Modern Steel Construction / November 2000

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TThe administration saw theBaruch College Academic Centeras an excellent opportunity to

obtain a variety of needed facilities.Sited on a midtown Manhattan block,the Center is an urban campus inthree dimensions. The building isorganized around an atrium thatextends upward, fifteen stories fromthe ground level, changing shape asit rises. Below grade rests a world-class athletic facility with two gymna-siums, a pool and racquetball courts.Also located below grade are a per-forming arts center with an auditori-um, a theater and a student center.The academic spaces reside on theupper floors and include classrooms,computer labs, lecture halls, facultyoffices and lounge. Elevators locatedon one end of the atrium and a mul-titude of stairways provide verticaltransportation for the three thousandstudents that will pass through thebuilding every day. The academicspaces are arranged so that studentswill use the elevators only whenarriving and generally be able to usethe stairs, climbing or descending nomore than two stories between theirclasses and offices during the day.

The atrium looks as if it had fol-lowed the path of a lightning boltthat cut through the building. Itpenetrates the south façade of thebuilding on the upper floors andextends down to the ground floor onthe north side, allowing daylight toflood the center of the 200’ by 330’building. Both the north and southsides of the building have verticallycurved facades that not only add tothe structural complexity of thebuilding but introduce visual excite-ment as well.

FFoouunnddaattiioonnThe site occupies a 197’ block-

front on Lexington Avenue between23rd Street and 24th Street andextends 336’ east toward ThirdAvenue. The western end of the siteis excavated 42’ down to providethree levels below grade. AnOlympic size swimming pool in the

Modern Steel Construction / November 2000

central part of the site extends a fur-ther nine feet below the lowest base-ment. The eastern end of the site,only excavated for two basement lev-els, avoids heavy rock excavation inthe vicinity of an existing residentialbuilding. In general, all footings areon rock with a 40-ton capacityexcept in the eastern end of the sitewhere the bearing capacity equals 20tons per sq. ft. Recognizing the sensi-tivity of the neighbors, vibrationmonitoring of the adjacent propertieswas instituted during the rock exca-vation phase of the project. Groundwater is located about 45’ belowgrade, which places it below the low-est basement but above the bottom ofthe pool. To counter the resultinguplift forces on the pool slab, rockanchors spaced eight feet on centerin each direction were provided.The lowest basement level, except forthe pool area, contains an under-drain system to catch any stray waterthat may seep through the rock.Sheeting with a temporary tie backsystem builds the basement slabs andwalls without the encumbrance ofinternal bracing.

SSttaaggggeerreedd TTrruussss SSuuppeerrssttrruuccttuurreeTo accommodate the various

aspects of the program, structuralspans varied from 30’ to 120’ andwere distributed both in plan andvertically throughout the building.Given that steel would provide theonly solution for this problem, a con-ventional approach would have beento design girders or trusses above thelongest span spaces. Such a solutionwas explored in the preliminarydesign phase of the project andshowed a severe penalty in tonnageof steel, therefore increasing the cost.Since the $168 million budget forthe project was fixed and very tightfor an academic building of 785,000-sq. ft., another approach was needed.Weidlinger Associates proposed reor-ganizing the building to achieve anefficient structure that included stag-gered trusses to accommodate thelongest span spaces. With only minor

revisions to the original layout, thearchitects, KPF, placed the longestspan spaces on the east side of thebuilding. The relatively smallertheatre and auditorium were placedon the west side, which has a typicalsteel beam and concrete slab onmetal deck framing system.Classrooms and offices, positionedthroughout the building, shifted asneeded to accommodate the newplan.

Vierendeel openings in story-high truss.

Perspective view of staggered trusses within building envelope.

To develop a space and cost-effi-cient frame for the long span side ofthe structure, Weidlinger engineersused the staggered truss concept veryefficiently. Originally developed forapartment buildings in the 1970s byMIT engineers, the concept intro-duces story deep, long span trussesarranged in a staggered pattern verti-cally. In this way, floors only have tospan from the top chord of one trussto the bottom chord of the next one,while the open space on each floor is

Matthys P. Levy, P.E. is a principalwith Weidlinger Associates in NewYork City.

Modern Steel Construction / November 2000

double the floor span. Viewed threedimensionally, the whole system pro-vides rigidity in the direction of thelong span trusses with the floors act-ing as diaphragms, transferring loadsfrom one truss to the next as lateralforces descend to the ground.

OOppeenn FFlloooorr PPllaannThe principal advantage of using a

staggered truss system lies in theability to create wide-open spaceswithin a restricted floor-to-floorheight. The disadvantage is the exis-tence of diagonal members crossingthe space along the truss lines. Toovercome this problem, Vierendeelpanels allowed corridors to passthrough. Since such Vierendeel pan-els introduce bending in the trussmembers, a penalty of increased steelweight occurs. To minimize thiseffect, the Vierendeel panels werepositioned as close to the center ofthe truss as possible where the bend-ing effect is the least. In addition tothe staggered trusses on the east endof the building, numerous other iso-lated trusses satisfy specific planningrequirements. For instance, a two-story high truss between the fourthand sixth floors carries the loads ofthe two floors below it with a hangersystem and also supports the floorsup to the penthouse above. Sincethis truss is on the curved façade ofthe building, it is sloped to match thefaçade contours. There are othertrusses at the ground floor over thepool, the gym and the theatre. In all,the project includes twenty trusses,most of which are a story high and120’ long.

LLaatteerraall SSyysstteemmSince the building’s structural grid

is so irregular, providing lateral forceresistance against wind and seismicforces proved to be a challenge. Inthe east-west direction, the staggeredtrusses provided a substantial portionof the lateral force resistance. In thenorth-south direction, vertical bracedframes were introduced for lateralstiffness. But, as nothing in this

structure is straightforward, the posi-tion of the atrium that virtually cutsthe building in half required theinteraction through the floordiaphragms to distribute the lateralforce around the atrium.

Finally, the sloped faces of thenorth and south facades above thesixth and eighth floors respectively,introduced horizontal forces that hadto be resisted by the vertical bracingsystem.

CCllaaddddiinnggTo support the façade of the

building that includes masonry andmetal panels, girts were placed out-side the columns at a vertical spacingdictated by the requirements of thepanels. This material was included inthe 6600 tons of steel provided bythe fabricator.

CCoonncclluussiioonnThe imaginative application of the

staggered-truss framing system to thisstructure allowed the owners to com-bine their varied program needs intoa mixed-use structure, maximizingtheir investment.

Circular stair on north side of building.

Structural Engineer:Weidlinger Associates, NewYork, NY

Architect:Kohn Pedersen FoxAssociates, P.C., New York, NY

Fabricator:Canron Construction Corp.

Software: STAAD, RAM Steel