Modern Steel Construction / December 2001

Marco J. Shmerykowsky, P.E.

All photos by

Marco J. Shmerykowsky, P.E., courtesy

of SCE/Shmerykowsky

Consulting Engineers.

The 5 Times Square tower is a 40-story high-rise

building located at the intersection of Seventh

Avenue and 42nd Street in the “Bow Tie” dis-

trict of New York City's Times Square. The

tower, which is approximately 565' tall, con-

tains 1,000,000 rentable sq. ft. of office space and

100,000 sq. ft. of retail space.

One of the major chal-lenges on this projectwas the foundation sys-tem. Just like manyNew York City towers,

the 5 Times Square site is bordered bythe New York City subway. The engi-neers must be very careful in the de-sign of the foundation system to avoidtransferring the tower's foundationloads into the subway structure. Thecloser the building and the deeper thesubway, the more of a challenge thistask becomes.

The 41st Street side of the 5 TimesSquare tower foundation wall is ap-proximately 2’-0” from the adjacentsubway structure. The bottom of thesubway, in turn, is approximately 60’-0” below grade level and 30’-0”below the second cellar level. Conse-quently, a deep foundation system wasneeded to safely transfer the towerloads from the tower columns to a levelbelow the subway structure.

The 5 Times Square Tower is fortu-nate to sit on Manhattan Shist bedrock.While it is good from a structural per-spective, it is very difficult to excavate.Furthermore, the proximity of the siteto the subway and a historical land-mark building on the west propertyline dictated that blasting could not beused. Consequently, the system wentthrough three phases of design evolu-tion. In the initial scheme, the intentwas to use 24” diameter caissons. Al-though these large caissons had ade-quate capacity, it became difficult toefficiently locate them while account-ing for New York City code and TransitAuthority’s placement requirements.The alternative solution was to exca-vate large concrete piers down tobelow the bottom of the undergroundsubway structure. While this was a di-

rect engineering solution, the highquality of the bedrock on the sitewould have made efficient excavationwithout blasting difficult.

The final solution consisted of highcapacity 10” diameter mini-caissons.These caissons were composed of a12” diameter steel shell, high strengthconcrete and a core of reinforcing bars.The small size of the caissons allowed

a placement pattern that minimizedthe eccentricity of each caisson grouprelative to the line of the load from eachtower column. Furthermore, the smallcaissons allowed the contractor to usefaster drilling equipment, which deliv-ered less energy to the surroundingrock.

The caissons are used along the en-tire 41st Street side with the exception

December 2001 / Modern Steel Construction

Left: Transfer truss on tower’s west side looking north.Above: View of the west face of the tower looking south.

of the southwest corner column adja-cent to the New Amsterdam Theater.Due to the high axial loads and its cor-ner location, the placement of an ade-quate number of caissons under thiscolumn was not practical without im-posing a large eccentricity on the foun-dation system. To address this issue, alarge concrete pier was built at this lo-cation down to the bottom of the sub-way. This direct load path allowed forthe tower’s column load to be trans-ferred below the subway, while avoid-ing the costly construction whichwould have been required to properlytransfer a large additional eccentricityin tight quarters.

Computer AnalysisAs with all modern complex proj-

ects, the design of 5 Times Square madeextensive use of computer analysis anddesign techniques. A key aspect to thiscomputerized work was to avoid plac-ing complete reliance on a single tool.In addition to finite element based pro-grams such as SAP90 and ETABS, sev-eral custom design programs andcommercially available structural pro-grams were used. This resulted in acompartmentalized approach to the

computer design and analysis and al-lowed the structural engineer to main-tain full control over the designprocess. This approach also allowed fora method of independent consistencychecking where the results of multipleprograms and hand calculations can becompared to ensure that the results arereasonable. For example, the prelimi-nary lateral analysis was accomplishedthrough the use of several two-dimen-

sional models using the SAP90 analy-sis program. The final analysis wasdone using a three-dimensional modeland the ETABS program. The initialtwo-dimensional analysis provided anexcellent benchmark for verifying theoverall accuracy of the more complexanalysis.

In addition to the various tools usedduring the design of the structural sys-tem, extensive use of the computer was

Modern Steel Construction / December 2001

Above: View of east side elevation.Left: Caisson installation at south end of tower.

also made in the detailing phase of thebuilding. In order to ensure that thebuilding properly “closed” and that thearchitect’s high complex multidimen-sional geometric parameters whereproperly achieved, the entire structuralframe was modeled in three-dimen-sions in the computer. This allowed thedetailer to identify all complex connec-tions in a virtual environment prior tocutting or drilling a single piece ofsteel. In the construction of a buildingwith complicated geometry such as 5Times Square, this approach was ex-tremely beneficial.

The Lateral Load Resisting SystemThe structural lateral load resisting

system selected for this building can bebest described as a “modified perime-ter tube system.” Traditionally, a build-ing of this size would use a centralbraced core as part of the lateral loadresisting. The costs for fabricating anderecting a braced system tend to bemuch lower than for a comparably per-forming moment frame system. Thedown side of such a system is that theamount of usable floor space is reduceddue to the large size of the bracingmembers, which must be installed inthe building’s core. Since rentablesquare footage is such a highly prizedasset in the New York City office mar-ket, minimizing the area needed for thelateral system and therefore reducingthe core size became a high priority andchallenge to the design team at the ini-tial stage of the project. By moving thelateral system to the perimeter of thebuilding, the design team achieved thegoal of maximizing the rentable squarefootage.

The system itself is a modification ofa traditional “tube” system, which usesclosely spaced columns, typically 6’ to12’ on center, and deep spandrel gird-ers around the entire perimeter of thestructure. The north and south faces ofthe lateral system are composed of W14columns, which are spaced at 10’-0” oncenter and are linked by W36 windgirders.

The “modification” to the tradi-tional tube system exists in the east andwest faces. These sides of the toweralso utilize W14 columns and W36girders. Since these sides are longerthan the north and south faces, it waspossible to increase the column spacingon the center three bays to 30’-0”. The

longer spans allowed the architects toutilize a larger column free viewingarea while still retaining many of thestructural benefits of traditional tubesystem.

Some of the columns at the lowerfloors are large built-up members.These sections ranged from 24” deepby 20” wide I-shapes composed of verythick plate material to four 30” deep by20” wide “box” columns. The boxcolumns are located at the northeastcorner of the site and work in combina-tion with the second floor perimetertruss system to transfer the tower’snortheast corner column at the secondfloor. The transfer makes possible thecreation of a column free space, which

looks out onto New York’s famousTimes Square.

The Core and Floor SystemThe selection of the perimeter lateral

system was not the only component,which contributed to the ability to de-sign such an efficient and tight core. Tominimize column sizes, the majority ofthe core columns consist of W14 sec-tions conforming to ASTM A572, grade65, while the perimeter columns areW14 and built-up sections conformingto ASTM A572, grade 50. Since thetower uses a perimeter lateral system,the controlling factors in the selectionof the perimeter moment frame mem-bers are lateral loads and drift control.

December 2001 / Modern Steel Construction

Foundation excavation looking at southeast corner of site.

Typical view of perimeter truss system between 2nd and 3rd floor.

Consequently, the perimeter lateralsystem columns and the interior grav-ity columns tend to work at differentstress levels. This in turn leads to dif-ferential shortening of the columnsunder load. In order achieve a desiredlevel floor upon the completion of con-struction, it was necessary to lengthencertain column lifts. This lengtheningwas designed so that as the building isgradually loaded, the beam-to-columnconnections at a given floor level willarrive at the same final elevation.

The typical floor system consists ofa 21/2” of normal weight concrete over3” composite metal deck, which spansbetween W18 floor beams. The maininterior girders, into which the floorbeams frame, were given special con-sideration in order to produce a cost ef-fective floor design. Since these girdersspan nearly 44’, deep W33 memberswere selected. In order to properly ac-commodate the mechanical system,which was installed in the ceilingspace, penetrations were provided inthe girders. The cost saving wasachieved by sizing the girders so thatcostly reinforcing of the openingswould not be required.

In addition to the typical floor con-struction, there are several areas wherea heavier floor system was imple-mented to meet the specific needs ofthe building. The most notable area isthe Con Edison transformer vaults.Due to the tight nature of the site andthe close proximity of the subway lines,it was not feasible to install the trans-formers in the more typical sidewalkvault spaces. The solution was to con-struct the six vaults and network pro-tector rooms at the southern end of thethird floor. The concrete vaults them-selves were specifically designed as perCon Edison requirements, which stipu-lated that in addition to other designloads, the concrete structure needed tobe capable of withstanding a heavyblast load should any of the transform-ers explode. This resulted in a massivereinforced concrete box structure sup-ported on a cradle of W16 and W21beams spaced 5’-0” on center. In addi-tion to the reinforced concrete vaults,various sections of the slab needed tobe constructed in “layers” to accommo-date the installation of electro magneticshielding and waterproofing. The basicfoundation of this multi-layered floorconstruction consisted of a 21/2” nor-

mal weight concrete on top of 3” com-posite metal deck.

The Transfer TrussAnother key aspect of the tower is a

full story high transfer truss whichspans nearly 90’-0” and picks up theload from three of the towers “bustle”columns, located on the west side ofthe building. Below the eighth floor,these “bustle” columns would have be-come interior columns. In order to cre-ate a cleaner column free space, a largetruss was built into the perimeter mo-ment frame between the eighth andninth floors. This allows the overallsystem to continue to benefit from themodified tube system while economi-cally transferring the loads.

This 90’-0” transfer truss was de-signed almost entirely of W14 wideflange sections. The top chord of thetransfer truss consists primarily ofheavy W14 sections of ASTM A572grade 50 steel. The vertical and diago-nal members range from small W14sections to large W14 members. Thebottom chord of the truss uses custombuilt-up reinforced wide flange sec-tions, sized to accommodate the truss’sclose proximity to the perimeter fa-cade. The nominal section falls withinthe dimensional parameters of a heavyW14 section. The center span of thebottom chord reinforces this built-up

section through the addition of 2” thickflange plates.

A similar system was used betweenthe second and third floors to transfercolumn loads. At this level diagonalmembers, typically consisting of four6x6x3/4 thick angles, were added to theperimeter frame to create a belt andsuspenders system consisting of mo-ment connections and a typical trusssystem. The members which create the“bottom” and “top” chords of this hy-brid system consisted of the W36 mo-ment frame girders which form themajority of the tower’s perimeterframe.

Transfer FloorIn addition to the transfer truss, the

tower has an additional transfer systemat the 25th structural floor level. At thislevel, 12 of the tower’s perimeter mo-ment frame columns at the southeastcorner are transferred through the useof a series of W40 and W36 girders,which average 42” in depth. Thelargest girder in this system is aW36x848. The horizontal shear forcesin the columns are transferred from theupper segment of the modified perime-ter tube to the lower segment of the lat-eral system though the use of ahorizontal truss system.

The modified tube system adoptedfor the 5 Times Square tower serves as

View of the north side elevation looking south.

a practical structural system for sup-porting the elegant lines, angles andfolds created by the architect whileminimizing the impact on rentablesquare footage. The construction of the5 Times Square Tower in one of theworld’s most famous and well knowncity centers is an example of the mar-riage between innovative engineeringand architectural design.

OWNER & DEVELOPER: Boston Properties, Inc., New York City

STRUCTURAL ENGINEER:Thornton-Tomasetti / Engineers,New York City

ARCHITECT: Kohn Pedersen Fox, New York City

STEEL FABRICATOR: Helmark Steel Inc., Wilmington,DE (AISC member)

STEEL ERECTOR: Helmark Steel Inc., Wilmington,DE (AISC member)

STEEL DETAILER: DOWCO Consultants, Vancouver,Canada (AISC & NISD members)

SOFTWARE: Sap90, Etabs, RAMFrame,RAMAnalysis

January 2001 / Modern Steel Construction / 29