long and 22.5 m wide, the  width being divided into two segments, one 9 m wide and the other 13.5 m wide. The floor framing system above grade consists of structural steel beams spaced 3 m apart along the 9 m segments and structural steel trusses along the 13.5 m segments. These steel trusses act compositely with metal deck slabs and lightweight concrete fill. Since mechanical, electrical, and plumbing conduits were to be located within the bays inside the 13.5 m wide segments, this arrangement created the most efficient floor systems, and the truss configurations made it possible for the ducts and other services to be located within the depth of the floor trusses. On the basis of this approach, the typical story height was minimized to 4 m.

The building is underlain by a rectangular four-story basement whose lowest slab is approximately 20 m below grade. Gravity framing in the basement takes the form of a conventional concrete beam and slab design. The structure is supported by a raft foundation, and where required, tie-down anchors are used to prevent hydrostatic uplift.

Two areas of the building’s structure required special engineering consideration. The first was the upper half of the south wing. To expose the atrium to direct sunlight from the south, steel columns in this wing do not continue below the 11th floor. The upper stories therefore form a “bridge” spanning between the cores and columns at the southeast and southwest corners of the building. To prevent brace elements from interfering with what otherwise would be unrestricted views from the office floors, the bridge structure is supported by structural steel Vierendeel trusses extending the entire height from level 11 to level 24. The bridge structure also forms part of the lateral-force-resisting system, acting together with the columns at each end as a “megaframe.” The bridge and floor slab diaphragms tie the three cores of the edifice together to form a monolithic structure, and the design was modeled and analyzed as such. Lightweight concrete fill was typically used on the metal deck floor slabs, but critical connecting diaphragms used a thickened slab made of concrete of normal weight.

The second area requiring special engineering treatment was the lantern, which protrudes from the southeast core into the main atrium. Its eight-story-tall cross-braced steel frame cantilevers 24 m from the building core beginning at level 2. There are no columns beneath the lantern. The tip of the cantilevered braced frame is effectively suspended by its connection to the same primary diagonal cables that simultaneously stiffen the cable net wall. The gravity-load-bearing elements supporting the weight of the lantern are the southeast building core together with the primary diagonal cables, which transfer those gravity loads back to the cores at the top of the building. To provide a redundant gravity load path in the event of damage to the primary cables, the braced frame of the lantern was designed to achieve a life-safety performance level when cantilevered from the shear wall core without the load-supporting benefit of the primary diagonal cables. Lateral forces in the lantern are resisted by the shear wall core at the southeast corner, which acts as a torsion box. The shear wall core is torsionally restrained by the ground-floor slab at level 1 and by its connection to the main building through concrete slab diaphragms located at level 11 and above. The lantern floor diaphragms transfer the lateral force to the core at each level.

Since Beijing is considered to be in a region of moderate to high seismicity, construction there is subject to the provisions of the Chinese code governing the seismic design of buildings (GB50011). That code requires that building structures that exceed the intended scope of the standard provisions undergo a special seismic review by a panel of Chinese structural experts. Structures that may require a special seismic review include those that exceed the height limits prescribed for each type of structural system or that have significant discontinuities in plan or elevation, for example, structures with uneven distributions of mass in plan or structures with discontinuous vertical structural systems. Reviews may also be required for “connected structures,” that is, independent buildings tied together above grade. Because of its unusual design, the New Beijing Poly Plaza project underwent a special seismic review.

To satisfy the review committee, an extensive additional analysis was performed to study the linear and nonlinear responses of the superstructure. Three-dimensional dynamic nonlinear analyses were performed independently by som and by the local associate design engineers, the Beijing Special Engineering Design and Research Institute, and the results confirmed an acceptable building response to a “rare” earthquake—one that had a 2 to 3 percent probability of being exceeded within a 50-year period.

Moreover, a 1:20 scale concrete and steel model of the entire building structure was constructed and subjected to a shake table test in Beijing at the China Institute of Water Resources and Hydropower Research to determine if any weak points in the structure could be discovered. The model included concrete formed elements for the shear walls and concrete-filled metal deck slabs. All openings in the walls were modeled as such. The structural steel columns and beams, including columns embedded in the concrete shear walls, were modeled using scaled steel H sections. The primary V cables and rocker mechanisms—pulleylike structures that support the cables—also were included in the model. While the test is in many ways subjective, the structure successfully passed the shake table test at the prescribed intensity. An additional test to failure was attempted but abandoned because the strength of the model exceeded the capacity of the testing equipment. Final seismic approval was obtained after increasing the design strength of certain critical-load-path and connecting elements of the structure to levels higher than normally required by the code.

While conceptually simple because of their minimal structure and large in-service deflections, cable net curtain wall systems may nevertheless be regarded as an exotic solution for the support of glass walls. The completion of several major walls around the world, however, has established a track record with respect to achievable scale and level of transparency. Planar two-way cable systems support and stabilize glass facades through the resistance to deformation of the two-way-pretensioned net. Gravity loads from the glass elements are carried through the attachment nodes to the vertical cables and in this way are taken up to a transfer structure in the main building above. Lateral deformations caused by wind and seismic loadings are resisted by the catenary action of the cables, that is, the tendency of each of the horizontal and vertical cables to return to its straight-line configuration between supports when subjected to an out-of-plane force. The flexible nature of a planar cable net under lateral loading means that the critical design goal typically is limiting deflection to a reasonable level by correctly selecting the axial stiffness and pretension of the cable elements.

In-service deflection limits in response to a wind loading condition with a 50-year return period are typically set in the range of L/40 to L/50, L corresponding to the shortest span between supports passing through the considered point. This will serve to protect the integrity of the glass and sealant and to minimize any perception of movement on the part of the building’s occupants. Often the controlling design criterion is the latter issue, occupant perception. A well-designed and properly detailed cable net wall system may be able to sustain the design-level wind condition and more without structural or sealant failure, but if the occupants of the building are uneasy in the vicinity of the cable net wall, the project cannot be deemed a complete success.

Within the past dozen years, several significant cable net wall systems have been installed around the world. Significant milestones in the development of wall systems supported by cable nets include the Hotel Vier Jahreszeiten Kempinski München, in Munich, Germany, and Chicago’s UBS Tower. The Kempinski hotel, completed in 1993, is generally considered to be the first application of cable net facade technology. Designed by the Chicago-based architect Murphy/Jahn, Inc., and engineered by Schlaich, Bergermann und Partner, of Stuttgart, Germany, the Kempinski hotel’s cable net wall is 40 m wide and 25 m tall and was designed with the principal cable pretensioning installed in the longer-span (horizontal) direction. The 22 mm diameter cables are spaced 1.5 m on center and are pretensioned to limit deflections to 900 mm.

The UBS Tower—completed in 2001 and believed to mark the introduction of this technology into the United States—utilizes glass walls supported by cable nets as the lobby enclosure at the base of the tower. The structure was designed by Lohan Caprile Goettsch Architects (now known as Goettsch Partners, with offices in Shanghai and Chicago) in collaboration with Advanced Structures, Inc., of Culver City, California. What appears to be a single cable net structure actually comprises seven smaller, similar nets constructed next to one another and divided by the tower columns. The segments are each 12.2 m tall and have widths reaching 13.7 m.

In comparison, the New Beijing Poly Plaza project includes a 21-story atrium enclosed by a cable net glass wall that is 90 m high and 60 m wide. The scale of this wall greatly exceeds anything built before, and it introduced unique challenges that are not faced in smaller walls. som’s preliminary analysis showed that the cable net spans were too large to be economically achieved using a simple two-way cable net design. However, the engineers determined that the solution could be achieved by subdividing the large area into three smaller zones by folding the cable net into a faceted surface and introducing a relatively stiff element along the fold lines. The faceted cable net solution allows the individual sections of the cable net to create a virtual boundary condition at the fold line, effectively shortening the spans. In place of a major beam or truss element to stiffen the fold line, a large-diameter cable under significant pretension is used.

The cable net wall system was designed to meet a span-to-deflection ratio limit of 45 when subjected to the service-level wind load condition (the 50-year wind event). The cables were designed to meet the requirements of ASCE 19-96 (Structural Applications of Steel Cables for Buildings). The design strength load factors given in this standard were increased from 2.0 and 2.2 (depending on load condition) to 2.5 to meet the additional requirements set by the committee of Chinese structural engineering experts that reviewed the project’s design. As a supplement to the increased load factors, the cable design forces were based on the internal forces resulting from a higher-level wind condition (a 100-year wind event).

A cast steel rocker mechanism acts as a pulley, allowing the relative lateral drift of the upper half of the main building to be distributed through the upper portion of the cable net wall. A full-scale model of the rocker mechanism was constructed to confirm its efficacy.

Cable net systems impart significant forces to the base building structure at their boundary conditions. This is a result of the significant pretension loads that are required in the cables to provide sufficient out-of-plane stiffness. Therefore the design of cable net wall systems must incorporate appropriate consideration of the design of these boundary conditions. Stiff, strong boundary conditions were provided at each side of the cable net wall as a result of the fact that the building cores are located at each of the three corners of the floor plan. A stiff boundary condition at the top of the cable net wall was provided by the use of a three-story-deep structural steel truss that spans 60 m between the building cores. Constructed at grade and placed in position between the two building cores, the three-story truss was jacked 90 m upward into position in a single 500 metric ton lift.

The largest of the four primary cables measures 275 mm in diameter and consists of 199 parallel strands measuring 15.2 mm in diameter. These are 1×7 strands, meaning they are each formed by twisting six wires around one central wire. The largest cable is pretensioned to 17,000 kN and experiences a maximum service loading of 18,300 kN during the 100-year wind event. Because of the faceted design solution, the size of the typical horizontal and vertical cables could be limited to diameters of respectively 34 mm and 26 mm. The horizontal cables are pretensioned to 210 kN and the vertical cables to 100 kN. The horizontal and vertical cables are spaced respectively 1,333 mm and 1,375 mm apart on center. Laminated glass panes made up of two 6 mm thick layers connected to a polyvinyl butyral inner layer are used to prevent glass from falling from the facade in the event of damage to any of the glass panels.

The four primary diagonal cables that support the weight of the lantern connect from the roof of the lantern at level 10 to the top of the atrium at level 22. Because the main building structure will drift under anticipated seismic loads, the cables will act as braces and attempt to resist this drift unless the force levels in the cables are limited in some manner. Designing the primary diagonal cables to resist these brace forces while maintaining an appropriate factor of safety would have significantly increased the primary diagonal cable sizes employed in the final design. This would also—to accommodate the additional brace demands—have resulted in the initial level of pretensioning in the primary diagonal cables representing a lower portion of the cable’s breaking strength. Pretensioned cable systems typically rely on a high initial level of pretension to maintain the desired architectural form in the permanent load condition. When cable systems are installed with only a nominal level of initial pretension, the tendency of that system to exhibit significant deflections in response to the weight of the cables is greatly increased. Therefore, it was determined that the design solution required that the primary diagonal cables (the only cables acting as braces) be decoupled from the lateral system of the base building structure.

This requirement complicated the design of the connection between the primary diagonal cables and the roof, as did the need to simultaneously provide a flexible wall system that would allow relative lateral movements between the roof of the lantern and the roof of the main building above the cable net. Several connection concepts were evaluated before the final solution was determined. One option connected the main cables to the lantern roof through a sliding connection. This solution was difficult to achieve because of the load path of the very large primary cable forces through the eccentric connection created when the connection was displaced. It also resulted in the upper half of the cable net moving with the roof of the building and in one course of glass at the roof of the lantern being required to accommodate the full drift between the roof of the building and the roof of the lantern. The result of som’s analysis of this solution revealed that this course of glass would probably fail given any significant lateral displacement of the building, creating a safety hazard in the atrium and street below.

A second concept connected the bottom of the V cables to the top of the lantern through a 4 m tall pinned link element. This solved the load eccentricity issue but still resulted in the relative lateral drift of the upper half of the cable net being concentrated in a small portion of the wall. This solution also induced additional tension in the main cables as the building drifts in response to the downward movement of the lowest point of the cables caused by the rotation of the link around its base. Furthermore, the concentration of a significant portion of the lateral drift of the building in a 4 m high zone resulted in a high likelihood that glass panels would be lost during the design-level lateral drift event, representing an unacceptable risk to the occupants of the building and adjacent outdoor spaces.

The solution that the engineers devised for the decoupling mechanism consists of the equivalent of a pulley at the lower point of the V cables. As the overall building drifts, half of the V tries to lengthen and the other half tries to shorten. Connected by means of a pulley or equivalent mechanism, these strains are able to offset each other without inducing additional load in the cables. A cast steel rocker mechanism was designed to perform the equivalent function of the pulley. By crossing the cables and connecting them to the rocker casting arms, the need to provide curved pulley surfaces and curved sections of the main cable was eliminated. (See the photograph on page 43.) The rocker mechanism allows the load path at the connection to be concentric and also allows the relative lateral drift of the upper half of the building to be distributed through the upper portion of the cable net wall. Small relative movements between adjacent nodes on the main diagonal cables and the cable net cables are accommodated using pinned tie-rod connections.

To evaluate the effectiveness of the design before completing a comprehensive analysis of the system, a physical model of the rocker mechanism was constructed, along with a model of the link concept for comparison. The models were installed in a pin-connected frame, with soft springs installed in series with the diagonal cables. By rocking the frame backward and forward, the relative effectiveness of the two concepts could be visually evaluated. The physical model test demonstrated significant extension in the springs using the link model and negligible extension in the springs using the rocker mechanism model, highlighting the ability of this connection to decouple the main cables from the base building lateral system.

The final design of the rocker mechanism included five large castings per connection. The main cable clevis castings are approximately 4 m in length. The clevis castings are designed to pass through one another to maintain concentric load paths through the connection.

In March 2005 the three-story truss was lifted to the top of the cable net wall, and the installation of the cable net itself then began. The first process was the installation of the rocker mechanisms and the diagonal, primary V cables. As is common in cable-stayed bridge construction, the installation of the parallel strand cables required a detailed sequence for tensioning the individual strands to ensure that the 199 individual strands equally shared the pretension load. After the main cables were installed, the smaller cables in the net were loosely installed and connected to the main cables through pin-ended connecting rods. All the final cable net tensioning was performed by pulling the horizontal and vertical cable ends from within dedicated tensioning spaces provided in the cores at each side of the cable net wall.

The 110 m tall structure is intended to visually unify the disparate interests of its owner, China Poly Group Corporation, which is involved in importing and exporting, real estate, and cultural and theatrical productions. The museum within the structure has as its goal the repatriation of China’s cultural antiquities through purchases at international auctions.

At 90 m tall and 60 m wide, the cable net system in the New Beijing Poly Plaza represents a significant step forward. It is believed to be approximately four times larger than any cable net wall system built previously. The engineering challenges of a cable net wall of this scale required creative approaches to solving issues that in all likelihood were not encountered in smaller projects. The rocker mechanism provides an example of an innovative design that employs conventional technologies to solve a truly unconventional problem.

The architecture and engineering design team quickly recognized the rocker mechanisms as the central components of a machine that serves as the structural system of the main atrium. Serving as true keystones through which the lantern and the cable net support each other in a symbiotic relationship, the rocker mechanisms are celebrated accordingly. Prominently located in the center of the atrium picture window, they are approachable from the lantern rooftop café and have become a focal point in the occupants’ experience of this building.  

Mark Sarkisian, P.E., S.E., M.ASCE, is the partner in charge of structural engineering at the San Francisco office of Skidmore, Owings & Merrill LLP. Neville Mathias, P.E., S.E., is an associate director, and Aaron Mazeika, P.E., an associate in that office. Mathias served as project supervisor and Mazeika as project engineer on the New Beijing Poly Plaza project.

Project Credits
Client: China Poly Group Corporation, Beijing
Architect: Skidmore, Owings & Merrill LLP, San Francisco
Structural engineer: Skidmore, Owings & Merrill LLP, San Francisco
Local associate architect/engineer (for the base building): Beijing Special Engineering Design and Research Institute, Beijing
Wind engineer: Shifu Gu, Mechanics and Engineering Science Department, Peking University, Beijing
Cable net contractor: Yuanda China, Shenyang, China, and Advanced Structures, Inc., Culver City, California