Whats New in Hi Rise by Mcnamara

7/29/2019 Whats New in Hi Rise by Mcnamara

http://slidepdf.com/reader/full/whats-new-in-hi-rise-by-mcnamara 1/5STRUCTURE magazine • September 2005

Systems - GeneralThe lateral load resisting systems for

tall buildings have been nicely organized

into efficient types, ranked according tooptimum performance for a given heightor number of floors. These configurationsare shown in the well-published diagramdeveloped by The Committee on TallBuildings and Urban Habitat, GroupSC, 1980 (Figure 1). Well known systemsapplicable to any structure occupy theleft side of the chart, and systems moreapplicable to tall and super tall structuresoccupy the right side of the chart.

This chart has provided direction to de-signers for optimum system selection and is

still very useful today. Most tall buildings, atthe time the chart was developed, had regu-lar plan layouts and a regular vertical façade without significant set backs. The lateralsystems were continuous vertically anddid not deviate from the simple diagramsshown in the chart.

Many of these systems, especially thoseleft of center of the chart, are well suitedfor approximate hand calculations. Designcriteria such as lateral deflection limitationsand inter-story drift indices were used inconjunction with the hand computationsto produce very economical designs.These lateral load resisting systems (twoto eight stories) usually contributed a small increment to the total structuralcost. When designing taller buildings, thelateral systems became a much larger part

of the structures cost and adherence touniformity implied in the system shownin the chart was mandatory. The approxi-mate analysis methods used also requiredthat the organization of the system notdeviate from the assumptions built intothe methods used to analyze and design it.

Some CurrenTrends in High Ris

Structural DesigMixed Syste

& Irregular Load PatBy Robert J. McNamara, P.E.

Figure 1

STRUCTURE magazine • September 2005



Figure 3a

20

With the availability of sophisticated struc-tural analysis and design programs, Engineersbegan to take liberties with the formal orga-nization of the standard systems presentedin Figure 1. Architectural forms began todeviate significantly from simple rectangularboxes. Plan forms no longer remained similarfrom floor to floor. Plan shapes became dis- jointed, wall surfaces faceted, curved, sloped,

etc., all of which demanded the structuralengineer create systems to accommodate thenew building envelopes. The simple orderly systems categorized in the chart no longer worked with the architectural envelopes be-ing developed for new buildings. One of themore important consequences of these new architectural forms was that the structuralsystems were no longer continuous verti-cally, and that systems that engaged the en-tire building geometry such as tubes becameruptured and discontinuous. The fundamen-tal principles guiding the conceptual orga-nization of the structure were not changed,but the load path which engaged the lateralsystems became much less direct than thatof the more traditional systems. Because of this, the usual analytical assumptions inher-ent in many of the commercially availablecomputer programs such as rigid floors areno longer accurate. The deformations in pre-viously assumed rigid elements have becomean important aspect in the overall behaviorand analysis of these elements, and must beaddressed in the design.

To accommodate these new forms, engi-neers developed their own array of mixed

systems. A blending of the various classicallateral systems to accommodate the irregulari-ties and discontinuities was developed with thehelp of the sophisticated computer programsand new design methods. Moment frames

were mixed with braced frames rather arbi-trarily to create structural systems which con-formed to the irregular architectural layouts. All of these systems work toward the same goalof delivering the lateral wind or seismic loadsto the foundations, albeit rather indirectly. A good example of the mixed systems composedof various classical parts is shown in Figure 2 .The structure is the First Bank Place in Min-

neapolis, MN designed by CBM Engineers,Inc. This structure is developed to accommo-date the various architectural plan and façadechanges with height. Clearly a very carefulconsideration of all the load path deforma-tions and corresponding analysis assumptionsis important for the actual structure to per-form as the designer intended.

Another very important issue influencing the selection of structural systems was thegradual inclusion of seismic design require-ments throughout the county. No longer isit appropriate for the designer to concludethat “wind governs” or “seismic governs” by comparing the gross base shear on the build-ing. The nature and treatment of seismic and wind forces are quite different. Current seismiccode design forces can only be rationalized by assuming damage to the structure. The actualforces generated by a seismic event are muchgreater than the code forces, and these forcesare assumed to be reduced by the energy dissipated in structural damage. This is notthe case for wind code forces. Wind codeforces are derived from the wind velocitiesexpected, and are to be resisted elastically anddo not assume energy is dissipated in their

development. The end result of this dichotomy for tall buildings is the total seismic designforce, which may be numerically less than the wind force. However, the assumption thatthe structure can dissipate energy implies

Figure 2

that the elements being required to disenergy meet critical detailing requiremDamage must occur and be contConnections now need to develop thcapacity of the member in order to dithe energy implied by the seismic procedures. In most cases, it is the yiof the connections that dissipates thessary energy. It should be noted thresulting damage from the 1994 NortEarthquake convinced many owners

need for structural systems which appthe seismic risk from a more logical dcontrol point of view. Dissipating the required by the code procedures, co with non-ductile connections, produ



Figure 3b

damaged structure that proved to be toocostly to repair for use after the Northridgeearthquake. This code design approach andrelated performance problem has led toan increased interest in energy dissipating systems, and to systems where the dissipat-ing elements are designed directly for theenergy to be dissipated and the structuralsystems are intended to have little or nodamage. It is with the introduction of thesecombined systems that the importance of the

deformations along the load path becomes of paramount importance.

This article examines several projects thatillustrate some of the structural solutionsused in today’s tall buildings. In most cases,the solutions derived for today’s new formsare not new, but an adaptation of the triedand true systems outlined in the early 60’sand 70’s and idealistically diagramed in thechart shown in Figure 1. The adaptationhas been made possible by the ease of useof structural analysis and design computerprograms available on the commercial market.

However, application of the analysis anddesign programs with a critical eye towardstheir inherent built in assumptions, such asrigid diaphragms, and deformations along the load path, must be a prime considerationin applying the programs to today’s systems.Common assumptions relating to simpleconcrete member properties, such as crackedor uncracked section properties, also has animportant effect on the overall behavior of the system.

Along with the current approaches comes a curious development of a new jargon, whichseem to derive from the fashion industry.

Hats describe outrigger systems, belts foroutriggers without diagonals, etc. Similarterms such as bustles, bandages and zippershave found applications to structural systemsand have become common nomenclature forthe new combinations of systems.

Systems – Fancy Hats An example of the variation on a tradi-

tional system is shown for a 30 story con-crete tower currently under construction.The program requirements from the ownerdemanded a concrete tower with minimalfloor to floor heights. A restrictive urban siteresulted in an “L” shaped plan, with park-ing below the housing units. Economy of theconcrete structure called for a gravity system with a minimum of beams. All of this meantthat the lateral system would be confined tovery slender shear walls of concrete surround-ing the elevator core and a few isolated shear walls. The height to width ratio of the corealone was over 20 to 1, resulting in an ex-

tremely flexible structure. In order to developan economical system with these constraints,the adoption of an outrigger system com-posed of deep concrete beams (albeit slightly irregular) mobilizing the core with exteriorcolumns to resist the lateral loads was devel-oped (Figures 3a and 3b). The outrigger is a “hat” girder. The complexity of the fancy hat

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girder is an extension of the outriggecept shown in Figure 1, although not able to hand computations. The complarrangement of the stiff outrigger girdthe roof provides the necessary engagof building columns resisting the oveing effect with the slender core and stifthe lateral system. The deformations floor system have an important impact

effectiveness of the hat system engagiexterior columns. The resulting forces shear walls, engaged exterior columngirders, foundations, etc. are very depeon the assumptions used in the analythose elements. The outrigger engagterior columns can be in net tension, significantly reduces their effective arstiffness in the lateral system and accorreduces the effectiveness of the systemdesign is best served using the hat systengage the heaviest gravity loaded excolumns to eliminate this undesirablesequence of the system performance highly dependent on the element proassumptions (cracked or uncracked). Tsue is also an important aspect in the sis of the concrete system under the of code seismic forces, where the selectthe response reduction factor is not obMost common computer programs weasily account for different element p



Y

X

Figure 4b

ties, which depend on whether the element isin tension or compression.

Another example of a similar concept isshown in Figures 4a and 4b for a project cur-rently in the design stage. Here, a series of deep beams serve as an outrigger system foranother very slender concrete core. Again, theirregular “hat” serves to mobilize a larger por-

tion of the overall structure, reducing upliftloads in the slender core and stiffening thestructure to minimize lateral deflections.

In Figure 5 , a circular plan for a 500-foothigh office building is shown with wide col-umn spacing around the perimeter, eliminat-ing the possibility of an economical perim-eter frame. The interior layout is that of a square central core. The core is a braced steelframe with a height to width ratio of 8. Thislarge slenderness produced a relatively heavy steel frame. Another fancy hat system wascreated with steel truss outriggers as shown

in the plan. The hat was also supplemented with outriggers near building midheight toadd additional lateral stiffness to the system.This hat and outrigger addition providedsignificant overturning resistance and lateralstiffness to the internal core. The odd geom-etry of the diagonal core intersection pre-sented interesting steel detailing problems,but overall the “Fancy Hat” resulted in a very economical structural steel frame. The con-nections in the exterior columns engaged by the hat trusses required special considerationalong with the detailing of the load path

through the hat/core interface.

Systems – Fancy Belts A concept which replaces the outrigger

idea, but behaves in a very similar manner,is to use a “belt” system which eliminates theoutrigger diagonal. The traditional outriggeris necessary to mobilize more of the building geometry to resist lateral loads, namely theexterior columns. When the outrigger must beplaced in the middle of the tower, the outriggerdiagonal is usually problematic with interiorplanning of the user space. Traditionally, the

outrigger has been restricted to mechanical

floors and the roof. Alternatively, the diagonalbrace can be replaced by a “belt” around theperimeter, which is a story deep vierendeelgirder around the perimeter. The belt needsto be very stiff to eliminate the need forinternal diagonals. The load path of theforces through the diaphragm floors into thebelt and the deformations of the floor are

complex and require a very a detailedanalysis. Only through the use of presentday sophisticated finite element computerprograms can this type of structure beproperly designed and detailed.

The tower shown in Figure 6 is currently

under design utilizing the belt concstiffen the structure and force the ostructure to act as a combined sheartube system. No internal diagonals orspandrel beams are used. In order to the effective height-to-width ratio ostructure, “belts” were required at intermediate floor levels and steel outr

were used at the top. All of this efforforce the entire structure to act as a unified “tube” system. The height-to-ratio of the core walls alone is 25, muslender for an economical structureheight-to-width ratio for the overall str

Y

X

Figure 4a

Figure 5



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is 8, still slender but economically possible. As an interesting side note, the tower

preliminary design was done utilizing the Natural Hazard Aerodynamic LoadsDatabase (NATHAZ) sponsored by theUniversity of Notre Dame. The databaseat this site was used to determine targettranslation periods to control the expectedmotion levels of the upper (and most

expensive) condo floors. The number andlocation of the belts in the structure werethen studied to produce building periodsmatching those derived from the NATHAZ website (http://www.nd.edu/). The design was then tested in a wind tunnel at theUniversity of Colorado, using a forcebalance model to predict the top floor ac-celerations. The approach using NATHAZ website methodology proved to be very useful in generating realistic preliminary

Figure 6

target periods for the building, satisfying the motion comfort criteria. As a result of these studies, preliminary planning now allows for a Tuned Mass Damper to beincluded in the early planning and costallowances. This tower is being designed inassociation with KLA of Colorado

Future Trends

The fragmentation and rupturing of thetraditional structural systems has producednew structural forms, which in many casesare the simple juxtapositions of classicalsystems. Frames are uncoupled and mixed with braced diagonalized systems. Systemsare discontinued and switched from onetype to another depending on the architectur-al forms. However, the nature of gravity andlateral forces of nature was not changed. A clear and distinct load path is still themost important aspect of any tall building’sstructural system. The forces for one systemtransferred to another must be carefully

traced and accounted for. The usual “rigiddiaphragm” and other similar assumptionsimbedded in today’s computer programsmust be carefully examined for each of thesenew structural systems. As designers, we mustensure the structure can do what it is being asked to do. The imbedded assumptions inthe sophisticated programs in common usetoday must be clearly understood, and any deviation from these assumptions and theireffects on the overall behavior of the struc-ture must be clearly known to the designer.▪

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Robert J. McNamara, P.E., S.E. has been practicing structural engineering in the Boston area for over thirty-five years and

serves as President of McNamara/Salvia Inc.He can be reached via e-mail at

[email protected]

Structural Systems for Tall Buildings ,by the Council on Tall Buildings and

Urban Habitat (1995), is published by McGraw-Hill Publishing Company.

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Whats New in Hi Rise by Mcnamara