eq resistant design for presentation for students

1Presented By:

D. R. Panchal

` Earthquake are natural Threats

` Todays technological advances allow us to control the consequences

` such advances rarely utilize in earthquake resistant design, in common building

` Recent trend in construction increase in number of multistory buildings

2005 NPEEE Earthquake Design Concept : Lecture 3: Basic Terminology & Consequences of EQ3/23








Basic Difference: Magnitude versus Intensity

Magnitude of an earthquake is a measure of its size. For instance, one can measure thesize of an earthquake by the amount of strain energy released by the fault rupture.This means that the magnitude of the earthquake is a single value for a givenearthquake.

On the other hand, intensity is an indicator of the severity of shaking generated at agiven location. Clearly, the severity of shaking is much higher near the epicenterthan farther away. Thus, during the same earthquake of a certain magnitude,different locations experience different levels of intensity.

To elaborate this distinction, consider the analogy of an electric bulb. Theillumination at a location near a 100-Watt bulb is higher than that farther away fromit. While the bulb releases 100 Watts of energy, the intensity of light (orillumination, measured in lumens) at a location depends on the wattage of the bulb andits distance from the bulb. Here, the size of the bulb (100-Watt) is likethemagnitude of an earthquake, and the illumination at a location like the intensityof shaking at that location.

3An increase in magnitude (M) by 1.0 implies 10 times higherwaveform amplitude and about 31 times higher energy released.For instance, energy released in a M7.7 earthquake is about 31times that released in a M6.7 earthquake, and is about 1000(3131) times that released in a M5.7 earthquake. (Did you know?The energy released by a M6.3 earthquake is equivalent to thatgy y q qreleased by the 1945 Atom Bomb dropped on Hiroshima!!)

Hazard Vulnerability

Indian Subcontinent: among the worlds most disaster prone areas

54% of land vulnerable to Earthquakes 8% of land vulnerable to Cyclones 5% of land vulnerable to Floods

Earthquakesq

12% land is liable to severe

earthquakes (intensity MSK IX or

more)

18% land is liable to MSK VIII

(similar to Latur / Uttarkashi)

27% land is liable to MSK VII (similar

to Jabalpur quake)

` Earthquake of Magnitude > 8-1819 Kutchh Earthquake (M8.3)-1897 Assam Earthquake (M8.7)-1905 Kangra Earthquake (M8.6)-1934 Bihar-Nepal Earthquake (M8.4)-1950 Assam Earthquake (M8.7)

4` India prone to Great Earthquakes

` NoM>8 earthquakes in last 50 years

` 2001 Bhuj (M7.7) , 2004 Sumatra (M9.3) and

2005 (M7.4) IndiaPakistan earthquakes to be

seen in this light.

Zone V MM IX or moreZone V MM IX or moreZone IV MM VIIIZone IV MM VIIIZone III MM VIIZone III MM VIIZone II MM VI or lessZone II MM VI or less

Area under the zones Area under the zones V V 12%12%IV IV 18%18%III III ~27%~27%

Total damageable area Total damageable area ~ 57%~ 57%

5ExampleExample

Ground Acceleration Time History

Mass = 10,000kg

Natural Period T=1 sec

Damping =5% of critical3mTime (sec)

n, g

Acc

eler

atio

n, g

Acceleration Response Spectrum for the above accelerogram for 5% damping (Fig. from Seed and Idriss, 1982)

From Response Spectrum:

Spectral Acceleration (for T=1sec) = 0.48 g

Max. Base Shear = Mass x Spectral Accln.=(10,000kg) x (0.48x9.81m/sec2) = 47,000 N = 47 kN

Max. Base Moment

=(47kN) x (3m) = 141 kN-m

Undamped Natural Period T (sec)

Max

imum

Acc

eler

atio

Components of EQ Acceleration

1)TwoHorizontalcomponentsofAcceleration2) One Vertical Component of Acceleration

3) Rotations3) Rotations

Gravity Loads

Inertia force due to vertical Acceleration

EQ force = inertia force

Resisting force offered by slab (diaphragm)

Forces on vertical Elements

` Increased Seismicity & Earthquakes areuncontrollable.

` CAPACITY > DEMAND

` Earthquake induces inertia forces F = ma

` For increased value of F, increase thestrength of building, the capacity must beincrease to avoid damage.

6Q4

Q3

Wihi2

Qi = VB n Wihi2j=1

EQ

Q2

Q1

` Continuity - redundancy

` Strong column - weak beam theory.

` Properly designed diaphragms` Properly designed diaphragms.

` Materials - ductile - energy absorption - good quality workmanship

` SHEAR WALLS

` BRACING SYSTEMS

` MOMENT RESISTING FRAME` MOMENT RESISTING FRAME

` TUBES

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` LARGE STRENGTH AND STIFFNESS WHICH REDUCESLATERAL SWAY OF THE BUILDING AND THEREBYREDUCES DAMAGE TO STRUCTURE AND ITS CONTENT.

` REINFORCEMENT DETAILING STRAIGHT FORWARDSO EASILY IMPLEMENTED AT SITE AND EASY TOCONSTRUCT.

` REDUCTION IN CONSTRUCTION COST.

` Isolation means the state of being separated.` The structure is separated from its foundation.` Concept in base isolation is to reduce the

fundamental frequency of structural vibration to avalue lower than the predominant frequencies ofp qearthquake ground motions

` Provide a means of energy dissipation with which toreduce the transmitted acceleration to thesuperstructure

` Hospital, school building, Fire station, Institutionalbuildings are called life line structure and thesestructure we need in working condition even aftermajor event took place.

` Isolator will produce new mode of vibration

` New mode will increase the fundamental period.

` Not add inter story drifty

11

` Decrease acceleration of structure.

` Reduce inter-story deflections.

D tilit i t i d d` Ductility requirement is reduced.

` Reduce torsion effects up to certain level.

` Avoid resonance.

` very similar to a shock absorber

12

` A well designed structure can withstand a horizontal force several times the design force due to: Overstrength Redundancy

Sudhir K. Jain, IIT KanpurE-Course on IS:1893 / January 2003

Lecture 3 / Slide 67

Redundancy Ductility

` The structure yields at load higher than the design load due to: Partial Safety Factorsx Partial safety factor on seismic loadsx Partial safety factor on gravity loads

Partial safety factor on materials



x Partial safety factor on materials Material Properties x Member size or reinforcement larger than requiredx Strain hardening in materialsx Confinement of concrete improves its strengthx Higher material strength under cyclic loads

Strength contribution of non-structural elements Special ductile detailing adds to strength also

` Yielding at one location in the structure does not imply yielding of the structure as a whole.

` Load distribution in redundant structures provides additional safety margin.

` Sometimes the additional margin due to



` Sometimes, the additional margin due to redundancy is considered within the overstrength term.

` As the structure yields, two things happen: There is more energy dissipation in the structure

due to hysteresis The structure becomes softer and its natural period

increases: implies lower seismic force to be resisted



by the structure` Higher ductility implies that the structure can

withstand stronger shaking without collapse

` Overstrength, redundancy, and ductilitytogether lead to the fact that an earthquakeresistant structure can be designed for muchlower force than is implied by a strongshakingshaking.

` The combined effect of overstrength,redundancy and ductility is expressed interms of Response Reduction Factor (R)

` See Fig. on next slide.

MaximumLoad Capacity

rizon

tal L

oad

Non linear Response

First Significant

Yield

Linear Elastic Response

Fy

Fs

Fel

Load at First Yield

Due to

Due to Redundancy

Due to Ductility

Maximum force if structure remains elastic

Total Horizontal

Load

)(F Force Design)(F Force Elastic MaximumFactor Reduction Response

des

el=

Design force

Tota

l Hor

Roof Displacement ()

max

Fdes

yw

Overstrength

0

13

` Load EL implies Earthquake Load in +X, -X, +Y,and Y, directions.

` Thus, an RC building with orthogonal systemtherefore needs to be designed for the following 13load cases: 1.5 (DL+LL) 1.2 (DL+LL+ELx) ELx = Design EQ load in X-direction 1.2 (DL+LL-ELx) 1.2 (DL+LL+ELy) ELy = Design EQ load in Y-direction 1.2 (DL+LL-ELy) 1.5 (DL+ELx) 1.5 (DL-ELx) 1.5 (DL+ELy) 1.5 (DL-ELy) 0.9DL +1.5ELx 0.9DL-1.5ELx 0.9DL+1.5ELy 0.9DL-1.5ELy

` When the lateral load resisting elements areNOT oriented along two perpendiculardirections

` In such a case, design for X- and Y-directionloads acting separately will be unconservativeloads acting separately will be unconservativefor elements not oriented along X- and Y-directions.

` A lateral load resisting element (frame orwall) is most critical when loading is indirection of the element.

` It may be too tedious to apply lateral loads ineach of the directions in which the elements

dare oriented.` For such cases, the building may be designed

for: 100% design load in X-direction and 30% design

load in Y-direction, acting simultaneously 100% design load in Y-direction and 30% design

load in X-direction, acting simultaneously

ELx 0.3ELx

0.3ELy ELy

Note that directions of earthquake forces are reversible. Hence, all combinations of directions are to be considered.

` Thus, EL now implies eight possibilities:+(Elx + 0.3ELy)+(Elx - 0.3ELy)-(Elx + 0.3ELy)-(Elx - 0.3ELy)+(0 3ELx + Ely)+(0.3ELx + Ely)+(0.3ELx - ELy)-(0.3ELx + ELy)-(0.3ELx - ELy)

1.5 (DL+LL)

1.2[DL+LL+(ELx+0.3ELy)]1.2[DL+LL+(ELx-0.3ELy)]1.2[DL+LL-(ELx+0.3ELy)]

1.5[DL+(ELx+0.3ELy)]1.5[DL+(ELx-0.3ELy)]1.5[DL-(ELx+0.3ELy)]1.5[DL-(ELx-0.3ELy)]1.5[DL+(0.3ELx+ELy)]1.5[DL+(0.3ELx-ELy)]

Thereforer, one must consider 25 load cases:

1.2[DL+LL-(ELx-0.3ELy)]1.2[DL+LL+(0.3ELx+ELy)]1.2[DL+LL+(0.3ELx-ELy)]1.2[DL+LL-(0.3ELx+ELy)]1.2[DL+LL-(0.3ELx-ELy)]

[ ( y)]1.5[DL-(0.3ELx+ELy)]1.5[DL-(0.3ELx-ELy)]

0.9DL+1.5(ELx+0.3ELy)]0.9DL+1.5(ELx-0.3ELy)]0.9DL-1.5(ELx+0.3ELy)]0.9DL-1.5(ELx-0.3ELy)]0.9DL+1.5(0.3ELx+ELy)]0.9DL+1.5(0.3ELx-ELy)]0.9DL-1.5(0.3ELx+ELy)]0.9DL-1.5(0.3ELx-ELy)]

14

` Note that the design lateral load for a building in the X-direction may be different from that in the Y-direction

` Some codes use 40% in place of 30%.

` In complex structures such as a nuclearreactor building, one may have very complexstructural systems.

` Need for considering earthquake motion in allthree directions as per 100%+30% rule. Now, EQ load means the following 24 combinations:x Elx 0.3ELy 0.3ELzx Ely 0.3ELx 0.3ELzx Elz 0.3ELx 0.3ELy

Hence, EL now means 24 combinations A total of 73 load cases for RC structures!

16

Thank youThank you

eq resistant design for presentation for students

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