ese e Eight residential towers and one hotel structure interconnected by elevated, public bridges constitute the Linked Hybrid mixed-use project in Beijing. This open city within a city also features a cinematheque, a kindergarten, parks, and other attractions. The structural and architectural features of this 220,000 m2 pedestrian-oriented development range from the practical to the whimsical and in many ways exemplify the various challenges and rewards that face designers working on major projects in China. • •• • • By Guy Nordenson, P.E., s.E., F.ASCE, and Erik Anders Nelson, P.E., s.E.

WORKING IN CHINA CAN BE A CHAlLENGE FOR ten went to cronies as u•ell as to e11trept-enetm. lu China, as engineers and architects, in part because ofthe contradictiom in other developing economies, valuable land or industries intrinsic in modern Chinese culture--co11tradictions that sometimes went to those with connections rather than to those were cleady appa·rent during the emergenee ofptvgressive ar­ possessing merit. As a result, there was a very uneven sense of chitecture there in the 1990s and the first decade ofthis centu­ the value ofcapital and natural resources. Wbile many engi­ ry. 'There has alu •ays been a strong design and craft cttlture in neers, physiciam, and other professi01zals may have c01zcenz China because ofthe country's historical ambivalence toward for the management ofland, resources, and public welfare, fine art in the Westem sense a11d because ofthe dominant role those to whom opportunities at-e simply given are often will- ofengineers in Communist society. In- CONCEPT SKETCH OF ing to spmdfreely and without much regard. deed, matty of China's cttrrent leaders THE LINKED HYBRID 'This particular combination offactors-professional are engi11eers who often embody the val- CO MPLEX'S 'FILMIC progressivism together u•ith ctwzy capitalism--can create a ues offmgality, resourcefulness, ingenu- URBAN SPACE' unique challenge for both Western and Chinese designers. In ity, and daring in the tradi­ China, it has been possible to design and con­ tion ofengineering. struct buildings that would be inconceivable else­ For the older gmet'tltion where, except perhaps in the Persian Gulf. M01·e­ of Chinese engi11eers, mrmy over, the expense ofthese spectacular projects, of whom lived and sttffet·ed especially 'll'ith respect to Stich resources as stee4 through the upheavals of the concrete, aluminttm1 glass, and other materials, Cultural Revolution and has been extrat•agrmt by any measure. 'These were at the vcmguard ofDeng material costs are offset by the ve1y low wages of XU/oping's tramformati01z of the migt-ant UJOrkers who mostly originate from the Chinese ec01zomy, the in­ the countryside ofChina and often live in tem­ herent contradicti011s ofany poraty dormitories at or near the constmctio11 absolute rationalism are quite clear. Yet they site until the project's completi01z. have embraced a progressive disposition to­ Indeed the most disturbing and tl'ard contemporary engineering and archi­ challenging fttct()l; despite the tecture, although tempe1·ed by a continui11g nominal Communism in Chi­ feeling for frugality. 'Their progressive atti­ ua, is the economic exploitation tude is inj/ttential became ofthe conttvl that oflow-wage migrant UJOrkers to 1nany of them-particularly tbe universi­ the benefit oftho se given the busi­ ty professors-have Otler the building per­ tress oppm-trmities. mit approval process. 'This opem the u•ay for Many Chinese engineers aud mm1y radical projects. arr:hitects recognize the complex­ Facilitated by the call first mark in the ities of this situation and con­ CONCEPT SKETCH OF 1960s by Zhou Enlai (Chou En-lai) for modernization front the difficult social mtd moral dilemmas THE COMPLEX'S MAIN in the four ctreas ofagriculture, i1tdustry, national (/e­ PASSAGE ROUTES at stake. For the most part this is reflected in fmse, and science and technology and stressed again in debates and criticaltt•ritings, but tbese mis­ the late 1970s by Deng Xiaoping, this progressivism was cou­ givings have been unable to slow the rapid pace ofdevelop ­ pledwith market liberalism. As was the case with the neolib­ ment. Because many designers express their reservatians ·with eralism that took root in many pmts ofthe world beginning courage and h01zesty, it t-emaim interesting to UJOrk in China, in the early 1980s, the opportunities for profit and wealth of as opposed to other t-egi011s ofexcess and extreme developtnent. I

[ SZ ] Civ il Eng ineering MARCH ZOll 0885-7024103·001 1·0052/$2!5.00 PER ARTICLE

HE UNKED HYBRID COMPLEX, in Beijing, is the gound garage that occupies the entire footprint of the site. The largest of four structural engineering projects that roof of the parking garage supportS both the cinematheque and Guy Nordenson and Associates has undertaken in a large reflecting pond in the center courtyard of the complex. China. It is also a project on which our firm worked Steven Holl has described the project as follows: "The on the design from its inception, in 2003, collaborating close­ 220,000 m2 Linked Hybrid complex, in Beijing, creates a po­ ly with Steven Holl Architects, which has offices in New York rous urban space, inviting and open ro the public from every City and Beijing, and with our collaborators in Beijing at the side. As a 'city within a city' the new place has a filmic urban China Academy of Building Research (CABR). The Linked experience of space-around, over, and through multifaceted Hybrid is a great example of the progressive, imaginative, and spatial layers. A three-dimensional public urban space, the daring architecture that has emerged in China recently. The project has programs that vary from commercial, residential, complex consists of 20 unique structures, including 8 resi­ and educational to recreational." dential towers (containing 644 apartments), a hotel, a cinema­ The key to this concept is the relationship between the theque, a kindergarten, and an underground parking garage. "courtyard" formed by the buildings-an allusion to that The 35m tall hotel and the eight approximately 60 m rail of the siheyuan, or quadrangle house type, common in Bei­ rowers are connected at their upper levels by a series of eight jing-and the openness of the building complex to the city bridges. These bridges are public spaces in which residents and around ir. Unlike many gated communities that have been visitors can walk and enjoy various pastimes and activities, and developed in Beijing, the Linked Hybrid complex is generally they accommoclate a care, a fitness room, a gallery, an auditori­ open ro the public. The bridges create both a landmark ~ um, and other functional spaces. One ofthe largest bridges even and an inextricable visual connection between the i contains a swimming pool. All of the residential towers and complex and the surrounding city. This balance of ; the other buildings can be accessed via the two-level under- community and citizenship is what makes this Unlike the many communi­ range in height from 2 stories to 20. The devel­ project unique among the many characterless ties in Beijing that are gated, oper, Beijing-based Modern Green Develop­ developments occurring in China. the Linked Hybrid complex ment Company, Ltd. (formerly Modern Group), The shape of the space berween the build­ is generally open to the has built several housing complexes in the im­ ings and the circulation loops provided by the public. The walk along the mediate area. It is among the major developers bridges were conceived before the buildings complete bridge circuit pro· and patrons of modern architectural develop­ and the bridges evolved as objects. The cinema­ vides an exhilarating and ment in Beijing. theque, the hotel, and the kindergarten were in- unique promenade in the air. The tower structures, which feature large eluded in the design after the central space, the cantilevered sections at their upper levels, were rowers, and the bridges had been developed as a composition. designed to be as simple and robust as possible. Each build­ The positioning of the eight main towers and the circuit of ing measures 30 by 30 m in plan and is typically divided into bridges that link them was skillfully calibrated to shape the four residential units. These typical plans change at the upper central shared space as well as the views to the city beyond. levels at the points where the bridges link to the towers. The The walk along the complete bridge circuit provides an ex­ bridges are located between the 12th and 18th floors of the hilarating and unique promenade in the air. towers. The cores, which house the elevators, the stairs, and The pedestrian-oriented complex is located adjacent to a the mechanical services, are all compact and enclosed in cast­ former city wall of Beijing, a choice site at the northeast cor­ in-place concrete shear walls. Four shear walls, cruciform in ner of the city's Second Ring Road where the highway to the plan, radiate from the central core to form the "party" shear airport enters the city. Similar to other parts ofBei­ walls for each unit, breaking the total plan into four quad­ jing, this is a dense urban area with buildings that rants. The floor systems feature cast-in-place beamless slabs with one large distinctive column in each unit to TRUSS cut the span from the core wall to the perimeter. CONFIGURATION The 900 mm deep perimeter moment-resisting frame consists of beams and columns of cast-in­ place concrete. The corners are open and free of columns, but the grid frame wraps continuously around to form a complete tube frame. The structural diagram is clear and easily adapt­ ed to numerous apartment layouts. The lateral­ force-resisting system comprises concrete shear walls, concrete moment-resisting frames, and concrete-en­ state. This would ensure that the structure would be repair­ cased diagonal steel bracing on the perimeter. This approach able after a moderate earthquake. The seismic action would be formed a redundant and very stiff lateral system that proved equivalent to the ground acceleration of the design intensity. to be of paramount importance in designing the bridges. It Furthermore, during this phase our collaborators at the CABR was also essential in winning the approval of the local build­ checked for plastic deformation in an extreme earthquake ing authorities for the radical concept of floating bridges and event to ensure that the structure would not collapse and that cantilevered rowers that in elevation form inverted L shapes. human casualties could be avoided. The seismic action was as­ The lateral-force-resisting system was designed to accom­ sumed to be 0.4g for this check and served to define the maxi­ modate China's national standards and codes. Following these mum displacements the building might experience. provisions, the wind load was calculated in a way similar to the The requirements of the seismic code considered, among procedure given in ASCE-7 (Minimum Design Loads fw Build­ other factors, the purpose of the buildings and their impor­ ings and Other Structures), although wind pressure, rather than tance. In this case, the rowers were designed ro remain elastic wind speed, was mapped. The Chinese codes were formulated tmder a moderate earthquake of0 .2g. This goal was used for the on the basis of a pressure from a 100-year event amplified by derailing reqLlirements for certain key members, including the the appropriate shape factor, ter­ perimeter diagonals and the elements that link the shear walls. rain roughness, and height. In For a rypical20-story tower, we found fundamental peri- addition to static amplification, ods in the range of0.7 co 1.1 seconds. The base dynamic analysis was required shear coefficients were determined to be 0.07g to calculate the wind vibration ro O.lOg for the design. Because the Chinese amplifier (which is a function codes require that 50 percent of a of the fundamental period). As building's live load be considered a result, the final wind pres­ in the seismic mass, the base shear, sures varied from 1.8 kN/m 2 at expressed as a percentage of the the bases of the towers to building mass, was on the order 4.1 kN/ m2 at their tops. of 20,000 kN, depending on For seismic designs, the the rower. Moreover, because Chi nese code refers to two phases the buildings had vertical and of analysis. The first phase involves de­ horizontal seismic irregulari­ signing within the elastic deformation ties and included che unusu- range for frequent minor earthquakes. al linkage features of the sky "t The goal of the first phase is to design 3 bridges, our design had to be ~ the structure so that there is no dam­ approved by a special structur- ~ age to the structural members during al engineering committee. ; a minor seismic event and so that the non­ O n the basis of static and ~ structural members also will avoid major dam­ J dynamic seismic analyses, ~ age. During this phase, the design team assumed which included both equivalent ; that any deformation would occur elastically and that static and response spectrum analyses, ~ the seismic action would be set equivalent to a ground ac­ we were able co determine the deflections of : 0: celeration of0.072g. the towers at the support points of the bridges. ~ During the second phase, the design team con­ THE GEOTHERMAL The Chinese code limits the maximum story de- E sidered elastoplastic deformations under a par­ HEATING AND flection to U800, which is approximately 4 mm ~ ticular design "intensiry"-in this case 0.2g- as when the floor heights are 3.05 m. Fortunately, .,~ COOLING SYSTEM ~ well as under extreme earthquakes. The goal was our design was well within this limit, averaging ~ to meet the code's criteria so that the structure would be re­ U 1,400. Under linear elastic analysis, the rower deflections i pairable if subjected to earthquakes of this greater intensity. were generally very small- less chan 4 em at the location of the ~ During this phase, the design team assumed that elasroplas­ bridge supports, which are roughly 50 ro 55 m above grade. ~ tic deformation would occur but that the key structural mem­ The deflections were still well within the code limits even ~ bers would continue to function elastically within the limit when the design iAcluded amplification arising from the ~

[ 56] Civil Engineering MARCH 2011 nonlinear and ductile behavior ofthe tower struc­ Steel braces were also installed in lo­ ture. The CABR completed a rime cations with large openings or where history analysis under the extreme the particular seismic forces necessi­ earthquake scenario of 0. 4g to better tated their use. These diagonal braces determine the minimum allowances fearure wide-flange steel members en­ for the bridge movements. cased in concrete and clad in alumi­ A major concern in de­ num. Vertical steel members also were signing the later­ embedded in some of the grid col­ al-force-resisting umns to resist rhe large system was the tension caused by rhe mass eccentricity cantilevers and the seis­ caused by both the mic effects. tower cantilevers and Much like the over­ the bridges. The design team all project, the strucrural had difficulty, however, in meet­ design of the Linked Hy­ ing the code requirement that the brid complex contrasts the maximum deflection on a particu­ regularity of the basic system lar floor not exceed the average de­ with the uniqueness of the ex­ flection by 40 percent or more. In order to ception. The balance of redundan­ meet this deflection criterion, which is in­ cy--core plus walls plus perimeter tended solely to minimize seismically in­ grid-and adaptability, that is, the use duced torsion, we had to strategically place of lighter steel cantilevers and the occa­ bracing members on the perimeters of the sional steel diagonal and vertical members, towers. In addition to the resistance provided enabled us to receive official approval from the by the concrete m oment-resisting frames on Chinese authorities for both the concept and the the tower exteriors, the diagonal steel bracing pro- detailed design quickly and smoothly. This contrast­ ;: vides stiffness to resist the'large torsional demands of ed with the difficulties that the designers of many ~ the cantilevered upper levels. STRUCTURE other challenging projects encountered in obtaining ~ As described below, the effects of the bridges ELEVATION their approvals during the same period, which was ~ were eliminated by placing both ends of the bridges just prior to the 2008 Summer Olympics. ':~ on isolators, which reduced to manageable levels the loads The eight bridges of the Linked Hybrid project, which were ~ rhar rhe bridges impose on the structure. But the cantilevers never part of the client's original program, constirute its most < ~ of the towers could not be isolated. Thus, their effects were radical aspect. Their presence in the final design arrests to the :;" mitigated by using a combination ofsteel framing with met- tenacity ofSteven Hall's vision and to rhe client's openness dur­ a :5 al decks and concrete fill to reduce the weight of the canti- z ing the design process. The bridges not only link the rowers but ; levered sections and by adding steel braces to rhe perimeter also incorporate spaces that can be shared by the residents, rein­ 0 o grid frame to carry the cantilever shear forces. forcing the sense ofcommunity, and they provide visual links ro the city at large. The bridges' functions are public and alternate between the practical and the whimsical, the spaces including a lap pool, a tea seating area, and a group exercise space. The glazed, transparent bridges span between 20 and 60 m, creating floating hallways oflight that contrast sharply with the heavy concrete of the towers. Designed as irregular and angular fo rms, the bridges are composed of steel units that were designed as pairs of parallel trusses-a hybrid of Vierendeel and Pratt trusses-in order to achieve the maxi­ mum transparency. The decision to use parallel truss chords was not easily made given the architect's original intent to install large steel truss m embers within the bridges. But the parallel truss chords resulted in an elegant design solution. The top and bottom chords of the trusses are composed of wide-flange steel shapes. Infill wide-flange beams sup­ port the metal deck and concrete floor system. The vertical

The structure's lateral-force-resisting system comprises concrete shear walls, concrete moment-resisting frames, and concrete-encased diagonal steel bracing along the perimeter. This solution is robust and redundant and is critical to the support of the high-level bridges.

MARCH 2011 Civi l Engineering [ 57 ] members consist of square built-up members, while the di­ the suspended pool will begin ro feel the earthquake motion agonal members have solid, cylindrical shapes. only well after the ground motion has ceased. To increase the transparency of the trusses, the geometry was Although the cinematheque is overshadowed by the row­ refined by varying the spacing of the vertical members so that ers that loom above it, this irregularly shaped structure at the the greatest number of them--as well as rl1e largest of the di­ center of the Linked H ybrid complex is a daring building in agonals--would be concentrated near the supports. Thus, we its own right, and its design proved ro be just as challeng­ were able co "nme" the bridge so that the tension in the diago­ ing as that of the rowers. The approximately 446 m 2 public nal profile was fixed. That is, the size of the tension rod bracing building's face ted design features a cinema and a vegetated members could remain the same throughout and remain as ef­ roof above the third floor. Especially critical was determining ficient as an equally spaced--and more typical-Pratt cruss. how to fit the cinematheque, which has no vertical colunms, Finally the hybrid trUSses were all designed with a so-called on top of the underground parking garage, which features a Vierendeel backbone that moment connected the vertical and regularly spaced column grid. Li Hu, the Steven Holl Archi­ horizontal members to one another, leaving the center one or tects partner in charge of the project, and his team consid­ two panels without diagonals. This backbone is also useful as an ered many iterations of the design before finding a solution added measure of redundancy. In effect, the backbone can sup­ whereby the structure of the cinematheque could be joined port the bridges in extremis, even though the diagonal mem­ to the regular column grid below without the use of trans­ bers are necessary for stiffness. fer girders. The structure comprises a series of Although the orientation of the diagonal tension ~--... - Design store diagonals composed of members in the Pratt tmsses minimizes the size of these F~~:;;:;::::::::-"7"o round hollow steel elements, large connections were required between the gusset plates and the nodes of the columns or beams. Viewing platform The bridges are Light and delicate. To protect them _ _, and art gallery from the effects of tower deflections at the upper floors un­ PE DESTRIAN '-....~ der an extreme seismic event, Xiao Congzhen, our brilliant CIRCULATI ON collaborator and a CABR structural engineer, suggested < DIAGRAM that we isolate them entirely. Initially we had imagined '-....y adding rollers, elastomeric bearings, or friction pendulum bearings to one side, leaving the other side fixed. But this fix­ ity at one end of the bridge would have translated into large torsional forces in the towers because of the eccentricity of the bridge mass. When Xiao suggested that we simply isolate the bn·dghes odn bodth sides hsoedthbey whould be ealntbirely fr~, we were_.. J.·. .···.. . >:: de1 tg te an as toms y t e practlC ut poeuc sug- . . · gestion. Isolating the bridges from the towers not only re- f"· ····1--····· duces the forces on the rowers but also protects the light ! ~ Public roof garden : F bridges from the movement of the towers. ! ~ Together with the CABR's engineers, we i ~ '~., . .) Escaljlt~~ _. ...- decided co use friction pendulum isola- :! • i~ •• •I tors, which were provided by Earth- ··.t I I I quake Protection Systems, Inc., : ·· .. I I I I ofVallejo, California. The iso- :::~;;;~· ;;;~~!;;: I I lators are shaped with a radi- ,.- I I I I us that can be customized I to meet the requirements ,I of the project. Friction pendulum isolators are unique in that they can be designed to a particular period by simply altering the radius of the isola­ cor, independent of the mass and the stiffness of the bridge. The bridges are perhaps the ---- Bridge level access highest base-isolated structures in and circulation a seism ic region anywhere in the ----- Access from B1 parking lot world, an achievement made pos­ -- Ground level circulation sible by the stiffness of the rower - Commercial stmctures. As a result of this isola­ lobby to apartment - Access lobby to bridge level tion system, a person swimming in

[ss ] Ci vil Engin ee ri ng lUR CH z o11 In the center of the complex's open court­ yard are various attractions, including parks, ponds, an irregularly shaped cinema­ theque with a vegetated roof, and a hotel that Is linked to the network of bridges.

section members that form both the lateral and gravity system From an engineering point of view, the structure was suc­ for the building. A single size of steel shapes was used for ev­ cessful also in part for the modesty of its underlying design, ery perimeter diagonal member, simplifying the connections despite the more dramatic cantilevers and bridges. The prima­ at the base of the structure. There was, however, considerable ry structural design, combining a stiff core with a perimeter complexity involved in the detailing of the point where three concrete g rid frame occasionally stiffened with bracing, was or four diagonal rubes meet at the center of the base plate. both redundant and very rigid. As befits its name, the project The cinematheque's vegetated roofwas framed using typi­ is indeed a hybrid, a dynamic combi­ cal composite beam construction with a composite concrete nation of residential and urban plan­ and metal deck. All of the spandrel beams in the cinema­ ning accomplished by progressive and theque featured a combination ofaxial and shear connections sound engineering design. CE and served as drag struts to carry the lateral load and gravity loads to the braces. The perimeter members at the roof acted Guy Nordenson, P.E., S.E., F.ASCE, is as an irregular tension ring and thus prevented the cop-heavy a partner ofGuy Nordenson and Associ­ structure from unfolding outward. ates, ofNew York City, arzd a professor of Composed of flat plates and concrete shear walls, the architecture and structural engineering at 12-srory hotel is probably the simplest structure on the site. It Princeton University. Erik Anders Nelson, features no columns or beams, which leaves space for floor-to­ P.E., S.E., was Guy Nordenson and Asso­ ceiling curtain walls and provides the guests with clear views ciates' principal engineer on the Linked Hy­ of the rest of the Linked Hybrid complex and greater Beijing. brid project; he is now a principal at Struc­ The cast-in-place, rwo-story concrete kindergarten has a tures Workshop, Inc., ofProvidence. sloped concrete roof that extends from the roof ro the ground. The roofhas a large round opening ro allow light to enter the PRO.IECT CREDITS C I i en t: classrooms. Modern Green Development Com­ The ground-level spaces within the complex also include pany, Ltd. , Beijing Architect: Steven restaurants, ponds, and small parks. Installed beneath the Holl Architects, New York City and complex is a series of more than 600 geothermal wells that Beijing (design architect: Steven Holl; design architect and descend to a depth oflOO m, helping ro cool the Linked H y­ partner in charge: Li H u) Associate architect: Beijing Capi­ brid buildings in the summer and warm them in the winter. ral Engineering Architecture Design Company. Ltd., Beijing The Linked H ybrid project has proven to be significant Structural enQineer: Guy Nordenson and Associates, New from a number of perspectives. While considered a landmark York City Associate structural enQineer: China Academy of by virrue of irs location and the uniqueness of its bridges, the Building Research, Beijing (structural engineer: Xiao Cong­ .. Linked H ybrid complex is organized more as a public space zhen) General contractor: Beijing Construction Engineer­ I ~ than as an iconic building, which has made it popular with ing Group, Beij ing Friction pendulum isolator consultant: o the city's inhabitants. Earthquake Protection Systems, Inc., Vallejo, California

MARCH 2011 Civil Engineer i ng [ 59 ]