High-Rise Buildings

In the last decade, significant developments in architecturalexpression and in increasing demandfor lighter and taller buildings resulted in a systematical evolutionof structural systems. Steel structures were predominant in thesedevelopments and profiting from theinherent properties of this material,new steel framing systems emerged.The developments in the steel industry contributed to the structuralefficiency of these new framing concepts by providing new heavysections and high-strength steelswith outstanding mechanicalproperties. The floor framing systems for high-rise buildings aregenerally identical to those used inmedium-rise structures despite theneed for larger spans and columntransfer facilities at ground level forlobbies or steel activities. The maindesign criteria for tall buildings aregoverned by the lateral stiffness inorder to resist wind and earthquakeforces. The following is a briefdescription of different structuralsystems related to the lateral stiffness in order to resist horizontal forces.

Citicorp at Court Square, New York, 1988Skidmore, Owings & Merrill, New York

Preface

Shear FrameMoment resistant beam to column connections createshear frames or Vierendeel frames which provide lat-eral stiffness in both orthogonal directions. The effi-ciency towards lateral stiffness is controlled by theindividual stiffness of the members depending on thesection and the length of girders and columns. Theresistance to sway deflection in mainly governed bythe bending of beams and columns due to windforces and less from column shortening or cantileveraction. The figure below shows the theoretical swaydeflection of this framing system under the action ofwind forces. Common bay dimensions rangebetween 6 m to 9 m.

The shear frames can be located only in the exteriorfacade or, for more efficiency, in all vertical sectionsaccording to the column grid. This frame system hasthe advantage of large rectangular openings in the

Shear truss-frameVertical shear trusses located around the inner core inone or both directions can be combined with perimetershear frames in the facade of a building. These frame-shear truss interacting systems are considered tobe the most economical steel systems for buildings upto 40 storeys. In some cases, the inner core can beexecuted using concrete shear walls, thus substitutingthe steel trusses in K, X or single brace form whichmay interfere with openings that provide access to, forexample, elevators. The K-form is preferred for trusses since in the case of X and single brace formbracings the influence of gravity loads is very important. In seismic areas, eccentrical bracing isused in order to enhance the damping behavior of theoverall structure. The interaction of shear frames andvertical trusses produces a combination of two deflection curves with the effect of more efficientstiffness.

Outrigger and Belt TrussesAnother significant improvement of lateral stiffnesscan be obtained if the vertical truss and the perimetershear frame are connected on one or more levels by asystem of outrigger and belt trusses. The outriggertruss leads the wind forces of the core truss to theexterior columns providing cantilever behavior of thetotal frame system. The belt truss in the facade

facade. However, the fabrication of moment resistant connections is expensive compared to othersystems. Additionally, steel consumption is ratherhigh and reduces the range of application to low-risebuildings.

Systems Evolution

Sway deflection under wind pressure

Frame-shear truss interaction

Free frame Free truss Combination

improves the cantilever participation of the exteriorframe and creates a three-dimensional frame bebehavior. The overall stiffness can be increased up to25%.

The efficiency of this system is related to the numberof trussed levels and the depth of the truss. In somecases, the outrigger and belt trusses have a depth oftwo or more floors. They are located in servicesfloors where there are no requirements for wide openspaces. These trusses are often pleasingly integratedinto the architectural conception of the facade.

The Framed TubeTo achieve cantilever action of the exterior frame, thecolumn spacings are reduced to 3 m - 5 m and areconnected by moment resistant joints to the spandrelbeams at the overall frame perimeter. Thus, the partof sway deflection due to the shear frame can bereduced to less that 25%. Such framed tubes that pro-vide only small openings for windows have a swaydeflection behavior similar to a hollow tube at theground.

First Wisconsin Center, Milwaukee, WisconsinSkidmore, Owings & Merrill, Chicago

Deflection curve1 Drift with belt truss2 Drift without belt truss

The framed tube system calls for heavy beam and column sections with moment resistant joints. In orderto avoid welding on site and to facilitate erection, column-trees are fabricated in the workshop.

The spandrel beams are bolted on site at mid-span. Thecolumn-tree system i s very efficient for structures ranging between 30 and 100 storeys. The erection of 3to 4 storeys a week is commonly achieved.The framed tube, due to the close spacing of thecolumns, has a determinative influence on thearchitectural treatment of the facade. If wider column

spacings are required and in case of the rectangular planconfiguration, the diagonalized tube system is oftenmore suitable. This system introduces in the John

Hancock building in Chicago provides the tube actionby an exterior diagonalized truss.The trussed tube with diagonals on all four buildingfacades emphasizes space frame action. The overallefficiency i.e. fabrication, erection and steelconsumption advantages have to struggle with the need

for a uniform and regular exterior architecture.

John Hancock Center, Chicago, Illinois, 1970Skidmore, Owings & Merrill, Chicago

Column-tree system 4 welded connection1 column section 5 continuity plate2 beam section 6 penetrations for3 bolted connection mechanical services

Specimen of a heavy jumbo section, successfully used forboth columns and beams

Developments in the Steel Industry

It is obvious that high-rise structures have tocope with enormous gravity loads compared toother structures. On the other hand, the overallstiffness of high-rise buildings under the actionof lateral forces, such as wind and earthquakes,is governed by the stiffness of the individualcomponents of the steel structure.

To increase the performance of hot rolled sec-tions, two interacting properties have to be con-sidered: stiffness depending on the geometricalsection and its influence on deflection andbuckling behavior on the one hand and the steelstrength controlling the load bearing capacityof beams and columns on the other hand.Furthermore, the mechanical properties as, forexample, the weldability have a determinativeinfluence on fabrication costs.

ARBED, one of the main suppliers of rolledsections recognized at a very early stage thechallenge to meet these requirements.

Tailor-Made Beams

For the design of high-rise buildings, the struc-tural engineer, demanding heavier sections asthose specified in the american standard forrolled sections (ASTM A6), frequently speci-fies expensive built-up sections. To offer avaluable alternative, the concept of tailor madebeams has been developed, enabling flangethicknesses of up to 140 mm. Some of thesetailor-made beams have been introduced inASTM A6-88. thanks to this new concept,rolled beams can be used where built-up sec-tions were previously required.

Tailor-made beams are rolled sections producedwith the standard sets of rolls, within the usualrolling cycles. Only the thickness of flangeand web are adapted to the customer’s needs.With reference to ASTM A6 where rolled H-shapes are denoted as “W” (plus nominalheight in mm), the tailor-made beams are called“WTM”. These heavy jumbo sections and alsothe W 920x420 series are successfully used forcolumns and spandrel beams in exteriormoment frame structures .

HISTAR- A New Generation ofRolled Beams

The new Histar-grades for rolled beamscombine properties such as high yieldstrength up to 500 MPa, excellent tough-ness at low temperatures and outstandingweldability, considered incompatible untilnow. This was achieved by an innovativeQuenching and Self-Tempering (QST)process which facilitates the cost-effectiveproduction of a new generation of beams.Even under extreme conditions, Histarbeams stand guarantors for space andweight saving design.

A Case for SteelThe case studies presented on the follow-ing pages will demonstrate the numerousadvantages of structural steel in the designof modern high-rise buildings.

Architectsand Structural Engineers:Skidmore, Owings & Merrill,San Francisco

Developer:Honorway InvestmentCorporation

Site DescriptionThe Building is shaped in plan like a tear drop. It rises from a trian-gular base that covers nearly the entire 2.100m2 site and includes 15floors of office and 63 condominiums. Excavation of the bay mudsite and the installation of a heavy internal shoring system was atricky question. The shape of the site created a prominent buildingwhere the center of rigidity and mass are eccentrically placed.

Framing SystemTo create a structural system that could provide the building withenough stiffness to minimize its natural tendency toward wild lateraland torsional displacement under seismic and wind loads, the engi-neers developed a multiple tube framing system.

Computer simulation of lateral and torsionaldisplacements under seismic and wind loads

In this project, a perimeter ductile moment-resisting space frame is connected to aheavy central frame along the axis wherethe triangular and circular portions of thebuilding meet. To strengthen the connec-tion between triangular and circular por-tions, a 9,50 m long tangential beam wasadded, spanning the columns between thecircular and triangular portions on floors3,18, 19, 20, 25 and 26. The central frameis the most important part of the structure,taking most of the shear forces. In fact, thebuilding is designed to withstand seismicbase shear forces 50% greater than thosespecified in the San Francisco buildingcode and to deflect only 230 mm eachway. On each floor, the central frame has arow of six columns. The two interior rolledcolumns are flanked by two 1000 mm deepbox columns, which are flanked in turn bytwo 640 mm deep box columns. Deeprolled girders span between the columns.

388 Market Street, San Francisco

Framing plan 4th to 17th floor1 common frame line2 WTM 10003 WTM 840/WTM 9204 columns in the frame tubes

consist of WTM sections(WTM = tailor-made beams)

Structural scheme

26th floor

Massive, 1000 mm deep,rolled-shape steel columnsand beams were specifiedfor the central frame divid-ing the triangular and cir-cular portions. To furtherstrengthen the connectionbetween the triangular and

circular portions, a 9,45m long tangential beamwas added, spanning thecolumns between the circular and triangularportions on floors 3, 18,19, 20, 25 and 26.

3rd floor

street level

53rd at Third, New York

Architect:John Burgee Architectswith Philip Johnson, New York, NY

Structural Engineer:The Office of Irwin G. Cantor P.C.New York, NY

Developer:Gerald D. Hines Interests,Houston, Texas andSterling Equities, Inc.,New York, NY

34th floor

27th floor

19th floor

1W 920 x 201 -W 920 x 446

2W 840 x 1933W 360 x 3824 1/2 W 360 x 147 -

1/2 W 360 x 2875W 610 x 92 -

W 610 x 1136W 690 x 1257W 760 x 147 -

W 760 x 2578 1/2 W 360 x 448 -

1/2 W 360 x 1349 1/2 W 360 x 179

ground floor

A unique architectural schemeThe elliptical office tower has an enormous skylineidentity because of its shape and the striking contraston the color of the polished and flame-finished redgranite alternating with the stainless steel bands andtinted grey glass. Setbacks at level 19 and 27 enhancethe building’s sculptural form. The open and grandand welcoming lobby is two storeys high and ringed by8,50 m tall columns. The building rises in three tiersof eighteen, eight and eight storeys.

A telescopic framing systemStructural steel was selected as it was best equipped tohandle the long spans as well as locate the wind systempartially in the core and partially at the exterior. Fourbuild-in jumbo steel columns in the core take 80% ofthe wind load and 50% of the gravity load on thebuilding. To get the wind loads to the core, the engi-neer designed a moment frame with partially bolted,partially semi-rigid connections for the structure’sperimeter girders. The wind load is transferred to thecore through the floor diaphragms. The semi-rigidconnections transfer half the gravity loads to the coreand help the building resist torsion. Because the ellip-tically shaped frame left little margin for error, allmoment connections were field-welded. Where thegirders were joined to a column flange (in the semi-rigid moment connection), the girder’s web wa boltedto the column’s shear plates through horizontal slotsand then the flanges were field welded. A loose topplate was butt-welded to the column flange and thenfillet-welded to a plate that was shop-attached to thecolumn. The welding was completed before the boltswere torqued into their final position.

Floor framing plans1 core trusses2 4 steel plates

seam-welded together in lower levels only

3 9-storey rectangular annex

4 column transfer

3rd floor

27th floor

Column transferat setbacks 19th and 27th floorscale 1:201 column above2 column below3 transfer girder

Architectsand Structural Engineers:Skidmore, Owings & Merrill,New York, NY

Developer:New York CommunicationsCenter Associates,New York, NY

A new landmark for NYC-skylineThe 53-storey commercial tower is separated from the 41-storeyresidential tower by a multi-level plaza. The commercialbuilding has four elliptical entrance lobbies, each set off by10,60 m high granite-clad columns. The spandrels in the glazedbrick exterior are set back to define the verticality of the shaft.

Exterior tube with tree-columnsThe two-storey high columns were fabricated with four andsometimes two half-span spandrel beams welded to them toform “trees”. These tree sections were shop-fabricated with1,50 m studs extending out either on both sides or on one side,depending on were to be placed. One advantage of the tower’sframing being a perimeter tube framing system was that its stiffness facilitated some of the column transfers taking place

Worldwide Plaza, New York

45th floor

39th floor

18th floor

16th floor

13th floor

6th floor

groundfloor

Structural frame elevation1 exterior tube, moment connected columns and

spandrel beams2 braced frame, frames terminate as elevators

drop off3 tube transfer at ground floor arcade4 braced roof frame supported on core5 space frame supporting skylight

The structural solution using irregular columnspacing resulted in a unique expression for atube-framed building and exemplifies thecooperation between structural and architectural disciplines.

throughout. This building project is unusual becauseboth the arcade at grade level and the copper-cladroof are open to wind forces on the interior walls.Some of the bracing stops at the 16th floor, somestops at the 40th and the rest continues to the 50thfloor. The exterior tube resists most of the overturn-ing moment and the braced core resists most of theshear forces. Tight bay spacing was needed at thecorners to help transfer the wind loads around thecorners of the building.

Framing plan 19th to 39th floor

|-1.67-| 1.67-|-1.67-|-1.67-| | |-1.67-|shop unit shop shop

unit unit

Typical shop-fabricated erection units

Architects:Johnson Fain and Pereira Associates

Structural Engineer:Albert C. Martin & Associates,Los Angeles

Developer:JBM/Urban Development, Co.Los Angeles

A highly articulated facadeThe 39-storey office tower with its dramaticcurve and highly articulated facade inVenetian Gold and Verde Fountain granite,with sapphire tinted glass creates a uniqueprofile on the Los Angeles skyline whileproviding 72,000 rentable square and spec-tacular views.One of the main features of the building isthe 26 m wide elliptic curved facade at thesouth-east corner. This facade spans unin-terrupted with columns at the first storey.The exterior columns at ground level forman arcade with an unbraced height of 12,20m. There is a setback at the 36th storey.The floor plate is column-free between thecore and the exterior glass line. However,due to the L-shaped core only one columnwas allowed to be used at the south-eastquarter of the building to carry the gravityloads. This column stops above the lobbyarea to accommodate the main cornerentrance. The column gravity loads aredelivered to a 14,00 m long, 3,25 m deep,tapered plate girder above the entrance ceil-ing.

A dual framing systemThe floor framing of the building consists ofpartial composite steel beams 3,00 m oncenter with 51 mm metal deck and 83 mmlightweight concrete fill. Below grade fram-ing system is reinforced concrete beams andslabs. Located in a highly seismic area, thetower had to be designed to withstand themaximum ground shaking predicted in thesite, as well as provide for wind inducedaccelerations, which may adversely affectoccupant comfort. Accordingly, a structuralsteel dual system was selected for finaldesign. This system consists of a perimetertube integrated with six interior core eccen-tric bracing frames. The perimeter tube con-sists of a ductile steel moment frame up tothe 36th storey. Above this level the tube isdivided into separate frames.

1999 Avenue of the Stars, Los Angeles

39th floor

The selected system hasan advantage from bothstiffness and ductilityaspects. In addition tothe ductile momentframes, the shear links ofthe eccentric bracingframes systems act asfuses for seismic energydissipation

20th floor

10th floor

street level

Structural frameelevation ofperimeter tube andframing plan7th to 13th floor1W 9202 WTM 9203 box column4 WTM 6905W 360(WTM = tailor-made beams)

Column-girder connectiondetails A,B scale 1:501 WTM 10002W 3603 WTM 9204 WTM 6905 box column(WTM = tailor-made beams)

-3

-3

4

1

3

5

2

Two-buildings-in-oneThe Swiss Bank Tower was placed to the rear of the historic department store and designed to respect the classical architecture ofthe Sak’s building while allowing the tower to rise above the new 9-storey store extension to Sak’s Fifth Avenue on axis withRockefeller Center. Extraordinary construction efforts werenecessary to satisfy two owners with radically different require-ments. The different column locations in the two-buildings-in-onerequired an innovative column-transfer system. While Saks wanted exceptionally long 18,60 m spans on its retain floors, in the SwissBank above, in contrast, in the same east-west orientation of theSak’s spans, the spans are only 14,00 m.

Architects:Lee Harris Pomeroy Associates/Abramovitz Kingsland Schiff,Manhattan, NY

Structural Engineer:The Office of Irwin G. Cantor P.C.,Manhattan, NY

Developer:The Swiss Bank Corporationand Saks & Company,Manhattan, NY

Swiss Bank - Saks, New York

West elevation

Structural varietyThe transfer system covers the central portion of theupper tower and supports the elevators and on theopposite side of the building, a 3-storey high portionof the Swiss Bank that cantilevers, 8,23 m over Sak’sescalator tower. The space within the 2-storey hightruss grid houses the building’s mechanical equipment.In the upper, Swiss Bank portion wind loads are resist-ed by a hybrid structural frame, consisting of exteriorbraced frame on the fourth side. The center of thewest face is cut out below the 10th storey. From the13th storey up to the top is another cutout. Because ofthe cutouts, the building has a hybrid wind framebelow the 14th storey.

East-west section

36th floor

The cutouts in thetower’s west face formost of its height compli-cated the handling of windloads. The center of thewest face is cut out belowthe 10th storey

12th to 13th floor

office area Swiss Bank

11th floor sky lobby

10th floor mechanical floor

2nd to 9th floor Saks

ground floor, entry level

basement loading area

Framing plan: 15th to 20th floor

Framing plan: 4th to 7th floor1 corner column2 existing buildings3 column-free space

The column transfersystem is made up of fivetrusses, 7.95 m high and19,50 m to 26,80 m longand one 105-ton plategirder, 3,66 m high by22,86 m long, tying thefour trusses together.

Architectural versatilityThe World Financial Center at Battery ParkCity is a group of architecturally coordinat-ed skyscrapers, landscaped plazas and win-ter garden. The four reflective glass andgranite office towers rise from a continuousgranite-sheathed base and make a transitiontowards entirely reflective glass.The tops of the towers are designed assculptured forms-spheres, pyramids andprisms. What has become to be called theBuilding B is unique in that it is the onlyone to feature a 45,72 m diameter copperdome supported by a series of steel trusses.Building B has a ten-storey podium stretch-ing from the tower to the nine-storey octag-onal structure of the gatehouse located tothe south and forming the main entry atLiberty Street.

A case for steelStructural steel was selected for Building Bbecause of several program requirements,including electrified floor system flexibilityto accommodate tenant requirements and atransfer girder that had to be accommodat-ed in the 91 cm ceiling sandwich. The 45-storey tower has a 4,57 m setback at the26th storey. Instead of carrying columnsall the way down and having a columnspacing of 4,57 m, the upper perimetercolumns were picked up on transfer girdersallowing a 9,14 m by 9,14 m grid to beemployed below the 26th storey.The dome framing consists of 48 arch-trusses spanning between a tension ring anda compression hub. The arch-trusses arespaced approximately 2,74 m on center atthe tension ring and converge at the com-pression hub. Eight rows of concentricbridging provide bracing for the arch-trussbottoms chords. Simple field connectiondetails connect the arch-truss to the tensionring and the compression hub.

Architects:Cesar Pelli & Associates,New Haven, Connecticut

Structural Engineer:Thornton-Tomasetti, P.C.New York, NY

Developer:Olympia & YorkEquity Corporation,Toronto, Canada

World Financial Center, New York

Layout World Trade Centerat Battery ParkA, B, C, D office towersE winter garden

Structural elevation of the twoconnected braced cores, line B

40th floor

30th floor

All lateral forces areresisted by the two con-nected cores acting asgiant vertical cantilever.The wind bracing consistsof K-type bracingarranged and designed tominimize the loss ofrentable area

Framing plan

20th floor

The general framing ofthe podium consists ofstructural steel beamsspaced 3,05 m on centerframing into girders andspanning 9,14 m. Thesame framing system isused for the 20th to 26thfloor. The framing abovethe 26th floor consists offloor beams spaced 3,05m on center and spanningapproximately 12,80 mbetween the spandrelbeam and center core.The beams are designedto act composite with theconcrete slab of the floorconstruction.

10th floor

Steel level

Detail A scale 1:201W 760 x 2572W 460 x 1773 4 L-sections4W-column5 composite slab

Architects:Kohn Pedersen Fox Associates,Manhattan, NY

Structural Engineer:Severud Associates,Manhattan, NY

Developer:Shearson Lehman HuttonManhattan, NY

Extraordinary dimensionsThe big building, not only in height, but in plan, measuring 81 x 50m is located on Manhattan’s Hudson River shore near the WorldTrade Center. There are entrances on the east, west and north sidesand the three lobbies are interconnected. These lobbies and inter-connected have about 213 running meters marble wall, necessitat-ing an extraordinary extensive sub-structure of miscellaneous ironframing to support the marble. Thanks to the use of steel, accom-modating changes late in the design was far easier, faster and atlower costs than would have been possible with concrete. Some ofthe major changes were the addition of one complete floor at themid-height of the building, an auditorium at the 26th floor withtransfer girders above the clear span space, a reinforced floor for anaerobic area, new openings for connecting stairs, framed pits at thekitchen and reframining supports for the exterior precast walls. The

Shearson Lehman Hutton Plaza, New York

35th floor

26th floor

17th floor

8th floor

ground floor

Wind bracing north-south: a bracing over 9 floorswith columns on the apex to reduce the span of thehorizontal girder. Vertical and lateral loads arepicked up at each floor by the truss and transferredthrough the A-frame diagonals to mega-columns atthe outer edges of the “A”.

general-use office is believed to be the largest inManhattan with raised floors throughout for com-puter and wiring.

The superiority of steelAlthough the building has characteristics thatwould have made concrete less costly than usual,for example, relatively close column spacing, rec-tangular floor plans, central rectangular core,moderate floor-to-floor heights and raised floorsfor electrification, a steel frame was fully justifiedfor its superior advantages. The problem of theheavy loads of the high speed double decked ele-vators was solved by including elevator supportbeams at two intermediate floors as well as at theroof. Further, the requirement for column-transferbeams for the setbacks at the upper floors werebest answered with steel. Both a braced core andmoment frames are used to resist lateral windload. The vertical trusses running full width andheight of the building were placed 4,57 m fromthe end walls, so as not to diminish the magnifi-cent view. Moreover, at this location the diagonaland vertical members of the truss could be hiddenby office partitions. To limit the number of verti-cal and diagonal members, the truss was designedas a simple “A” frame. The wind load in the longdirection of the building is resisted by conven-tional crossbracing in the cores plus momentframes.

Details A, B, Cscale 1:1001W 760 x 3142W 3603W 6904W 610

Each truss consists of four9-storey A-frames, 38,00m wide at the base.Girders at each floor con-nect the frame’s 39,62 mlong diagonals. A 4-storeycolumn extends upwardsfrom the apex of each A-frame, reducing the girderspans in the lower floorsof the A-frame above.

Horizontal girders at eachfloor are used as crossmembers to tie diagonalstogether. Girder to A-frame connections arerigid. Vertical and lateralloads are picked up ateach floor by the truss andtransferred through the A-frame diagonals tocolumns at the outeredges of the “A”.

Framing plan 19th to 22nd floor1 wind bracing north-south2 bracing core

Architects:Kevin Roche John Dinkeloo& Associates, Hamden, Connecticut

Structural Engineer:Weiskopf & Pickworth,Consulting Engineers, Manhattan, NY

Developer:Solomon Equities, Inc.New York, NY

750 Seventh Avenue, New York

A tapering spiral shapeThis 35-storey tower features setbacks that spiral aroundthe building and culminate at the top with a 36,58 m highspire. The only way to satisfy the architectural design andthe owner’s requirements for column-free space was touse transfer girders. While three or four or even six orseven transfer girders are not uncommon, the tower has 84column transfers on 26 perimeter columns. The use ofcolumn transfers led to complicated column connections,whose complexity is increased by the fact that manycolumns switch from W 920 at the exterior of the buildingto W 360 as they become interior columns at the slopingsetbacks. At a single location, some columns had to bedesigned for a change in section, a spandrel moment con-nection, a moment connection to the girder that frames atright angles to the spandrel and horizontal X-bracing.Where ductwork had to pass through the girders, the manypenetrations make the girders look like “Swiss cheese”.Especially near the building core, these conditions meantgirder penetrations as large as 1,82 m wide and 406 mmhigh through a 1,067 m deep girder.

A challenging structural designFrom outside the building, large vision panels are separat-ed by opaque grey glass bands that closely shadow thelocation and dimensions of the structure’s exterior tubeframe. The horizontal opaque bands are 0,91 m deep, thedepth of the floor-ceiling sandwich; the vertical opaquebands are 1,52 m wide, little more than the width of the914 mm columns that are spaced on 4,57 m centers alongthe building perimeter plus the column covers. Thisbuilding is one of the first in New York City where theglass in the curtain wall is attached to its supporting mem-bers solely by adhesive structural silicone.

Column layout including core1 six interior non-core columns on lower levels2W 920 section3 braced core

What challenging conditionsstructural steel can be designedto handle is exemplified in thecolumn transfers at the set -backs, where many columnsswitch from W 920 at the exte-rior of the building to W 360 asthey become interior columnsat the sloping setbacks.

Typical column transfer1 WTM 9202W 3603W 920(WTM - tailor-made beams)

Design Architect:I.M. Pei & Partners, New York, NY

Executive Architect:Welton Becket Associates, Los Angeles

Structural Engineer:CBM Engineers, Inc., Houston

Developer:Maguire Thomas Partnersand Pacific Enterprises, Los Angeles

The tallest tower in LARoughly circular in plan at the base, thebuilding changes shape five times as it stepsback at the 47th, 56th, 60th, 69th and 73rdfloor. A three level difference in elevationsbetween streets on north and south sides ismade up with beautifully landscapedSpanish style steps, which are seismicallyisolated from the main tower structure andfountains. Called a signature building forthe city of Los Angeles, the granite cladbuilding with its faceted exterior will con-tain about 1.4 million square feet of officespace.

Conflicting structural demandsTo arrive at the most economical design forthe steel structure, due consideration had tobe given to both seismic and wind-loadingfactors. The conflicting demands of optimalseismic vs. optimal high-rise design resultedin a structure that is really two structuresinteracting with each other, with enoughductility to accommodate the randomlyvariable jerks of an earthquake yet, stiffenough to ensure occupant comfort duringhigh winds. The dual system that wasselected consists of steel columns ringingthe building’s perimeter creating a ductile,moment-resisting tube. It is the 6,80 m2core, formed by four massive 1,20 x 1,20 msteel core columns connected by chevronbracing that provides stiffness up to the 54thfloor. At the 54th floor, the chevron bracingdrops out of the core. At this point, thereare no columns between the core and theperimeter tube and the perimeter columnsact as the main lateral bracing mechanism.The core structure is supported on a con-crete mat foundation. The perimeter framecolumns are supported on circumferentialstrip footings that tie into the core mat.Curtain walls were designed to sustain prac-tically no damage during earthquakes. Thetrusses that support the cladding weredesigned to move with the maximum inter-storey drift.

First Interstate World Center, Los Angeles

Floor framing plans (selection)1 perimeter of 8th to 46th floor2 perimeter of 57th to 59th floor3 floor framing plan 70th to 71st floor4 braced core lower levels

The perimeter structure is a ductile, moment resisting frame. The interior core is 22,55 x22,55 m wide and 68 storeys high. Box columns art each corner of this core have designgravity loads of 10.000 t. Lateral support is provided by a series of 2-storey chevronbraces, spanning each of the four sides of the core. The chevron braces are being used ina high seismic region for the first time. At the 54th floor, the chevron braces are replacedby free-spanning Vierendeel girders. Between the core frame and the perimeter frame,concrete floor slabs on steel beams span up to 16,76 m.

75th floor

68th floor

Dual frame structure of core Aand perimeter tube B

60th floor