EARTHQUAKE EFFECTS ON STRUCTURES...Earthquake effects on Buildings (Analysis and Design) Vertical...
Transcript of EARTHQUAKE EFFECTS ON STRUCTURES...Earthquake effects on Buildings (Analysis and Design) Vertical...
EARTHQUAKE EFFECTS ON STRUCTURES
Dr. G. P. ChandradharaProfessor of Civil EngineeringS. J. College of EngineeringMysore- 570 006
E mail : [email protected]: 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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