1. Student name
Moslem Wazir Bdaiwi
EARTHQUAKE EFFECTS ON
STRUCTURES
University of Baghdad
Al-khwarizmi Engineering College
Mechatronics Engineering Department

2. Vibration effects on Structures
Structures are subjected to Two types ofLoads
 Static loads
 Dynamic Loads
Equation of Static Equilibrium
F = KY
F - External Force
K – Stiffness of Structure
Y – Resulting Displacement
The restoring Force (KY) resists the applied force
Equation of Dynamic Equilibrium
Ma + Cv + Ky = F(t)

3. There are two additional Forces that resist applied force Along
with the restoring Force ( KY)
Ma – Inertia Force Cv
– Damping Force
These two force are resulting from the induced Accelerationand Velocity in
the Structure.
Under the action of dynamic loads the structure vibrates, that is
1- the structure develops significant level of inertia forces.
2- significant level of mechanical energy is stored as kinetic energy.
a) b)

4. 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.
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.

5. Dynamic response of the structure depends on
(i) dynamic load
(ii) dynamic properties of the structure itself (ex: natural period)

6. 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.
SEISMIC EFFECTS ON STRUCTURES

7. 2. 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.
3 . Horizontal and Vertical
Shaking

8. 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
X
Y Z

9. • The lateral inertia forces are transferred by
the floor slab to the walls or columns, to the
foundations, and finally to thesoil 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
4. Flow of Inertia Forces to Foundation
House Elements Resist & Horizontal Forces

10. 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)
Up/Down
Acceleration

11. Effects of Earthquakes
on Stress distribution
Change in Stress
Change in Moment
Change in Load
Distribution in Portal
Frame

12. RESONANC
E
 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