5 New Trends in Earthquake Engineering
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Transcript of 5 New Trends in Earthquake Engineering
CHAPTER 5
New Trends in Earthquake Engineering.
Passive and Active Control
Structural control for civil engineering appeared as a necessity imposed by special,
longer, or taller constructions spread allover the world. The passive approach was
adopted already in many cases. For active control there are experiments showing
good results. Hybrid solutions are under investigations by many researchers.
However, the field is very large and civil engineer designers do not have a
clear image of this scope.
This chapter is showing the aim of structural control. It follows a short image
of passive control and it is then stressing on active control. Theoretical aspects,
devices used, and practical applications of active control are presented.
A critical comparison between the two types of structural control is intended
to introduce the reader into the complexity of the problems involved by the control
implementations. Computational means helping the study of the field are also
reviewed.
5.1 INTRODUCTION
The Civil Engineering field is now undergoing important changes in philosophy and
practice due to dramatic evolution recorded in other branches of human activity like:
electronics, automatics, computer science, robotics, new materials and technologies,
etc. At the same time, it should be mentioned major differences between Civil
Engineering and other engineering fields.
It is specific for constructions to use materials with high costs, on large
surfaces and volumes, and to need huge energy during construction, especially in the
case of long span bridges and very tall buildings.
Life of many people and vital social activities depend on the well functioning
of civil structures during and after important earth shakings. There are also civil
engineering structures with inestimable material and spiritual values, as historical
buildings.
NATURAL ACTIONS STRUCTURES
PROTECTION
Passive Control
Active Control
Typhoons
Earthquakes
Tall
Buildings
Long Span
Bridges0 10 20 30 40 50 60
-800
-600
-400
-200
0
200
400
600KOBE NS 1995
818 gal
acce
lera
tion
(ga
l)
time (s)
Figure 5.1 Relation between actions, structures, and control
Protection of some structures as those reminded above is a very important problem for
human communities. A solution for avoiding the harming effects of strong
earthquakes or strong winds is the structural control. It is using specific means and
procedures that lead to reduction in intensities for actions and change the way they act
on civil engineering structures. It changes also the structural response to the actions
and lowers the induced energy. In Figure 5.1, the existing relation between natural
actions, structural control, and built constructions is shown. This figure highlights the
reduction in the input due to control means.
5.2 SEISMIC RESPONSE CONTROL
In order to determine the ways to control the structures acted by seismic forces, the
equation of energy balance may be written
E E E E Ek s h d (5.1)
where: E is the energy induced by seismic shaking; Ek is the kinetic energy; Es is the
elastic strain energy; Eh is the energy dissipated by the structural system due to
inelastic behavior or other causes; Ed is the energy dissipated by supplemental
damping devices.
From Equation (5.1) it can be deduced that the control means play the role of
taking a part of the energy induced into the structure. However, it should be observed
that there is a dependency between the structural characteristics, e.g. the natural
period of vibration, and the amount of induced seismic energy. Therefore, the seismic
behavior of civil engineering structures is very complex in its nature and its
complexity is amplified through the presence of control means, which are actually
modifying the structural characteristics.
There are many ways to accomplish a seismic response control. According to
Professor Kobori, these ways are:
1. Cut off the energy transmission of the earthquake ground motion to a
structure.
2. Isolate the natural period of the structure from the predominant frequency
domain of the ground motion.
3. Achieve the non-stationary and non-resonant state by providing nonlinear
characteristics.
4. Apply control force such as mass damper/driver or tendon.
5. Utilize the energy absorption mechanism.
If the first method is achievable, then the second and the fifth method are not needed.
However, the first method is difficult to achieve, therefore it is recommended that at
least two of the above methods to be combined.
Table 5.1 Classification of structural protective systems
Seismic
Isolation
Passive Energy
Dissipation
Active
Control
Elastomeric bearings Metallic dampers Active bracing systems
Lead rubber bearings Friction dampers Active mass dampers
Elastomeric bearings with energy
dissipating devices
Viscoelastic dampers Active variable stiffness or
damping systems
Sliding friction pendulum Viscous dampers Pulse systems
Flat sliding bearings with restoring
force devices
Tuned mass dampers Electro-rheological active
dampers
Lubricated sliding bearings with
energy dissipating devices
Tuned liquid dampers Aerodynamic appendages
Table 5.1 shows a classification of protective systems. It provides a part of the many
control devices existing now. Depending on the location of the control devices other
classification can be stated:
a) protective devices located into the structure
b) protective devices located at the base of the structure
c) combination of a) and b).
Each of the above can be either passive or active.
The passive control is based on the nature of the materials they include
(rubber, lead, steel, viscous materials) and especially on supplementary adding in
damping and ductility. Some of passive devices rely on the use of friction forces.
The active control is using external energy for reducing, even minimizing the
seismic response. The controlled structure becomes active, with different
characteristics and different behavior compared to the initial non-controlled one, even
in case of strong actions.
5.3 PASSIVE CONTROL. DEVICES AND PRACTICAL APPLICATIONS
Passive controlled structures are widely spread all over the world. Table 4.3, at the
end of this chapter presents a list of isolated structures and isolation devices used in
different countries.
Also at the end of the chapter there is the Table 4.4 showing some passive
devices used in Japan for seismic isolation of bridges. These devices are the objects of
specifications in a design guide named “Menshin”. Other developed countries try to
setup similar specifications.
Top steel plate
Steel plates
Rubber
Holes for bolts
Bottom steel plate
Figure 5.2 Elastomeric bearing
A rubber bearing with top and bottom steel plates is presented in Figure 5.2. Other
type is that from Figure 5.3. It contains steel plates integrated in rubber. These plates
have the role to limit lateral displacements. The system includes a lead kernel, which
can assure an increased ductility, but it can determine permanent displacements after
strong earthquakes. However, the lead core can be replaced when needed.
Dowel holes
Lead plug
Rubber
Steel plates
End steel plates
Figure 5.3 Lead-rubber bearing
The passive means devices range is considerable large. One important type, named
Tuned Mass Damper (TMD) is based on moving masses. Figure 5.8 shows an Active
Mass Damper. If the active part (the actuator which generates the force ua) is
removed, a TMD is obtained. Such systems determine changes in structural dynamic
characteristics. Especially the frequency response function is lowered if the dynamic
properties of the TMD are close to that of the main structure and if the mass of the
TMD is enough large (between 1% to 4% from the mass of the main system).
In a similar manner is acting a TLD, Tuned Liquid Damper. One TLD is
shown at the fourth level of the structure from Figure 5.9. The mass value, the natural
period of vibration, and the liquid viscosity are the main elements that make the
system to be efficient.
Articulated slider
Supporting
column
Bearing material
Spherical surface
Cylinder
Seal
Figure 5.4 Friction Pendulum System Bearing
Figure 5.4 presents other passive control system based on a sliding bearing moving on
a spherical surface. An important advantage of this device type is that it can assure a
return to the initial position under the structure’s weight.
5.4 ACTIVE CONTROL. THEORETICAL ASPECTS
Every construction suffers changes during its life. At the same time, the environment
where the structure is placed is changing, too. Therefore one could compare existing
constructions to living beings. However, the most common way a civil engineering
structure overcomes external loads is to resist to them. The living beings not only
resist but also adapt to the environmental aggressiveness, responding in a different
manner to different actions or intensities. Adapting to external loads and to structural
changes is a basic idea in active structural control.
In 1972, Prof. James T.P. Yao, through his paper “Concept of Structural
Control”, is defining the start for this new branch in structural synthesis. Figure 5.5
shows a feedback system as J.P.Yao viewed it. The author describes a structural
controlled structure as an error-activated structural system the behavior which varies
automatically in accordance with unpredictable variations in the loading as well as
environmental conditions and thereby produces desirable responses under all possible
loading conditions.
From the point of view of theoretical studies and application methodologies,
there are two main approaches in structural control:
i. LQG (Linear Quadratic Gain) control, based on time domain
ii. H and -Synthesis control, based on frequency domain.
These two ways are very developed in many sub-methods and versions. Meanwhile,
additional tools are added to the main methods: fuzzy sets analysis and neuronal
networks.
Input, r variable, c
Control
Actuating signal, e
Error or
Forward Path Elements
- error sensing device
- compensating network
- amplifier
- servo-motor-
+
Feedback
Path
Elements
Command or
Reference
Figure 5.5 Closed-Loop Control System
From the first category of control, LQG, very popular are: pole assignment, optimal
control, instantaneous optimal control, modal control, critical modal control, and
sliding modes control method.
Majority of these methods is based on rewriting the structural dynamics
classical and familiar system of equations
M z + C z + K z = fs s s (5.2)
in the form of state equation
x = Ax + Bf
y = Cx + Df (5.3)
In the Equation (5.2), Ms, Cs, and Ks are the mass, damping, and stiffness matrices of
the structure; z is the vector of the generalized displacement vector, and f is the vector
of the external forces.
A
C B+
+ xf x
+
+ y
D
Figure 5.6 System described by Equation (5.3)
In the Equation (5.3), A is the system matrix, B is the load location matrix, C is the
measurement matrix, and D is a matrix showing the influence of the input, f, to the
output, y. Equation (5.3) is described by Figure 5.6.
If all the states, x, are known (measured), a state feedback, u, can be
introduced, and the control problem is reduced to finding a gain matrix, K, so that:
x = Ax + B(f - u)
u = Kx (5.4)
and a graphical representation of the Equation (5.4) is given by Figure 5.7.
A
Bx
u
f x
+
+
-
+
K
Figure 5.7 System described by Equation (5.4)
As an example of control method, the optimal control method is presented next. For
this case, the objective is to determine the gain matrix, K, under the condition that a
quadratic index, J, defined by Equation (5.5) should be minimized.
J dtt f
x Qx u Ru0
(5.5)
In the Equation (5.5), Q and R are weighting matrices representing the importance
given to reducing the structural response and the importance given to use less external
energy for obtaining the control, respectively.
This method leads to a Riccati matrical equation
PA PBR B P A P 2Q 011
2 (5.6)
with the matrix P being unknown. After solving, the gain matrix is obtained from the
next equation:
K R B P11
2 (5.7)
Replacing the control force vector, u, into Equation (5.4) one could observe that the
feedback system is transforming the original uncontrolled system by changing the
system matrix A, such that it will respond to the requirements of Equation (5.5).
m2
k1
c1
ua
k2
c2
m1
x x x 2 2 2
x x x 1 1 1
xg
Figure 5.8 Active Tuned Mass Damper
5.5 ACTIVE CONTROL. DEVICES
In structural active control various devices are used. Between them, Active Tuned
Mass Dampers (ATMDs) are widely used and studied. A system with one degree of
freedom equipped with an ATMD is shown in Figure 5.8. It can be seen from this
figure that an ATMD is formed from an additional mobile mass attached to the system
through an actuator, generating the force ua. The active forces values are generated
from the on-line measurements and employing control algorithms.
In Figure 5.8 additional springs and dampers, tuned to dynamic characteristics
of the structure, link the main system and the secondary one. This control system is
also called Hybrid Mass Damper (HMD) because it can be seen as a combination
from a pure active system Active Mass Damper (AMD), and the elements of a passive
Tuned Mass Damper (TMD).
m Active Tuned Mass
Damper, ATMD
Tuned Liquid
Damper, TLD
Active Braces
System, ABS
Active Variable
Stiffness, AVS
Active Tendon
System, ATS
Active/passive base
isolation systems
Figure 5.9 Use of some structural control systems
In order to have a better look about passive and active control systems, Figure 5.9
shows some of them located on a building. The figure is only a representation, not a
true solution, because employing many device types in the same building is not
common. Usually, real applications involve only one type of control system.
However, for hybrid applications, the base isolation is utilized together with some
active devices.
At the top of the building from Figure 5.9 one can observe an ATMD device
and one floor lower, a Tuned Liquid Damper (TLD). At the third floor, there is an
Active Brace System (ABS) which is principally made from a piston and a servo-
valve acting on the diagonal of that floor.
The system from the second floor of the building from Figure 5.9 is named
Active Variable Stiffness (AVS) and was introduced by Professor Takuji Kobori and
Kajima Corporation of Japan. It is made from two very stiff inclined beams moved to
the left or to the right by the upper active piston and therefore minimizing the relative
floor displacement and changing the floor stiffness.
The first floor from the building in Figure 5.9 is equipped with an Active
Tendon System (ATS). This system is based on active diagonals consisting in tendons
having the role to provide the limitation of relative floor displacements.
At the base of the building from Figure 5.9 passive isolators are installed. In
order to prevent too long displacements of the building’s base, an actuator and,
eventually, a spring and a damper are employed. For the base actuators, the problem
of generating huge forces could be prohibitive.
Table 5.2 Buildings with structural active control in Japan
Active Control System's
Name
Developer Building's Name Year
AMD (Active Mass Driver) Kajima Kyobashi Seiwa Bldg. 1989
AVS (Active Variable System) Kajima KaTRI No.21 BLDG 1990
AMD Takenaka Sendagaya INTS 1992
AMD Takenaka Hankyu Chayamachi Bldg. 1992
HMD (Tuned Active Damper) MHI, Yasui A&E Kansai Airport Control Tower 1992
HMD (Hybrid Mass Damper) Shimizu ORC200 Symbol Tower 1992
HMD (DUOX) Kajima Ando Nishikicho Bldg 1993
HMD MHI Landmark Tower 1993
HMD Nikken, Prof. Fujita Long Term Credit Bank Bldg. 1993
HMD Takenaka KS Project 1993
HMD (TRIGON) Kajima, IHI Shinjuku Park Tower 1994
HMD MHI, Nihon Sekkei ACT Tower 1994
AMD (AVICS-1) Obayashi Riverside Sumida 1994
HMD MHI, Nikken Sekkei Osaka WTC Bldg. 1994
HMD Shimizu Hotel Ocean 45 1994
HMD (DUOX) Kajima, KRC Dowa Kasai Phoenix Tower 1995
HMD MHI, Nikken Sekkei Rinku Gate Tower 1995
HMD Fujita, Prof. Fujita Hirobe Miyake Bldg. 1995
5.6 ACTIVE CONTROL. PRACTICAL APPLICATIONS
Most of the practical active control implementations exist in Japan. Table 5.2 shows,
in chronological order, the Japanese buildings equipped with active devices, the name
of the company that developed them and the devices’ types.
As a practical example, in Figure 5.10 the ATMD used in Landmark Tower
Yokohama, the tallest Japanese building (296 m), can be seen. Two identical devices
with that from Figure 5.10 are installed on top of the building, at floor 70. The moving
mass of such device is 170 tones out from 250 tones of the ensemble (it should be
noted that the whole building mass is evaluated at 223,000 tones). Maximum
displacement of the mass is of 1.70 m. The device has the in-plane dimensions
4.90 4.90 m and a height of 9 m. It is expected an efficiency of 60-70% in
acceleration’s decrease during strong typhoons.
Figure 5.10 Hybrid Mass Damper
For protection against winds of the towers during construction some Japanese bridges
had been equipped with active/passive systems: Akashi-Kaikyo Bridge (the longest
world’s suspension bridge, with 1990 m central span) and Tsurumi Tsubasa Bridge
(the longest world’s one plane cable stayed bridge, with 510 m central span).
Figure 5.11 Active Variable Damper
An active system for bridges is under intensive studies at the Public Work Research
Institute Tsukuba, Japan. This device, shown in Figure 5.11 is to be placed between
the infrastructure and the superstructure of highway bridges for limiting the
displacements that could harm the functionality of those bridges during strong
earthquakes.
5.7 COMPARISONS BETWEEN THE TWO CONTROL CATEGORIES
There is no doubt that the active control systems can provide better results when
compared to the passive control. Following a step by step analysis, the displacements
at the top of a structure equipped with a TMD compared to them for the case of an
ATMD are plotted in Figure 5.12. A comparison in frequency domain for acceleration
for the same two cases may be seen in Figure 5.13.
Figure 5.12 Time-history displacement response
Next follows the main reasons for and against choosing passive or active control for
civil engineering structures:
passive control means are cheaper and easier to maintain. However in some
cases the number of needed devices can become extremely high. For example, in each
building of The World Trade Center in New York, USA, 10,000 viscoelastic dampers
had been installed.
passive devices’ condition is or might be damaged by time, environment
and load intensity.
passive devices’ characteristics cannot be easily adjusted, as is the case with
active devices.
for the time being, active means have poor reliability: they still present
malfunctions and, on the other hand, it is hard to obtain for them high power supply
capable to properly work during strong earthquakes.
energy consumption for active systems, even in stand-by state, makes their
maintenance extremely expensive.
for both passive and, especially, active control systems, response time lag
leads to difficulties in strategy control analysis.
when structural control, either passive or active, is to be applied, a very
precise knowledge of the structure is needed. Therefore intensive and expensive
studies have to be done to identify it.
in order to perform an efficient active control, monitoring of the structural
response might lead to highly costing equipment (sensors, computers, etc.).
there is a danger of spillover phenomena (amplification of the response) in
structural active control as a result of poor structural identification, long time lag,
malfunctions in equipment, or due to extremely high external actions.
To the above aspects it should be added another one: the effectiveness of passive
control is still difficult to prove. An example is the stiffness to be chosen for the case
of isolators used as passive devices. If the isolators are too soft they might lead to very
large displacements. If they are too stiff, they will have reduced effects and will
transmit more energy.
Passive Control
Active Control
0 5 10 15 20 25 30-0.2
-0.15
-0.1
-0.05
0
0.05
0.1
0.15
0.2
Top Displacement, El-Centro NS
Time (s)
Dis
pla
cem
ent
(m)
Figure 5.13 Acceleration frequency response
Other problem, linked with the first one, is that if the passive devices can come back
to the initial position, after an earthquake. If some deformations are permanent, then
the system could become unmanageable.
However, the most worrying aspect is that of the contradictory responses
shown by isolators and passive controlled structures: to some earthquakes they
perform very well but in some other cases their effects are null or even amplifications
of the responses might occur. This can be explained by the narrow frequency limits of
the effectiveness for the passive control. Therefore, only the frequency response of the
isolators is shown to be very good. The non-stationary characteristics of the
earthquakes make, in many cases, the isolators behavior to be unexpected.
5.8 SPECIFIC COMPUTER PROGRAMS
Structural computational and seismic analysis methods for structures employing
control means cannot be the same as for common civil engineering constructions. It is
almost obvious that a simple spectral analysis, as is specified by majority of
Earthquake Engineering national codes, is not satisfactory.
For passive controlled structures especially, and also for many active
controlled structures, it is necessary to adopt models that accept non-linearities for
materials and structures, large displacements, P- effect, soil-structure interaction,
different foundation condition, and asynchronous excitations at construction base.
Even if the step-by-step analysis remains a main analysis procedure, controlled
structures must be analyzed using other approaches as linear or non-linear stochastic
analysis, or frequency domain analysis. It is not possible to state that an analysis
method is superior to other analysis method. They just reveal different aspects of the
control system and complete the image of the structural response.
In order to perform analysis for passive controlled structures, majority of finite
element computer programs (ANSYS, ADINA, NASTRAN, ABAQUS, I-DEAS,
ALGOR, etc.) can be used.
Other category of programs is specialized in dealing with non-linearities for
civil engineering structures. For example, IDARC2D, offers a step-by-step analysis
that manipulates the degrading of structural elements, showing the mechanisms that
affects the structural behavior. Also it provides a push-over analysis. Including
Passive Control
Active Control
0 2 4 6 8 100
2
4
6
8
10
Frequency (Hz)
Res
ponse
(m
/s2)
FRF of acceleration
hysteretic damping devices, viscoelastic elements, and friction devices is possible.
Contribution from nonstructural elements can be taken into consideration. Similar
capabilities are offered by other available computer programs: DRAIN, SARCF,
ANSR, SAP2000, ETABS.
Commercial programs are available for solving active control problems. One
of them, MATLAB, is built around a kernel that offers an integrated environment for
various applications in mathematics, statistics, matrix computation, graphical
simulations, signal processing, neuronal network, nonlinear programming, control
systems, etc. MATLAB needs a relatively hard training to be learned and it is limited
to small structural control applications.
However, to obtain very efficient computation for active structural control,
one should think to use specialized computer programs that would intensively employ
all computer resources and to adapt to civil engineering problems. Though large
construction companies surely possess such kind of computer programs, commercial
software is not yet available.
5.9 CONCLUSIONS
Studying the structural active/passive control responses of civil engineering
structures, one can see that there is an obvious superiority of the active control
compared to passive control. Improvement of the structural response is observed in all
analysis types: time domain, frequency domain, stochastic or spectral.
However, active control is, for the time being, very expensive and unreliable.
In this case there are unsolved aspects linked to time delays and instability due to
control devices. Structural system identification is still a complicated task and
therefore the structural control, based on it, is negatively influenced.
For the moment, practical application of active control looks more likely to
generate more problems than the problems it solves. This does not mean that the
future will reject this idea. It is especially a technological matter that slows the
advances in this field. The need of longer/taller and safer bridges/buildings will surely
accelerate the theoretical and practical works of structural active control.
As a small example, one could remember what was happening only 30 years
ago compared with nowadays. At that time, a computer performing similar tasks that
are performed today by a workstation was maybe thousands times more expensive.
The cost for maintenance (mainly because of the need for special rooms highly
conditioned and because of their poor reliability) was huge. Today’s workstations
have prices close to the first personal computers and sooner will be affordable even
for home use. Their maintenance cost is almost zero and their reliability is
outstanding. It is somehow obvious that automatic systems for other fields of activity,
e.g. structural active control, will become more and more reliable at higher
performance/cost ratio.
Table 5.3 Base isolated structures allover the world
Country Type of Structure Number of Structures Type of Isolation Systems
Former Soviet
Buildings over 200 Sliding Bearings
Rocking Columns
Pile-in-sleeve systems
Union Bridges n.a. n.a.
Other structures n.a. n.a.
Buildings 6 Rubber Bearings
France Bridges n.a. n.a.
Other structures 2 Nuclear Power
Plants
Neoprene Bearings
Buildings 9 + several apartment
houses of the Italian
Navy
High Damping Rubber
Bearings
Neoprene Bearings
Italy
Bridges 156 (total length =
150 km)
Sliding Bearings
Rubber Bearings
Lead-rubber Bearings
Various Hysteretic
Damping Devices
Other structures ... ...
Japan
Buildings 67 Rubber Bearings & Energy
Dissipaters
Lead-rubber Bearings
High Damping
Rubber Bearings
Sliding Bearings
Bridges over 100 partially
isolated
Sliding Bearings
Lead-rubber Bearings
High Damping
Rubber Bearings
Other structures Radar Tower Sliding Bearings
Buildings 6 Lead-rubber Bearings
Pile-in-sleeve systems
Lead-rubber & Sliding
Bearings
New Zealand Bridges 37 Lead-rubber Bearings
Various Energy
Dissipating Devices
Other structures Industrial Chimney Rocking Foundation
Buildings 24 Lead-rubber Bearings
High Damping
Rubber Bearings
Friction Pendulum System
Springs & Viscodampers
United States Bridges 54 (total length = 11
km)
Lead-rubber Bearings
Sliding Bearings
Other structures 2 Tanks
Heavy Equipment
Friction Pendulum System
Lead-rubber Bearings
High Damping Bearings
Low Damping Bearings
Table 5.4 "Menshin" passive control devices
Type Menshin Device
High Damping
Rubber Bearing
Slide Friction
Rubber Bearing
Steel
Damper
Roller Menshin
Bearing
Viscous