Joseph Burns, P.E., S.E., A.I.A., Carol Post P.E., S.E., Garret Browne, S.E. and Suzanne Provanzana

BraceYourself!

Modern Steel Construction May 2002

Steel framing provides flexible layouts inthis recent mid-rise residential condo-minium project in Chicago. The buildingfacade features exposed steel elementsthat mirror the structure behind the sleeksteel and glass curtain wall.

Erie on the Park is one of newest mid-rise residentialcondominium buildings in Chicagos popular RiverNorth neighborhood, an area populated with mid-risecondominium buildings and lofts converted fromwarehouses. Located at 510 W. Erie Street, the 25-

story steel-and-glass-clad structure presents a unique faadeamong its concrete and masonry neighbors.

The building site is in the shape of a parallelogram, with ex-isting neighboring buildings built up to the property line on theeast and west sides, a major street on the south side and an alleyto the north. The dimensions of the site, and consequently thelargest floor plates, are approximately 90 between the existingbuildings and 120 between the street and the alley. The buildingconsists of three concrete stories at the base topped by 22 storiesframed in wide-flange shapes with steel joist floors. The typicalfloor-to-floor height is 10-8.

The lateral system is comprised of concrete shear walls at thebase and three-story steel mega-braces in the steel stories. Thefoundation system consists of grade beams and caissons. Unliketypical mid-rise construction, there is no basement. Conse-quently, a structural slab was designed at the base of the buildingto act as a rigid diaphragm and transmit the base shear to all ofthe caissons.

PRIMARY OBJECTIVESThe primary objective of the building program was to opti-

mize the space within the tight confines of the site. As a result,the 275 high condominium building stretches to the maximumheight allowed for the area. The maximum square footage al-lowed for the buildings footprint is 266,000 sq. ft. and is accom-plished by extending the floor plates to within inches of the lotline on three sides. The owner wanted the building to encom-

pass a range of floor plansand unit sizes, including largepenthouse apartments with ter-races. Twenty-three different unitlayouts were designed creating theneed for varying floor plate sizes andsetbacks at the upper levels. These set-backs create space for terraces for theupper level residences. Equally impor-tant to the owner was a 9-0 clear floorto ceiling height. Access to the resi-dences is provided by two elevators,located in a small elevator core (26-0in the north-south direction, 26-4 inthe east-west direction) located in thecenter of the floor plate. As with somany projects, the owner required thedesign solution meet a compressedconstruction schedule.

CHOOSING STEELWhen schematic studies began, the

design team believed the owners crite-ria lent itself to a concrete building de-sign. The typical bay size was 26 by26, making flat plate construction anoption, provided that no interior beamswere necessary. This solved the gravitysystem; however, the lateral systemwas more of a challenge. Due to thelength and the geometry of the floorplate and the location of the small,square core, the analysis of wind pres-sures showed the inadequacy of ashear wall system, as it did not providesufficient torsional resistance, and mo-ment frames would have been requiredat the building perimeter.

Next, the design team studied steelschemes. Two proposals were pre-sented to the owner: (1) compositerolled shape members and (2) rolledshape members in the east- west direc-tion supporting steel joists in the per-pendicular direction. Composite steelrolled shapes and metal deck were notshallow enough to provide the 9 clearfinished floor to ceiling heights desiredby the owner and the architect. How-ever, closely spaced joists supported onrolled shape girders resulted in 9 ceil-ing heights, while also allowing longerspans between columns than couldhave been achieved with concrete.

Having concluded that the gravitysystem was reasonable, the next stepwas to tackle the lateral system. It wasclear the lateral system could not work

as a brace system in only the core forthe same reasons the shear wall systemfailed to perform. However, collabora-tion with the architects helped Thorn-ton-Tomasetti conceive of a veryefficient use of steel bracing by usingtwo 90 dividing walls that spannedacross the floor plate in the east-westdirection. It was immediately apparentthat within these walls was the perfectplace for three story high mega-braces.

To solve the lateral wind pressuresin the north-south direction, the archi-tect agreed to move the mega braces tothe exterior of the building, and theowner was pleased by the notion of ex-pressing a structural steel brace as partof the exterior cladding. The braces inthe north-south direction are alsothree-story, but unlike those in the di-viding wall, only occupy a 52 width.

The final decision to build the struc-ture as a steel building was made bythe owner/contractor for this develop-ment. The owner strongly felt thatchanging concrete formwork fromfloor to floor in 25-story building, par-ticularly considering the non-uniformfloor plates of the upper levels, wouldcause unaffordable delays.

The owner liked the expression ofsteel on the exterior and truly believedthat at the scheduled time for construc-tion, steel was a faster end date thanconcrete. By the selection of materials,the engineer found opportunities incollaboration with the architect to use

May 2002 Modern Steel Construction

West elevation of 510 W. Erie. The steel andglass curtain wall stands out against thesurrounding masonry buildings.

Top floor plan. Floor plates are in theshape of a parallelogram to fit within theboundaries of the lot.

dividing walls to camouflage the steelbracing on the buildings interior andexpose the bracing at the exterior, solv-ing any torsional issues.

In addition to the superstructurelimitations, the owner required that atleast one garage space be provided perunit, since parking is at a premium indowntown Chicago. Parking was ac-commodated in the concrete base of thebuilding and in the first two levels ofsteel framing, which are framed withrolled shapes and 3 composite deckwith 41/2 of normal weight concretetopping. Four levels of a six story pre-cast concrete parking garage adjacentto the building connect to the parkinglevels of the main building.

JOISTS AND SHAPESAs previously described, the typical

floor plate of the building is in the

shape of a parallelogram. The gravitysystem consists of W12 beams andgirders and 14K joists. The typical baylength is 26 and the joists are spaced atabout 2-7 on center. Half-inch con-form deck with 2 of normal weightconcrete is used to create a total floorsandwich thickness of 17. The use ofjoists allows the sprinkler piping andmechanical ducts to run through theframework and raises the soffit towithin an inch of the bottom of thejoists. As the girders supporting thejoists are depressed 21/2, it was possi-ble to make them composite girders,which helped to limit the depth of themembers to 12. A 2 composite deckwith 3 normal weight concrete top-ping is used in the core, where joists areomitted from the framing. At the me-chanical penthouse level, 18K joistsspanning 26 at 2-7 on center withrolled shape girders varying from 16 to24 are used due to the heavier loadsand snowdrifts. The floor deck at themechanical penthouse level is a 2metal deck with 21/2 normal weightconcrete topping. The roof consists ofW12 rolled shapes and 3 roof deck.The columns are W14s, and the typicalfloor-to-floor height is 10-8. RAMStructural System was used to modelthe gravity system. Joist floor vibra-tions were checked using Steel Joist In-stitutes Steel joist vibration program.

Joists and rolled shapes used in thetypical floor framing made the struc-ture very light, which was beneficial inthe column design but provided an ob-stacle in the column base plate design.Since the tower was very tall but light,large tensile forces developed in two ofthe columns at the base of the steelframing due to overturning moment.The tension was resisted with large ten-sile base plates anchored into the con-crete shear walls. Another obstacle wasencountered in the construction of thebuilding due to the light joist construc-tion. In typical steel construction, thesteel for the floor level to be con-structed is picked and laid on the floorimmediately below. However, with thesteel joists and half-inch conform deck,the floor system was not strong enoughto hold the weight of the steel for thenext floor. Although each steel pickwith the crane came from the ground,resulting in a slightly longer than antic-ipated time to place the steel, this didnot seem to impede the speed of erec-

tion. The building was constructed atan average rate of one floor per week,including the architectural steelcladding.

LATERAL SYSTEMThe lateral system comprises three

story high chevron mega-bracedframes. Each chevron consists of a 52spandrel beam at the base, two 42 di-agonals and a 33 column bisecting thechevron. Four chevrons form a rectan-gular box at the center of the building,with two legs of the box extending inopposite directions at the exterior ofthe building. There are seven three-story high mega-frames and one two-story mega-frame for a total of eightchevrons stacked vertically. By usingthe steel mega-braces and pushing twoof the braces to the outer edges of thebuilding, both torsional accelerationsand lateral drift are well under the ac-ceptable limits. Eliminating the needfor a concrete core, the architectsgained more flexibility in laying out theunits. ETABS software was used tomodel the lateral system. Due to the ir-regular shape of the building, the cityof Chicago required that a wind tunneltest be conducted. Wind pressures andbuilding accelerations were deter-mined by Rowan, Williams, Davies &Irwin, Inc. from Ontario, Canada. Nounusual wind pressures were found,and torsional accelerations were withinacceptable limits.

EXPOSED STEELThe 510 W. Erie building is clad in

glass and architecturally exposed steel.The architecturally exposed steel mim-ics the braces on two of the buildingsfaces. The exposed steel reveals compo-nents of the buildings structure whilehiding the fireproofing required for thestructural steel. The architecturally ex-pressed chevrons differ from the struc-tural chevrons in beam size and infabrication. The cladding chevrons arecomprised of W10 and W16 spandrelswith W12 columns and W21 diagonals.The structural chevrons have W12spandrels, W14 columns, and W8 andW10 diagonals. To meet an AESS Spec-ification for fabrication tolerances ZalkJosephs, the fabricator, built a jig to as-semble the architectural chevrons intheir shop. Since they were to be usedas cladding, the frames were weatherwelded and shipped as modular units

Section showing both structural and archi-tecturally exposed steel. The 1-8 separa-tion was held constant around the building'sperimeter.

Modern Steel Construction May 2002

to the site, and the windows were fit-ted within these frames. The structuralmega-braces were erected in a conven-tional manner at the site. Bolted con-nections were used for the structuralbraces to save time and cost. Spandrelsare wrapped in architecturally exposedW21s to mask the interior floor systemcreating a ribbon of steel around thebuilding at each level. All of the archi-tecturally exposed steel is painted withwhite TNEMEC paint system. Theoverall steel tonnage for the buildingwas approximately 2,005 tons, includ-ing the architectural steel, and the totalcost of the project was roughly $25 mil-lion.

XSTEELAt the early stages of design, Thorn-

ton-Tomasetti suggested to the ownerthat a steel drafting/fabrication pro-gram might be a useful tool to reducetime in creating shop drawings. Thepremise was that since steel sizes aredrawn by design engineers first in 3-Dmodeling programs used for analysis(i.e. ETABS, RAM Structural System),why not use a program that allows thegeometry and the members to be ex-ported straight from the analysis andinto fabrication. This process wouldsave time and reduce potential errorswhen transferring information fromanalysis programs translated into the 2-D in AutoCAD environment. Also,since the building is three dimensionalin reality, a 3-D modeling tool could beuseful to spot problem areas or difficultconnections before being sent to thefabricator and would eliminate theneed for steel shop drawings. In an at-tempt to accelerate the steel produc-tion, a 3-D model of the building inTekla Corporations Xsteel was created,excluding the bottom three concretestories. Thornton-Tomasetti Engineerscreated the Xsteel model because of thedifficult geometry involved in thebuilding because it made sense for theengineers already familiar with thebuilding to construct the model. Thebuilding is clad in architectural steel,and the program allowed Thornton-Tomasetti to model the architecturallyexposed structural steel along with thestructural steel to see how the twolooked together. It was very importantto the owner that the architectural steelconceal the gusset connections betweenthe braces, columns and beams.

The Xsteel program was used byDowco, the projects steel detailer, todetail the steel connections of thebuilding. The computer program cre-ated a 3-D model that included an ex-truded image of each piece of steel. Thedetailers used the model in Xsteel tographically add their connections. The3-D model was then be sent to the steelfabricator, who fabricated the steel byreading the information directly fromthe model, eliminating the need forsteel shop drawings. Additionally, thearchitects used the model to see howthe structure fit within the architecture.

Overall, using Xsteel for the firsttime in Thornton-Tomasettis officewas a positive experience in spite ofsome challenges along the way. The ar-chitectural design was not finalized atthe time the model was created, so sub-sequent changes to the architecture(such as the core dimensions) had largeimpacts on the model and were time-consuming to remedy. Using Xsteel as adesign tool has the potential to helpoverall discipline coordination on com-plicated projects and is clearly thewave of the future.

The use of steel both structurallyand architecturally allowed the designteam of Erie on the Park to create a dis-tinctive building. While steel joists androlled shapes are not typical in residen-tial mid-rise construction, this combi-nation of materials proved to be aneconomical and efficient way to framethe structure. It gave the developer andthe design team the opportunity toachieve an aesthetic structure, unlikeany other that could have been createdwith other structural materials.

Joseph Burns, P.E., S.E., A.I.A., servedas the Principal in charge of the 510 W.Erie project. Carol Post P.E., S.E., is a Se-nior Associate, and Garret Browne S.E. isan Associate; both served as Project Man-agers. Suzanne Provanzana is a Senior En-gineer and design team member. All arewith Thornton-Tomasetti Engineers inChicago, IL.

OWNER AND DEVELOPERSmithfield Properties, Chicago, IL

STRUCTURAL ENGINEERThornton-Tomasetti Engineers,Chicago, IL

ARCHITECTLucien Lagrange Architects, Chicago, IL

STEEL FABRICATORZalk Josephs Fabricators,Stoughton, WI (AISC member)

JOIST FABRICATORCanam Steel Corporation, Washington, MO (AISC member)

STEEL ERECTORArea Erectors, Rockford, IL (NEAmember)

STEEL DETAILERDowco Consultants, Ltd., Vancouver,BC, Canada (NISD member)

GENERAL CONTRACTORWooten Construction, Ltd., Chicago, IL

SOFTWAREXsteel, RAM Structural System,ETABS

3-D rendering from Xsteel showing bracing.

3-D rendering from Xsteel showing a por-tion of floor framing and structural bracing.

May 2002 Modern Steel Construction