EARTHQUAKE EFFECTS ON STRUCTURES

Dr. G. P. ChandradharaProfessor of Civil EngineeringS. J. College of EngineeringMysore- 570 006

E mail : chandu_gpc@yahoo.comMobile: 094482 46425

Vibration effects on Structures

Structures are subjected to Two types of Loads

Static loads

Dynamic Loads

Equation of Static Equilibrium

F = K YF - External Force

K – Stiffness of Structure

Y – Resulting Displacement

The restoring Force ( KY) resists the applied force

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Vibration effects on Structures

Equation of Dynamic Equilibrium

There are two additional Forces that resist applied forceAlong with the restoring Force ( KY)

Ma – Inertia ForceCv – Damping Force

These two force are resulting from the induced Acceleration and Velocity in the Structure

y

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VIBRATION EFFECTS ON STRUCTURES

Under the action of dynamic loads the structure vibrates, that is,

(a) the structure develops significant level of inertia forces

(b) significant level of mechanical energy is stored as kinetic energy

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Building at RestBuilding at Rest Ground Accelerates to LeftGround Accelerates to Left

Ground Accelerates to RightGround Accelerates to Right Ground & Building at RestGround & Building at Rest

Direction of Inertia Force

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Representation of Inertia Force

F is known as an inertial force,

◦ created by building's tendency to remain at rest, in its original position, although the ground beneath it is moving

F

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What is Dynamic

Dynamic load is one whose magnitude or direction or position varies with time.

Dynamic response of the structure are displacement, velocity, acceleration, stresses etc are.

Dynamic response of the structure depends on –

(i) dynamic load

(ii) dynamic properties of the structure itself

(ex: natural period)

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1. Inertia Forces

Ground acceleration at the base of structure Structural vibration due to inertia force

Force generated at the floor mass level

Need to be transferred to ground safely

•When the ground moves, the building is thrown backwards, and the roof experiences a force, called inertia force•The walls or columns are flexible, the motion of the roof is different from that of the ground (F=M x a)

•More mass means higher inertia force. Therefore, lighter buildings sustain the earthquake shaking better.

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SEISMIC EFFECTS ON STRUCTURES

SEISMIC EFFECTS ON STRUCTURES2. Effect of Deformation in Structures

The inertia force experienced by the roof is transferred to the ground via the columns, causing forces in columns. The columns undergo relative movement (u) between their ends horizontal displacement u, larger is this greater the internal force in columns. Also, the stiffer the columns are, larger is this force. these internal forces in the columns are called stiffness forces. the stiffness force in a column is the column stiffness times the relative displacement

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SEISMIC EFFECTS ON STRUCTURES3. Horizontal and Vertical Shaking

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Earthquake Motions

◦ Three components Two Horizontal

Longitudinal (X) & Transverse (Y)

One Vertical (Z)

◦ Vertical component Add or subtract the gravity effect

Nor critical to structures

◦ Horizontal components Inertia force & lateral displacement

Critical for structures performance

Need adequate load transfer path

XY Z

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Effects of Horizontal Shaking

Direction offorces on Building

Movement of building

U-DEW

NS

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SEISMIC EFFECTS ON STRUCTURES4. Flow of Inertia Forces to Foundation

The lateral inertia forces are transferred by the floor slab to the walls or columns, to the foundations, and finally to the soil system underneath.

So, each of these structural elements (floor slabs, walls, columns, and foundations) and the connections between them must be designed to safely transfer these inertia forces through them

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Shear WallShear Wall

FoundationFoundationFloor

DiaphragmFloor

Diaphragm

Roof DiaphragmRoof Diaphragm

House Elements ResistHorizontal Forces

f1

f2

f3

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Earthquake effects on Buildings(Analysis and Design)

Vertical Acceleration – Significant near epicenter

(Adds/Reduces to the gravity forces, Large balconies)

Horizontal Acceleration – produces sway( Effect of Inertia , distribution of lateral forces)

Effect of Resonance - Excessive deflection

( Natural frequency coincides with Earthquake frequency

weq/wnat

Ymax

10 2

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Vertical Acceleration

Up/DownAcceleration

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Large Span Cantilevers

Up/DownAcceleration

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Effects of Earthquake on Stress Distribution

Lateral Displacement &

Overturning

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Effects of Earthquakes on Stress distribution-

Change in Stress

Change in Moment

Change in Load

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Bending Moment Distribution in Portal Frame

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RESONANCE

frequency content of the ground motion is close to building's natural frequency

◦ tends to increase or amplify building response

◦ building suffers the greatest damage from ground motion at a frequency close or equal to its own natural frequency

• Example: Mexico City earthquake of September 19, 1985 – majority of buildings that collapsed were

around 20 stories tall

– natural period of around 2.0 seconds

– other buildings, of different heights and different natural frequencies, were undamaged even though located right next to damaged 20 story buildings

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