Hearst Tower Feb 06

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STRUCTURE magazine February 2006

New YorkHearst Towe

A Restoration, an Adaptive Reand a Modern Steel Tower Rolled Into O

IIn the late 19th century, William Randolph Hearst envisioned a headers building for his newspaper empire and began acquiring realin and around 57th Street and Eighth Avenue. The site was origintended to hold a two-story, mixed-use structure with stores, offica 2,500 seat auditorium.In the 1920s, a six-story structure was commissioned to house offices f

Hearst Corporations twelve magazines. Located between 56th and 57th Streehorseshoe shaped structure contains 40,000 square feet and was originally n

the International Magazine Building. The building was designed in 1926 by JUrban and George P. Post & Sons to accommodate seven additional floors

were never built.The building included an

auditorium and features sixsculptural groups executedat the buildings corners,main entrance on Eighth

Avenue and the 57th Streetentrance which waslater altered for commercialuse. The precast limestonefaade is comprised of a four-story setback above a two-story base. Its

consists of columns and allegorical figures representing music, art, commerindustry. The main entrance is flanked by Comedy and Tragedy on tand Music and Art on the right. Sport and Industry are above the cor56th Street and Printing and the Sciences are located on the buildings corner at 57th Street. Construction began in 1927 and was completed inat a cost of $2 million.

The building was designated as a Landmark Site by the Landmarks Pretion Commission in 1988 and was considered to be an important monin the architectural heritage of New York.

In early 2001, the Hearst organization commissioned Foster and ParArchitect and Cantor Seinuk, Structural Engineer, for the design of itheadquarters at the site of its existing building. The new headquarters isstory office tower, approximately 600 feet tall with 856,000 square-feet oand two underground levels.

By Ahmad Rahimian, Ph.D., P.E., S.E., and Yoram

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Landmark FaadeOne of the major design requirements was the preservation of the

six-story landmark faade and its incorporation into the new towerdesign. The existing six-story building had a horseshoe-shaped floorplan with a footprint of approximately 200 by 200 feet. The newdesign was based on removing all the existing construction, exceptthe landmark faade wrapped around the three exterior faces of thebuilding at 56th, 57th Street and Eighth Avenue.

The design called for a new tower, 44 stories above ground leveland with a footprint of 160 by 120 feet. This was to be situated onnew foundation rises behind the existing six story landmark faade.The new design also required a seven story high interior atrium,formed by the existing faade and the tower above.

Maintaining the existing faade without the existing supportingstructure meant a larger unbraced height, a feature not originally de-signed for. This necessitated a new framing approach for the structur-al stability of the existing wall, addressing the new design conditionas well as construction phase issues. In addition, the existing faade isreinforced and upgraded for new seismic requirements contained inthe current New York City Building Code.

Along with the mason-ry faade, the supportingperimeter steel columnsand spandrel beams

were also maintained.The spandrel beams andcolumns provided full

vertical support for thefaade system. The lat-

eral stability and seismicrequirements for the fa-ade construction werestudied, and as a resultan additional grid of

vertical and horizontalframing was providedbehind the existing fa-ade. The new and ex-isting framing are inturn laterally supportedby the new towers 3rdfloor framing system, as

well as skylight framingsystem at the top of theseventh level which co-incides with the top ofthe existing faade.

FoundationThe boring tests indicated a sharp drop in the elevation

rock at the site. The rock elevation varied from a few feet feet below the basement level. Therefore, almost half of the tosupported on spread footing on rock and the other half on caof equivalent strength embedded into rock below.


A diagrid systemwrapping around all fourfaces of the tower was

proposed...

StructureThe building utilizes a composite steel and concrete floor wi

foot interior column free span for open office planning. The has two distinct zones. The office zone starts 110 feet abovelevel at the 10th floor rising to the 44th level. Below the 10th

the building houses the entrance at street level and lobby, cafand auditorium at the 3rd floor with an approximately 80-foointerior open space. At the seventh floor elevation, the toconnected to the existing landmark faade by a horizontal sksystem spanning approximately 40 feet from the tower columthe existing faade.

Lateral systemOn three sides, the building is open to streets; however, o

west side it has a common lot line with an existing high rise ing. Therefore, from the standpoint of interior layout efficienc

The nodes are allprefabricated and installedat the site using all bolted

connections.


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STRUCTURE magazine February 2006STRUCTURE magazine February 200627

...an important monumentin the architectural heritage

of New York.

service core zone is placedasymmetrically towardthe west side of the tower.However, this reduces thestructural benefit of utiliz-ing the core as the mainspine of the tower due tothe eccentricities inherentin the location. To ad-

dress the general stabilityof the tower, the designteam decided to focus onthe opportunities that theperimeter structure couldprovide. This resulted inthe conceptual design go-ing through an evolution-ary process by evaluatingthe effectiveness and ben-efits of various systems. Adiagrid system wrappingaround all four faces of thetower was proposed andselected for its multitudeof merits.

DiagridThe diagrids form a network of a triangulated truss system inter-

connecting all four faces of the tower, thus creating a highly efficienttube structure. The diagrid nodes are formed by the intersection

of the diagonal andhorizontal elements.These nodes are oneof the key design ele-ments both structur-ally and architectur-ally. Structurally, theyact as hubs for redi-recting the memberforces. Architecturally,they were required tonot be larger than thecross dimension of thediagrid elements inorder to maintain thepure appearance.

The nodes in thisproject are on a 40-footmodule and placed at

four floors apart creat-ing the diagrid system.The chamfered cornerconditions, which arecalled Birds mouth,

were the natural evo-lution of the refine-ment of the structuraland architectural op-tions. This not onlyaccentuates the aes-thetic character of thediagrid but also solves

an otherwise structural vibration concern of having 20-foot caver conditions at each corner every eight floors.

The diagrid members are typically wide flange rolled sections. The nodes are all prefabricated and installed at theusing all bolted connections. Typically there are two types of nthe interior and corner nodes. The interior nodes are planartransfer the loads in two dimensional space, whereas the cnodes transfer the loads in three dimensional space and thus a more complicated arrangement. The nodes were designed d

the Conceptual Design phase since the actual dimension onodes, although maybe an issue for Detail Design phase, chave significantly impacted the viability of the overall conceaddressing issues such as cladding, aesthetics, and ultimatelystructural system.

NodesThe alternate study of the corner nodes was an interesting pr

Even with all the available computer software, it was much rewarding when we switched to a time tested method of mmaking. The key to the design of the node was to have a less intensive design, even at the cost of marginally increased mater

The outcome satisfied not only the structural and architecrequirement, but also the fabrication requirement in such a waythe design concept was wholly accepted by the steel contractor.

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...a new framing approachfor the structural stability of

the existing wall...

...the need for asecondary lateral systemconnected to the common

diaphragm fl oors.

The inherent lateral stiffness and strength of the diagrid provideda significant advantage for the general stability requirement for thetower under gravity, wind and seismic loading. This resulted in ahighly efficient structural system that consumed 20% less steelmaterial in comparison to conventional moment frame structures.

While diagrid systems have inherent strength and stiffnesscomparable to a triangulated structure, the diagonal elements arerequired to be braced at the floor levels between the nodal levels.This necessitates the need for a secondary lateral system connectedto the common diaphragm floors. The secondary lateral systemshould account for the stabilizing requirements, considering the

total gravitational loads at each level and the customary inter-story construction tolerances. The secondary lateral system in thisproject is defined by a braced frame at the service core area.

The highly redundant diagrid system provides a structural net-work with multiple load paths that provides resistance to progressive

collapse. This structure provides a higher standard of performanceunder extreme stress conditions that national and internationalcodes are striving to achieve. At the 10th floor, the diagrids are sup-ported by a series of mega columns around the perimeter. The lat-eral system below the 10th floor is achieved by a robust compositecore shear wall comprised of steel braced frames encased in rein-forced concrete walls. The core wall lateral stiffness is enhanced bythe two sets of super-diagonals.

Mega ColumnsThe typical office tower starts at 110 feet above ground, at the 10th

floor. The design calls for an interior open space between the 3rd and10th floor with a height of approximately 80 feet.

The structure below the 10th floor is designed to respond tlarge unbraced height by using a mega column system arounperimeter of the tower footprint, supporting the tower perimstructure. Mega columns are primarily made out of built-uptube sections and strategically filled with concrete. In ordcreate the interior open space, two of the tower interior coluare also transferred out to the perimeter of the tower via a sersuper diagonals below the 10th floor.

Additional interior super-diagonals are provided below thefloor to assist the composite core wall system for general stabMega columns are continued down to the foundation.


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DesignThe structural software used was ETABS, RAM and SAP

The controlling lateral load was primarily wind. Seanalysis was in accordance with the New York City BuCode, Seismic Zone 2A.

ConstructionThe existing faade was laterally reinforced and is suppor

the 3rd

and 7th

level which is equivalent to the top of the wall new tower structure. However, to erect the new tower, the exstructure which provides stability for the existing faade neebe removed. Therefore, the retained faade had to be stabiliztemporarily keeping the first bay of the structure all arounperimeter, including its columns and floor framing, as a rinment. This also provides a working platform for existing faadreinforcing.

Nevertheless, the analysis of the one bay ring structure the temporary loading condition showed that it also temprequired to be laterally stiffened. This necessitated placing bmembers within the temporary remaining one bay ring strprior to removal of the balance of the existing building. temporary bracings remained in place until the major permstructural work was completed up to the 10 th floor, and thstability of the existing faade wall was restored.

The structure is planned to open in June 2006.

Ahmad Rahimian, PhD., P.E., S.E. is president of WSP Cantor Seinuk, Structural Engineers, New York division of WSP Group. He is aninternationally recognized expert in tall buildings. Dr. Rahimian is the recipient of numerous awards from engineering societies for various

exemplary projects that he has engineered, including the ENR Excellence Award as one of the Top 25 Newsmakers of 2003 and 2005 ASCE-CERCharles Pankow award for innovation. Dr Rahimian holds a US patent for seismic protective design, has authored numerous articles and lectur

widely on the design of tall buildings in professional societies and universities.Yoram Eilon is an Associate with WSP Cantor Seinuk StructuraEngineers, New York division of WSP Group. He has designed office, residential, stadium, industrial and parking structures in United States an

overseas. Mr. Eilon is currently completing work on the Hearst Building and working on the design of the Freedom Tower in New York City.

Project TeamOwner

Hearst Corporation

Development ManagerTishman Speyer Properties

ArchitectFoster and Partners

Associate ArchitectAdamson Associates

Structural EngineerWSP Cantor Seinuk

MEPFlack & Kurtz

Construction ManagerTurner Construction Co.

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Hearst Tower Feb 06