Design Philosophy (Module III)

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Prof. Utsav KoshtiAlpha college of Engineering and TechnologyDesign Philosophy(Module-III)

INTRODUCTION Shaking of earth due to movement of rocks along a fault.

Associated with faulting or breaking of rocks

Rocks under stress accumulate strain energy and breaks.

WHAT IS AN EARTH QUAKE?

Earthquake Engineering

Vaibav Your ppt will start from hereExplanation: All of you know what an earth quake isbut it is defined as shaking of earth due to movement of rocks along a fault which results in breaking of rocks. Rocks in earth quake due to stresses accumulate strain energy and breaks along the fault plane which causes movement of rocks

Let us see what is focus and epicenter

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EarthquakesAround 500,000 earthquakes occur each year, detectable with current instrumentation. About 100,000 of these can be felt. Human activities that produce minor earthquakes: Storage of large water behind a dam, Injecting liquid under high pressure into wells (fracking to extract natural gas), Coal mining Oil drillingEarthquake Engineering

Causes of EarthquakesEarthquake Engineering


VARIOUS THEORY FOR EARTHQUAKE OCCURANCE

Elastic Rebound TheoryThis theory was discovered by making measurements at a number of points across a fault.

Prior to an earthquake it was noted that the rocks adjacent to the fault were bending. These bends disappeared after an earthquake suggesting that the energy stored in bending the rocks was suddenly released during the earthquake.

Elastic Rebound Theory

Elastic Rebound

Sequence of elastic rebound: Stresses

Sequence of elastic rebound: Bending

Sequence of elastic rebound: Rupture

Sequence of elastic rebound: Rebound

Sudden Slip by Elastic Rebound Stresses (force/area) are applied to a fault.Strain (deformation) accumulates in the vicinity of friction-locked faults.Strain accumulation reaches a threshold and fault slips suddenlyRupture (slip) continues over some portion of the fault. Slip is the distance of displacement along a fault.

Plate tectonics- Alfred Wegener , 1912


Cross-section of Earth

Though we have explored Space above ground extensively, we could go only about 7.6 miles below ground! Russian geologists started drilling into the Kola Peninsula, near Finland, in 1970 and after 22 years could not proceed further.

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Plate tectonics (PT)Earlier theories assume gradual shrinking (contraction) or gradual expansion of the globe. PT is based on continental drift & developed in early 20th centuryLithosphere is broken up into 7-8 major tectonic plates, and numerous smaller platesTectonic plates move because lithosphere has a higher strength and lower density than the underlying asthenosphere- Dissipation of heat from the mantle is the source of energyLateral relative movement of the plates- 0 to 100mm annually


Three types of plate boundaries exist


EARTHQUAKE TERMINOLOGYEarthquake Engineering

Focus & EpicenterFOCUS:The point within Earth where faulting begins is the focus, or hypocenter

EPICENTER: The point directly above the focus on the surface is the epicenter

FOCAL DISTANCE:Vertical distance from focus and epicenter Earthquake Engineering

As on the slide focus is the point along the fault plane where the earth quake startsEpicenter is the point above the focus on the earth surface.you can clearly watch it on the imageFocal distance is distance from the focus and epicenterits a vertical distance19

Location Of EPICENTER

Three seismograph stations are needed to locate the epicenter of an earthquake

A circle where the radius equals the distance to the epicenter is drawn

The intersection of the circles locates the epicenter

To locate the epicenter of an earthquake three seismic stations are requiredOn each station a circle is drawn with a radius equal to distance is drawn The intersections of the circles is the epicenter as shown here on the arrow

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AftershocksAn aftershock is an earthquake that occurs after a previous earthquake, the mainshock. It occurs in the same region of the main shock but always of a smaller magnitude. If it is larger than the main shock, the aftershock is redesignated as the main shock and the original main shock is redesignated as a foreshock. Formed as the crust around the displaced fault plane adjusts to the effects of the main shockThey are dangerous - usually unpredictable, can be of a large magnitude, and can collapse buildings that are damaged from the main shockEarthquake Engineering

Magnitude

Richter scale measures total amount of energy released by an earthquake

Amplitude of the largest wave produced by an event is corrected for distance and assigned a value on an open-ended logarithmic scale

Richter scale measures total amount of energy released by an earthquake

As shown in the figure the amplitude, magnitude and distance between p-s waves is co related and magnitude is find out..The amplitude and distance is known from seismogram which u can see in the coming slides

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IntensityModified Mercalli Intensity Map 1994Northridge, CA earthquake, Magnitude 6.7Intensity refers to the amount of damage done in an earthquake

Mercalli Scale is used to express damage

Intensity refers to the amount of damage done in an earthquake

Mercalli Scale is used to express damageImages shows mercalli intensity map during an earth quake

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Intensity of earthquakesModified Mercalli Intensity scale (MMI) and MSK scale (Appendix D of Draft IS 1893)Initially developed early last century by Giuseppe Mercalli.Both have twelve levels of intensityLevel I least perceptive Level XII most severeEarthquake Engineering

MEASUREMENT OFEARTH QUAKE

Earth quake is measured in Magnitude and intensity 25

Normal type seismographDigitalize type seismographSeismographVertical typeHorizontal type

Seismograph is the instrument used to measure the earthquake magnitude..There are mainly two types of seismograph vertical and horizontal used for various purposesNow the digitalized seismograph is available which can give directly the magnitude..

Normal type of seismograph consists of drum, marker, suspended mass cable and support26

Seismograph

Seismogram is the graph that is given by the seismograph As I said already amplitude and distance between p and s waves are shown here clearly used to get the magnitude by co relation..As seen in the graph p waves reach first, s waves reach next and body waves the last

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DEMO OF SEISMOGRAPH

Now just have a look at seismograph working..See earth quake occurs at a point and reaches the station and the waves are recorded in the seismogram28

Characteristics of an EarthquakeEarthquake Engineering

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Seismic wavesThe waves that causes vibrations on earth are seismic waves

Types of seismic waves

1. Body Waves travel through interior

1.1 Primary or "P" Waves:Highest velocity Causes compression and expansion

1.2 Secondary or "S" Waves:Slower than P waves but faster than surface waves. Causes shearing of rock perpendicular to direction of wave

Seismic waves are the responsible waves for all the damages done during earth quakesThey r of two typesBody waves and Surface wavesBody waves are also dividend into two types 1. primary waves 2. secondry wavesConsider the a undisturbed material as shown in the figureThe primary waves causes compression and expansionSecondry waves travel perpendicular to the surface making shearing of rocks30

S & P Waves Trend

Let us now watch how actually the s and p waves travel

As I said before p waves travel in compression and expansion you can see the trendAnd also s waves which travel in the perpendicular direction which makes the wires break 31

2. Surface Waves or "Love" (L) Waves Travel on surface of earthCause vertical & horizontal shakingCause maximum damage

Second type of waves are surface waves which travel on surface of earth and are the main cause for damageThey r also divided into rayleigh wave and love wavesPatterns are seen in the images..32

DESIGN PHILOSOPHYNeed:Severity of ground shaking at a given location during an earthquake can be minor, moderate and strong. Average annually about 800 earthquakes of magnitude 5-5.9 occur in the world, while the number is only about 18 for magnitude 7.0-7.9. What should be done?Should we not design building for earthquake effects ? Or Should we design the building to be earthquake proof for strong but rare earthquake shaking ?Earthquake Engineering

DESIGN PHILOSOPHYEarthquake proof vs Earthquake resistance design:Design of building wherein there is no damage during the strong but rare earthquake shaking is called earthquake proof design.The engineering intention is to make building earthquake resistance, such building resist the effects of ground shaking, although they may get damaged severely but would not collapse during the strong earthquake.


EARTHQUAKE DESIGN PHILOSOPHYEarthquake EngineeringThe seismic design philosophy as per IS 1893(part 1) is: Minor and frequent earthquakes should not cause any damage to the structure < DBEModerate earthquakes should not cause significant structural damage but could have some non-structural damage = DBEMajor and infrequent earthquakes should not cause collapse > MCEHence design is done for much smaller forces than actual seismic loads. Note that this approach is different than that adopted in the case of wind, dead, live and other loads, where the structure is designed for the actual loads.

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Earthquake design philosophyEarthquake Engineering

EARTHQUAKE DESIGN PHILOSOPHYEarthquake EngineeringDefine:DBE (Design basis earthquake)MCE (Maximum considered earthquake)

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EARTHQUAKE DESIGN PHILOSOPHYEarthquake EngineeringDefine:DBE (Design basis earthquake)MCE (Maximum considered earthquake) Design basis earthquake is defined as the maximum earthquake that reasonably can be expected to occur at least once during the design life of the structure. Maximum considered earthquake is the earthquake corresponding to the ultimate safety requirement.

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Four virtues of Earthquake Resistance StructureEarthquake EngineeringLateral StrengthAdequate stiffnessGood ductilityGood structural congiuration

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The two functions of shear wall are explained clearly.it stops the side sway and resist the shear40

LOCATION OF SHEAR WALL

The location of the shear wall should be symmetrical so that the box action occur to resist the seismic waves41

Lateral Force Resisting SystemsEarthquake Engineering

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Ductile and Brittle performanceEarthquake Engineering

DuctilityIt is the capacity of the structure to deform in-elastically without significant loss of strength

Ductility ConsiderationPlain concrete is brittleSteel is ductileR.C. is not ductileDuctile detailing introduces ductile behavior

Better Performance in Earthquakes

Have simple and regular Plans


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Collapse of L-shaped building in Ahmedabad, 2001


Avoid Irregular Configurations


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Avoid Novel Structural Features(If their EQ behavior is not known)Earthquake Engineering

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Geometric vulnerabilities- CCTV Tower, China


Seismic Codes of IndiaIS: 1893 (Part1-5): 2002 Criteria for Earthquake Resistant Design of Structures, BIS, New DelhiIS: 13920:1993 Ductile Detailing of Reinforced Concrete Structures Subjected to Seismic Forces, BIS, New DelhiIS: 4326:1993 Earthquake Resistant Design and Construction of Buildings, BIS, New Delhi

IS 4326 1993 Earthquake resistant designand construction of buildingIS 13827 1993 Improving earthquakeresistance of earthen buildingIS 13828 1993 Improving earthquakeresistance of low strength masonry bldgIS 13920 1993 Ductile detailing of reinforced concrete structures subjected toseismic forcesIS 13935 1993 Repair and seismicstrengthening of buildingsCont.

Basic Intent of Seismic CodeThe structures are able to respond without structural damage to shocks of moderate intensities and without total collapse to shocks of high intensitiesCodes are made for normal structuresSite-specific study for special structuresIt is not the intent of code to provide regulations that the structure shall suffer no damage during earthquake of all magnitudes

Basic Principles of Modern Codes(Seismic Design Philosophy)Serviceability limit state: Structures must resist low-intensity earthquake without structural damage. Elastic behaviour is required during small and frequent earthquakesUltimate limit state: Structures should withstand an earthquake of moderate intensity (design earthquake having PGA with 90% probability of not being exceeded in 50 years) with repairable damage in structural as well as nonstructural elementsCollapse limit state: Structures should withstand high intensity earthquake with a return period much longer than design life without collapse

Design Philosophy IS: 1893 (Part1):2002The structure should possess at least a minimum strength to withstand minor earthquakes without damageThe structures should resist moderate earthquake without significant structural damage, some non-structural damage may occurThe structures withstand a major earthquake without collapse

Design Philosophy for Special Structures

Low structural redundancy in structures like, Chimneys, Cooling towers, Water towers, Cantilever bridge piers, Core suspended structuresEnormous risk that a possible failure can cause to nuclear power plants, dams

1962 -Seven Seismic Zones1975 - Five Seismic Zones2002 - Four Seismic Zones II, III, IV, V

Zone I merged with Zone II

Seismic Zone Map

SEISMIC ZONES OF INDIA


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59Load combinations 1.5 ( DL + LL )1.2 ( DL + LL EL)1.5 (DL EL )0.9 DL 1.5 ELSubstitute WL for EL in case of windLimit State Design

60The design horizontal seismic coefficient Ah Z I Sa Ah = -------- 2 R g Z = Zone factor I = Importance factor R = Response reduction factor Sa / g = Average response acceleration coeffDesign Spectrum

Response Acceleration coefficient as given in IS 1893 (Part 1)-2002

Smoothened Elastic Design Acceleration Response Spectrum (SEDRS) for 5% damping.For Steel structures use 2% dampingEarthquake Engineering

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62This is for MCE (maximum considered earthquake).

The factor 2 in the denominator is used to reduce the factor for DBE (design basis earthquake) Zone Factor ( Z )

63When depth > 30 mhorizontal acceleration spectrum value=Ah/2

When depth is between ground level & 30 m design horizontal acceleration spectrum value linearly interpolated between Ah and 0.5Ah

The design acceleration spectrum for vertical motion Av = 2 / 3 Ah Underground Structures and Foundations

64Spectral values for different soils, having 5% damping isSpectral Values ( Sa / g )

Type of soilSa / gFor values of TRocky or hard soil1 + 15 T0.00 to 0.102.500.10 to 0.401.00 / T0.40 to 4.00Medium soil1 + 15 T0.00 to 0.102.500.10 to 0.551.36 / T0.55 to 4.00Soft soil1 + 15 T0.00 to 0.102.500.10 to 0.671.67 / T0.67 to 4.00

65For other values of damping, spectral values are multiplied by factors as below Multiplying Factors for Obtaining Values for other Damping

66The minimum value of importance factor, I, for different building systemsImportance Factor ( I )

67Response Reduction Factor ( R )

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Cont.

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Cont

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Design Imposed Loads The earthquake force shall be calculated for full DL plus the percentage of LL

The imposed load on roof need not be considered.

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Seismic Weight For each floor = Full DL + LL

For the whole building = sum of seismic weights of all floors

The weight of columns and walls to be equally distributed to floors above and below

weight supported in between storeys shall be distributed to floors above and below in inverse proportion to its distance.

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Design Seismic Base Shear ( VB )The total design lateral force or design seismic base shear

VB = Ah W

Ah - design horizontal acceleration spectrum value based on fundamental natural time Ta

W - seismic weight of building

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Fundamental Natural Period ( Ta ) Ta = 0.075 h0.75 for RC frame building, without brick infill panelsTa = 0.085 h0.75 for Steel frame building, without brick infill panels 0.09 hTa = for all other frame buildings, d with brick infill panelsWhere h is the height of the building in m

d

d

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Distribution of Design Forces The base shear VB along the height Wi hi 2Qi = VB -- --n----------- Wi hi 2 i =1Qi = design lateral force at floor iWi = seismic weight of floor ihi = height of floor i measured from base, n = number of storeys in the building The shear at any floor i = nVj = Qi Where Vj is shear in jth storey i = j

Equivalent Lateral Base Shear Force ProcedureEarthquake EngineeringEquivalent Lateral Base Shear Force :

Where Z= zone factor, I = importance factor, and R= Response reduction Factor

I = 1.5 for largely crowded and imp. Buildings, and equal to 1.0 for other buildings.

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Approximate Fundamental PeriodEarthquake EngineeringThe approximate fundamental natural period of vibration for a moment resisting frame without brick infill panels is :Ta = 0.085 h0.75 in secondswhere h = height of the building in m For all other buildings, including moment resisting frame buildings with brick in-fill,

Ta = 0.09h / din seconds

where d = base dimension of the building at the plinth level, along the considered direction of the lateral force, in meters.

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Equivalent Static Method (seismic coefficient method)Earthquake Engineering

Total design seismic base shear if determined by VB = Ah WAh = Design horizontal acceleration spectrum value W = Seismic weight of the building

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Fundamental natural periods ofstructures differ over a large rangeEarthquake Engineering

Adapted from: Newmark, (1970), Current trends in the Seismic Analysis and Design of High Rise Structures, Chapter 16, in Wiegel, (1970), Earthquake Engineering, Prentice Hall, USA.

Distribution of Base Shear to Different Levels of the BuildingEarthquake EngineeringAfter the base shear force VB is determined it should be distributed along the height of the building (to the various floor levels) using the following expression:

After the Base shear is distributed, the frames may be analyzed by any standard computer program to get the internal forces!

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Seismic zoneIIIIIIVV

Seismic intensityLowModerateSevereVery Severe

Z0.100.160.240.36

Damping %

02571015202530

Factor

3.21.41.00.90.80.70.60.550.5

Sr

NoStructureImportance factor (I)

1Important building 1.5

2All other buildings1.0

SystemLoad resisting systemR

Building frame systems A. Ordinary RC moment resisting frame (OMRF ), detailed as per IS 456 or IS 800 or SP 6 (6)3

B. Special RC moment resisting frame (SMRF ), detailed as per IS 13920 5

C. Steel frame

a) Concentric braces4

b) Eccentric braces5

D. Steel moment resisting frame designed as per SP 6 (6)5

System

Load resisting systemR

Building with shear wallsA .Load bearing masonry wall building

a) Unreinforced1.5

b) Reinforced with horizontal bands as per IS 43262.5

c) Reinforced with horizontal RC bands and vertical bars at corners of rooms and jambs of openings as per IS 43263

B. Ordinary reinforced concrete shear walls. These are prohibited in zones IV and V3

C. Ductile shear walls, detailed as per 139204

System

Load resisting systemR

Buildings with dual systemsA. Ordinary shear wall with OMRF3

B. Ordinary shear wall with SMRF4

C. Ductile shear wall with OMRF4.5

D. Ductile shear wall with SMRF5

Imposed Uniformly Distributed

Floor Loads kN / m2 ( LL )Percentage of

Imposed Load

Upto and including 3.025

Above 3.050

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Design Philosophy (Module III)