Hybrid Design Reduces Costs(2)

Switching from aconcrete frameto steel beamsand concrete

columns provedadvantageous

WHEN DESIGN FIRST BEGANON THE 30-STORY IJLFINANCIAL CENTER INDOWNTOWN CHARLOTTE, the pro-ject team assumed that thebuilding would be cast-in-placeconcrete. However, AISC ActiveMember SteelFab, Inc., with thehelp of Nucor-Yamato Steel, pro-posed a steel alternative to theowner with a guaranteed maxi-mum cost.

While the proposal includedsome common steel design con-cepts, such as cambered beamsand the use of LRFD, it alsoincluded some creative methodsto minimize steel weight.SteelFab proposed framing steelbeams spaced 15 on center intoperimeter columns in four placesat each level. A 3 lightweightstructural concrete slab wasplaced over a 3, 19-gage compos-ite steel deck. While the 3, 19-gage composite steel deck is typi-cal, it was not adequate for the15 span. Instead of reducing thedeck span by adding anotherbeam, the deck was increased toa 16-gage deck in these spans, aswell as one adjacent span.

FABRICATOR INPUTAfter reviewing SteelFabs

proposal, the owner decided thatstructural steel was the mosteconomical material for thebuilding. Steven Hamvas, P.E.,with Stanley D. Lindsey andAssociates, Ltd., said it was thepush by SteelFab and Nucor-Yamato Steel, which providedmaterial pricing and availabilityinformation, that persuaded the

Modern Steel Construction / June 1998

HYBRID DESIGNREDUCES COSTS

owner to switch from concrete tosteel. The effort put forth bythe steel fabricator and steel pro-ducer in convincing the owner touse structural steel should serveas an example for future pro-jects, Hamvas added.

The lateral support system forthe structure comes from ahybrid system consisting of steelspandrel wide flange sectionsand concrete columns. However,the integration of the concretecolumns with the structural steelpresented a challenge since theconnection between the steelspandrel beam and the concretecolumn had to be designed as arigid joint. Closely spaced col-umn ties within the depth of thejoint were required to confine theconcrete and vertical reinforcingbars. Holes were provided in thewebs of the steel spandrelbeams, within the depth of thejoint, to allow for two diame-ter reinforcing bars to passthrough where the bars could belapped for continuity.

The structural engineerworked closely with the steel fab-ricator to assure that at eachlocation there would be a hole inthe steel spandrel beam web.Another complication was thatvertical reinforcing steel for theconcrete columns could not bespaced evenly at the perimeter ofthe column, a typical method forconcrete construction, but had tobe placed strategically to avoidboth the steel spandrel beamsand the floor beams framing into the joint.

FAST ERECTIONAnticipating that steel would

rise faster than concrete, thecontractor wanted to construct10 levels of structural steelahead of the concrete columnconstruction. W14x109 steelerection columns were construct-ed to support 10 levels structuralsteel, steel deck and concreteslab. The rigid connectionsbetween the steel erectioncolumns and the spandrel beamsenabled the columns to providesome of the temporary bracingfor the structure during the con-


Tower Framing Plan

-Level 30Tower Fram

ing Plan Levels 4-12, 14 & 15


struction.Another difficult challenge of

the project was to calculate theamount of differential columnshortening that would occurbetween the interior steel corecolumns and the perimeter con-crete columns. Steel columnsshorten due to elastic shorten-ing, which is related to the mag-nitude of the load. Concretecolumns shorten due to elasticshortening as well as creep andshrinkage strains. In order tomake an estimation of the axialshortening in the steel erectioncolumns, it was important toknow how many levels of slabswould be placed ahead of theconcrete columns. The contractorintended to place concrete on thesteel deck just seven levelsbehind the steel erection andplace concrete for columns ninelevels behind the steel erection.It was determined that theperimeter concrete columnswould shorten about 1 morethan the steel columns. In orderto counterbalance this difference,the steel columns at the 10thlevel were fabricated 5/8 longer,and at the 19th level they were3/8 longer than the theoreticallength.

Typical steel beam to concrete column connection detail

Typical column tie detail

Typical column tie detail

TREE COLUMNSA concept discussed early in

the projectand ultimatelyadoptedwas the use of prefab-ricated tree columns consist-ing of 36 deep spandrel beamsand W14x109 erection columns.The two-story tree columns con-sisted of 10 long spandrel pieces,one full story erection columnand two half-story erectioncolumns. The tree columns wereconnected to the top of the lowertree columns, and then fill-inpieces of spandrel beams wereplaced between the spandrels ofthe tree columns and weldedtogether to make a continuousspandrel beam at the perimeterof the building. Unfortunately,the full benefit of the treecolumns could not be realized.

While the steel erection wasfast, the concrete column con-struction lagged behind to thepoint where temporary suspen-sions of steel erection wererequired in order to allow theconcrete construction to catchup. Despite these delays, thestructural steel for the buildingwas completed in 24 weeks.

The initial design of the corecolumns indicated that columnsections as large as W14 x 730would be necessary. One of thekey elements to guaranteeingthe cost was that the steel had toprovided by Nucor-Yamato,which at the time of constructiononly produced members up toW14X398.

If we used something larger,we would have to have gottensomething from overseas, Ham-vas said. If the shapes had to beobtained from overseas, the fab-ricator would not have been ableto keep to the projects guaran-teed price. Instead, it was agreedthat a built-up box section con-sisting of plates welded to theflanges of the W14x398 would beused for any column required tobe larger than a W14x398.

The use of structural steelgoes beyond just the buildingsframe. The foundation systemfor the building also utilizesstructural steel. It consists of

HP14x89, grade 50, piles with acapacity of 200 tons driven intopartially weathered rock. Again,the economical value of usingsteel prompted the owners deci-sion.


Project: IJL Financial Center

Charlotte, NC

Owner:201 North Tryon Acquisi-

tion Co.Atlanta

Structural Engineer:Stanley D. Lindsey and

Associates, Ltd.Atlanta

Architect: Smallwood, Reynolds,

Stewart, Stewart & Asso-ciates, Inc.

Atlanta

AISC Member Fabricator & Detailer:

SteelFabCharlotte

General Contractor:Beers Construction Co.

Atlanta

Hybrid Design Reduces Costs(2)

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