SCALE FORM

Sensory fields, self-reflection and the future Mark Sarkisian is the structural and seismic engineering partner

SOM.

Key to the development of new and the mainte nance of existing infrastructure, neighbour-

hoods, campuses and cities is the balanceof resiliency, self-sufficiency and regeneration, To avoid the depletion of natural resouroes,

structures must be designed to be durable and

adaptable, capable of coexisting with imposed environmental conditions while accommodating changes in use.

neighbours. Advances in energy storage wi te developed to bridge periods of limited cr no power derived from solar exposure, while on-c water reclamation, purification and reuse w reduce demand on our most important resource

or

Beyond sustainability, resiliency leads to envir- onmentally sensitive buildings consisting of reused materials that are capable of adaptingto future conditions such as climate change.

Systems within these buildings require a design ethos based on performance whereby every

component has multiple uses: structural systemscapable of heating and cooling, exterior wall

systems capable of absorbing and storing energy and building systems capable of oper- ating with site-based water collection, power

generation and distribution. Buildings must be

completely self-sufficient, without relying on their

Morphogenetic planning of the future wil con. sider weighted parameters for design beyona individual buildings (Fig. 3.2). Form. Duiding material, embedded and operationai carcon daylight, use efficiency, site placement and other important parameters wili be consiaerea even on the district or city scale. at earty concec tual stages. The abundance of data will intorm

sensory fields, where the magnitude and drec tion of oncoming environmental changes can te

anticipated, gathered, reported back and used to inform optimally performing structures. Struc tures will become self-refiective, capable of

undergoing state changes of materials. alowng

component properties to be temporarily altered to efficiently resist abnormalities in ioaaing. Rne

ological systems will use the fiow of materals

3.1 Sensory field data mapp:ng of

San Francisco 3.2 Digital image of San Francisco

33 Example of advanced analyss investigatung the nituence o'

the sensory ieid on spec:ic

structirai response pararneters

46

Ory til f elio.ton l,r

3.2

triggered by advanced analysis models inter-

connected with the building and the region's

sensory field. Structural systems and all building components will be designed to behave natur ally in the environment, free of the potential

for damage from extreme conditions such as

seismicity. Uitimately, structures will exist in a true state

of eauilibrium in which umbilical reliance on

services from other sources is eliminated and

Interactively linking mathematical models to information souroes fed from a sensory field would enable structures to respond inteligently. perhaps systematically changing internal prop- erties or triggering active mechanisms (Fig. 3.3). Material or motion sensor systems would be placed within structures to evaluate in real time

their behaviour under load, correlating actual and predicted performance and also providing

a map of stress states of materials that could

have experienced plastic stress states or per

manent deformations. regeneration of resources is possible; structures

will contribute to the environment rather than

challenge it. This goal will only be achieved

through innovative processes of coliaboration,

invention and integration. Lotte toer diagrid argie Zoneof max1mum structural eticenc

Zone of structural neticiency Sensory fields The response of structures within the urban con text is most significantly affected by imposed

load and material characteristics, including stiff-

Shallow angles resist wind load

450

ness, mass, shape and the connection to the

earth. Mathematical models accurately depict the characteristics of these structures. However,

oading magnitudes and directionalities are

ypically enveloped in the design, leading to Conservatively estimated structural demands and, consequently, a waste of materials. Analyt ICal models stand atone and are systematicaly Subjected to different, imposed enveloped force COnditions on a case-by-case basis. Defining9 the precise magnitude and direction of imposed oads results in optimal performance and the east amount of materials required tor tnis

Tesponse.

360

315

Diagrid nmeriiber #8 180 DIOIOgically based sensory systems used to monitor specific site conditions on a district or Cny Scale can inform buildings systems of a

equired response to imposed demands from

1atural events. A deterministic assessment or

SEISInIC grOund motions, for instance, resuls a definition of force vectors and energy asSO

ed with an imminent earthquake. A similar

ecinique could be used to evaluate wind con dtons. A field of sensors, including acceleroners and anemometers, could pinpoint natura erce fiows on a district or city-wide grid (rIg

35

Steep angles resist gravity load

S0 55 6O E5 70 75: 80 85

Column angie i 3.1).

SCAL tORM

3.4 Huawei Technologies Corpo- rate Campus, Shanghai (CN)

2007

Self-reflection relatively stiff earthquare-resiStng comoorars such as shear walis or steel-oraced trames usually experience significant ductity cemand in severe earthquakes and undergo damage while dissipating energy.

Lengthening a structure's characteristic (funda

mental) period results in less demand from ground motions caused by earthquakes. Main- taining material elasticity is essential to achiev- ing the highest performance with the minimum

damage and the greatest chance of the building

being put back into service following an extreme event. Seismic isolation is a technique of artifi-

cially lengthening a structure's period by de- coupling the building's base from the ground. The rotation or "fusing" of joints accomplishesthe same goal. Cast-concrete roof truss ele- ments incorporating steel pinned connections at the research buildings of the five-storey Huawei Technology Headquarters (Fig. 3.4) unify the buildings into a complex with a consistent and

lasting identity. The trusses are designed to separate or "fuse" at their apexes in major earth-

quakes, breaking the modules linked across the atria into smaller, simpler, stable structures with cantilevered trusses, and reducing potential damage (Fig. 3.5). Link elements that connect

3.5 Fused-pin linking trusses, Huawer Technologies Corp0-

rate Campus 3.6 Frame with pin-fuse joint

connections 3.7 Link-fuse joint connection detail

Link-fuse and pin-fUse joints Shear is typically the criticai force comooner link elements. The link-fuse joint was deeco to withstand severe seismic events ty dssc ing energy through friction siip at a oreset sn force level without experiencing significanor joint yielding damage (Fig. 3.7). By incorporating a joint that "slips" in cron at force levels just shy of potentially damagn shear forces, the link-fuse joint protecs tre

integrity of the link beam components connas preventing yielding and damage in the comoo- nents and, ultimately, costly post-earthauake replacement of the damaged elements. Tne ged of the link-fuse joint is to postpone inceoror of this flexural yielding by introducing mechanca

energy-dissipating friction "siip" prior to oastc hinging of the beam at its ends. Yielaing ana damage thus occur at drift levets signficantiygreater than in structures with traditional ink ele ments. The lateral system is allowed to behae elastically (no sip) in a trequent event (such as

a 25-year event). In a severe event (sICh as a

2475-year event), the link-fuse joint can sip to

accommodate larger displacements, with Sigrit cant energy dissipation due to tricton. This P'o vIdes a high performance level with litle or ro

structural damage and with minimal requiremetor post-earthquake repairs.

Additionally, there is the prospect of being aet to loosen slipped link fuse pins and utilise eas tic energy stored elsewhere in the structure to recentre it if it has experienced permanent o' in a severe earthquake.

Pin-fuse systems remain fxed during wind au

ioderate seismic events, and rotate or slide

when subjected to high demand (Fig. 3.0 Damping systems can also be used to prore

structures and mitigate violent responses i wind or seismic conditions by lessenirng tne dynamic response and inherent inposed

Sensory fields, sel rollectoft a te fuhr

Wall

Link-fuse joint The link-fuse fort fivolves a pretonsioned pir that clamps tv halves c! a irk lcOping) Dearn tOgother unti the stiear tore in hg

hearn exceeds a Certain trreshcid

Beam Connection plate

As the lateral load Cortiriues to increase, the pin traverses te hA

length of the siots irn a evere eurt-

quake and evertiualy re ergago

the wo haives o the bearms. Oniy Embedded Steel nut plate then do tne bearm alves beyand

the fuse attract addtona: torc

and eventually yieldin flexure. ne

detaiis of a typicai lrk-fute int utilise conventiona structurai stee

Comiponernts.

3.6 3.

Informing the structure

Similar to the way fwO plants of the same spe-

cies react difterently to different placements

in the environment with variations in growth

patterns, or the way luminary control devices determine required light levels for supplemental

lighting, structures should be designed to be

environmentally reactive, dynamic and self-

reflective. Sensory information flows would inform structures of anticipated demand and

allow an interactive response. Analytical math ematical models for the structure would be

therefore the capability of force transmission between the ground and the structure. A peri-meter membrane restraining system could be used to limit the possibility of uncontrollable property changes. The viscosity change in the soil would be similar to that of ketchup, which

undergoes liquid property changes through shear thinning when shaken. The fixity of certain joints within the structure could also be modified on demand. For instance, if clamping forces could be temporarily relieved, the stiffness of the frames would be lowered,

directly linked to the imposed magnitude and

direction of load and inform structures of the next steps in response. These next steps could include activating strategically-placed behav- iour-controlling devices or mechanisms that alter

the natural periods of vibration lengthened and the attracted inetial loads reduced. This could be achieved by introducing energy in the

form of heat into these joints, perhaps through fastenings, which wouid increase their lengths due to heat expansion and reduce the clamping forces. To control joint behaviour and allow for structural recentring, joint sinews could be introduced using counteractive high-strength strands or shape memory alloys such as nickel- titanium (NITI), which would use the inherent elastic properties of the materials, or heat is applied, then cooled, changing the characteris-

tics of the material through a martensitic trans-

joint or base connections.

For instance, pneumatic dampers that incorpor ate compressed air or viscoelastic fluid would be

activated and tuned by interactively correlating the actual dynamic response with the predicted

mathematical dynamic response. Air is intro- duced into the handrail to change the dynamic behaviour of a long-span carbon fibre ribbon Dridge to prevent vibration resonance. Damper actvation would be introduced sequentialy, focused on critical areas of the structure where

formation.

Reducing the mass A reduction in mass results in a reduction in their participation could be most beneficial. inertial forces caused by seismicity. This reduc-

tion in mass results in less demand on vertical in the case of strong ground motions, a more effective and sophisticated response would De to create complete separation of the input SOurce from the structure. Temporary evitation created by electromagnetic flow. or air cushions Simuiar to the technology used by Poma Otis for

tne Skymetro train line in Zurich, would provide ICtionless seismic isolation by separating the Superstructure from its foundations. Soil lique- action under foundation systems is typicaly Tgated (for instance by adopting deep foun- 0ations) to control superstructure respornses Curing strong ground motions. but perhaps vnage should be taken of this soil behaviour

uC tne subgrade rheology by using the ground

Ons to reduce soil shear strengths and

load-carrying elements, and usually also in a

reduction of required lateral stiffness. Less mass

results in a lower fundamentai period; however,less stiffness results in a longer fundamental

period (dynamic period is proportional to the

square root of mass divided by stiffness). As the

period of the structure lengthens, less ground

acceleration is felt by the structure. It is known

that approximately 25% of the concrete placed in conventionally constructed buildings is not needed for strength but nerely increases mass

and demand on vertical load-carrying elements

such as walls and columns. For example, most

of the concrete placed in centre spans of struc-

tures is not required and is placed there for ease

49

SCALE M

Recycied inc.lusinn

TTTT

Parametric city model This model interfaces with programs Such as Grasshopper, used to

define geometry. combined with tools such as Galapagos and Karumba. wnch alow generic algo

future. The parametric city model cornbines weighted importance of form, structure, emce ded carbon and efficiency of space use whle considering orientation, inciuding expoeure to daylight and solar gain. Ihe model is also caca ble of evaluating the embedded carbon mpac: of construction with regard to matenial type (stee concrete, wood, masonry, etc.). fabrication and

of construction. In addition, the environment is

becoming overburdened with waste materials

that do not decompose and are not recyclable.

Materials such as lightweight waste plastics and polystyrene could beneficially reduce mass if

strategically introduced into structures where

concrete is not needed. The Sustainable Form Inclusion System (SFIS; Fig. 3.8) - originally

conceived for creating air voids in structures by

placing capped, empty plastic beverage con tainers into structural systems achieves these

goals. More practically, the system can utilise

bricks composed of ground and formed plastics

or waste foamed polystyrene (such as styro-

foam) cast into a lightweight mortar. Environ

mental responsibility could be taken further by

using zero-cement concrete with the use of products such as Greencem, whereby cement

is essentially eliminated and waste blast furnace

slag used as a replacement.

rithms and structures to be defined.

he modet accesses a database of

hundreds O previoUSiy designed and but structures. The recorded

data inciudes structural require-

Tents relative to height, material

type and Ste location (seismicity

and wind conditions) along with

space requrements for building Systems, such as vetical transporta

tion and mechanical systems.

transportation of these materials, construction

time and required equipment, and the numcer

of construction workers and their transportation

to and from the site. With the requirements for the structural and mechanical systems knowr the commercial value of the net availabie space

can be assessed based on its location witnin

the building (such as floor level). access to da light and views. The model is also capable of evaluating the environmental and financial bene fit of incorporating advanced seismic systems

into structures through the reduction of lifecycie

carbon and anticipated damage over tme. and

the cost-benefit of addressing those nsks at the

time of construction. For slender structures or

Morphogenetic planning for the future Evaluating multivariable parametric building models on a district or city scale can be used

to identify the best planning strategies for the structures with complex geometries, parameters can be interactively evaluated with regara to the

advantages of interlinkages or other geotetic

modifications.

These models can be translated into more

sophisticated structural anaiyses to determune where structural material should be placed m

order to ensure that the least amount of ener gy

IS expended when work is done to resist ioad.

In minimising the energy. forces and detcrma ions should be distributed as evenly as pOssibe

throughout the structure by a synergetic place ment of mater ial. Forces will tlow through trhe

easiest, shortest and most naturai toad pah of

the structural form. Topographical optnisatic techiniques are used to map the structurai

response and define the most efficient piace

ment of materials.

On a district or city scale, the environmerntd"

pact of planning can be interactively evaluate

Dased on proposed or anticapated buiding

material type, geometry and site conditoi3

The type of use plays a sign1ficant :0le Ht t

38 Susta.raoa

torm-.nctusion systems (SFIS)

39 Parameric City mode'. iustrating darker areas where

OcCuDed ettiCiency talis below targets

2:h Construct on, Poiy International Piaza, Beijing (CN) 2015

Corrsud 311 Dagram or acade, Foly international Plaza

3.10

overall plan as the requirements for building

systems and structure vary when comparing,

for example. office, residential and mixed use

Occupancies (Fig. 3.9).

The concept of flow can be further developed into structures that are interactively monitored

for movement. Through the measurementof imposed accelerations due to ground motions

or wind, structures could respond by changing the state of the liquid within the system. For

instance, the structure could use endothermic Rheological buildings for the future

The envelope enclosure for structures repre-

sents the single greatest opportunity to consider

flow and interaction between architectural, struc-

tural and building service systems (Figs. 3.10

and 3.11). Hundreds of millions of square metres

of occupied area are enclosed each year with

systens that essentially provide protection from

the elements, safe ocCupancy and internal com-

fort. A closed-loop structural system integrated into an exterior wall and roof system that incor-

porates liquid-filled structural elements could Drovide a thermal store that heats up during the

day and could be used for building service sys tems Such as a hot water supply or heat for

OCCupied spaces during the evening hours.

reactions to change liquids to solids within the

closed network. Sensor devices could inform structural elements of imminent demand and

initiate a state change in liquids that could be

subjected to high compressive loads during

which buckling could occur.

Magnetorheological or electrorheological (ER) fluids could be used to change the viscosity and

therefore the stiffness of closed vessels and

their damping characteristics. When subjected

to a magnetic field, magnetorheological fluids

greatly increase their apparent viscosity and

can become viscoelastic solids. When subjected

to an electrical field, ER fluids can reversibly

change their apparent viscosity quickly, transi

tioning from a liquid to a gel and back again. A SOlar collection system could be integrated into the network and incorporated into double

walil systems, where it could be used to heat he internal cavity in cold climates. Transparent photovoltaic cells could be introduced into the giass and spandrel areas to capture more of the

energy of the sun.

Vihen storing liquids in very tali structural sys ems. pressures within the networked vessels Decome very large. With this level of pressure, Tor example. water could be supplied to the Sructure or to neighbouring structures of lesser

e'gnt without requir1ng additional energy to eit. The energy required to store the water tuSy Is minimised if water was colected at upper levels of the building. particularly root and upper exterior wall areas. A continuous low Veiocity flow or a liquid with a low freezing poinn assing through these systems would keep it m reezing. Liquid in tuned-liquid dampers *n the networked system would conro otion, with fiuid flovw act1ng to dampen the Scture when subjected to lateral loads from ind and earthquake evenis. 311

SCALE + FORM

Wind Gravity load

diagram overturning diagram

Combined gravty wind load dagram:

Structural design of tall buildings bidirg one wird direction

The support of the gravity loads is. of course also very significant. For an economical ta building, it is important to use the vertcal struc ture needed for gravity loads to resist the atera forces as well. This also decreases the tensior in the superstructure from wind loads and tre potential uplift at the foundations. At SOMs is often referred to as "managing gravity" anc

involves moving the gravity torces (and struc ture) to positions where they can also be used in the lateral systerm. The system used to resst

shear can be readily used to move gravty icas to optimal locations. In general, the vertca

elements should be arranged to maximise the

flexural stiffness (a function of the moment of inertia) of the building, assuming that the shear system can move the forces to the coumns and walls. In fact, the further the vertical elemens are from the centre, the lower the uplift for ces

will be (decreasing inversely with distance from the centre) and the higher the stitness

(increasing with the square of the distance rom

the centre; Figs. 4.3 and 4.4)

Bill Baker is the structural engineer ing partner for SOM. Jim Pawikowski is an associate director for SOM.

Tall buildings are one of the great achievements of structural engineering. The desire to build tall

has long been a fundamental human aspiration, as evidenced by the Great Pyramids, the story

of the Tower of Babel and medieval ltalian tower

construction (to name but a few examples).

However, only in the last century has the aspira- tion been truly realised. The development of modern building materials, but more importantly, the birth of the discipline of structural enginee ing- through which structures can be conceived and designed using the applied principles of

mathematics and physics allowed humans, for the first time in history, to construct buildings that reach extraordinary heights. Many of the new tall building systems that have allowed and

continue to allow ever-higher construction were and are created by SOM engineers. The defining characteristic of a tall, slender

building is its lateral system. It is the tall building acting as one giant vertical beam and its resist- ance to lateral loads, such as wind or seismic forces and the lateral destabilising forces of

gravity, that dominate the design (Fig. 4.1). It is the slenderness, even more than the height, that makes the design problem different from

other structures. Slenderness (height divided by the width at the base) makes the structure more sensitive to movement caused by the

axial deformation of vertical elements, such as

free wind

load end

fixed base In the Burj Khalifa structure, the eiongated trree

Wing shape greatly enhances stability and str

ness by engaging the columns and wails at re

tups of each of its long wings. A similar strategy

was employed in the design of the Nanning

Wuxiang Tower to maximise the distance Of ne

vertical elements from the centre of the tower

4.2

Using beam theory to understand

tall building behaviour it is often useful to think of a tali

building as a giant beam canti

levering from tho ground. As vith beams, the deflection of tall build ings is caused by flexure, shear

and torsion. Flexural behaviour and

Columns and walls.

Engaged nose columns shear behaviour are highty inter related. In common beam theory, it

IS gererally assumed that "plane sections remain plarie, implying that

shear detormation eftectS (shear lag)

for increased moment Hammstd

hig of inertia

do not significantly affect the flexural deformations. Furthermore, all verti-

cal elements ol a cartievering beam

are assummed to contribute based on

Wals ; she

ther distance trorm the neutal axis. Because ot the geoITetry and scale

of tall builcdings. there are shear lag

effects that can b8 quite large and

cause the behaviour to deviate irorm

that of a purely flexual beam. In

Hexagonal central core, high torsional stiftness

extreme cases, the vertical elements

do not act together but behave as a Colection of 'ndividual elements,

and the singie giant beam concept

es e

s not realsed.

4.3

52

Siructural desugn of tall baldiriggs

Mega-column

Outrigger

1 Development

of a potential tall building form

4.2 Diagram

of verlical cantileverbeam

43 Gravity load flow, Burj Khalita, Dubai (UAE) 2010

44 Lateral system description which makes the analogy

between a wide-flange beam and a closed shape the

buttressed core exhiDIts the dest traits of each of these

structural shapes, Burj Khalifa

45 Structural components, Nanning Wuxiang ASEAN Tower.

Nanning (CN), antucIpated completion 2019

4.6 Diagram, Nanning WUxiang ASEAN Twer

4.7 Floor plan, ASPIRE TOwer, Jeddah (KSA), design 2009

4.8 ASPIRE TOwer

Reinforced Concrete wall

Moment frame column

4.5 Commonly used laterail shear

systems Common lateral shear systerms inciude braced frames. wal sys tems, moment frames and core-

The Nanning Tower uses a three-wing core and

plan configuration reminiscent of Burj but adds

three pairs of mega-columns at the tips of each

wing. The mega-columns are optimally posi-

tioned at the extremities of the floor plate to

maximise overturning resistance and stiffness.

The stiffness contribution of the mega-columns

s proportional to the square of the distance of

the columns from the centre of the floor plate thus even a small increase in the structural "foot-

and serviceability. The strength requirements not only consider issues of members and con nections but also global behaviour, such as Overturning due to wind or seismic loads.

The ASPIRE Tower (Fig. 4.8) takes the concept of using gravity loads to assist in resisting lateral loads one step further. The tower was an entry

in a design competition for a kilometre-tall build-

ing and utilises a strong central core as the only vertical structure - the core serves as both the

outrigger systems (Fig. 25. 44

Braced frames utilising diagcnai or

X-bracing can be very efiicient. as can wal systems. Moment Tames

are less efficient Dut can nave arcn

tectural advantages: ine cioSer the columns and he deeper ihe

Connecting spandrei dearms. Te

more efticient the moment rame

print can have a large impact on the system's stiffness (Figs. 4.5 and 4.6).

becomeS. A core-and-cutrgger

system is popular because r he

way it pemits an open perimeter.

The walls of the core are very am-

cient in shear, but the core s too

t is clear that creating a stiff shear system that

can deliver the overturning forces to the vertical elements is crucial. One way to measure the eficiency of a system is to look at how much it

would deflect if it were a purely flexural beam with no shear deformation. For an efficient tower

this value should be at least 70% of the actual deflection. In extremely efficient towers, it can achieve 90%.

gravity and lateral system. Floor framing is canti

levered off the central core by post-tensioned, cantilevered concrete girders without the use of any columns (Fig. 4.7). In this manner, all

gravity load is directly placed within the lateral

system; since there are no columns, there are

no transfers, outriggers or beltwalls required

to tie the structure together. Not only does this

provide an efficient load path, it also helps facili-

tate construction because every floor can be

treated as a typical floor - there are no atypical

elements, such as Outriggers, to interrupt and

slow the construction process.

slender in ilexure to resist tne Ove turning moments Over the tul heignt

The outnggers then create zones

of high shear in the core and out reaching arms in order to move the

Overturning forces from the core to

the perinmeter columns much further from the centro:d Cores can aise De

used with penneter oracea trames

or moment frames to create a com

bined system

orsional effects are not often discussed, but a torsionally flexible building can have major problems affecting both strength and service- ability. Fortunately, adequate torsional behav OLur can be provided in the form of a structural

uDe. which can usually be created by the core

or other elements of the iateral system. oT Uctures must be designed for both strengin

Wind issues for very tall buildings

Serviceability in the wind is a major concerr

The aspects of serviceability for tall buildings are

generally related to motions and the perception

of motion. The cladding, fit-out and building

services have to be designed and detailed to

1 Composite metal deck slab at

center coro

2 Structural steel colurns and

framing at central core

3 Reiniorced coICrete ink beam

4 Reinforced concrete cHcular

core wal

5 Reintorced concrete one-way slab

6 Post-tensioned cantiiever boamm

7 Open voia

48

53

sCALEFORM

Vortex shedang torco

Wwind Wind Wind Wind

Rapd vortex shedding Moderate vortex shedding Slower vortex sheddin9 Improved vortex shedding Excalert ortex tesg

the harmonics, mass and damping of the accommodate the nmotion of the building without damage. The perception of motion is much more

complicated. Motion can be perceived in many ways: visually,

audially and inertially. Visual detection of motion is uncommon, unless there are adjacent build-

ings of comparable height or the building twists in the wind. These conditions allow the occupant to detect motion through the movement of

outside objects relative to objects inside the building. Audial detection of motion is common. The non-

tower structure.

Vortex shedaing

Typical building shapes are not streaml1ned objects; the wind cannot fiow around them like t

does an aircraft wing or a racing car. Buiicings are complex objects in a highiy turbuient flow

that comes from variable directions The wind

flow tends to detach or separate at edges and corners to form a region of separated flow and a turbulent "wake". This vortex shedding 's a

common phenomenon in fluid mechanics and happens in air flow at scales as smail as a wire

(the sound of an Aeolian harp is caused by vortex shedding) to as large as a mountain. in tall buildings, vortex shedding is extremely

important because the vortices cause alterrat ing zones of low pressure which tend to rock me

building from side to side in a rhythmic manner (Fig. 4.11). If the frequency of these pulses is close to the natural harmonics of the buiiding. t

structural elements can creak and groan as they try to move with the structure. This can be par-

tially ameliorated through details that permit the relative motions to take place quietly or by using structural systems that minimise shear-racking. Inertial perception occurs through the detection of inertial forces on the occupant. This sensation can be experienced in the inner ear or feeling the need to balance. The inertial forces and

frequencies can be estimated based on the dynamic properties of the building in conjunc- tion with wind-tunnel testing. Even though the perception of inertial forces is highly variable and depends on the individual, there are indus- try guidelines (for example, by the Council on Tall Buildings and Urban Habitat (CTBUH), ISO, and the Architectural Institute of Japan- AlJ) for evaluating a design. They provide guidance on acceptable accelerations and velocities that building occupants will tolerate associated with various return periods. The less frequent the "event", the higher the notion or accelera- tion tolerated. Although these guidelines and methods of estimating motion are very approx imate, they often control the design of tall

buiidings. The engineer nay need to address movement issues by adjusting the shape of the tower or the structural properties (mass, stiff-

can resuit in very large forces perpendicuiar to

the direction of the wind (in the across wnd or

lift direction), and are ofton much larger than the

forces in the direction of the wind (drag drecton

In order to access the sensitivity of a buiding to

vortex shedding øarly in the design, SOM eng eers tabulate plots ot "critical building widh tor

various wind speeds. The trequency of vortex shedding can be estimated using hand calcua tions taking into acocount the wind speed and the

building shape (see sidebar "Strouhal number he buitding width at which the trequency ot vortex shedding wouid match the structures fundamental trequerncy of vibration is reterred to as the "critical width" because it could resuit n

resonance. Overlap between the critical wIaui

plots and the actual buitding width ndicate potential for undesirable resonance and po to means of tuning the structural design. etet

by changing the building shape or the dyatt

properties. The SOM design team of engriedts and architects often goes into the wind tuwet very early in the design process to assess t terent tower geometries (Fig 4.9.

Strouhal number The rate at which wind vortices

occur is described by a mathem

atical function calied the Strouhal

number S. The Strouhal number is

a dimensionless parameter and

relates the size of the building and the velocity of the wing to the tre- quency at which the vortices are

created SB/Vx T

S Strouhat nummber, B = width of

the bulding: V = mean hourly speed of ar; T - period of vortex shedding

(1/trequency} The Strouha! number has a tariy

narrow range tor commion Duilding

shapes For mostshapes tihe rânge

Is from Ü.11 isguare) to 0.125 (Octagon. A notewortny exceptior

S a Circe witi a Strouhai nunber

of approximateiy 0 20.

ness, periods, mode shape and damping). One method commonly used to address these ISSues is to reduce the forces on the building (which also results in a reduction of motion). and to improve the aerodynamics of the tower. Key items to consider are the rate of vortex

shedding. the directionality ot the wind and

trg lesn tall tiaeiri

yar w

off 10X0 year irx

100X)e wir 10.000 year rrd BNcirng irth5 Rare wnd with hugher erergy mpacta smai portions the buatcng at kow hegrits

for minimsrng overturrinig

moments

shcTg Te 't hreo "nd

reprtat mortes Rir

rexaggeratsri jatrre

Shape Su Phatfs, J

60 80 100 120 20 Building width Im)

4.10

see if there are unusual aerodynamic sensitivi-

ties (Fig. 4.10). Tne wind speed is a product of nature and is

taken as a given. It varies in magnitude along

the height of the tower, with slower speeds near

the ground because of upstream ground rough-

ness. Normally, wind speed is assumed to vary

in the ratio of (z/z,)1/a where z is the distance

rom the ground and Z, is a reference height.

In many cases, a is taken as 7. In general, fre-

quentily ocCurring wind speeds such as those

with return periods of one year, five years and

10 years are considered for issues of occupant

comfort. Return periods of 25 or 50 years are

often considered for tower movement effects

Depending on the harmonics of a tower, reson-

ance caused by vortex shedding can occur

at relatively frequent return periods, so the

designer should determine the structural forces

at several wind speeds.

The strength of the pulses from vortex shedding

is related to the air flow separation. If a tower

has a sharp corner, the flow separation of the

air from the boundary of the tower will occur at

the corner, and the forces can be quite large. If

the corner is rounded, notched or canted, the

on tems such as cladding and partitions. The

return periods for strength considerations vary,

0epending on the design codes and load

actors (factors of safety) used. These return

periods might be 50, 100, 1000 or 1700 years.

SOM often looks at 10,000-year wind events to

point of flow separation may not always occur at

the same location, and the forces will generally

decrease, often substantially. Other treatments

such as blades, vents or other physical changes

to the shape (including the plan proportions of

Mode 2 Mode 2 Mode 1

Translation Translation P 10 Sec.

Translation

P 4 sec. P = 11 seBc.

Confusing the wind

Key lo suKC'BSsulty naraJ4 g he

wind is to tnuse t ty ar Cu

ding cusau yanised vovhsa shettng

Over the henght i re iiwer

meri¥ y et 'ech ijds nchude

lir ten t1s ii Crientng the Duikntg

vitl y ghe isiLatiiai he ioorS

hamor 3.

ior pialt) at d keyt err1s. A

suro plan witi srarg ctmers

cari hav iar Je ac*USS Win0 torces

A rectanJular shdD8Jt ndve

pe hefits E Cortun ind recions

tasg het &'tuií s by tounGIg

Other te "irtLues e reatrng erts, holes i "pleet ir tnat vai

inhp the fomatcn vces, taerd shape paOLS t fr.

str ahes 4nd out ot-aignnent moe

411

SCAL f ORM

4.12 4.13 blowing onto the corner of a square tOwer

(diagonal direction) is much less than the wind blowing onto the face of the tower. In general a square tower has four critical wind directiona onto the four faces. If the faces do not align with the dominant wind directions of the iccal climate, the probable maximum wind forces will be less than if they are in alignment (see sidetar Strouhal number).

the floor plate) - can also have remarkable

effects on the across-wind forces. The circular tower shape as compared to a square or octagonal shape deserves some

discussion. The circular shape has the disad vantage of shedding vortices aimost twice as fast as a square, which results in resonance

occurring during more frequent wind events. Also, the tower will generate the same strength of across-wind forces regardless of the wind direction. This is partially offset by lower drag forces and somewhat weaker vortex shedding forces.

Structure harmonics and damping Because of the speeds of normally ccuring winds and the shape and size of normal tower floor plates, it is not unusual to have wind storms generating vortices at rates that aporoach or match the harmonics of the tower (periods. mode shapes and generalised mass).

Wind directionality Depending on the climate, the directionaity of the wind can have a degree of variability. This can be used to the advantage of the building. For example, the across-wind effects of the wind

The harmonics of the tower can be manipulated to help mitigate wind forces. The effects of

having forces occurring at the harmonics of a tower can be understood by observing a cnila on a swing. The child is able to swing to great

heights just by kicking its feet in time with the natural period of the swing. This height repre sents the large amount of dynamic energy being generated by the harmonic forces of the child's feet. The analogous event is the large structura forces generated by the puls1ng of the wind

at the natural frequencies of the tower. fo help

address these issues, SOM has deveBoped computer programs that give guicdance on

the proportioning of a tower to acheve targe dynamic properties (Fig. 4.11. p. 55).

atprer1 Yrm

Charnere:rd nea,

An important parameter in wind dynammcs is the

damping of the building. Shock absorbers on

an autornobile are dampers that dissipate tha

dynamic energy caused by bumps in the road

Vibrating the vehicle. In buildings. there are low

levels of natural damping in the structure ard Contents of the building. In a property designed buiiding, this is usually enough to limit the viDrdr

tions. If more damping is needed, devices suc as tuned-mass dampers. piston damperS O water-sloshing dampers can be added. One tnese devices are expensive and need specia

Stat to operate and maintain the dampers. " 4.14

56

tutural deetn ttal tyulrg

4.15

alnost all cases, SOM has been able to avoid

the devices by designing the building based on

an understanding of the wind.

4.12 Hybrid tied braced frae TBF

megaframe system. Larsherg

Taryuan (CN 2012 idesign

a Concentr:caly braced

megaframe b Introduction of eccentric

shear yielding - and the latest results from

SOM research into optimal bracing patterns (Fig. 4.12). In much the same way that SOM engineers decades ago created a new mega

braced frame tube system with the John

Hancock Center, the mega TBF frame pro-

posed for the CITIC Financial Centre, Shenzhen

represents a fundamentally new way of pro

viding ductility and stiffness to supertall build-

ing frame systems (Fig. 4.15). The CITIC tower

brace patterns are based, in part, on Michell

truss forms. Brace angles change continuously and morph over the height of the building as a

direct reflection of the optimal brace geometry. The result is at once a striking geometric dis-

play of the underlying structural system and

a fundamentally new high-rise brace typology

(Fig. 4.16).

brace frame iEBF C introducion of te columnsS Seismic issues for very tall buildings

Seismic issues for very tall buildings are funda-

mentally diferent to wind issues, and the desired

solution is often diametrically opposite. For

most tall buildings, the higher the stiffness, the

lower the dynamic wind forces, but the higher

the seismic forces. In tall buildings, the higher

dynamic modes are paticularly important for

seismic design, but are not dominant in wind

design. While ductility is always desirable, the

member forces due to wind do not approach the yield point of the materials, while in seismic

design, ductility is a key consideration in limiting

the forces and absorbing energy. In tall build-

ings located in a high seismic region, a good wind system, such as concentric bracing, is not

d Inclusion of ductie links at

all levelss 4.13 Linked hybrid megafrarme

system, Liansheng 4.14 Schematic design or Ci!

Financial Cente, Shenzher

(CN). antiCipated comoletor

2018 4.15 Brace geomety of CIC

Financial cente (0) as an

evolution from John Hancock

Center, Ghicago ja) 4.16 Evolution of brace geomary

optimal truss geometry apcied

to tower, CTIC Financia Cente

desirable because of issues of stiffness and P/2 ductlity, unless a seismic fuse system can be

ntroduced to limit the forces on the tower 1o address the sometimes opposing needs 1or

ductility and stiffness on the Liansheng project, SUM engineers developed a mega-braced frame system that is inherently stiff, but aug9 mented it with an embedded tied braced frame BF, Fig. 4.12) with ductile "fuses" that can be TLuned to allow a specific level of seismic energy OSSipation. The TBF system acts similarly toa

SiEeleccentrically braced frame (EBF); but in E Liansheng project, the ductiie link zones were tied with vertical elements to allow redis UOi of ductility to multiple levels. The linked wh tower design expands the TBF frame con

p to the macro level by linking the twin towers 0 enectively form one giant stiff but ductie System (Fig. 4.13).

2B-

P

pma' truss Optimal

geometric

proportions

Design domain goomotry

DSeauent to the Liansheng tower desigl. UM engineers have further advanced ine ga lBF ductile frame concept to incorporate

Oth the ductility aspects of Liansheng otably ihe corner fuses that limit seIsitno torres by d

Sipating seismic energy throug

57