Download Resistance of buildings during an earthquake

Guillaume M. Melin (177626)
VIAUC Campus Horsens – 02.12.2013
BACHELOR OF ARCHITECTURAL
TECHNOLOGIES AND CONSTRUCTION
MANAGEMENT
7th SEMESTER
DISSERTATION
Author :
Guillaume MELIN (177626)
Consultants :
Professor Bixiong Li
Dissertation realized in partnership with Sichuan
University in Chengdu, CHINA.
Guillaume M. Melin (177626)
VIAUC Campus Horsens – 02.12.2013
Acknowledgement
This dissertation has been written in order to achieve the 7th and last semester
of my Bachelor of sciences in Architectural Technologies and Construction
Management in VIA University College in Denmark.
I want to thank particularly the Sichuan University (SCU) in Chengdu, in
Sichuan, a province of China, which took me as student for a period of 6 months.
Thanks to them, I have been able to pick this subject, very important in China,
geologically less important in Denmark. VIA University College gave me the
opportunity to come in China for that, and I am appreciative.
Thanks to my Danish and Chinese consultants who guided me in the good
directions: Mr Laurids Green (head of department of Constructing Architect in VIA
UC), Mrs. Xia Wang (lecturer in the architecture dept. of SCU) and Mrs. Bixiong Li
(head of department of Civil Engineering in SCU).
My work on this dissertation has been mainly done by reading books, to have
an overall knowledge of the subject, and then some articles and some universities
or specialized centres websites helped me to be more meticulous on certain
points.
Number of pages (2400 characters): 30 pages. - Characters : 71886 All rights reserved – no part of this publication may be
reproduced without the prior permission of the author.
NOTE: This dissertation was completed as part of a
Bachelor of Architectural Technology and Construction
Management degree course – no responsibility is
taken for any advice, instruction or conclusion given within!
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Guillaume M. Melin (177626)
VIAUC Campus Horsens – 02.12.2013
Abstract
The subject that is scrutinized in this dissertation is the seismic design. Since I
am in China, I have decided to choose this topic to understand another aspect of
design, a more restrictive one, where technology’s needs exceed the onlyaesthetical design to create another type of architecture.
My problem statement is the following one:
“How can buildings structures resist on a seismic tremor?”
In order to reply to this problematic statement, I divided my research in a few
others formulations, those research questions, are the following:
-
How, structurally, are the buildings made of?
-
What are the consequences of an earthquake?
-
Which structural systems could prevent different levels of earthquake to get
a building down?
-
How do engineers size seismic-resisting buildings over the world?
The books I have read helped me to get a general idea on some of the
earthquake specificities and on seismic design over the past 40-50 years with the
birth of new technologies and the unceasing inspections and studies from the past
earthquakes to evolve towards a quake-proof construction world.
To get more specific, internet (articles, online courses of some universities,
specialized centres on earthquakes, codes …) provided me the rest of information
I needed.
All resources used are usually listed at the end of each part and at the end of
the dissertation - List of references. The illustrations used are also listed - List of
illustrations- in the following page.
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Guillaume M. Melin (177626)
VIAUC Campus Horsens – 02.12.2013
List of Contents
ACKNOWLEDGEMENT................................................................................................... 2
ABSTRACT .................................................................................................................... 3
LIST OF CONTENTS........................................................................................................ 4
LIST OF ILLUSTRATIONS ................................................................................................ 7
CHAPTER 1. INTRODUCTION........................................................................................ 8
CHAPTER 2. THE STRUCTURAL DESIGN OF CONSTRUCTIONS ...................................... 10
2.1 THE STRUCTURE AS THE BUILDING’S SKELETON ................................................................. 10
2.1.1 Structural engineers .......................................................................................... 10
2.1.2 The structural engineer/architect relationship ................................................. 11
2.1.3 Static equilibrium .............................................................................................. 12
2.1.4 Codes and standards ......................................................................................... 12
2.2 BUILDING MATERIALS .................................................................................................. 13
2.2.1 Material density ................................................................................................ 13
2.2.2 Internal forces ................................................................................................... 13
2.2.3 Material extension ............................................................................................ 14
2.2.4 Most common structural material .................................................................... 14
2.3 FINANCIAL INFLUENCES................................................................................................ 16
2.3.1 The client ........................................................................................................... 16
2.3.2 Country specificities........................................................................................... 16
2.4 STRUCTURAL DESIGN HISTORY EXAMPLES ........................................................................ 17
2.4.1 Before Antiquity................................................................................................. 17
2.4.2 Romans .............................................................................................................. 17
2.4.3 Traditional Chinese architecture ....................................................................... 18
CHAPTER 3. THE SEISMIC RISKS ................................................................................. 20
3.1 EARTHQUAKE CHARACTERISTICS .................................................................................... 20
3.1.1 Geology.............................................................................................................. 20
3.1.2 Type of waves .................................................................................................... 21
3.1.3 Peak ground acceleration .................................................................................. 22
3.1.4 Forces involved. ................................................................................................. 22
3.1.5 Extra-consequences ........................................................................................... 24
3.2 A YOUNG TECHNOLOGY ............................................................................................... 24
3.2.1 The 20th century................................................................................................. 24
3.2.2 Retrofitting ........................................................................................................ 25
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VIAUC Campus Horsens – 02.12.2013
3.3 EARTHQUAKE IN L’AQUILA, CENTRAL ITALY (2009) ........................................................... 26
3.3.1 Characteristics ................................................................................................... 26
3.3.2 Geographic area and history ............................................................................. 26
3.3.3 Damage ............................................................................................................. 27
3.4 EARTHQUAKE IN WENCHUAN, SICHUAN PROVINCE, CHINA (2008)...................................... 28
3.4.1 Characteristics ................................................................................................... 28
3.4.2 Geographic area and history ............................................................................. 28
3.4.3 Damage ............................................................................................................. 29
CHAPTER 4. EARTHQUAKE-RESISTANT DESIGN .......................................................... 31
4.1 K.I.S.S. PRINCIPLE ..................................................................................................... 31
4.2 CAPACITY DESIGN ....................................................................................................... 32
4.2.1 Ductility ............................................................................................................. 33
4.2.2 Hierarchy of strength ........................................................................................ 33
4.3 RESISTING EARTHQUAKES’ FORCES ................................................................................. 34
4.3.1 Horizontal planned resistance ........................................................................... 34
4.3.2 Vertical planned resistance ............................................................................... 35
4.4 MAIN STRUCTURAL SYSTEMS ........................................................................................ 35
4.4.1 Shear walls ........................................................................................................ 36
4.4.2 Braced systems .................................................................................................. 36
4.4.3 Moment resisting frames .................................................................................. 37
4.4.4 Mixed systems ................................................................................................... 37
4.5 COMMON ISSUES TO AVOID .......................................................................................... 38
4.5.1 Structural discontinuity and off-set ................................................................... 38
4.5.2 Soft storey ......................................................................................................... 38
4.5.3 Short column ..................................................................................................... 39
4.5.4 Torsion ............................................................................................................... 40
4.5.5 Infill walls........................................................................................................... 40
4.5.6 Buildings pounding ............................................................................................ 40
4.5.7 Re-entrant corners............................................................................................. 41
4.6 PARTICULAR SYSTEMS ................................................................................................. 41
4.6.1 Seismic separation gap...................................................................................... 41
4.6.2 Stairway ............................................................................................................. 41
4.6.3 Bridge between buildings .................................................................................. 42
4.7 NEW TECHNOLOGIES ................................................................................................... 42
4.7.1 Seismic proof constructions ............................................................................... 42
4.7.2 Dampers ............................................................................................................ 42
4.7.3 Carbon fibres ..................................................................................................... 43
4.7.4 Innovative structural configurations ................................................................. 43
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VIAUC Campus Horsens – 02.12.2013
CHAPTER 5. THE DESIGN PROCESS AND THE BUILDING STANDARDS OVER THE WORLD
45
5.1 PERFORMANCE-BASED DESIGN ...................................................................................... 45
5.1.1 Performance level.............................................................................................. 45
5.1.2 Hazard level ....................................................................................................... 46
5.1.3 Probability of the earthquake ........................................................................... 46
5.2 THE DESIGN PROCESS .................................................................................................. 47
5.2.1 The building ....................................................................................................... 47
5.2.2 Peak ground acceleration.................................................................................. 47
5.2.3 Building response spectra ................................................................................. 48
5.2.4 Seismic force ...................................................................................................... 48
5.3 COMPARISON BETWEEN CODES AND STANDARDS .............................................................. 48
5.3.1 Factors ............................................................................................................... 48
5.3.2 Intensity scale .................................................................................................... 49
5.3.3 Probability of the earthquake ........................................................................... 49
CHAPTER 6. SUMMARY ............................................................................................ 50
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VIAUC Campus Horsens – 02.12.2013
List of Illustrations
FIG. 1 : WILLIS FABER DUMAS HEADQUARTERS BY N. FOSTER AND A. HUNT ........................................ 11
FIG. 2 : FREE-BODY DIAGRAM...................................................................................................... 12
FIG. 3 : INTERNAL FORCES........................................................................................................... 13
FIG. 4 : IMPROVED WOODEN PROFILE ........................................................................................... 15
FIG. 5 : GENERAL IMROVED PROFILES ........................................................................................... 15
FIG. 6 : PIT HOUSE CROSS SECTION ............................................................................................... 17
FIG. 7 : TEEPEE HOUSE STRUCTURE ............................................................................................... 17
FIG. 8 : ROMAN ARCH FORCES PATH ............................................................................................. 18
FIG. 9 : CHINESE WOODEN STRUCTURE.......................................................................................... 19
FIG. 10 : TYPICAL CHINESE TEMPLE CROSS SECTION .......................................................................... 19
FIG. 11 : SEISMIC WAVES ........................................................................................................... 21
FIG. 12: MAGNITUDE, RICHTER SCALE GRAPHIC REPRESENTATION ...................................................... 22
FIG. 13: INTENSITY, AMERICAN SCALE........................................................................................... 23
FIG. 14 : ISOSEISMAL MAP ITALY EARTHQUAKE ............................................................................... 26
FIG. 15 : BUILDING COLLAPSED IN THE CITY OF L’AQUILA ................................................................... 27
FIG. 16 : SAN FRANSISCO CHURCH CLOSE TO L’AQUILA WITH THE ROOF COLLAPSED ............................... 27
FIG. 17 : ISOSEISMAL MAP CHINA EARTHQUAKE .............................................................................. 28
FIG. 18 : APOCALYPTICAL SURROUNDING IN BEICHUN COUNTY AFTER EARTHQUAKE ............................... 29
FIG. 19 : BAILHUZEN MIDDLE SCHOOL AFTER EARTHQUAKE ............................................................... 29
FIG. 20 : UNREINFORCED MASONRY IN SICHUAN PROVINCE. ............................................................. 30
FIG. 21 : STEEL BEHAVIOUR WHILE STRESSED .................................................................................. 33
FIG. 22 : LOCATION OF HINGE JOINTS IN A MOMENT RESISTING FRAME. ............................................... 33
FIG. 23 : BOND BEAMS-CHORD .................................................................................................... 35
FIG. 24 : SIMPLIFIED BUILDING WITH SHEAR WALLS ON EACH SIDE TO RESIST EVERY HORIZONTAL FORCES .... 36
FIG. 25 : SIMPLIFIED STEEL FRAME WITH BRACING ........................................................................... 36
FIG. 26 : SIMPLIFIED MOMENT-RESISTING FRAME BUILDING .............................................................. 37
FIG. 27 : SIMPLIFIED BUILDING WITH BRACING AND SHEAR WALLS ...................................................... 38
FIG. 28 : SOFT STOREY EXAMPLE .................................................................................................. 39
FIG. 29 : SHORT COLUMN EXAMPLES WITH SLOPED GROUND OR WITH A MEZZANINE FLOOR .................... 39
FIG. 30 : EXAMPLES OF ISOLATION SYSTEM. ON THE LEFT, RUBBER PLATES, ON THE RIGHT SLIDING BEARING43
FIG. 31 : THE TOWER TAIPEI 101 - IN YELLOW THE TUNED MASS DAMPER ........................................... 44
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Guillaume M. Melin (177626)
VIAUC Campus Horsens – 02.12.2013
Chapter 1. Introduction
This dissertation has been written within the context of the 7th semester of the
bachelor in Architectural Technologies and Construction Management of VIA
University College, in the city of Horsens, Denmark. To fulfil this last semester
composed of a dissertation and a final project, I chose to go abroad in exchange
with the Sichuan University in Chengdu, which is a relevant location in order to
write down those pages with professors that are specialized within the field I had
picked, but also people leaving here who feel the tremors regularly and that can
share their personal experience with me. The general subject I have been
intrigued by is the earthquake in the construction field. Myself simply wondering
how could they stand with such a massive force shaking the whole ground.
Earthquake is one of the most disastrous events that can happen to a building
or a civil construction, its enormous energy coming out of it is able to destroy
entire cities, or to create a tsunami taking over everything on its way. Being a
student in Denmark is not the best way to get in touch with those things since the
seismic risks are negligible over there. My travel in China, in a region of the centre,
in the province of Sichuan - next to the highest mountain chain in the world, the
Himalaya - known for earthquakes offered me the possibility to study those famous
- unfortunately for bad reasons - tremors, coming out of the ground to make us
thrill. Before coming to Denmark, I was a student in the University of Strasbourg,
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France, where I studied civil engineering and have been graduated with a
Bachelor on the relevant field, the earthquake being relatively low in the area - 2-3
on the Richter scale at most -, me and my classmates haven’t been taught a lot
about it. Therefore that was a reason now to get more knowledge on seismic risks
and seismic design.
My problem statement is: “How can buildings structures resist on a seismic
tremor?”
In this dissertation we will investigate on how the buildings are made of,
without any earthquake considerations, the materials that our civilizations have
used and how they do stand in usual circumstances, that is to say the way we
have built towers, industrial hangars, stadiums, etc... After this recap, we will turn
on the big problem for edifices that has existed since the very beginning of our
planet, earth movements and their consequences on our societies. After the
problems, come the solutions and the different structural systems that we can
meet to prevent the buildings to collapse. Finally, we will take a general look on
the design process and give examples on codes differences or similarities around
the world.
As a student in the Constructing Architect programme, my goals are not to go
deep into the engineer calculations, even though it surely is very interesting to dig
the possible differences on calculations or diagrams. Nevertheless that will not be
relevant for this dissertation. Therefore, and as stipulated by the name of the
programme, I will limit myself around the architectural technologies against
earthquakes going a bit further on some points, with the codes and standards for
example.
My resources have been mainly books, to get an outline of the seismic design
of the last decades and nowadays. Articles and specialized websites helped me to
go deeper in the understanding on specific parts of the dissertation.
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Chapter 2. The structural design of constructions
This chapter 2 will introduce the real content of this
dissertation. The structure in architecture is an important thing, it
is merely the concept that make the building standing. Therefore
we will firstly take a look of this field, on the engineers, on the
building industry and on their way to work. Besides, a recap of
structural design history will end this chapter relating materials
and construction technics.
2.1 The structure as the building’s skeleton
2.1.1 Structural engineers
In a construction process, the client is the first person involved in the project,
since he/she or a moral person like an institution decides either to create a new
project, and therefore a new building or to renovate a building that needs it for
different reasons; could be for another function of the construction and then
another architectural form would be more appropriate or to upgrade the buildings
to the last building codes released, which is an interesting subject, a normal
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refurbishment, or in case of seismic resistant structures, a retrofitting The
architect, the person in charge legally to respond to the client needs will create the
outline, and so on.
The structural engineers can enter the process at several different moments,
theoretically and what is usually represented in schools is the engineer starting to
work on the project after the outline being finished and after tender bid accepted. It
could also be in a very technical construction the engineer being called by the
architect to work on the outline and give some advices. In some civil construction,
a structural engineer could be the only person in charge.
Unfortunately for the engineers, architects are usually more known than them,
there is even a specific term for a widely-known architect, and it is a “starchitect”.
The
engineer
stays
almost always in the
shadow of the architect.
Almost, because some
engineers are still, wellknown in the building
industry and they are
always linked to famous
architects as well – e.g.
Anthony Hunt that has
worked with few of
Norman
Foster
and
Richard Rogers’ projects.
FIG. 1 : WILLIS FABER DUMAS HEADQUARTERS BY N. FOSTER AND
A. HUNT
They are the people
sizing buildings and they
also get their words to
say on the precise components of the construction. They are the people taking
care of the respect of the seismic codes acting in the country the project is settled
and are directly responsible for any damages any deviances.
2.1.2 The structural engineer/architect relationship
Engineers and Architects are two complementary actors of the building
industry. Beyond the fact that they might have some contentious relationships,
their work as one team is important on several points. First of all, the client could
lose his money caused by the time schedule postponed - mistakes on the
structural design, etc... -, the building could be less sustainable, or could have
bigger issues. This relationship becomes even more important when the project is
located in a seismic region, the risks may directly be connected to humans’ lives.
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2.1.3 Static equilibrium
This sub-part of the chapter talks about the exact role of the engineer in the
sizing process of a building. The static equilibrium is the key in order to define the
actions on each component of the structure and size them. The concept of
equilibrium is that the external forces acting on the structure cancel each other; the
system is thus in equilibrium. If the system is not static - i.e. in equilibrium - it is
called to be a mechanism.
A free-body diagram is a 2D view that allows an engineer to simplify and find
out the value of the forces acting on the structure. Those forces will be matched
with the loads acting on the building; vertical loads - i.e. dead loads and imposed
loads - and horizontal loads - i.e. the wind being important for long-spanning
structures like bridges or high-raise
buildings but also seismic resulting
forces in certain regions on earth. The
path that forces take through the
different components is an important
aspect of sizing elements, however
some structure can be harder than
others to represent schematically. The
reason for that is that the force paths
is not simple to analyse, too many
possibilities
are
possible.
Nevertheless,
computer-based
FIG. 2 : FREE-BODY DIAGRAM
software help engineers with those
structures
and
avoid
many
complicated calculations that moreover take time.
(Mcdonald, 2001, p. 9)
2.1.4 Codes and standards
The way to size structural components or the solutions that can be brought
within the structure are stipulated in the respective codes of the country the project
is built in. Those minimum requirements help the building industry to be at a
certain level of efficiency towards the resistance of the construction and safety for
the people working in, living in, or simply as users of those superstructures.
Each country and government has their own standards that have to be
respected by any company. This will be discussed on the dissertation’s part 5.
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2.2 Building materials
Each material that are used as a construction material or even that can be
found anywhere on this planet for whatever reason have their specific
characteristics. They allow the construction, to look different but also to act
different predictably enough on a structural point of view thence there are more
optional ideas to raise an edifice.
2.2.1 Material density
One of the most common characteristic is the density, calculated by the weight
or the mass (depending on the units we want to involve in the calculations) divided
by the volume. The density of the construction materials are directly linked to the
total mass of the building, this mass is a very important factor in the seismic
design that we will look deeper into in the following part. Indeed, the mass is linked
to inertia, the Newton’s second law of motion establishing that the force equals to
the mass times acceleration - F = m x a. We will go through it in the next chapter.
(Title 3.1.4, p.18)
2.2.2 Internal forces
FIG. 3 : INTERNAL FORCES
Materials are generally
submitted to constraints
and very important ones
quantitatively
if
the
components are used in
the building industry. Their
functions have to be
completely exploited, to do
so,
architects
and
engineers need to know
what kind of forces they
must resist to.
Several forces are at skate and cause intrinsic stress in the structure, the
normal force is acting perpendicularly to the plane cross section, and therefore
longitudinally on the component. The latter force is currently called axial force; we
can find two sorts of thrust in the materials fibres, the compression and the
tension. The shear force on the opposite is acting perpendicularly to the length of
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an object, or, to be more specific, in the same plane than the component cross
section.
The bending is a combination of the 3 kinds of forces mentioned above,
compression, tension, and shear. The result is a bend created on the longitudinal
plan of the component. It creates on one of the extreme fibres a high compression
and on the opposite side a high tension. Therefore, to resist a bending, the
element has to be as resistant in tension as in compression to be completely
effective and not oversized. The last force that can occur is the torsion; this is a
particular one caused by the forces mentioned above when the centre of the thrust
is not adequate with the inertia centre of the plan they act on.
2.2.3 Material extension
The expansion of a material can be generated by natural circumstances or by
mechanical constraints. Thermal expansion, known by everyone, is the dilatation
of elements provoked by a certain difference of temperature. It can be a structural
problem if the system is indeterminate. An indeterminate system is the opposite of
the determinate system, its particularity is that the unknown forces are too
numerous to use the simple method with the free-body diagram. It brings more
difficulty on the calculations but also with extensions of elements; indeed
theoretically the forces don’t allow any displacement in the system. In order to
work fine, it has to consider potential extension within the joints’ areas.
The mechanic strain, on the other hand, depends on the forces that apply on
the object. The difference of size is the result of the material elasticity against
stress. There are two sorts of behaviour while speaking of strain; the elastic mode
is defined by a level of strain proportional to the stress applied; the second one,
called plastic mode has a more complicated mathematical definition. The latter
increases the resistance of the material before yielding.
2.2.4 Most common structural material
Timber is one of the most used materials for construction, especially for lowraise residential building. Its characteristics in compression, tension and bending
are good depending on the fibres’ directions; it allows a low-medium span
definitely enough for a single family house scale. However, timber has some
inconvenient defaults, since wood is a raw natural material, the inner structure of it
is not regular, wood fibres can be affected by any parasites or we can also find
nodes in it. This will affect the resistance of the material; therefore most of the
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structures in timber we deal with
are
made
with
improved
methods in factories. Its density
being quite low, it becomes even
more efficient.
Masonry is as the wood, a
very common material for a very
FIG. 4 : IMPROVED WOODEN PROFILE
long time now. It is defined by
assembled blocks - concrete
blocks, bricks … - linked together by a mortar, a joint between all the blocks that
sticks them together. Although masonry is very suitable in term of compression
forces, tension is not resisted enough to be chosen for that perspective. Most of
the constructions in the roman period were using masonry, with active-forms like
arches, vaults, domes... Nowadays, masonry is used for the vertical structure and
it is mixed with another type of material for horizontal structure in order to resist
bending of slabs, lintels and so on. Usually reinforced concrete fulfils this role.
Steel is either a main structural material from
buildings, either a complement to the other ones,
and that is because of one reason; steel has the
best capacities to resist compression and
tension. We can find structural steel structures in
industrial buildings or in civil constructions as
bridges or skyscrapers. The major default of
steel is its density, therefore improved shapes I-shape, H-shape, Hollow structural section… are needed to reduce the volume of steel, which
means decreasing the weight and the price, and
not reducing the webs’ inertia of the beam
resisting the bending.
FIG. 5 : GENERAL IMROVED PROFILES
Nowadays, reinforced concrete might be the most common structural material,
indeed it is used for almost all kind of buildings, resisting all kinds of forces. It is
due to concrete resisting very well in compression, and steel helping concrete to
reinforce itself while in tension. Skyscrapers, bridges, dams, single-family houses,
multi-storeys residential buildings, etc... Improved techniques are common in
precast concrete. In Denmark, hollow-core slabs are more efficient because of the
structure has less weight, dead loads are minimized then. Also, different
techniques of casting exist, they have different consequences on the structure steel frame as the framework to cast concrete, and thus it brings an additional
structural reinforcement. (Mcdonald, 2001, pp. 22-36)
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2.3 Financial influences
2.3.1 The client
The client is the one who invest in the building, according to the latter’s
reasons or goals; he might want to choose other technics that are demanded by
architects and engineers; the main argument being the cost of those products. It
can be dangerous at first sight, even more when seismic design is at play but
fortunately codes are made for that kind of cases.
When it is a public client, financial mediums are regulated by the government
of the country with its budget. However, public institutions are on one hand,
imposing the codes and standards on their territory, on the other hand that would
be impossible to not be aware of those ones.
Private clients might not be aware of the regulations and codes, nevertheless
the rules have to be respected, but the client can still change his mind and bring
extra cost to the building construction.
Nowadays, saving money on every aspect of the construction has become an
international sport; a bad thing would be the construction not being able to fulfill its
mission, in other words, not being functional anymore. The worst case, particularly
during an earthquake, is the structure collapsing and having a contingent of people
in danger or lost.
2.3.2 Country specificities
Several differences between countries can be guessed, the location is
different, and so there are not the same reliefs, the same resources from the
ground. That has an impact on the material cost and therefore on the way to
construct and neither the same seismic risk. As a consequence of the lack of
resources, industries are missing, the country cannot be developed enough; and
then constructions might not be strong enough for cause of budget limit.
Finally, codes and standards are usually able to prevent structures to collapse
and stay functional; however some countries have not official standards on their
own even though those countries form a minority among the international
community.
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2.4 Structural design history examples
2.4.1 Before Antiquity
Structural design relatively exist
since the humanity does; starting with
the hut from the Kung people in Africa around 60000BC -, from the Gravetians
in Europe - around 28000 BC - or the
Tipis in America. Those housings were
done with sticks made of timber and
ingeniously linked together to restrain a
tensile membrane made of plant, or
animal skin. Some were more
FIG. 7 : TEEPEE HOUSE STRUCTURE
sophisticated than others, it also
depended on the function of the tent;
some people were moving regularly in
order to survive, while others used to
stay at the same location near their
vital needs. On a seismic point of view
their houses were perfect; those tents
are made of an active-form – defined in
FIG. 6 : PIT HOUSE CROSS SECTION
the next parts - and work in tension of
the membrane, the only rigid
component is the wooden skeleton of the tent which is light-weight, earthquakes
had very small influences on our ancestors’ homes.
(Mcdonald, 2001, p. 2) (Ching, et al., 2011)
2.4.2 Romans
The romans are known for their engineering; especially for their remarkable
public works. The water supply network built at that period of time can compete,
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on the amount of water furnished, against the biggest cities’ networks from our
contemporary time, which says how improved it was. The system used to provide
the famed Romans thermal baths as well as the improved toilet system and the
aristocracy villas with water.
In order to do so, they improved a lot of construction technics like the semicircular arch, but didn’t invent so much of those latter. Understanding the assets of
it, they have built aqueducts, bridges, temples, houses and thus very spacious
constructions could have been realized, for example the domes of some
cathedrals still standing nowadays. In order to do so, they had noticed the
importance of decreasing the weight of the structure - e.g. use of hollow bricks.
Their construction materials
were first the hydraulic cement
made with volcanic ashes, and
then later they have started to
build
with
concrete.
This
concrete mix had lime and
volcanic
sand
as
main
components. They have also
used masonry bricks, copper
and bronze in their architecture,
those last two were not, by all
means, structurally oriented.
Roman
concrete
was
remarkably
stable
under
FIG. 8 : ROMAN ARCH FORCES PATH
earthquakes, the non-intentional
variations in the density of the
concrete allowing disruptions of
seismic waves. Their constructions, for example the coliseum in Rome and or the
aqueducts in south Europe still exist and show their brilliant engineers’ minds.
(Raucci & Jewell, n.d.)
2.4.3 Traditional Chinese architecture
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Traditional constructions in China are made
of timber. they developed the timber
construction
process,
the
felling,
the
transportation and the processing being more
and more convenient.
FIG. 10 : TYPICAL CHINESE TEMPLE
CROSS SECTION
FIG. 9 : CHINESE WOODEN STRUCTURE
Concerning
the
structural
components of the building, the
latter stands thanks to a post and
beam structure with bounds, tenons
and mortises.
Besides, those links between components have a relatively small freedom in
their movement, that plus the fact that wood is a good material on a damping
perspective, the building lower the risk of damages if horizontal loads apply.
The traditional architecture in China is symmetrical; it uses rectangular shapes,
circular, hexagonal or octagonal ones. The components are oversized and
therefore stronger. A concrete made of lime and earth is use as foundations.
(Zhang, s.d.)
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Chapter 3. The seismic risks
The Chapter 2 dealed brought up the way engineers are in
the building industry and also the way building have been built.
The seismic risks were already there obviously and some seismic
design considerations can be observed in some historical and
traditional architecture.
This part of the thesis will introduce the phonemenon that is
an earthquake, stopping by the geology of our planet until the
seismic waves hitting the building. Then, 2 examples will be
scrutinized, an earthquake in the center of Italy in 2009 and one in
Sichuan province in 2008.
3.1 Earthquake characteristics
3.1.1 Geology
Our planet is made out of solid and liquid core. Even though the centre is a
solid core made of heavy metals, The crust, the external part of our globe is
composed of light materials as basalts and granites and exists as plates covering
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the planet’s surface. The outer core is liquid and the mantle has its own internal
flow. That is the latter that creates tectonic movement.
The phenomena of the earthquake come from an energy liberated by the
friction between the tectonic plates. The elastic strain generated by the plates
rubbing next to each other, will concentrate a massive amount of energy. The
latter being released will cause a fault in one of the plate and at the same time an
earthquake. This concept is schematically explained by the lateral force exceeding
the friction force of the two plates. Andrew Charleston compares it with our fingers
snapping. We apply a normal pressure directed on each other on both fingers, and
then we apply at the same time a lateral force that will create the snap.
(Lindeburg & Baradar, 2001, pp. 1-8) (Charleson, 2008, pp. 4-11)
3.1.2 Type of waves
A quake will generate several kinds of waves. Some will be called surface
waves while the others are called underground or body waves.
When the fault appears, body waves are transported from that point until the
surface of the earth, pushing and pulling the soil particles - P-waves - and moving
them side to side - S-waves. The P-waves are the fastest to get to the surface; it
will reach the building before any others. Primary - P-waves - and secondary - Swaves - are usually reflected back at
the surface and back again towards the
earth surface. This fact brings a
stronger ground shaking - by up to
twice as much than in the ground.
FIG. 11 : SEISMIC WAVES
Surface waves are on the other
hand a consequence of the body
waves. Body waves will hit the surface
of the earth at the epicentre. Then,
surface waves are created from this
latter position going all around. Surface
waves are composed of two different
ones. Love waves are going sideways
in the horizontal plane, this wave is
similar to the S-waves unless there are
no actions directed up or down. And
Rayleigh waves which create an
elliptical movement in the vertical plane.
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Those waves will therefore create movements which will act on the building
with its inertia mainly in a horizontal plane. (Lindeburg & Baradar, 2001, pp. 1-8)
(Charleson, 2008, pp. 4-11)
3.1.3 Peak ground acceleration
The PGA value defines the shaking; it is calculated by seismometers or
accelerometers, parts of the seismograph. The unit, used in general all over the
world is linked to the unit g, the gravitational acceleration. It can also be measured
in gal – cm/s² - or in m/s² or in the imperial unit system - ft/s² or in/s².
The acceleration of the ground is related to the tremor’s intensity, the more
acceleration, the higher will be the intensity and consequently the more damage. If
the designs of constructions are the same, as well as the geographical region, we
can observe a relatively clear correlation between the intensity and the PGA. The
“reference peak ground acceleration” is used to design the buildings within the
codes. National zoning maps with RPGA exist and represent a certain earthquake
with a certain occurrence, defined in the codes as well.
(Lindeburg & Baradar, 2001, p. 12)
3.1.4 Forces involved.
The
magnitude
of
an
earthquake is defined by the
“size” of the latter. Each quake is
attached to a value on the
Richter scale - named after the
American seismologist Charles
Richter – for the low and
moderate earthquakes and with
the moment magnitude scale for
the
strong
and
severe
earthquake since the Richter one
is mathematically not precised
enough on this range. The scale
is growing on an exponential
range. Indeed a difference of 1
FIG. 12: MAGNITUDE, RICHTER SCALE GRAPHIC
unit on the scale equals a 10 times
REPRESENTATION
difference on the wave’s amplitude
and 31 times difference on the energy released. The fault being more or less deep
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in the ground, the latter is similar to an insulation layer reducing for us, fortunately,
this massive difference. So that, when it reaches the external surface of the earth,
the difference will not be as big as it was.
Following Newton’s
law of motion, we
have “F” equalled to the
inertia force; “m” which is
FIG. 13: INTENSITY, AMERICAN SCALE
the mass of the building;
and “a”. This part of the
equation represents the acceleration generated by the shaking. It is especially
expressed in function of “g” - universal gravity acceleration - in seismic design - in
a way that 0.5g is equal to 4.90 m/s².
2nd
When a quake has been detected, seismographs can detect the intensity of
the quake, there are different scales representing it, each of them are going from I
to XII - roman numbers; XII being the most severe. For example, the “Leidu scale”
is in use in CHINA and the “European Macroseismic Scale” in Europe. Three
characteristics are considered, the animals’ perception of shaking, the humans’
one and the surrounding impact. On the opposite of the magnitude, intensity is
different at every location where earthquakes have been felt. This value will
depend on the layers of soil, on some probable phreatic table and so on, and
particularly on the distance between the fault - or also the epicentre - and the
location aiming to be measured. A geographic plan is generally made to see the
different area of intensity called “isoseismal map” like a topographic plan, but
instead of altitude lines, we see intensity lines.
As it is brought up on one of the paragraph above, the inertia force of the
building is linked to the building. The weight is linked to the inertia; therefore the
weight has a connection with the force applied to the building. The heavier a
building is, the stronger it has to be to resist the resulting inertia force. When
earthquake’s waves reach the foundations, those latter will follow the shaking
randomly in the 3 spatial dimensions - x, y, and z. The building will tend not to
move considering its weight, this force resulting to keep the building from not
moving is called inertia force. And it is this resulting force that is applied to the
building during a tremor.
The shaking of the building varies according to the soil. The nature of the soil
will have a consequence on the magnitude and on the frequency of the quake.
With a soil type made of soft sediments, the magnitude will increase and the
duration of the shaking as well. On the contrary, with a soil made of rock, the
shake will last a shorter time and the magnitude with be smaller.
(Charleson, 2008, pp. 15-24) (Murty, n.d., p. Tip 03)
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3.1.5 Extra-consequences
Earthquakes are not only tremors which provide structural problems. Indeed a
couple of other consequences not related to the superstructure can happen. The
quake may partly damage the building services, generating electricity sparks, gas
leaks .A major problem in the last decades and centuries was the subsequent fires
after earthquakes. In 1906, San Francisco was battling against fires for 3 entire
days after the earthquake destroying around 90% of the city or more recently, the
Northridge quake in 1994 in California.
Ground shaking might if some circumstances are gathered lead to dangerous
incidents. If the epicentre is located in the ocean and relatively close to the coast –
a few hundreds kilometres – this can create a tsunami with the consequences we
all know, floods. Identically, the shaking can destroy civil constructions like dams
or levees which will cause floods as well.
In some regions, earthquake can generate landslides, it happens when the
inertia force from the tremor exceeds the intrinsic strength of the soil, it creates a
rupture and a part of the land moves over.
Dangerous for the stability of buildings, liquefaction happens when the ground
is made of a layer of sand or loose soil, a water table high enough to submerge
this layer and a ground shaking putting a sufficient pressure. Although this
situation is dramatic, the ground losing its ability at bearing structures, the kinds of
soil which are subject to such a characteristic are rare. Indeed both characteristic
have to match immediately while the ground shaking is still running. The concept
is similar to the sand at the beach, having our feet in the sand, and when the wave
goes over the sand; our feet sink in the ground.
Those consequences are more severe if specific buildings are touched,
hospitals, schools, and in another perspective, nuclear power plants - for example,
the earthquake in Fukushima, Japan in March 2011 – or some refineries like in
Tomakomai, Japan in 2003.
(Charleson, 2008, pp. 113-119) (Michigan Technological University, 2007)
3.2 A young technology
3.2.1 The 20th century
The 20th century is known for major changes, mainly in technology, the
architecture has changed and become high in altitude but also new shapes
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appeared, pleasing architects and residents, bringing challenge to engineers, all
that to respond to new society demands. It has in the building industry, been a
century of analysis on quake resistance.
Seismic technologies are relatively new. It has started in the beginning of the
century with an earthquake in San Francisco in 1906 and another one in 1908
in the south of Italy; both of them killing around tens of thousands of people. Those
2 events has sounded the death knell for people to notice that improvements were
necessary. Consequently, in response to that, instruments were created in order to
calculate the ground shaking.
20th
After two other earthquakes in 1923 in Kanto – 140,000 victims – and in 1925
in Santa Barbara, the building industry at that time decide to evolve towards
seismic design. Laboratory tests are running and in late 20’s to early 30’s, the first
building codes with seismic design is introduced on the municipal territory of Los
Angeles.
Until the 1950’s, Engineers and architects developed seismic design around
strength and stiffness, but evolving with the time, they came up with another
concept, named ductility which responded to what used to be their problem
statement: “what happens to a structure if its inertia forces exceed those for which
it has been designed?. (Charleson, 2008, p. 35)
But it is around 1970 that a group of engineers from New-Zealand introduce a
specific design approach to resist tremors. Called Capacity design, this latter is
based on specific ductility needs in the building. This approach significantly
changes the way to build. Made with this system, a building is judged to be 6 times
more resistant than a common one.
(Charleson, 2008, pp. 34-35)
3.2.2 Retrofitting
To retrofit a building is defined as the renovation of the latter up to seismic
codes and standards.
Apart maybe of the new developed countries and considering the cities that
have built a lot during this century, a major problem appears. Since engineers and
architects assume the lifespan of constructions around 50 years depending on its
function, a lot of them have been thought and erected before or during those
seismic studies. Therefore, a major part of buildings in cities all over the world is
not designed to resist earthquakes, or at least severe ones. Then the need to
retrofit those constructions is undeniable. Unfortunately, that renovation has a
cost; private investors might not consider that risk as far as a quake did not
happen yet or they merely can’t afford it.
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Also being able to happen, is a relatively important earthquake in a low-risk
seismic region, especially with buildings conceived in the second part of the
century where constructions were not as able as today to handle it.
(Charleson, 2008, pp. 187-209)
3.3 Earthquake in L’aquila, central Italy (2009)
3.3.1 Characteristics
The earthquake happened on the 6th of April in 2009, in the region of L’aquila,
name of the biggest city around, and particularly touched by the tremor since the
epicentre was a few kilometres from it. Its magnitude was over 6 on the Richter
scale, precisely 6.3 so that we can define the earthquake as strong. The depth of
the fault was 8.8 km and the biggest intensity detected was of VII.
(USGS, 2012)
3.3.2 Geographic area and history
The area of L’aquila is located
in central Italy on the North-East of
Rome and surrounded by a
mountain belt called the Apennines.
The city of L’aquila (about 75,000
inhabitants) was situated at 5 km
from the epicentre, Pizzoli 6km,
Tenni 59 km and the capital city
Rome at 85 km. The quake has
been triggered by a normal fault
rupture, this region is seismically
active; in 1997, an earthquake of
magnitude 6.4 was recorded with a
prior period of 2 months recording 8
earthquakes of more than 5.0 on
the Richter scale; finally a few years
later in 2002, an earthquake of
magnitude 5.9 shook central Italy.
FIG. 14 : ISOSEISMAL MAP ITALY EARTHQUAKE
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3.3.3 Damage
Concerning the damage of facilities,
the region is not urban and has a
medieval past accordingly to the
buildings
still
present.
The
constructions, built a few centuries ago
down to the 13th century have suffered
from the shaking. In the cities around
the epicentre, 25% to 50% of the
edifices have been severely damaged.
However L’aquila city centre, being the
closest to the epicentre, handled the
FIG. 15 : BUILDING COLLAPSED IN THE CITY OF
L’AQUILA
quake relatively well, since rare buildings
have been found completely collapsed.
Most of the latter are only partly destroyed; it is due to the good material used
likely hosted by the ancient aristocracy. Therefore, buildings of good quality has,
with no surprise, resisted better.
The intensity of the earthquake has been very different within close areas, so
that some cities have suffered from a violent shaking. The region has very different
sediments layers on the valleys, and that may have caused differences. The peak
ground acceleration – i.e. PGA – exceeded 0.35g while the average design level
of the constructions was at most 0.25g
from the 2003 codes upgrade for this
region. That is only for the newest
buildings that represent a few percentage
of the constructions, before that, the
buildings were designed at 0.23g.
FIG. 16 : SAN FRANSISCO CHURCH CLOSE TO
L’AQUILA WITH THE ROOF COLLAPSED
Critical buildings have also been
damaged, the main hospital of the region
had to be evacuated following to some
apparent cracks at the top of the column
on the ground floor. More dramatically, a
whole student dormitory collapsed in
L’aquila killing the students inside – the
earthquake happened during the night.
The Italian government condemned seven members of a commission
responsible of the buildings’ situation in the region, charged for reckless
homicides.
(EERI, June 2009)
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3.4 Earthquake in Wenchuan, Sichuan province, China
(2008)
3.4.1 Characteristics
The Wenchuan earthquake was recorded on the 12th of May in 2008. Its
magnitude was 7.9 on the Richter scale which can be defined as a major
earthquake. It killed 69,000 people, 370,000 injured, 1,5 million Chinese citizens
had to be relocated. About the constructions, 216,000 buildings were damaged
with 6,900 of them being schools. The intensity of the earthquake was VII in
Chengdu, VIII in Mianyiang and IX in Tianpeng area. (USGS, 2013)
3.4.2 Geographic area and history
The epicentre was situated close
major cities like Mianyang – by 150
km - and the Sichuan province
capital city, Chengdu – by 80 km.
The rupture depth was 19 km,
and the area covered by the shaking
was enormous - i.e. 260,000 m². The
reason for that is the length of the
fault, which is 270 km long. The
earthquake is due to the reverse
thrust of the Tibetan plateau - which
represents a soft crust - against the
Sichuan basin – the strong crust.
With its magnitude and intensity,
that
is
strongest
and
most
devastating earthquake from the
FIG. 17 : ISOSEISMAL MAP CHINA EARTHQUAKE
current 21st century. The Longmen
Shan fault is a very active one,
earthquake with a 4,0 magnitude are common. It can be explained by the
characteristic of the fault – the fact that the Tibetan plateau is going up, those are
very active situations.
(USGS, 2013)
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3.4.3 Damage
The predominant structure in Sichuan is the
unreinforced brick masonry, especially in rural
areas which are not trustworthy during an
earthquake. In the cities the buildings resisted
pretty well, the majority of them being pretty
young. Most of the ones that collapsed were
built with the codes from 1976 and 1989
upgrades.
The peak ground accelerations were
recorded by 3 stations, the first one was 957 gal
– gal is a geodesy unit related to the movement
FIG. 18 : APOCALYPTICAL
of the ground, 1 gal = 1 cm/s², so that 981 gal =
SURROUNDING IN BEICHUN COUNTY
AFTER EARTHQUAKE
1g – at a distance of 22 km form the epicentre
with 60 seconds of shaking duration. The second
one recorded a peak at 802 gal at 88 km from the epicentre but 1 km from the fault
with a shaking that lasts 90 seconds. The third one recorded 550 gal at 150 km
form the epicentre, 75 km from the fault and lasts 150 seconds. Those peaks
define a very brutal earthquake which left no chances for the buildings without
seismic design.
Therefore, the disaster happened in the rural zones, close to the fault but also
where the last seismic codes were not applied. The constructions being already
not strong enough, the shaking triggered several landslides where the conditions
were gathered and formed
quake-lakes. However some
buildings
avoided
those
critical regions and resisted
structurally well close to the
fault, it is the case of
Bailuzhen middle school or
Tongji middle school done
with moment resisting frames
– defined in the next part.
The
shaking
had
FIG. 19 : BAILHUZEN MIDDLE SCHOOL AFTER
consequences on dams and
EARTHQUAKE
bridges since Sichuan province
had an important amount of
them. Some bridges’ spans collapsed and some issues with dams happened, for
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example, the Zipingpu dam area located at 17 km from the epicentre has recorded
some displacements on the parapet wall part around 10cm. As a result, the dam
has been entirely drained a few days later in order to solve safety issues detected.
Moreover, 30,000 km of
pipelines in the region had to be
restored; the stations recorded a
PGA of 0.1g and a bit more near
them. Consequently, although
Chengdu hasn’t been touched
severely by the quake regarding to
the buildings’ structures, the water
supply network related to the city
FIG. 20 : UNREINFORCED MASONRY IN SICHUAN
was damaged. The authorities
PROVINCE.
even distributed pills all over the city
to disinfect the water. Power outage
touched all the regions around but Chengdu, the other regions were without
electricity for a period of 10 to 20 days depending on the difficult accessibility of
the different areas.
(EERI, October 2008)
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Chapter 4. Earthquake-resistant design
We have observed in the prior chapter that earthquake needs
to be considered by everyone has a big risk for the human lives, bu
also for the economy of a country.
In this part we will talk about the solutions. Studies and
researches have been conducted during the whole 20th century and
continue to be at the beginning of the 2000’s. Engineers and
architects have successfully invented methods to avoid
constructions to collapse and therefore to save humans’ lives,
technologies improving all the time, an earthquake can occur
nowadays and keep edifices intact, without putting in troubles their
functionality. Consequentely we will go through all the problems
that a quake generates and how buildings can be designed and
built to resist.
4.1 K.I.S.S. Principle
This principle is a basis to design constructions in a seismic region. KISS Keep It Simple and Symmetrical - is a way to think in order to create a building
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resisting shaking well. On a design perspective, the structure of the building has to
be stable; in such a way that floor plans are coherent and homogenous, and the
cross sections as well. This philosophy facilitates the design of buildings that can
be submitted to complex force patterns due to the random shaking, and thus
thwarts those edifices from main structural issues.
(Charleson, 2008, p. 126)
4.2 Capacity design
Capacity design is a way to conceive a building with an increased aptitude at
standing against a quake. It has been developed by a research team from NewZealand and nowadays, all the buildings are created by their concept. The latter
rests on the ductility and the damping efficiency of the structure in itself - i.e. the
absorption of the shaking within the structure. The deflection of the buildings is
therefore controlled.
The method deals with the reinforced concrete, the concrete cannot enter in a
ductile state, but steel bars can. In order to get so, they have to reach the plastic
range of the steel. The characteristic of the steel defines for this metal, contrarily to
the concrete, several states under axial forces. The steel has an elastic range –
the strain being proportional to the force applied – and after a certain state called
the yield point the steel becomes plastic. The particular effect of that plastic range
is that the steel will be more flexible within its intrinsic crystal structure – strain
hardening - and the most important be still resistant.
In the plastic range, the element will strain more than in the elastic one,
therefore this strain, will act like a mechanical damper with concrete keeping the
whole building component united.
Capacity design particularly takes care of the vertical components of the
constructions, indeed, in order not to collapse those ones need to keep bearing
the loads in the same axis – Dead loads, imposed loads. This technology allowed
architects and engineers to build better high-raised buildings within medium and
high-risk regions in the last quarter of the 20th century.
(Murty, n.d., pp. Tips 09-17)
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4.2.1 Ductility
When earthquake’s waves hit the building, it
will create a deflection explained by the inertia
force in a prior part of the report and that we
can analyse in the period of vibration of the
building. This deflection need to be controlled,
in order not to break, the structure needs to be
ductile. Although, only some critical parts need
that characteristic, these are the bottom of
building on the vertical structural elements –
columns, shear walls – and on horizontal
FIG. 21 : STEEL BEHAVIOUR WHILE
elements close to the junction with the vertical
STRESSED
ones – i.e. in a cross section, two parts that are
ductile on each side on the horizontal
element.
Those critical parts need to create
sort of a hinge joint, and are called
structural fuses. Their locations allow
the building to be more flexible, and
therefore to direct the earthquake
power into those hinge joints.
(Murty, n.d., pp. Tips 08-09)
FIG. 22 : LOCATION OF HINGE JOINTS IN A
MOMENT RESISTING FRAME.
4.2.2 Hierarchy of strength
In a practical way, a structural fuse is built by creating a hierarchy of strength.
This hierarchy puts on the top of it, the force that will generate the fuse. If another
force is too strong and break the component before the fuse is active, then the
building will collapse. A major work will be done to counter those other forces. In
other words, the fuse must be active before the other forces put the building down.
During an earthquake shear force might contribute to a failure in the vertical
elements; steel reinforcements have to be replaced by others. Since it is
perpendicular reinforcements – called ties – that handle shear force, the diameter
of those bars can be increased for a bigger strength, the spacing between them
should also be decreased.
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The footing of the foundation shouldn’t overturn, if it does, important damage
could appear. Even if that issue may bring big issues, it is easy to counter, the
footing will be dimensioned according to the final designed bending strength and
then, that would not be a problem.
Over-reinforced concrete has also to be considered as a risk; by adding bigger
diameter and more reinforcements the latter will not yield and therefore will not
become ductile. The concrete not being strong enough would break into pieces;
the solutions are to increase the depth of the element or to take off some of the
steel.
(Murty, n.d., pp. Tips 17-18)
4.3 Resisting earthquakes’ forces
This part will deal with the force paths coming from the earthquake fault,
passing by the inertia force of the building, to the foundations resisting stress. The
seismic forces go up-down and side-to-side, the vertical resultant force is
insignificant though, first of all because the building is designed to resist dead
loads and imposed loads with a safety coefficient, and second of all because the
vertical movements don’t provoke any direct risks for the building to collapse –
nevertheless an exception occurs for constructions having long distance spanning,
which is not the case for usual buildings.
During an earthquake, the issue comes from the horizontal plan – inertia force
-; it is the one disturbing factor for the stability of constructions by deflecting them.
Buildings must resist this force.
4.3.1 Horizontal planned resistance
The inertia force is coming on the horizontal plan; theoretically, the point of
action will be the centre of inertia of the building. Here, engineers consider a
centre of inertia on each storey with a different force; the inertia force may be the
same, but the moment of inertia is not, and that is the cause of the deflection – the
distance between the foundations and the storey height.
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To get a good structural behaviour, hinge
joints should be present on both side of the
slab in order to absorb the deformation
submitted by the vertical components on the
horizontal diaphragm. As a consequence, this
latter’s design is specifically thought to
improve its bending – horizontal bending. (as
shown in Fig. 22.) Similarly to an I-shape
beam, the diaphragm is made of chords going
around the slab and a web. The web merely is
the centre of the slab and the chord is the
component which penetrates the wall
structure with specific reinforcements to
improve the moment of inertia and likewise be
FIG. 23 : BOND BEAMS-CHORD
stronger. The chord is a bond beam cast in
the wall-slab junction in a concrete structure a usual beam in wooden or steel
structures.
To sum up, horizontal diaphragms need to transmit the inertia force to the
vertical structure, however by doing that, internal forces will occur in the horizontal
structure, strength and ductility is therefore demanded.
(Charleson, 2008, pp. 126-140)
4.3.2 Vertical planned resistance
Inertia force acts on the horizontal plan but for the building to stand this
horizontal force needs to be resisted by the foundations which is usually the only
support of any construction. The force paths must go through the vertical
components until the foundations. Those structural parts of the construction are
the critical ones. Without them, the slabs cannot be supported; moreover the
vertical components are the ones that counter the cantilever effect from the
earthquake, the inertia force.
Those components are detailed in the next part (part 4.4).
(Charleson, 2008, pp. 144-154)
4.4 Main structural systems
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When clients ask to build in an area known to be shaken by the tectonic plates’
movement, architects and engineers have several solutions. This part is about the
differences between them.
4.4.1 Shear walls
This system is by far the most
efficient. As it has been said on the
main part about structural design at
the beginning of this paper;
monolithic walls resist very well to
horizontal force applied in their
longitudinal direction.
FIG. 24 : SIMPLIFIED BUILDING WITH SHEAR WALLS
ON EACH SIDE TO RESIST EVERY HORIZONTAL FORCES
The shaking of the quake
generates a random inertia force,
but this latter can be projected on
two axis – e.g. x and y - on a 2D
plan of the construction. If shear
walls are built with their directions
following the axis, all the forces no
matter which directions will be resisted.
Even though It might be the best choice structurally speaking, shear walls
systems propose a very closed design, indeed, the shear walls can be highly
stressed – especially at the bottom of the construction – openings are not
recommended and are avoided. Fortunately, shear walls can separated by
coupling beams – i.e. a beam which very high and connect two shear walls – or
they also can be built on just a part of the building’s length.
(Murty, n.d., p. Tip 23) (Charleson, 2008, pp. 66-75)
4.4.2 Braced systems
Also seen in the first main part about
structural design, they are used in light
industrial buildings particularly and on
low-rise buildings. The braced system
uses triangle geometry with braces in
tension to resist forces coming – as the
shear wall – from the perpendicular
plane to its length.
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FIG. 25 : SIMPLIFIED STEEL FRAME WITH
BRACING
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Braced-systems are made of steel, as mentioned on prior parts of this paper,
ductility, a major piece of capacity design, is brought in reinforced concrete by this
specific material. Therefore, we can also create hinge joints in braced systems, in
order to so, the bracing bar is placed eccentrically; that would create the structural
fuse and absorb the tremors resultant forces.
This system stays a light weight construction method, foundations needs to be
controlled to be sure they won’t overturn under the seismic cantilever reaction;
tension piles can be built to prevent that to happen
(Charleson, 2008, pp. 76-80)
4.4.3 Moment resisting frames
This is the system used with columns and beams; it has to fulfil 3 criteria. First
of all, the columns have to be deep enough to resist significant bending moments,
second of all the beams and columns need to have similar depths, even though
the column is usually recommended to be stronger and then having a bigger
section; and finally a rigid connection between the column and the beam.
The beam needs to be smaller at
some critical points to create hinge joints
in the beam by focusing the weak part of
the structure on the horizontal structure’s
member.
On a design point of view, moment
frames have almost no limits on the
buildable shapes and opening for natural
lights are not an issue.
(Charleson, 2008, pp. 83-88)
FIG. 26 : SIMPLIFIED MOMENT-RESISTING
FRAME BUILDING
4.4.4 Mixed systems
This system is defined by the use of 2 or more of the systems mentioned
above on the same axis. If engineers and architects mix systems, they will not
respect the KISS principle – Keep It Simple and Symmetrical -, the force path will
be difficult to understand and computer-based programs will be needed to
apprehend all load cases.
It is a system that should be avoided, however shear walls and moment
frames systems can be complementary on a certain kind of building; the high-rise
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buildings. The deflections of the 2 systems
are totally different, and a good use of
them can improve the seismic design of
this kind of construction.
In certain case, it can allow the failure
of some components on purpose with a
sort of backup structural system. The
earthquake force will focus on the weaker
system like it does with the structural fuses.
(Charleson, 2008, p. 90)
FIG. 27 : SIMPLIFIED BUILDING WITH
BRACING AND SHEAR WALLS
4.5 Common issues to avoid
4.5.1 Structural discontinuity and off-set
The vertical continuity of the structure during an earthquake is fundamental;
this concept integrates several things to avoid in order not to face a discontinuous
structure, which leads to the building’s failure. Firstly, the building vertical load
system made of different components at each storey needs to be well connected
together; the force path will therefore go directly to the foundation and possibly in
the ground. A homogenous structure means also a continuity, for example if a
shear wall is present on the 2nd floor, this component should be there on all the
other floor to form one continuous entity; important issues could happen if this is
not respected and we will go through it in the next paragraphs.
The fundamental statement is that the vertical pattern of the building must be
homogenous from floor to floor and well connected.
4.5.2 Soft storey
As it has been adduced above, if the structure is not homogenous, it creates
issues. A soft storey is merely a storey that is weaker than the others. The
structural fuse is similar to it, the earthquake is going to focus on the weaker part
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of the construction, with a structural fuse,
everything is done for that to happen and then no
risk is generated on the building’s ability to stand. A
soft storey will be weak but it will not absorb
anything, in that particular case, all the forces will
be concentrated on this latter and will cause a
failure. A failure caused by a soft storey is
disastrous; the whole building is strongly affected
and may collapse entirely or partly according to its
architectural features.
(Murty, n.d., p. Tip 21)
FIG. 28 : SOFT STOREY EXAMPLE
4.5.3 Short column
2 types of short column exists, on one hand there are columns that find
themselves shorter than others in a moment resisting frame and on the other
hand, the column that don’t fulfil their structural function by being brittle because of
external matters.
FIG. 29 : SHORT COLUMN EXAMPLES WITH SLOPED
GROUND OR WITH A MEZZANINE FLOOR
The first ones can be placed in a
certain way that their length is
shorter than the others with still the
same function to do. An easy
example of that is when an edifice
has to be constructing on a slope.
The foundations can be on a
different level, which brings more
complications or can be on the same
level. In both case one column will
be shorter than the other. The
stiffness of the column will not be
enough – the stiffness of a column is
proportional to its length cubed: L3.
The main problem is the column blocked by the soil; therefore to outwit that,
the solution is to build the foundations on the same level with a pile serving as a
column with a movement gap that parts the soil and the vertical structure. The
structure is therefore equal on both sides.
The other ones can be brittle because of masonry. In a moment resisting
frame, the gaps between columns and beams can be filled. And similarly to the
soil, this can be an issue. If the masonry does not fill the entire wall, then part of
the columns are weak and break under the massive shaking.
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(Murty, n.d., p. Tip 22)
4.5.4 Torsion
The torsion is caused when the centre of resistance of the construction does
not match the centre of mass. In other words, if the building is not symmetrical
along the inertia force on the x or y axis, the building will be submitted to torsional
thrusts. However, the centre of resistance can be further than the centre of mass if
the structural frame is capable to support the torsion with a specific sizing.
4.5.5 Infill walls
A moment resisting frame is very opened, the columns and beams being
literally a skeleton. Masonry walls, here without any structural function helps to
infill the gaps. The problem occurs because the blocks prevent the building from
deflecting correctly, the walls are weaker than the force coming from the
earthquake, infill walls will suffer damage and will cause damage on the structure.
If it does happen in the ground floor, it can result as a short storey.
A stiffer structure or separation gaps – on the top and on the sides -would be
appropriate to solve those issue.
The masonry needs also to get some vertical reinforcements going through it
with a short and regular spacing between the components, in view of a strong
earthquake the masonry could part horizontally by not following the exact force
applied to the building.
(Charleson, 2008, p. 159)
4.5.6 Buildings pounding
This issue concerns the medium and high rise buildings; those buildings have
to respect a certain distance from each other. The vertical cantilever effect cause
by the earthquake will deflect the constructions, if they touch each other, the
resonance generated will cause important damages.
The solution is therefore to create a seismic gap and stiffer the construction to
minimize the deflection. A separation gap on the roof level can be around 700mm
long.
(Charleson, 2008, p. 137)
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4.5.7 Re-entrant corners
In case of a building with a C-shape or an L-shape, the building will suffer
torsion. An irregular re-entrant corner is approximatively declared when B > 0,15 A
where A is the façade of the building and B is the re-entrant façade.
Consequently, either the formula in the prior sentence should be considered,
either a separation gap between the two parts of the building should be created.
(Charleson, 2008, p. 132)
4.6 Particular systems
4.6.1 Seismic separation gap
Separation gap in constructions are usual, they are required for the dilatation
of the material - a characteristic seen in the first part – or to separate different
building structure parts. In a seismic region, those specific gaps get some
changes. Because of the deflection, and in order to prevent the buildings to pound
each other, structure needs to be stiffer, overlapping cantilevers with sliding joints
could be used to handle the relatively long spanning gap.
4.6.2 Stairway
The stairway needs to stay functional during and after the quake, they are the
escape routes, in case of fire after the tremor, their usability will be vital for the
residents or users in the construction.
Stairways links two different storeys, because of the deflection, the slabs have
not the exact same movement; the one above will move on a longer distance. As a
result, monolithic stairway should get specific systems to behave well during an
earthquake.
In order to limit the effects of the shaking on the component, some mechanical
translation support can be added. Sliding joints – Teflon strips - at each floor allow
drift in any direction, components are pin or casted on the level above and allowed
to translate on the inferior level.
(Charleson, 2008, p. 168)
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4.6.3 Bridge between buildings
A normal fitted bridge between two buildings during a quake will fail very easily
and the consequences would be terrible.
When a component as a bridge is connected to two different buildings, with as
a result a different period of vibration, the latter must be suited to rotate in plan and
to slide on elevation. The sliding must be allowed for one connection only.
(Charleson, 2008, p. 140)
4.7 New technologies
4.7.1 Seismic proof constructions
This technic is also called base-isolation; the goal is to part the foundations
with the ground and the superstructure. Hence, the waves coming from the ground
have no influence on the building; so that the seismic isolation decreases the peak
acceleration of the building. This technic has been used “over 2000 buildings in
the world since the late 70’s … 1500 are in Japan”. Critical buildings are subject to
it due to their function; for example, hospitals that needs to stand against any
earthquake intensity.
However, the seismic-proofing has limits. The building shouldn’t be too flexible,
that is to say buildings that have more than ten storeys and more than 1.0sec of
period of vibration. The soil will be considered as bad if it is soft, for the same
reason, soft soil increase the period of vibration of the building. The wide
separations gap needed could possibly be a problem.
It is the best method to outwit a quake; the building avoids theoretically any
contact with the waves.
(Charleson, 2008, p. 218) (Lindeburg & Baradar, 2001, p. 169)
4.7.2 Dampers
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Seismic
prrof
constructions
are
possible thanks to those
components.
The
dampers are similar to
shock absorbers in a car;
within the construction
FIG. 30 : EXAMPLES OF ISOLATION SYSTEM. ON THE LEFT, RUBBER
they are located in the
PLATES, ON THE RIGHT SLIDING BEARING
moment resisting frame
within frames in diagonal; a sort of moment-resisting frame with a bracing-system.
But instead of being just that, which would not be good for the structure and
particularly its deflection, the bracing is composed on one side of a damper. This
side will absorb the shock and reduce the horizontal drifts; slender structures are
therefore possible for designers.
(Lindeburg & Baradar, 2001, p. 169)
4.7.3 Carbon fibres
Nowadays, new construction materials are available on the market like the
fibre reinforced composite materials. They are made of carbon fibres, which are
around ten times stronger than mild steel – steel with low percentage. It can be
used to reinforce walls or columns by wrapping of bandaging them. They are stuck
to the wall with s special synthetic resin.
This method is interesting while retrofitting constructions since it does not
include any demolition processes.
(Lindeburg & Baradar, 2001, p. 173)
4.7.4 Innovative structural configurations
Some structural features can decrease the inertia force acting on the building.
The concept is to have a part of the building connected to the main structure with
dampers. The different parts of the building will not vibrate on the same basis,
which is supposed to cancel a certain amount of the resulting force by absorbing
one another. They are called tuned mass damper or harmonic absorber, one of
the most well-known example is the Taipei 101, located in Taiwan.
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FIG. 31 : THE TOWER TAIPEI 101 - IN YELLOW THE
TUNED MASS DAMPER
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Chapter 5. The design process and the building
standards over the world
The prior part summarized all the possibilities that exist nowadays to
counter an earthquake. From a very simple structural choice or by
extreme and innovative ‘tricks’ like the tuned mass damper ine the tower
Taipei 101.
In this part 5, that is to say the last one, we will deal with the design
process of the constructions in a seismic area, obvisouly this process is
dfferent. The codes and standards will also be adduced in thispart and
especially the difference on certain point between the chinese standards,
the american, and the european ones.
5.1 Performance-based design
5.1.1 Performance level
Codes and standards focused nowadays on the potential victims of quakes, by
trying to decrease them. As it has been mentioned on the second part about the
seismic risks, institutions responsible for building codes went slowly into that goal
by passing from different phases in the 20th century. Every buildings created today
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shouldn’t theoretically collapse entirely under an earthquake, allowing significant
damage though – thanks to the generalised application on capacity design on the
worldwide building codes.
For example, the Eurocodes stipulate 4 performance-levels:
-
Operational
-
Immediate occupancy
-
Life safety
-
Near collapse
(Fardis, 2009, pp. 1-36)
5.1.2 Hazard level
The hazard level is complementary to the performance level. If we follow the
theory, an operational performance should be fulfil for a building returning
frequently; respectively, immediate occupancy with an occasional occurrence of
the earthquake, life-safety with a rare one and near collapse with a very rare one.
5.1.3 Probability of the earthquake
The non-collapse requirement is defined by the value called “design seismic
action”. It represents a 10% probability of exceeding during a 50 years span, which
represents the usual span the buildings are constructed for. The occurrence of this
quake should be 475 years which also means a 0.2% exceeding probability in a
single year. Those values represent the performance level of life safety.
Of course, the client can decide a better performance for his building to reach,
with those performance levels he knows the risks and consequently the designer
is also protected by that if something happens to the client’s property. As a result,
the client can’t sue the designer or engineers if all the codes have been respected,
whatever the outcome of the catastrophe.
For other performance requirements, we can note the damage limitations,
which keep the building in the elastic range, with an earthquake return period of 95
years and still an occurrence probability of 10%
The design peak ground acceleration is calculated by the reference peak
ground acceleration – that is found on the hazard zonation maps - times the
importance factor of the building
Indeed, the buildings are submitted to importance factors, the life-safety being
the goal. Therefore more the building is a risk for humans’ losses, more the factor
will be important.
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A few examples, still with the Eurocodes, an ordinary building will get his factor
equalled to 1.0. if the building is aimed to receive an important amount of people
indoor, the factor goes up to 1.2; it is the case for schools. For critical buildings,
that need to resist for public needs after an earthquake like a hospital, the factor is
1.4. On the opposite, a building with low-importance for humans’ lives has a factor
of 0.8.
(Fardis, 2009, pp. 2-9) (Lindeburg & Baradar, 2001, pp. 13-14)
5.2 The design process
In this sub-part, we will dwell on the design of the building from the beginning
of the project until the dimensioning of the constructions.
5.2.1 The building
All the buildings and projects are different one another, that is to say each
building will behave with different manners while the seismic waves will hit it.
The period of vibration of the building is one of its most important
characteristic; like any physical object, buildings have an intrinsic vibration
frequency and as a result a period as well. Some characteristics of the building,
architectural and structural features, have an influence on it. The height is the
most influent among them, along with the weight and the main structural system in
use.
As a result, the building will oscillate on a given rhythm when the waves will
reach it. The oscillation is outlined by the modes of vibrations. There are as many
modes as storeys in the building, the first ones – usually first three – being
considered within the building codes because they are submit to the majority of the
dynamic energy.
(Charleson, 2008, pp. 18-23)
5.2.2 Peak ground acceleration
The reference peak ground acceleration is defined by the history on
earthquakes on a specific region, zonation maps with this value are done to know
what kind of ground acceleration the earth is able to provide. As it has been said in
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the prior sub-part, this reference ground acceleration represents the ground
acceleration on rock, the soil if it is softer, would change this acceleration, as well
as the importance factor of the building. A peak ground acceleration, in order to
design the building will come out.
(Lindeburg & Baradar, 2001, pp. 17-19)
5.2.3 Building response spectra
Those diagrams were firstly done practically, with shaking table and different
buildings, captors were recording the response of the building and were point out
to form a curve. Nowadays, with computer-based technologies gathering all kinds
of software, the response spectra are mathematically modelled.
Therefore we can see the building’s acceleration in function of the period of
vibration. The beginning of the curve starting at 1.0 represents the design ground
acceleration. And we observe that if the period of vibration is too low, the buildings
acceleration is increased. The phenomenon is called resonance. Those curves are
different if the soil changes; the kind of soil adds a factor on the equation of the
curve.
(Charleson, 2008, pp. 21-22) (Fardis, 2009, p. 10) (Lindeburg & Baradar,
2001, pp. 20-22)
5.2.4 Seismic force
With the acceleration of the building we can calculate the inertia resulting from
it and size the building.
While dimensioning the elements strength will be consider in order to resist the
bending moment and the shear force that seismic force generates. As well as
stiffness, present with the maximum storey drift requirements. Capacity design has
a role in those sizing, and as it has been seen, particularly in the critical areas of
the building, plastic hinges.
5.3 Comparison between codes and standards
5.3.1 Factors
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The factors used in general are a bit different or the same. The philosophy
stays the same but the methods change. The institutions in charge in their
respective countries can have a look on the standards of their neighbor, the only
things that separate them is the habits of their engineers and architects working
within the country. The different codes and standards arrange themselves to be
close to each other – probability of exceeding occurrence which is the same – but
has still deep differences.
5.3.2 Intensity scale
The intensity scale is a way to quantify the impact the earthquake had over the
surroundings and over people. Characterized by the humans feeling and animals
feeling during the quake and on the structures and general facilities damage, it is
quantified by a 12 grades scale different all over the world.
In Europe, the scale is called “European macroseismic scale”, while in China,
they call it “Leidu scale”. Small differences can be noticed accordingly to the
subjectivity of the institutions making them, but the philosophy is the same. The
grades are both noted in roman numbers and the XII grade is the most severe. the
intensity is usually correlated pretty well with its distance from the earthquake
epicentre.
(Fardis, 2009)
5.3.3 Probability of the earthquake
As it has been mentioned in the prior paragraph (5.1.3, p.37), the codes
assume a certain earthquake to occur.
The US codes are not so much different compared to the Eurocodes, both aim
towards a life safety statement. The probability of occurrence of the seismic design
required is the same, that is to say 10% with 475 years of return period.
The US codes are established with a MCE value – maximum considered
earthquake – which serves as a base for the different performance level. It
represents the usual earthquakes that occurred on the specific region with an
increasing factor of 1.5.
For building of ordinary risks, MCE will be multiplied by a factor of 2/3. As for
the Eurocodes, the other risks are stated as large occupancy buildings and safety
critical facilities, respectively multiplied by a factor of 5/6 and 1.0.
(Fardis, 2009, pp. 2-8)
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Chapter 6. Summary
In the history of architecture and structural engineering, the materials used by
humans to build their homes, their cult places like temples or else are the same as
today. They are basically materials that we find in nature, wood, rock, earth, water,
or metal have been used to build constructions. As civilizations and societies, we
have been , we are and we will be able to improve our technics.
In a simplified version of the building industry today, a structural engineer and
an architect work together to create a project and to build it. This relationship is
important in order to complete the latter in the best conditions, those two
professions are complementary and must work well together in critical structure –
i.e. seismic structure.
Earthquakes produce a significant amount of energy aimed to move and shake
the ground, buildings on the other hand are aimed to stay stable and not to move.
Obviously, with this amount of energy, humans and mechanical systems can’t fight
against it, we must outwit the earthquake in order to save lives and goods. A
seismic design with particular technics is needed to limit ruination of constructions
and avoid any human loss.
The systems and the way to design buildings to facilitate the integration of an
inertia force in the building have been thought since the second half of the 19th
century until nowadays. The two examples of earthquake – in Italy 2009, and in
Sichuan, China 2008 - in this dissertation shows that whatever the magnitude of
the tremor, damage can appear, it does depend only on the mistakes done during
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construction. The future is therefore oriented towards an earthquake proof world
with isolation systems.
Engineers and architects need still to follow standards and codes. About them,
we can observe that they are very similar. That is explained by the fact that
institutions all over the world, that is to say, universities, laboratories, centres of
researches are helping each other, sharing their information. On the other hand,
countries have different landscape and risks, in response factors change without
surprises. Earthquake-resistance is definitely a worldwide community working
together to avoid those past natural disasters.
Although, a lot of safety are imposed on new constructions, old ones have kept
their issues with the time and that is one of the important to do in critical regions,
retrofit!
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LIST OF REFERENCES
Charleson, A., 2008. Seismic Design for architects: outwitting the quake. Charon Tec Ltd éd.
Oxford: Elsevier Ltd.
Ching, F. D. K., Jarzombek, M. & Prakash, V., 2011. A global history of architecture. 2nd ed. New
Jersey: John Wiley and sons, Inc..
EERI, June 2009. Special Earthquake Report - Abruzzo, Italy, Earthquake of April 6, 2009, s.l.: s.n.
EERI, October 2008. Special Earthquake Report - Wenchuan, Sichuan Province, China, Earthquake
of May 12, 2008, s.l.: s.n.
Fardis, M. N., 2009. Seismic Design, Assessment and Retrofitting of Concrete Buildings. Patras:
Springer.
Lindeburg, M. R. & Baradar, M., 2001. Seismic design of building structure: a professional's
introduction to earthquake forces and design details. 8th ed. Belmont, CA: Professional
publications, Inc..
Mcdonald, A. J., 2001. s.l.:Reed Educational and Professional Publishing.
Michigan Technological University, 2007. What Are Earthquake Hazards?. [Online]
Available at: http://www.geo.mtu.edu/UPSeis/hazards.html
Murty, C. V., n.d. Earthquake Tip, Kanpur, India: Indian Institute of Technology.
Raucci, S. & Jewell, T., n.d. Engineering in Ancient Rome, s.l.: Trustees of Union College.
USGS, 2012. Magnitude 6.3 - CENTRAL ITALY. [Online]
Available at:
http://earthquake.usgs.gov/earthquakes/eqinthenews/2009/us2009fcaf/us2009fcaf.php
[Accessed 31 October 2013].
USGS, 2013. Earthquake hazard archive - Magnitude 7.9 - EASTERN SICHUAN, CHINA. [Online]
Available at: http://earthquake.usgs.gov/earthquakes/eqinthenews/2008/us2008ryan/
[Accessed 29 October 2013].
Zhang, Z., s.d. Traditionnal Chinese buildings and their performance in earthquake, s.l.: s.n.
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Number of pages (2400 characters): 30 pages. - Characters : 71886 All rights reserved – no part of this publication may be
reproduced without the prior permission of the author.
NOTE: This dissertation was completed as part of a
Bachelor of Architectural Technology and Construction
Management degree course – no responsibility is
taken for any advice, instruction or conclusion given within!
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