Download Earthquake (地震)

Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University
Introduction to Earthquake Resistant
Design of Buildings
建筑抗震設計簡介
presented by
Ir Dr Y. L. Wong 黄玉龍博士
Associate Professor
Dept. of Civil & Structural Engineering
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University
Content (內容)

Earthquakes（地震）
What causes earthquakes and how do they happen?

Earthquake Resistant Buildings （抗震設計）
How could engineers design buildings that survive in
earthquakes?
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University 2
Acknowledgements (致謝)

Most of the materials in this presentation were
extracted from websites of “Introducing and
Demonstrating Earthquake Engineering Research in
Schools” (IDEERS) of Taiwan National Center for
Research on Earthquake Engineering (NCREE) and
University of Bristol.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University 3
Earthquakes in South China 華南地震
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University 4
Recent Major Earthquakes near Hong Kong 香港鄰近地震
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University 5
PGA Zonation Map of South China南中國地震設計加速圖
HK: 0.15g for 1 in 475 years
(中國地震局製作)
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University 6
Earthquake (地震)
Earth‘s Structure (地球構造)
Tectonic Plates (構造板塊)
Faults (斷層)
Causes of Earthquakes (地震的成因)
Seismic Waves (地震波)
Where Earthquakes Occur? (地震發生的地點)
Size of Earthquakes (地震的大小)
How Often do Earthquakes Occur? (地震發生的頻率)
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University 7
Earth‘s Structure(地球的構造)
The inner core, with a radius of 1,370 kilometers,
is believed to be a solid metal body.
The outer core is a 2,000 kilometer thick layer
believed to be a liquid metal layer.
The mantle is 2900 kilometers thick.
The crust is the outer layer of hard rock, ranging
in thickness from 4 to 60 kilometers
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University 8
Tectonic Plates (構造板塊)
World map showing
the tectonic plates
with only the larger
plates labelled
The surface of the earth is made up of 21 tectonic plates, some large
and some small, that are constantly moving.
As the plates are forced against each other, they deform, and eventually
they crush and fracture. The sudden fracture of the rock sends out a
shock wave that causes the earth's surface to shake. This is one way
earthquakes can happen.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University 9
Fault (斷層)

Over millions of years the earth's tectonic plates have been moving continuously
and pushing against each other. These movements have forced them to deform
producing mountains and valleys in the earth's surface.

Sometimes the rocky surface of the earth has just been bent and folded (Fig.1).
Sometimes the movements have caused the rock to deform so much that they
fracture. These fractures are called faults (Fig. 2). When the rock fractures, its
sudden movement causes an earthquake as shock waves spread away from it.
Fig. 1 How Rocks are bent and folded
Fig. 2 How Rocks Fracture
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University10
Faults in earthquake (斷層實例)
This fence crossed the San
Andreas Fault. In 1906, the fault
ruptured causing the Magnitude
8.3 San Francisco Earthquake.
The horizontal movement of the
ground caused the fence to move
by about 2 metres
The fault ruptured the ground surface
passing right through this school in Wu
Feng. It broke through a running track.
(Chi-Chi earthquake, Taiwan, 1999).
The Shih-Kang Dam is
constructed from concrete. The
thrust fault broke right through it.
In this picture taken on its
downstream side, you can see
where the right hand side of the
dam rose 10 metres compared to
the left hand side (Chi-Chi
earthquake, Taiwan, 1999).
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University11
Causes (地震的成因)

Earthquakes can be caused by natural events or human
activities. Here are some of the different causes of
earthquakes.
1. Tectonic Plate Movement- the most common cause
2. Volcanic Activity
3. Explosions
4. Collapsed Mines
5. Water Pressure in Reservoirs
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University12
Causes－Tectonic Plate Movement （構造板塊的移動）



The most common cause of earthquakes.
The earthquake occurs when the pressure that has built up in
tectonic plates causes the rock to break suddenly.
This usually occurs at the boundaries of tectonic plates and along
existing faults.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University13
Seismic Waves (地震波)

When the rock breaks, there is a sudden release of energy. Shock
waves spread out through and around the earth in all directions,
starting from the focus of the earthquake. At the earth's surface the
ground vibrates as the waves pass through it. The way the waves
spread is a bit like the ripples spreading on a pond when a stone is
dropped into it.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University14
Seismic Waves (地震波)

Energy spreads out through the earth in three different
wave types :
P-wave （P-波）
•a longitudinal wave
•travel through rock, liquid and the air
•the fastest traveling seismic wave
S-wave （S-波）
•a transverse wave
•travel through rock, but not through liquid and the air
•slower than a P-wave, but faster than a surface wave
Surface wave: （面波）
•the slowest traveling seismic waves
•Their movement is greatest at the earth's surface, and gets smaller
deeper below the surface
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University15
Where Earthquakes Occur (地震發生地點)
This map shows the distribution of the world's earthquakes that happened
during the 1980s. Each red dot represents an earthquake. It is clear that
earthquakes happen more often in some places than others.
the edges of tectonic plates
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University16
Size of Earthquake (地震的度量)

Magnitude (量級)- the amount of energy it
releases into the earth's crust.

Intensity(烈度)- the amount the ground
shakes.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University17
Size of Earthquake (地震的度量)

Magnitude(量級):

It gives an idea of the
strength of an earthquake.
For each unit on the scale,
the energy released by an
earthquake is about 30
times greater than the unit
below. So, a magnitude 6
earthquake releases 30
times as much energy than
a magnitude 5 earthquake,
and 900 times as much
energy as a magnitude 4
earthquake.

Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University18
Size of Earthquake

Intensity(烈度)：

For the same earthquake, its
intensity will vary from place
to place. Usually, it is
greatest near the epicentre,
and it gets smaller further
away.
Intensity is not measured on
instruments. It is worked out
by considering the effects on
people and buildings.

This is an isoseismal map for the
magnitude 4.2 (ML) Warwick
earthquake of September 23rd 2000, in
England, showing isoseismals from
intensity 2 to 5.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University19
How Often do Earthquakes Occur (地震發生的頻率)

This table shows how often earthquakes of different
magnitude occur, world-wide.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University20
What does a Seismogram tell us (地震記錄)




Size of ground motion at the measurement station
When the different type of waves, i.e. the P-waves, the Swaves and the surface waves, arrived at the measuring
station
What sort of rock they passed through on the way there
Information to calculate the magnitude of the earthquake
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University21
Earthquake Resistant Buildings
（結構的抗震設計）
How could engineers design buildings that survive in earthquakes?
To explain how buildings:




Vibrate (振動) during earthquakes,
are strengthened (加固) to resist earthquakes,
can be isolated (隔震) from the shaking ground,
use dampers (阻尼器) to reduce vibrations from earthquakes.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University22
Vibrating Buildings (建筑物的振動)

To understand the way buildings behave during
earthquakes, you need to know:


How the ground moves. (地面運動)
How buildings vibrate naturally. (建筑物的自由振動)
How vibrations die out. (振動的耗散)

Then, you can know

How earthquakes make buildings vibrate. (建筑物在
地震作用下的振動)

Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University23
Ground Motion (地動) –Introduction (簡介)

During an earthquake, the motion of the ground at any
location is very complicated, as the ground is shaking in
all directions. This motion can be described more simply
as a combination of different motions all happening at
the same time. They are:
Horizontally, side to side (Animate1)
Horizontally, backwards and forwards (Animate2)
Upwards and downwards (Animate3)
Rotating backwards and forwards (Animate4)
Rotating from side to side (Animate5)
Twisting (Animate6)
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University24
Ground Motion (地動) -A Single Simple Wave (正弦波)

Amplitude: (振幅)
Think of a point on the ground vibrating to
and fro. If it kept moving the same distance
each way, it would have a constant
amplitude of vibration. If it moved 10 mm
each way, its amplitude of vibration would
be 10 mm.

Frequency: (頻率)
If the number of times it moved to and fro
every second remained the same, it would
have a constant frequency of vibration.
Each to and from movement is called one
cycle of motion. If the patch of ground made
5 cycles every second, its frequency of
vibration would be 5 Hertz.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University25
Ground Motion (地動)

Combination of waves (波的組合)
Real ground motion during an earthquake is made up of many
waves of different amplitudes and frequencies. The main
shaking of the ground that is felt has frequencies up to 20
Hertz. (Animate)
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University26
Single Degree of Freedom System-Static Analysis
(單自由度系統-靜力分析)
x
F

System (系統)

Equation of Equilibrium (平衡方程)
m
Kx  F

Stiffness (剛度) is the resistance of an elastic body to
deflection or deformation by an applied force.
F
K
x
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University27
SDOF System–Free Vibration
(單自由度系統-自由振動)

x
m
Undamped system (無阻尼系統)
K

Equation of motion (運動方程)
mx  kx  0

Natural frequency (自振頻率)
n 
k
m
Note: It depends only on the system mass and the spring stiffness
(i.e. any damping will not change the natural frequency of a system).
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University28
SDOF System–Free Vibration
(單自由度系統-自由振動)

Damped system (有阻尼系統)
x
m
K
C

Equation of motion (運動方程)
mx  cx  kx  0

C : Damping ratio (阻尼系數), a measure of the damping of
the system, expresses the damping of the system as a ratio
of the critical damping level .
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University29
SDOF System–Free Vibration
(單自由度系統-自由振動)

Comparison of Sample Time Behaviors (兩種系統的反應時程比較)
Undamped System
Damped System
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University30
SDOF System–Dynamic Response
(單自由度系統-動力反應)
x'
x
 Earthquake Excitation
 Sample Response History-Displacement
m
(地震動激勵)
K
C
(時程反應樣本-位移)
a g  xg
 Equation of Motion (動力方程)
m( x  xg )  cx  kx  0
mx  cx  kx   mxg
Effective earthquake force (等效地震力)
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University31
MDOF System-Natural Vibrations
(多自由度系統-自由振動)

Earthquakes can cause buildings to vibrate. There are
two basic concepts on natural vibrations of a building:
1.
Mode and Mode Shape:(模態和振型)
Every building has a number of ways, or modes, in
which it can vibrate naturally. In each mode, the
building vibrates to and fro with a particular distorted
shape called its mode shape.
Frequency of vibration:(振動頻率)
The number of times it vibrates to and fro every second
is the frequency of vibration for that mode.
2.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University32
MDOF System-Natural Vibrations
(多自由度系統-自由振動)

The Fundamental Mode (基本模態)

Imagine you could push a building sideways at its top and
then let go so that it swayed naturally. The number of times it
swayed to and fro every second would be the fundamental
frequency of vibration of the building.

If you repeated the experiment, but pushed the building a little
harder or lighter, the fundamental frequency would stay the
same.
(Animate the fundamental mode of vibration of a 2-story typical frame building)
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University33
MDOF System-Natural Vibrations
(多自由度系統-自由振動)

Higher Modes (高階模態)

The building could be made to sway at other frequencies of
vibration and with other mode shapes by pushing it at lower
floor levels.
Animate the second modes and third modes of vibration
of a 2-story typical frame building
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University34
MDOF System-Natural Vibrations
(多自由度系統-自由振動)

The Natural Frequency of a Building (一棟建筑物的自振頻率)

The natural frequency for each mode of vibration follows this rule:
f = natural frequency in Hertz.
K = the stiffness of the building associated with this mode
M = the mass of the building associated with this mode

Buildings tend to have lower natural frequencies when they are:

Either heavier (more mass)
Or more flexible (that is less stiff).


One of the main things that affect the stiffness of a building is its height.
Taller buildings tend to be more flexible, so they tend to have lower natural
frequencies compared to shorter buildings.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University35
How Vibrations Die Out-Damping
(振動的耗散-阻尼)

The vibrations die out because of damping which removes energy
from the moving building.

The damping can be caused by
Friction (摩擦) as different parts of the building move against each
other.
Internal friction in the materials (材料的內部摩擦) making up the
structural members and other parts of the building.
Damage in the building (建筑物的破壞), for example, cracking in
concrete or brickwork or permanent distortions in steel.





What can engineers do?
Engineers can design buildings to have extra damping, by adding
dampers (阻尼器) to the structural frame. The dampers absorb
energy from a vibrating building, so that its movement is not as
violent.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University36
Resonance in Buildings (建筑物的共振)
The picture shows two buildings. Imagine the
tall building has a fundamental frequency of
5.5 Hertz and the small building has a
fundamental frequency of 7.5 Hertz.



If the ground moved to and fro with a frequency of 5.5 Hertz, the tall
building would vibrate strongly, or resonate, while the short building
hardly moved at all. (Animation)
If the ground moved to and fro with a frequency of 7.5 Hertz, the
small building would resonate while the tall building hardly moved at
all. (Animation)
During an earthquake, the ground shakes with a mixture of
frequencies of vibration. Q: If the frequencies ranged between 5.0
and 6.0 Hertz, which of the two model buildings would you expect to
vibrate most?
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University37
Strengthening Buildings for Earthquakes (結構的抗
震加固)

Horizontal structural systems (floors and
roofs)

Diaphragms (樓板)
Trussing (桁架)


Vertical structural systems (columns, beams,
walls and bracing)

Braced frames (帶斜撐的框架)
Moment resisting frames (抗彎矩框架)
Shear walls (剪力墻)


Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University38
Horizontal structural systems-Diaphragms
(水平結構系統-樓板)

Horizontal diaphragms are usually floors and roofs. They are
made up from a horizontal frame covered by a floor or roof
deck.

When a diaphragm is stiff enough in its horizontal plane. it can
share the sideways earthquake forces on a building between
the vertical structural members, e.g. the columns and walls.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University39
Horizontal structural systems-Trussing
(水平結構系統-桁架)

Horizontal trussing is usually used in roofs where there is not
enough deck to allow the roof to act as a stiff horizontal
diaphragm.

The trussing transfers the sideways earthquake forces on a
building to its vertical structural members e.g. the columns
and walls.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University40
Vertical structural systems-Braced Frames
(豎直結構系統-帶斜撐的框架)

Single Diagonals (單斜撐)
If a single diagonal, or brace,
is used, it must be able to
resist tension (stretching)
and compression (squashing)
caused by sideways forces
in both directions on a frame.
Single diagonals in a 3-storey frame
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University41
Vertical structural systems-Braced Frames
(豎直結構系統-帶斜撐的框架)

Cross Bracing
(交叉斜撐)
If two diagonals are used, in the form of crossbracing, they only need to resist tension.
This is because one brace is in tension for the
sideways force in one direction on the frame,
while the other brace is in tension when the
force is reversed.
Steel cables can be used for cross-bracing, as
they can be stretched, but not squashed.
Cross-bracing in a 3-storey frame
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University42
Vertical structural systems-Braced Frames
(豎直結構系統-帶斜撐的框架)

Miscellaneous Methods (混合方法)
Inverted V Bracing
Knee Bracing
K Bracing
V Bracing
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University43
Vertical structural systems-Moment Resisting Frames
(豎直結構系統-抗彎矩的框架)

In moment resisting frames, the joints, or connections,
between columns and beams are designed to be rigid.
This causes the columns and beams to bend during
earthquakes. So these structural members are designed
to be strong in bending.

Moment resisting frames simply means frames that
resist forces by bending.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University44
Vertical structural systems-Shear Walls
(豎直結構系統-剪力墻)

Shear walls are vertical walls that are used to stiffen the
structural frames of buildings. They help frames resist
sideways earthquake forces.

It is better to use walls with no openings in them. So, usually
the walls around lift shafts and stairwells are used. Also, walls
on the sides of buildings that have no windows can be used.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University45
Isolating Buildings –Introduction
(隔震結構-簡介)


Normally, a building is supported
directly on its foundations. When base
isolation is used, special structural
bearings are inserted between the
bottom of the building and its
foundation. These bearings are not very
stiff in the horizontal direction, so they
reduce the fundamental frequency of
vibration of a building. The frequency
becomes so low that the building does
not vibrate as strongly during an
earthquake.
During an earthquake, a fixed-base
building can sway from side to side.
When a base isolation system is used,
the sideways movement occurs mainly
in the bearings, and the building hardly
distorts at all.
Click the figure
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University46
Isolating Buildings – Bearings(隔震結構-支撐)

Rubber Bearings (橡膠
支撐)
Layers of rubber
+
thin steel plates between them
+
a thick steel plate on the top and
bottom

Click the figure
Friction pendulum Bearings
(摩擦擺錘支撐)
two horizontal steel plates that can
slide over each other because of
their shape
+
Click the figure
an additional articulated slider.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University47
Adding Dampers –Introduction
(阻尼器-簡介)

Dampers can be installed in the structural frame of a building to
absorb some of the energy going into the building from the
shaking ground during an earthquake. The dampers reduce the
energy available for shaking the building. This means that the
building deforms less, so the chance of damage is reduced.

There are many types of dampers that can be installed in
buildings. Here are some of them:

Metallic Dampers (金屬阻尼器)
Friction Dampers (摩擦阻尼器)
Viscous Fluid Dampers (粘滯阻尼器)


Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University48
Adding Dampers-Metallic Dampers
(阻尼器-金屬阻尼器)

There are different types of metallic damper. One type, the X-shaped
Plate Damper, is used where two braces meet. As the building vibrates,
the braces stretch and compress, pulling and pushing the damper
sideways and making it deform. They are designed to deform so much
when the building vibrates during an earthquake that they cannot return
to their original shape. This permanent deformation is called inelastic
deformation, and it uses some of the earthquake energy which goes
into building.
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University49
Adding Dampers-Friction Dampers
(阻尼器-摩擦阻尼器)

Friction dampers are designed to have moving parts that will
slide over each other during a strong earthquake. When the parts
slide over each other, they create friction which uses some of the
energy from the earthquake that goes into the building.
Click the figure
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University50
Adding Dampers-Viscous fluid dampers
(阻尼器-粘滯阻尼器)


They consist of a closed cylinder containing a viscous fluid like oil. A
piston rod is connected to a piston head with small holes in it. The
piston can move in and out of the cylinder. As it does this, the oil is
forced to flow through holes in the piston head causing friction.
When the damper is installed in a building, the friction converts some of
the earthquake energy going into the moving building into heat energy.
Click the figure
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University51
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University
Thank you!
(謝謝！)
Dept. of Civil & Structural Engineering, The Hong Kong Polytechnic University