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1. Unit-3
Wind & Seismic Effects on Behaviour of Tall Structures
1.Characteristics of wind load
2.Design considerations and Codalwind loads
3.Cladding pressures on behaviour of tall buildings
4.Tall building behaviour during earthquakes
5.Seismic design philosophy.
1

2. Characteristics of wind load
•The lateral loading due to wind is the major factor that causes the design of high‐rise buildings
to differ from medium‐rise buildings.
•The taller the building is, the more sensitive it is to wind loads.
•Structures respond to the effects of wind by buffeting, vortex shedding, flutter and galloping.
•Building design must consider these factors for the safety and stability of tall buildings.
•Climatic changes are unpredictable, wind conditions at the time of design may vary markedly
from the wind conditions during the lifetime of the structure.
•Wind load analysis attempts to factor all these conditions into the design of tall buildings that
are sturdy enough to withstand the effects of strong winds.
Wind: Wind is composed of a multitude of eddies of varying sizes and rotational characteristics
carried along in a general stream of air moving relative to the earth’s surface.
2

3. •Tall building structures have become more efficient and lighter-more prone to deflect and
Sway to wind loading.
•Approaching winds, the building’s geometry and the closeness and geometry of buildings
nearby can influence the wind pressure on a structure, greatly fluctuate and are difficult to
calculate.
•Average wind speed increases with height, and the gustiness, or different combinations of
eddies (circular movement of wind), decreases with height.
•Turbulence buffeting (strong, repeated assaults of wind) can affect tall buildings or towers.
•Buffeting, galloping and vortex shedding may cause cross-wind responses.
•Galloping tends to occur in tall buildings and consists of self-induced vibrations produced as an
effect of wind load. This galloping can alter the direction of winds on such buildings.
3

4. Eddies
•Eddies give wind its gusty or turbulent character. The gustiness of strong winds in the lower
levels of the atmosphere largely arises from interaction with surface features.
•The average wind speed over a time period of the order of ten minutes or more, tends to
increase with height, while the gustiness tends to decrease with height.
•Dynamic loading on a structure depends on the size of the eddies. Large eddies, whose
dimensions are comparable with the structure, give rise to well correlated pressures as they
envelop the structure.
•On the other hand, small eddies result in pressures on various parts of a structure that become
practically uncorrelated with distance of separation.
4

5. Vortex shedding
•When winds flow unsteadily around the surfaces of a tall building, a pattern of swirling vortices
are created.
•As the wind flows past the building, the vortices are shed, resulting in a reduction in wind
pressure at regular intervals. These changes in pressure, result in a lateral force that is at right
angles to the direction of the wind.
•When the wind speed increases, the vortex shedding frequency attempts to match the
building’s natural frequency. This could lead to a degree of building sway which could result in
dislodging windowpanes.
5

6. Handling vortex shedding through Building configuration
1.Corner Softening
2. Tapering
3. Setbacks
6

7. •4. Twist
•5. Cut-outs
•6. Dampers-(from upper level, Heavy load, counteract,Eg: Taipei 101,World Tower)
7

8. Dampers-wind load
•In the case of wind sensitive structures such as tall buildings, dampers reduces motion, making
the building feel more stable to its occupants.
•Controlling vibrations by increasing the effective damping can be a cost effective solution. It is
practical and economical means of reducing resonant vibrations.
a)Examples of passive dampers : Tuned Mass Damper (TMD) ,Distributed Viscous Dampers,
Tuned Liquid Column Dampers (TLCD)(Liquid Column Vibration Absorbers-LVCA), Tuned
Sloshing Water Dampers (TSWD), Impact Type Dampers, Visco-Elastic Dampers, Friction
Dampers
b)Examples of active and hybrid dampers : Active Tuned Mass Damper (ATMD), Active Mass
Driver (AMD)
c)Examples of semi-active dampers: Variable Stiffness Dampers, Hydraulic dampers,
Controllable Fluid Dampers, Magneto-Rheological (MR) Dampers, Electro-Rheological (ER)
Dampers, Variable Friction Dampers
8

9. Major wind effects :
•Effectonenvironment—changesinwindflowsfromanewbuildingcanaffectthe
surroundingenvironment.Fortallbuildingsorskyscrapersincities,theimpactofwindon
pedestrians,vehicles,fountains,etc.inthevicinityoftheproposedstructuremustbe
effectivelyassessed.
•Effectonfaçade—theeffectofwindpressuresonthebuilding’sfaçadecladdinghastobe
assessed.Assessingdesignloadsoncladdingcanminimiseinitialcostsandavoidcostly
maintenancebillsresultingfromleakage/structuralfailure.
•Effect on structure —the wind load will affect the lateral load on the structural system of the
building.
9

10. Wind loads on tall buildings
Classification:
•Building location and environment (to decide wind gust speed)
•Importance factor of the building (standards are stricter for critical buildings, such as hospitals)
•Building’s geometrical shape and traits (to decide the right pressure calculation method)
Criteria:
•Effect of surrounding bodies on wind flow —Hills and other buildings can affect wind loads on
a tall building. Wake characteristics, formation of vortices, tunnel throttling, wind speeds and
wind directions can be predicted and their effects measured
•Effects of turbulence and vortex formation —Building corners, uneven surfaces and high wind
speeds can alter wind flow.
Wind speed:
Atgreatheightsabovethesurfaceoftheearth,wherefrictionaleffectsarenegligible,air
movementsaredrivenbypressuregradientsintheatmosphere,whichinturnarethe
thermodynamicconsequencesofvariablesolarheatingoftheearth.Thisupperlevelwindspeed
isthegradientwindvelocity.
10

11. •Structural collapse due to wind was the Tacoma Narrows Bridge which occurred in 1940 at a wind speed
of only about 19 m/s. It failed after it had developed a coupled torsional and flexural mode of
oscillation.
•Slender structures are likely to be sensitive to dynamic response in line with the wind direction as a
consequence of turbulence buffeting. Transverse or cross-wind response is more likely to arise from
vortex shedding or galloping but may also result from excitation by turbulence buffeting.Flutter is a
coupled motion, often being a combination of bending and torsion, and can result in instability.
•An important problem associated with wind induced motion of buildings is concerned with human
response to vibration and perception of motion. Humans are sensitive to vibration to the extent that
motions may feel uncomfortable even if they correspond to relatively low levels of stress and strain.
11

12. DESIGN WIND LOADS
•The characteristics of wind pressures on a structure are a function of the characteristics of approaching
wind, geometry of the structure under consideration and the geometry and proximity of the
structures upwind.
•The pressures are not steady, but highly fluctuating, as a result of the gustiness of the wind and
because of local vortex shedding at the edges of the structures. The fluctuating pressures can result in
fatigue damage to structures, and in dynamic excitation, if the structure happens to be dynamically
wind sensitive.
•The pressures are not uniformly distributed over the surface of the structure, but vary with position.
•The maximum wind loads experienced by a structure during its lifetime, may vary widely from those
assumed in design. Thus, failure or non-failure of a structure in a wind storm can not necessarily be
taken as an indication of the non-conservativeness, or conservativeness of the Wind Loading Standard.
•The Standards do not apply to buildings or structures that are of unusual shape or location.
•Wind loading governs the design of tall buildings and slender towers. It is better to use experimental
wind tunnel data in place of the coefficients given in the Wind Loading Code for these structures.
12

13. Design Criteria
• Stability against overturning, uplift and/or sliding of the structure as a whole.
•Strengthofthestructuralcomponentsofthebuildingisrequiredtobesufficienttowithstandwind
loadingwithoutfailureduringthelifeofthestructure.
•Serviceabilityforexampleforbuildings,whereinterstoreyandoveralldeflectionsareexpectedto
remainwithinacceptablelimits.Controlofdeflectionanddriftisimperativefortallbuildingswiththe
viewtolimitingdamageandcrackingofnonstructuralmemberssuchasthefacade,internalpartitions
andceilings.
Theultimatelimitstatewindspeedisadoptedbymostinternationalcodestosatisfystabilityandstrength
limitstaterequirements.Inmanycodessuchaspeedhasa5%probabilityofbeingexceededinafifty
yearperiod.
Anadditionalcriterionthatrequirescarefulconsiderationinwindsensitivestructuressuchastall
buildingsisthecontrolofswayaccelerationswhensubjectedtowindloadsunderserviceability
conditions.
Acceptabilitycriteriaforvibrationsinbuildingsarefrequentlyexpressedintermsofaccelerationlimitsfor
aoneorfiveyearreturnperiodwindspeedandarebasedonhumantolerancetovibrationdiscomfortin
theupperlevelsofbuildings.Theselimitsarealsodependentonbuildingswayfrequencies.
13

14. •Windresponseisrelativelysensitivetobothmassandstiffness,andresponseaccelerationscanbe
reducedbyincreasingeitherorbothoftheseparameters.Loadsareminimisedinbuildingsbyreducing
boththemassandstiffness.Increasingthedampingresultsinareductioninboththewindand
earthquakeresponses.
StaticAnalysisandDynamicAnalysismethods:
•Thestaticapproachisbasedonaquasi-steadyassumption,andassumesthatthebuildingisafixedrigid
bodyinthewind.Thestaticmethodisnotappropriatefortallstructuresofexceptionalheight,
slenderness,orsusceptibilitytovibrationinthewind.Staticanalysisisnormallyappropriatefor
structuresupto50metresinheight.
•Thedynamicmethodisforexceptionallytall,slender,orvibration-pronebuildings.
•TheCodesprovidesomedetaileddesignguidancewithrespecttodynamicresponse,mentionthata
dynamicanalysismustbeundertakentodetermineoverallforcesonanystructurewithbothaheightto
breadthratiogreaterthanfive,andafirstmodefrequencylessthan1Hertz.
•InWindloadingcodes,windforcesarerelativelyconstantwithtime.Inrealitywindforcesvary
significantlyovershorttimeintervals,withlargeamplitudefluctuationsathighfrequencyintervals.
(dependentonmanyfactorsassociatedwithturbulenceofthewindandlocalgustingeffectscausedby
thestructureandsurroundingenvironment)
14

15. Static Analysis
•This method assumes the quasi-steady approximation. It approximates the peak pressures on
building surfaces by the product of gust dynamic wind pressure & mean pressure coefficients.
•The mean pressure coefficients are measured in the wind-tunnel or by full-scale tests and are
given by p
bar. The implied assumption is that the pressures on the building surface follow the
variations in upwind velocity. Thus, it is assumed that a peak value of wind speed is
accompanied by a peak value of pressure on the structure. In static analysis, gust wind speed V
z
is used to calculate the forces, pressures and moments on the structure.
Advantages:
• Simplicity, Continuity with previous practice
• Pressure coefficients should need little adjustment for different upwind terrain types
• Existing meteorological data on wind gusts is used directly.
Disadvantages:
• not suitable for very large structures, or for those with significant dynamic response.
• The response characteristics of the gust anemometers & natural variability of the peak gusts
tend to be incorporated into the wind load estimates.
• does not work well for cases where the mean pressure coefficient is near zero.
15

16. Dynamic Approach
•Suitable for exceptionally tall buildings(highly slender) or susceptible for more vibration‐prone
buildings.
•Such buildings may be defined in a more rigorous way according to the natural frequency and
damping of the structure, as well as to its proportions and height
•Effective wind loading in the building may be increased by dynamic interaction between the
motion of the building and the gusting of the Wind.
•The best method of assessing such dynamic effects is by wind tunnel tests in which the
relevant properties of the building and the surrounding countryside are modeled.
•Few alternative dynamic methods estimating the wind loading by calculation have been
developed.
a)The wind tunnel experimental method
b)The dynamic calculation methods.
16

17. Wind loads-along and cross-wind loading
•The wind approaching a building and the flow pattern generated around a building are
complicated by the distortion of the mean flow, flow separation, the formation of vortices, and
development of the wake.
•Large wind pressure fluctuations due to these effects can occur on the surface of a building. As
a result, large aerodynamic loads are imposed on the structural system and intense localized
fluctuating forces act on the facade of such structures.
17
•Under the collective influence of these fluctuating
forces, a building tends to vibrate in rectilinear and
torsional modes.
•The amplitude of such oscillations is dependent on
the nature of the aerodynamic forces and the
dynamic characteristics of the building.

18. Along-Wind Loading
•Thealong-windloadingcanbeassumedtoconsistofameancomponentduetotheactionofthemean
windspeedandafluctuatingcomponentduetowindspeedvariationsfromthemean.
•Thefluctuatingwindisarandommixtureofgustsoreddiesofvarioussizeswiththelargereddies
occurringlessoftenthanforthesmallereddies.
•Thenaturalfrequencyofvibrationofstructuresishigherthanthecomponentofthefluctuatingload
effectimposedbythelargereddies.i.e.theaveragefrequencywithwhichlargegustsoccurisusually
muchlessthananyofthestructure'snaturalfrequenciesofvibration.Theloadingduetothoselarger
gustscanthereforebetreatedinasimilarwayasthatduetothemeanwind.
•Thesmallereddiesinducethestructuretovibratenearoneofthestructure'snaturalfrequenciesof
vibration.Thisinducesamagnifieddynamicloadeffectinthestructurewhichcanbesignificant.
•Theseparationofwindloadingintomeanandfluctuatingcomponentsisthebasisof"gust-factor"
approach,whichistreatedinmanydesigncodes.Themeanloadcomponentisevaluatedfromthe
meanwindspeedusingpressureandloadcoefficients.Thefluctuatingloadsaredeterminedseparately
byamethodwhichmakesanallowancefortheintensityofturbulenceatthesite,sizereductioneffects,
anddynamicamplification.
•Thedynamicresponseofbuildingsinthealong-winddirectioncanbepredictedwithreasonable
accuracybythegustfactorapproach,providedthewindflowisnotsignificantlyaffectedbythe
presenceofneighbouringtallbuildingsorsurroundingterrain.
18

19. Cross-Wind Loading
•The slender structures are susceptible to dynamic motion perpendicular to the direction of the wind.
Tall chimneys, towers and cables frequently exhibit this oscillation which is significant if the structural
damping is small.
a)Vortex Shedding: If the natural frequency of the structure coincides with the shedding frequency of
the vortices, large amplitude displacement response may occur and this is often referred to as the
critical velocity effect. If the structure is flexible, oscillation will occur transverse to the wind and the
conditions for resonance would exist if the vortex shedding frequency coincides with the natural
frequency of the structure. This situation can give rise to very large oscillations and possibly failure.
b)The incident turbulence mechanism:change in wind speeds and directions induce varying lift and
drag forces and pitching moments on a structure over a wide band of frequencies. The ability of
incident turbulence to produce significant contributions to crosswind response depends on the ability
to generate a crosswind force on the structure as a function of longitudinal wind speed and angle of
attack. Streamline bridge deck section or flat deck roof may experience this effect.
c)Higher derivatives of crosswind displacement: There are three displacement dependent excitations,
‘galloping’, ‘flutter’ and ‘lock-in’, which are dependent on the effects of turbulence in as much as
turbulence affects the wake development and, hence, the aerodynamic derivatives. Many formulas
and computational fluid dynamics techniques have been used to evaluate these effects.
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20. WindLoadsonCladding
•Windloadingcriteriaspecificallyrelatingtoexteriorwallelementshavereceivedattentionin
buildingcodesaroundtheworldinthepastfewyears.
•Windtunnelmodelstudiesofbuildingcomponents,bothstructuralandexteriorfacade
elements,theyweregenerallyperformedforspecialbuildingstructures.
•Suchtestprogramsutilizestaticpressuremodelsforinvestigatingthewindpressureconditions
effectingtheexteriorwallcomponents,andmorecomplex,aeroelasticmodelstostudythe
dynamicresponseofafewveryspecialstructures.
•Recently,"forcebalance"proceduresarebeingpursuedtoobtainmoreaccurateresponse
predictionsfortheprimarystructuralform,whilestillutilizingthesamemodelbeingemployed
fortheexteriorfaçadepressuretesting.
•Althoughthewindtunnelinvestigatorycommunityperformedsuchtestingandgenerated
wealthofdocumenteddata,Littleofthisdatahasbeenassimilatedintodesignguidelinesfor
windloadingwithinthevariouscodes
•A.N.S.I.code(1982)differentiatingtherequirementsinwindloadingbetweentheprimary
structureandexteriorfaçadesystems.
20

21. •Windtunnelinvestigationshaveshownthattheeffectsandfactorsproducingwindloadingdesign
criteriaforexteriorwallcomponentscanbesignificantlydifferentfromcriteriafortheprimarystructure
eventhoughtheybotharederivedfromthesamewindenvironment.
•Thisdifferenceisdirectlyrelatedtothebehavioralresponsecharacteristicsofeachsystem.Thehighly
redundantprimarystructurefeelslittleofthespecificeffectsoflocalizedpeakpressuressuchasmay
occuratbuildingcorners,setbacks,parapetsandotherchangesinbuildingconfiguration.
•Theexteriorwallcomponentswhichusuallyexhibitlowdegreesofstructuralredundancy,canbe
significantlyimpactedbysuchlocalpeakloadconditions.Thisistheprimaryfactorinacknowledging
thattheextensivewindloadingcriteriafortheprimarystructure,weredevelopedbasedona
philosophywhichrecognizestheinherentredundancyofthestructure.
•Inmanycasessuchstructuralbuildingcriteriamayleadtounconservativeloadingconditionsifapplied
directlyasthewindloadingcriteriafortheexteriorfacadeelements.
•Althoughinthedesign,windloadsaretreatedasstaticloadevents,theactualwindanditsapplication
tothebuildingsurfacesarealwaysdynamicinnature,andthisactualresponsebehaviorneedsto
alwaysbeconsidered.

22. •Itshouldbenotedthatalthoughstructuralwinddesignloadingsfortheprimarystructuralsystems
generallydecreaseatthelowerelevations,thatduetogroundturbulenceeffects,andsignificant
buildingconfigurationchangestothefacadeatthelowerpartsofthebuilding,thefacadesystem
designpressuresmaynotdecreasenearlyassignificantly.
•Generally,thedesignconcernsforindividualcladdingsystemcomponentsrelatetowindpressure
conditionsperpendiculartothesurfaceplane.
•Theinterfacecompatibilityissuesbetweentheattachedcladdingsystemsandtheprimarystructure
generallyrelatetothedeformationsintheexteriorplaneofthestructure.
•Themostcommoneffectneedingconsiderationisthe"shearracking"orhorizontaldistortionofthe
structure'sbeam-columnframesattheexteriorofthebuildingduetolateraldeformationofthe
structure.
•Theattachedexteriorcladdingsystemsattempttorespondtothedeformedshapeofthesupporting
structureinducingin-placedeformationswithincladdingsystemswhich,ifrestrainedwithoutrelief
mechanisms,generatesignificantforcemechanismsleadingtocomponentdistressorfailure.
•Structuraldeformationsduetolateralloadsproducehorizontalandverticaltranslations,androtational
movementswhichneedtobeabsorbedwithinthefacadesystemandwithinitsanchorageelementsto
theprimarystructure.
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23. •Theresponsetosuchdeformationsystemsbythefacadesystemcomponentsismosteasilyachievedby
utilisingsmallersizesforlessductilecomponents,andallowinglargersizeswhenusingmoreductile
elements.
•Thegreatestdegreeofsusceptibilitytodistressisfoundamongthoseelementssuchaslargepanelsof
glasswhichexhibitlowlevelsofinplaneductility.
•Althoughsuchhorizontalwrackingoftheexteriorfacadesystemsisnormallylimitedinmagnitudefor
typicalfloor-to-floordimensionsbythelimitsofacceptableperformanceforstructuralbehaviouror
humanphysiologicalresponse,specialtallfloorsor"softstructure"zonescanproduceunacceptable
responserangesforsomeofthefaçadesystemscomponents.
•Itcanbeobservedthattheresponsetothe"shearracking"effect,withrespecttothedifferential
deformationcompatibilitybetweenexteriorfaçadeelementsandsupportingstructuralsystems,would
generallybemoresevereforthelowerdeflectionratiosofh/400-h/500.
•Itshouldagainbenotedthattheoptimizationoftheprimarystructure'sdesignutilizingthestronger,
moreductilesteelmaterialsversusthelessstrong,lessductileconcretematerials,producesagreater
contrastwhenconsideringtheinterfacedeformationcompatibilityconditionsoftheattachedsystems.
•Thedifferentialaxialdeformationsofthestructure’sexteriorcolumns,duetothe"cantileverbehaviour“
ofthestructureunderwindloading,resultsinafurtherseriesofdifferential,deformationdesign
considerations,withthesimilarcombinedeffectsofshrinkageandcreep.
23

26. 26

27. 27

28. Wind tunnel Experimental Method
•Wind tunnel testing is a powerful tool that allows engineers to determine the nature and intensity of
wind forces acting on complex structures.
•Wind tunnel testing is particularly useful when the complexity of the structure and the surrounding
terrain, resulting in complex wind flows, does not allow the determination of wind forces using
simplified code provisions.
•Wind tunnel testing involves blowing air on the building model under consideration and its
surroundings at various angles relative to the building orientation representing the wind directions. This
is typically achieved by placing the complete model on a rotating platform within the wind tunnel. Once
testing is completed for a selected direction, the platform is simply rotated by a chosen increment to
represent a new wind direction.
28

29. 29

30. 30

31. Wind tunnel testing
•For average size tunnel testing of tall buildings, the 1:400 (1:200 or 1:100) scale model, the natural wind
is usually generated using large-scale turbulence using devices such as trip boards
•Carpet or roughness blocks are used along the fetch length to generate the required velocity profile. In
order to use wind tunnel results to aid in the prediction of wind forces acting on full-scale structure, the
behavior of the natural wind must be satisfactorily modelled by the wind tunnel.
•Mean longitudinal wind velocity, standard deviation of velocity fluctuations, frequency andpower
spectral density of the velocity fluctuations, measurement of length, lengthscale associated with the
modelled building and natural wind and time scale are important factors
•To model the natural wind and maintain similarity between model and full-scale results, the non-
dimensional parameters (velocity profile, height of the building, the turbulence intensity and the
normalisedpower spectral density) are kept as constant between the natural wind and the wind tunnel.
•To relate wind tunnel pressure measurements to full-scale values, length and time scales must be
noted.
•The time scale depends only on the length scale and the ratio of mean wind speed at the top of the
model building to mean wind speed at the top of the full-scale building.
•The design wind speed is based on meteorological data for city which is analyzed to produce the
required probability distribution of gust wind speeds. By appropriate integration processes and
application of necessary scaling factors, directional wind speeds for the wind tunnel testing can be fixed.

32. 32

33. The following conditions should be satisfied while conducting Wind-tunnel tests:
1.Thenaturalatmosphericboundarylayerhasbeenmodeledtoaccountforthevariationofwind
speedwithheight.
2.Thelengthscaleofthelongitudinalcomponentofatmosphericturbulenceismodeledto
approximatelythesamescaleasthatusedtomodelthebuilding.
3.Themodeledbuildingandthesurroundingstructuresandtopographyaregeometricallysimilarto
theirfull-scalecounterparts.
4.Theprojectedareaofthemodeledbuildinganditssurroundingsislessthan8%ofthetestsection
cross-sectionalareaunlesscorrectionismadeforblockage.
5.Thelongitudinalpressuregradientinthewind-tunneltestsectionisaccountedfor.
6.Reynoldsnumbereffectsonpressuresandforcesareminimized.
7.Responsecharacteristicsofthewind-tunnelinstrumentationareconsistentwiththerequired
measurements.
33

34. Simple Static Approach-IS 875 part3
•Themodemstaticmethodsofestimatingwindloadingaccountsfortheeffectsof
gustingandforlocalextremepressuresonthefacesofthebuilding.Also
accountsforlocaldifferencesonexposurebetweentheopencountrysideanda
citycenter.
•TheVitalfacilitiessuchashospitals,firestationsandpolicestations,whosesafety
mustbeensuredforuseaftertheextremewindstorm.
•The design wind pressure is obtained from the formula as per IS 875 part 3
34

35. 35
Map of India showing
Basic wind speed

36. Wind Load Calculation as per IS 875 part 3:
Example:1
•No. of storeysn : 10
•Plan dimension : 24 m x 15 m
•StoreyHeight : 3.0m
•Parapet wall height : 1.0m
•Width= 4 Bays each 6 m length =24m
•Breadth = 3 Bays each 5m length =15m
•Location : Mumbai
•Topography: plane
•Design Life of the structure : 100 yrs
36

37. 37
Check : (45)
Height/Minimum Lateral Dimension = h/b =31/15=2.07 <5 -----(1)
Frequency = 1 /0.1n = 1/0.1 x 10=1 > 1 Hz -------(2)
We can follow Static method

38. 1.Design wind speed
V
z= V
bk1 k2 k3 k4
•Vb= Basic wind speed (m/sec)
= 44 m/sec as per Appendix A (51)
•k1=Risk coefficient (Table 1)= 1.07(7)
•k2= Terrain & height factor (Table 2)(8)
Terrain category 3 and height 31m
•k3=Topography factor
For slope 3
◦
< Ꝋ≤17
◦
(Annex C) (54)and Clause 6.3.3 (8),
value of k3 can be taken as 1.0.
•k4 =Importance factor for Cyclonic Region
Distance of the site from nearest sea coast =65 km
Nature of site : Non cyclonic, All other structures
k4= 1 (clause 6.3.4)(8)
Design Wind speed Vz= 44 x 1.07 x k2 x 1.0 x 1.0 =47.08 k2
38

39. 2.Design wind pressure p
d (9)
Design Wind Pressure, p
d= K
d K
aK
cp
z where
•K
d= Wind directionality factor for buildings as per clause 7.2.1=0.9 (rectangle)
•K
a=area averaging factor (10)
Tributary area of a panel TA= Column spacing x storeyheight=6 x 3=18m
2
For TA =18m
2
(Table 4) Ka=0.9
•Combination factor = Kc (clause 7.3.3.13) for frames over the building envelope when roof is subjected
to pressure and internal pressure is suction or vice-versa=0.9 (16)
p
d = 0.90 x 0.90 x0.90 x p
z=0.729 p
z
p
d> 0.7 p
z
•The wind pressure at any height z is given by p
z = 0.6 Vz
2
(9)
•Where Vz= Design wind speed at height z in m/s.
•Pz = 0.6 x (47.08 K
2)
2
= 1330 K
2
2
•pd= 0.729 x 1330 K
2
2
•=969.57 K
2
2
39

40. •Table 2
40

41. 3.Design wind load (F)
41
PLAN

42. 3.Design wind load (F)
42
PLAN

43. •The total wind load acting on the building is given by F= C
fA
ep
d(32)
C
f = Force coefficient (Fig 4)
A
e = Effective frontal area
p
d=Design wind pressure
For h/b=31/24=1.29 a/b= 15/24=0.625
•C
fvalue from Fig 4(a) =1.22 (35)
•A
e =Effective frontal area
•Width of tributary area supported by the frame =6m
•A
efor 1m width may be taken as centreto centreof frame x 1m height.
•F= CfA
epd
=1.22 x (6.0 x1.0) x p
d
=7.32 p
d 43

44. Design wind load per metreheight (table2)
44

45. Calculation of wind load -point loads
45
To convert the uniform loads into point loads, the loading at the top and bottom of each area in charge of nodes should be
determined and assumed as uniform over the area .
The loads at the centreof these areas are calculated from design wind load by interpolation.
•Wind load at 10.25m
10m → 5.88
15m → 6.68
F =5.92
•Wind load at 16.25m
15m → 6.68
20m → 7.24
F =6.82
•Wind load at 22.25m
20m → 7.24
30m → 7.97
F =7.40
•Wind load at 28.25m
20m → 7.24
30m → 7.97
F =7.24 + 0.60 =7.84

46. 46

47. 47

48. EARTHQUAKE RESISTANT DESIGN OF TALL
STRUCTURES
48

49. Why earthquakes are important to Civil engineers?
People get killed due to collapse of buildings
The ground motion generated during an earthquake contains wide range of frequencies from 0-
50 Hz.
Most of our engineering structures have resonant vibration frequencies in 0.1 Hz –10 Hz range.
NATURAL FREQUENCY = 1/(0.1N), N=number of stories
A structure is most sensitive to ground motions with frequencies near its natural resonant
frequency

50. Tall building behaviour during earthquakes
•The seismic motions can damage a building by internally generated inertial forces caused by the
vibration of the building mass -vibration problem.
•Increase in mass, can increase in the force
it can cause buckling or crushing of columns and walls when the mass pushes down on a member
bent or moved out of axis by the lateral forces. (PΔ effect) -greater the vertical forces, the greater
the movement due to PΔ.
•It is always the vertical load that causes buildings to collapse in earthquakes.
•Seismic design concern-Distribution of dynamic deformations caused by the ground vibrations
Duration of motion
•Tall buildings respond to seismic motion differently than low-rise buildings.
•The magnitude of inertia forces induced in an earthquake depends on (1) Building mass, (2)
Ground acceleration, (3) nature of the foundation (4) dynamic characteristics of the structure.
50

51. 51
•Usually, buildings have certain flexibility, the force tends to be
less than the product of buildings mass and acceleration.
•Tall buildings are more flexible than low-rise buildings, they
experience much lower accelerations than low-rise buildings.
•Flexible building subjected to ground motions for a prolonged
period, experience much larger forces if its natural period is
near that of the ground waves.
•The magnitude of lateral force is not a function of the
acceleration of the ground alone, but is influenced by the type
of response of the structure and its foundation as well.
•This interrelationship of building behavior and seismic ground
motion depends on the building period.
building & its
foundation
infinitely rigid

52. •On the basis of time period, building may be classified as rigid(T< 0.3 sec), semi –rigid(0.3 sec
< T < 1.0 sec) and flexible structure (T > 1.0 sec)
•Buildings with higher natural frequencies, and a short natural period, tend to suffer higher
accelerations but smaller displacement
•In the case of buildings with lower natural frequencies, and a long natural period, this is
reversed: the buildings will experience lower accelerations but larger displacements
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53. Building height and Spectral acceleration
53
•Shorter buildings are generally expected to experience proportionally larger forces than taller buildings
when subjected to earthquake ground shaking.
•Modern buildings have reduced vulnerabilities due to better designs and lower collapse risks, the
possibility of demolition of these structures due to excessive residual, (2010-2011 Canterbury
earthquake sequence in New Zealand) can still contribute significantly to the expected losses
Low rise
Medium rise
High rise
Sky scrappers

54. 1. Soil Properties
•Attenuation occurs at a faster rate for higher frequency (short-period) components than for lower
frequency (long-period) components.
•Tall building-although situated farther from a fault than a low-rise building, may experience greater
seismic loads because long-period components are not attenuated as fast as the short-period
components.
•Therefore, the area influenced by ground shaking potentially damaging to, say, a 50-story building is
much greater than for a 1-story building.
•Natural periods of soil are in the range of 0.5–1.0 s. Therefore, it is entirely possible for the building and
ground it rests upon to have the same fundamental period-(collapse of particular range of storeys).
•An obvious design strategy is to ensure that buildings have a natural period different from that of the
expected ground vibration to prevent amplification.
•Soft soil amplifies seismic forces on taller buildings with long fundamental periods. It has the potential
to lose its capacity to support buildings through a process called liquefaction.
•Period of firm ground-0.2-0.4Sec , 2 sec and more for soft soil
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55. 2.Damping
•Damping-depends upon the construction materials, the type of connections, and the influence of
nonstructural elements on the stiffness characteristics of the building. Damping is measured as a
percentage of critical damping.(min amount of damping necessary to stop vibration).
•No numerical method-experiments
•The damping of structures is influenced by 4 major sources.
1. External viscous damping caused by air surrounding the building. (viscosity of air is low-negligible).
2. Internal viscous damping (material viscosity)-increases in proportion to the natural frequency of the
structure.
3. Friction damping (Coulomb damping), occurring at connections and support points -constant.
4. Hysteretic damping that contributes to a major portion of the energy absorbed in ductile structures.
•For analytical purposes-single viscous damping-percentages based on material-(1-10%)-5%, 2%, etc
•Dampers
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56. 3.Building Motions,Building Drift, Pounding And Separation
•Earthquake-induced motions-more violent, Wind-not much,
Earthquakes occur much less frequently than windstorms
Duration of an earthquake is very short.
•Lateral deflections that occur during earthquakes should be
limited to prevent distress in structural members and architectural
components.
•Non load-bearing in-fills, external wall panels, and window glazing
should be designed with sufficient clearance or with flexible
supports to accommodate the anticipated movements.
•Comport to occupants-structural –non structural
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57. •Drift (the lateral displacement of one floor relative to the floor below)should be limited to
avoid damage to interior partitions, elevator and stair enclosures, glass and cladding systems.
•Stress or strength limitations in ductile materials do not always provide adequate drift control,
especially for tall buildings with relatively flexible moment-resisting frames or narrow shear
walls.
•Total building drift is the absolute displacement of any point relative to the base.
•Adjoining buildings or adjoining sections of the same building may not have identical modes of
response, and therefore may have a tendency to pound against one another.
•Building separations or joints must be provided to permit adjoining buildings to respond
independently to earthquake ground motion.
•Pounding damage-proximity to adjacent Buildings
•Building pounding can alter the dynamic response of both buildings, and impart additional
inertial loads to them.
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58. SEISMIC DESIGN CONCEPT
1.Selecting an overall layout of a lateral force–resisting system that is appropriate to the anticipated
level of ground shaking.
-providing a redundant and continuous load path to ensure that a building responds as a unit when
subjected to ground motion.
2. Determining code-prescribed forces and deformations generated by the ground motion, and
distributing the forces vertically to the lateral force resisting system. ( structural system, configuration,
and site characteristics)
3. Analyzing the building for the combined effects of gravity and seismic loads to verify that adequate
vertical and lateral strengths and stiffnesses are achieved to satisfy the structural performance and
acceptable deformation levels prescribed by codes.
4. The structure must have sufficient inelastic deformability to undergo large deformations when
subjected to a major earthquake.
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59. 1.Response of a structure
The inertia forces, F = ma,generated by the horizontal components of ground motion require greater
consideration for seismic design since adequate resistance to vertical seismic loads is usually provided by
the member capacities required for gravity load design. In the equivalent static procedure, the inertia
forces are represented by equivalent static forces.
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3.Response of non-structural elements
Pipe lines, machineries, electrical fittings, Heavy furniture, etc-rigid/rigidly attached
2.Load path-continuous
The inertia forces must be transmitted to the lateral force–resisting elements, through the diaphragms and
then to the base of the structure and into the ground, via the vertical lateral load–resisting elements.
4.Near-by Buildings
•Buildings with same height and have matching floors are likely to exhibit similar dynamic behavior. If
the buildings pound, floors will impact other floors, so damage usually will be limited to nonstructural
components.
•When floors of adjacent buildings are at different elevations, the floors of one building will impact the
columns of the adjacent building, causing structural damage.
•When buildings are of different heights, the shorter act as a support for the taller one. The shorter
building receives an unexpected load while the taller building suffers from a major discontinuity that
alters its dynamic response. Strong column-weak beam

60. 5. Irregularities in Buildings-Building Configuration
1.Strength and stiffness
2.Mass Irregularity
3.Vertical geometric Irregularity
4.Torsion Irregularity
5.Re-entrant corners
6.Diaphragm discontinuity
6. Lateral Load resisting systems:
1.Moment resisting frame-ordinary, special, Intermediate
2.Shear wall/Bearing wall
3.Buildings with Dual system
7. Ductility-absorbing energy by deforming in the elastic range-IS13920-Ductile Detailing-Provide joints
with sufficient abutments that can adequately confine the concrete –permitting it to deform plastically
without breaking
8.Seismic Weight-seismic forces are proportional to building weight and increases along the height
9.Redundancy 10.Foundations 60

61. 61

62. Base Isolation
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•Thesuperstructure is separated from the base by introducing a
suspension system between the base and the main structure.
Few base isolation devices which are widely adopted for seismic
base isolation are
1. Elastomeric Bearings
2. High Damping Bearings
3. Lead Rubber Bearings
4. Flat Slider Bearings
5. Curved Slider Bearings or Pendulum Bearings
6. Ball & Roller Bearings
Advantages:
1.Reduced the seismic demand of structure, thereby reducing the
cost of structure.
2. Lesser displacements during an earthquake.
3. Improves safety of Structures
4. Reduced the damages caused during an earthquake-
maintaining the performance of structure after event.
5. Enhances the performance of structure under seismic loads.
Elastomeric Rubber Bearings
Roller and Ball Bearings Spring Isolators
Sliding Bearing

63. Earthquake resistance methods of tall building
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1.Taipei 101, Taiwan-used tuned mass damper (TMD)-Hangs a ‘ball
of steel’ weighing 730 tonnesacting as a centralized pendulum that
is designed to oscillate away from the lateral bend of the building
to neutralize the effect of the earthquake.
2.Utah State Capitol building, USA-Base isolation system bears 281
lead-rubber laminated base isolators attached to the building
foundation with the help of steel plates
3.Petronas Twin Tower, Malaysia-The two glass towers are
connected with a centralized bridge. Designed to slide in and out of
the building every time there seem to be substantial lateral loads
acting upon the building.
4.Burj Khalifa, Dubai-mass dampener/harmonic absorber
5.The Yokohama Landmark Tower-a Hybrid mass damper (a
combination of tuned mass damper and an active control actuator),
“bandage pillars”-designed with the help of resin fibresthat
essentially may allow some chunks of the pillar to fall off but
prevent it from collapsing in case of an earthquake.

64. 6.Citigroup Center-410-ton concrete tuned mass damper
7.One Rincon Hill South Tower, USA-tuned liquid mass damper at top the 60
storeystructure. It is a 5 feet tall tank filled with 50,000 gallons of water that
flows the opposite side of the sway to decrease the impact on the
inhabitants.
8.SabihaGökçenInternational Airport, Turkey-The computer-simulated triple
friction pendulum isolators help the structure to stay aloft in the event of an
earthquake
9.The Transamerica Pyramid, USA-designed in such a way that it reflects
some sunlight to its neighbors, considering the tiny heights of the buildings
around -rest on a steel and concrete foundation that is engineered to move
along with the earthquake giving subtle sways to the structure itself.
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