Proceedings of the 17th International Conference on Nuclear Engineering
ICONE17
July 12-16, 2009, Brussels, Belgium
ICONE17-75022
SEISMIC ISOLATION OF NUCLEAR STRUCTURES BY A NON-OSCILLATING SYSTEM
SERB TYPE
Viorel Serban
Adrian Panait
Department of Safety Division
Manager
Marian Androne
George Alexandru Ciocan
Department of Safety Division
Department of Safety Division
Subsidiary of Technology and Engineering for Nuclear Projects
Bucharest-Magurele, Romania
ABSTRACT
The paper presents the advantage of non-oscillating
systems – SERB type – for the seismic isolation of buildings
compared with other solutions for seismic protection of the
buildings which are now employed worldwide.
The non-oscillating isolation devices – SERB type – have a
capsulated structure and are capable to overtake large
compression and tension loads on vertical direction with very
small deflection and cut off the seismic action in horizontal
direction.
The stiffness of the devices is zero for the translations on any
direction in horizontal plane upto a pre-set displacement which
usually, is between 0.6 - 0.8 of the maximum seismic soil
displacement. After having reached that value, a non-linear
increase of the device stiffness is occurring on the horizontal
direction in order to provide a non-shock limitation of some
random seismic displacements higher than the maximum
displacement accommodated by the device. The motion inside
the device is controlled all time by the dynamic friction and
restricted by nonlinear stiffness and damping material after a
pre-set displacement.
On vertical direction, the stiffness of the devices is big in order
to avoid the additional stresses on the structural elements of
the isolated suprastructure, due to the differential settlement of
the devices on the suprastructure permanent loads that could
appear. The system provides very small deflection of the
device on vertical direction with high damping. On request,
SERB isolators can be so fabricated to have the desired
stiffness on vertical direction.
Standard SERB devices 960x960x230 are capable to overtake
a permanent compression load of 3000KN and tensile load of
1500KN, respectively, over which dynamic loads of similar
values may overlap.
The maximum seismic acceleration transferred to the isolated
structure on horizontal direction is ranging between 0.03g and
0.05g depending on the friction coefficient. The maximum
relative displacement between the isolated suprastructure and
the infrastructure embedded into the ground, is about 3 times
smaller than the maximum seismic displacement showed by
the oscillating systems employed today and smaller then the
maximum seismic soil displacement.
In series SERB device 960x960x230 allows for a maximum
seismic displacement of 230mm.
An application of the non-oscillating SERB devices
960x960x230 as the solution to seismically isolate the heavy
water detritiation system building at Cernavoda NPP, is
presented.
1. GENERAL PRESENTATION
The construction of buildings capable to withstand seismic
actions can involve various techniques. Such techniques
depend both on the type of the building and the characteristics
of the seismic movement. Considering the fact that the seismic
action on the building is dependent on the type of the
earthquake focus, the distance between the building site and
the earthquake focus, the local geological conditions, it results
that the design solutions for buildings to withstand future
earthquakes need to consider the above mentioned parameters.
The current trend to apply the same design considerations to
all types of seismic actions for all types of buildings
irrespective of the kinetic characteristics of the ground seismic
movement, of the local geological conditions and of the type
of the building [1, 2] is an error which will lead to great
material and human losses with the future earthquakes. In
order to get a quantitative understanding of the differences
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Copyright © 2009 by ASME
among various building seismic design solutions, herein below
there is a brief presentation of the various alternatives.
The great difference in the building behavior during an
earthquake results from the fact that a building (along with its
foundation ground) make-up an oscillating system which
represents a BUILT-UP of kinetic and potential energy of
repeated seismic ground oscillations. The oscillating system
MAY or NOT OVERTAKE and BUILT-UP the seismic
energy from each soil oscillation, function of the location of
the important eigen vibration periods of the building
within the spectral component of the seismic action.
Related to the ground response seismic spectrum, the
important eigen vibration periods of the building may fall-in
the left part, in the bearing area of the seismic action (transfer
without energy built-up), on the right , in the isolation area of
the seismic action (non-transfer and non-build-up area of
energy) or in the resonance area of the seismic action
(maximum transfer and maximum built-up area of energy).
Among these areas, there are transition areas in which the
seismic energy transfer and built-up phenomena evidence
different weights.
In order to specify such dynamic behavior regimes of the
buildings, the seismic action defined by the seismic ground
response spectrum is divided into 5 areas which may be
defined as follows for a seismic action with only one dominant
harmonic component of T period:
Area a) resonance area, between 0.707T ÷ 1.41T;
Area b) isolation area, between 3T ÷ ∞;
Area c)
bearing area, between 0 ÷ T/3;
Area d) transfer area1, bearing-resonance, between T/3 ÷ 0.707T;
Area e) transfer area2, resonance-isolation, between 1.41T ÷ 3T.
Buildings, which in point of the dynamic behavior fall-in areas
‘c’, ’d’ and ‘e’, are building-up seismic energy during an
earthquake which leads to the amplification of the building
seismic response. The buildings falling-in areas ‘a’ and ‘b’ do
not build-up seismic energy and additionally, in area ‘b’ the
energy transfer from the ground to the building is not totally
accomplished.
For real seismic actions, which are different due to their design
ground response spectrum, the 5 areas may be defined as
follows (see fig. 1.1):
Area a) resonance area, between
&
Area b) isolation area, between 3TC & ∞;
Area c)
bearing area, between 0 &
;
;
Area d) transfer area1, bearing-resonance, between
&
;
& 3TC.
TB and TC may be defined by the possible dominant periods of
the seismic action on that site, namely, Tmin and Tmax.
Considering the large variety of the kinetic characteristics of
the seismic actions, a proposal of classification in function of
their spectral component, is presented below:
a) “fast”earthquakes are the earthquakes having the dominant
repetition periods of ground movement below 0.5 seconds.
These earthquakes are generated by surface foci (shallow
earthquakes) on hard and average soils;
b) “slow”earthquakes are the earthquakes having the
dominant repetition periods of the ground movement
higher than 1 second. These earthquakes are generated by
intermediate and deep foci on soft and average soils;
Area e) transfer area2, resonance-isolation, between
c) “moderate” earthquakes are the earthquakes having the
dominant repetition periods of the ground movement
ranging between 0.5 seconds and 1 second. This is also the
class of the earthquakes generated by surface and
intermediate foci in function of the site soil characteristics.
Area a)
Resonance
Area d)
Area e)
Bearing resonance
Resonanceisolation
Area c)
Area b)
Bearing
Isolation
Figure 1.1. The main 5 areas that define the dynamic
behavior of the building. Acceleration response spectra for
MEXICO CITY earthquake, Sep, 19, 1995.
The main problem that needs to be solved by the seismic
design of buildings consists in the transfer of a minimum
quantity of seismic energy from the ground to the building
and in so that the transferred energy should not build-up in the
building-ground oscillating system.
Herein below all the measures which make-up and
demonstrate the increase of a building capacity to withstand a
seismic action is called: ”seismic qualification”.
2.
POSSIBLE SOLUTIONS FOR MITIGATION OF
SEISMIC ACTION
2.1 Alternative 1. Elastic behavior of the building (classic
solution)
In this case, dynamically, the building is usually located in the
resonance or in the transfer area of the response spectrum,
function of the spectral area in which the dominant vibration
period of the building-foundation ground assembly is situated.
The seismic energy transferred to the building per an
oscillating cycle, may be built-up in the building-ground
oscillating system when its eigen vibration period is in the
transfer or resonance area of the seismic movement. A small
part of the seismic energy built-up by the building is
consumed due to the low damping capacity of the system,
leading to the limitation of the seismic response amplitude of
the building at large values.
With Alternative 1, the amplifications of the seismic
acceleration in a building are 4-6 times higher than the
maximum ground acceleration, function of the damping
capacity, especially of the non-structural elements of the
building.
If the building is located in the bearing area, though the
building takes seismic energy with each seismic action
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repetition cycle, the oscillating system is not building it up.
The relative movement of the building is in-phase with the
seismic action and the seismic energy per one cycle is equal
with the kinetic energy of the building (the system potential
energy is actually zero). In case that the building is in the
resonance area with the seismic movement, there occurs an
out-of-phase of the building seismic response with the seismic
action of about 90 degrees and the system is building –up
kinetic and potential energy. The maximum value of the builtup energy depends on the system damping capacity.
Seismic qualification of buildings as per Alternative 1 is made
by the proper sizing of the structural elements so that they may
overtake the permanent loads and seismic loads without the
occurrence of plastic hinges.
2.2 Alternative 2. Plastic hinges in the building (classic
solution).
In this case, dynamically, the building may be located in the
transfer or resonance area in its non-damaged condition but,
due to damaging, it may pass to the isolation or resonance
area, function of the type of the seismic action (i.e. fast or
slow or moderate) and the building characteristics (Park &
Pauley).
It is aimed that the reduction of the building seismic response
be accomplished by the occurrence of some structural local
damages (i.e. in the location of beam-column joints and
foundation-column joints) during a big earthquake, generically
called: ”plastic hinges”. The damage is modifying the
dynamic behavior of the building in two directions:
Direction 1 – Due to the friction developed in the plastic
hinges, a further dissipation of the seismic energy built-up in
the building is occuring, leading to the reduction of seismic
energy built-up in the building-soil oscillating system and
implicitly to a reduction of the seismic response which may
get decreased by 20% -30% as to the alternative 1. From that
viewpoint, Direction 1 is equivalent to inserting some
hydraulic dampers into the building (Alternative 3).
Direction 2 - Due to the damage of the connection between
beams-columns and columns-foundation, an important
reduction of the local building stiffness is occurring making
the damaged building eigen vibration period increase locally
(a large part of the building elements remains in the elastic
behavior range which make the structure, as a whole, behave
like an oscillating system).
In this case the dynamic behavior of the building in the
condition of a forced excitation of the seismic movement, is
substantially modified. The seismic energy transferred to the
building per one oscillation cycle may be smaller or bigger
than with the case of the non-damaged building and the
maximum energy which is built-up by the building- ground
oscillating system may be smaller or bigger.
Dynamically, the seismic response of the structure in direction
2 may fall-in the two cases:
Case 2a – the building enters the isolation area due to
damaging.
By the increase of the damaged building eigen vibration
period, the building passes from the transfer or resonance area
to the isolation area. Due to that phenomenon, a reduction of
the building seismic response by 2-4 times as to the situation
in which, dynamically, the building remains in resonance with
the seismic action, is occurring. The reduction depends on the
distance from the resonance area due to the increase of the
vibration period of the damage building. In this case, Direction
2 is equivalent to the building seismic isolation with
oscillating system (alternative 5). In fact, the damage is
leading to a self - isolation of the building against the seismic
movement. This method was developed in Japan and U.S.A.
where most of the recorded earthquakes have the dominant
repetition period of the ground movement below 0.5 seconds.
Alternative 2 of seismic qualification with platic hinges may
be successfully applied to “fast” and “moderate” earthquakes,
which is the case of the most earthquakes in the world.
For the seismically qualified buildings in case 2a there is the
economic disadvantage that the buildings may need to be
strengthened after any important earthquake that affect it.
Case 2b – the building enters or stays in the resonance area
due to damaging.
By the increase of the building eigen vibration period due to
damaging, the building passes to the resonance area and it
cannot shift out because the relative level deflections are
increasing so much that the building is destroyed before
entering the isolation area. This case may be frequently met
with “slow” earthquakes whose dominant repetition period of
the ground movement is greater than 1 second or even with
moderate earthquakes, function of the type of the building.
The relative level deflection of the building that needs to
increase very much so to reach the condition in which the
damaged building eigen vibration period may be greater than
the dominant period of the seismic movement, cannot be
obtained. If the damaged building cannot reach an oscillation
period greater than the repetition period of the seismic
movement, the phenomenon to reduce the seismic response
cannot occur and thus, the seismic energy built-up in the
partial-damaged building is occurring with the amplification
of the seismic response rather than the reduction of the seismic
response , because the energy dissipation due to the increase of
damping, cannot compensate the energy built-up in the
oscillating system which is not shifted out of the resonance
area of the seismic movement.
By the reduction of the building assembly stiffness and the
increase of the eigen vibration period, case 2 b is similar with
alternative 5 only if the damaged building eigen vibration
period exceeds (preferably 3 times so that the effect be
significant) the dominant repetition period of the seismic
movement (period may be considered Tc of the ground
response spectrum if other information is not available). For
“fast” earthquakes this phenomenon is usually occurring
without reaching an advanced damaged condition of the
building, condition which might jeopardize the building safety.
With “slow” earthquakes at which T C > 1 second the increase
of the damaged building eigen vibration period so that
dynamically the building may reach the isolation area, T>3T C
≥ 3 seconds, is practically impossible because the relative level
distortions should increase 10 times as to the values
corresponding to the elastic behavior of the building,
distortions which usually cannot be achieved with maintaining
the building stability in case of slow earthquakes.
The seismically qualified buildings in case 2b may get very
damaged during an earthquake.
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The seismic qualification of the building as per Alternative 2 is
obtained only in case that the building, through damaging, is
reaching the seismic isolation range; for all the other cases the
building reaches overloading and important damages which
jeopardize the building safety.
2.3 Alternative 3. The control, limitation and damping of
relative level deflections by hydraulic dampers or
mechanical devices with damage (modern solution).
In this case, dynamically the building may be in the all ground
response spectra area. The effect is maximum for the
resonance area and the transfer areas near-by the resonance
area. In the isolation area the increase of damping leads to the
increase of the building response in accelerations and the
decrease of the response in displacements.
If the building is situated in the resonance and transfer area
the seismic response in accelerations decreases with the
increase of damping. In the bearing area, the damping has no
effect on the building seismic response.
With this alternative (compared with alternative 1) there is a
reduction of the seismic response of the building structure due
to the controlled dissipation of a large quantity of seismic
energy by the hydraulic dampers or the mechanical devices
with damage.
The building structure need to stay in the elastic behavior area
for an efficient control of its behavior. If the building shifts to
the plastic strains area, the hydraulic dampers or the
mechanical devices with damage, might have a negative effect
on the building safety because the forces developed in the
devices oppose the revert of the building to the un-distorted
position. In this case, the cooperation between the dampers
with the building is altered.
Specially after the earthquake in Kobe in 1995 when many
buildings which were designed according to alternative 2 with
plastic hinges, were destroyed, in Japan, a change in the
seismic design happened, [4], in the sense that the occurrence
of plastic hinges is no longer acceptable and an increase of the
building damping capacity is obtained by the insertion of
specialized mechanical devices in braces (alternative 3).
The seismic qualification of the building as per Alternative 3 is
obtained by the installation , in the building, of an enough
number of hydraulic dampers or mechanical devices with
damage and by the proper sizing of the structural elements and
anchoring, also maintaining them in the linear range of
behavior.
2.4 Alternative 4. The control, limitation and damping of
relative level deflections by telescopic devices with nonlinear strengthening and large damping (innovative
solution).
In this case, dynamically, the building may situate in all the
areas of the ground response spectrum. Additionally,
compared to alternative 3 , for buildings one may allow a
slight exceeding in the non-linear area of behavior.
This alternative is different from alternative 3 because instead
of the hydraulic dampers or mechanical devices with damage
in braces, use telescopic devices with strengthening and high
damping, as for example SERB telescopic devices, are
installed. As per this alternative 4 one may control the relative
level deflections of the building and limit these deflections to
pre-set values because of the elastic non-linear forces
developed in the telescopic strengthening devices.
The telescopic devices provide a great dissipation of the
seismic energy including the case of very small relative level
deflections. The telescopic devices have a force-distortion
characteristic – hysteresis type - with strengthening in order to
limit the relative level deflections to pre-set values. The
devices are capsulated and pre-tightened in their boxes in
order to provide a satisfactory operation with the building
structure on any load level without introducing additional
forces in the building due to pre-stressing, also eliminating the
effects of shocks.
As per this alternative 4 the seismic behavior of the building is
controlled by elastic and damping forces developed in the
telescopic devices providing thus both the reduction of the
seismic response and the reduction of the building relative
level deflections to pre-set values. The exceeding of the elastic
limit of the building structural material may be allowed
because the elastic forced developed in the mechanical devices
are capable to make the building revert to its undistorted
position. The amplification of the seismic accelerations and
displacements, according to this alternative 4, is smaller than
with alternative 3 reducing thus the maximum seismic
accelerations and displacements in the building by about 2-3
times than with alternative 1, [5].
The seismic qualification of the building as per Alternative 4 is
obtained by the installation of a quite small number of SERB
devices in the braces and by the proper sizing of the device
anchoring and structural elements of the building. As per this
alternative 4, the linear range of behavior of the elements may
be exceeded due to the capacity of SERB telescopic devices to
revert to the undistorted condition (see figure below).
2.5 Alternative 5. The seismic isolation of the building by
oscillating devices (modern solution).
In this case, dynamically the building may situated only in the
isolation area of the ground response spectrum.
The seismic qualification of the building is so done that the
seismic action given by the ground movement is partially
transferred to the building and the building-isolator oscillating
system is building-up a small quantity of seismic energy, [6,
15-17]. With this alternative the building structure is divided
in 2 parts: the infrastructure which is embedded in the ground,
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on which the oscillating devices are installed; and the suprastructure installed on the oscillating isolation devices. This
solution was firstly implemented in New Zeeland. The seismic
energy associated to the ground oscillating cycles is
transferred in a small percentage to the isolated supra-structure
and the oscillating system made-up of the supra-structure and
the isolation devices build-up a small quantity of seismic
energy from the infra-structure seismic movement, function of
the value of the ratio between the building eigen vibration
period and the dominant period from the seismic movement
and function of the isolation system dissipation capacity.
For the system to satisfy the safety requirements for buildings
and the isolation for the seismic action transferred to the
building, the system has to be very stiff on vertical and
flexible on any horizontal direction. The separation surface
between the infra-structure and the supra-structure has to
remain plane and undistorted on permanent loads imposed by
the supra-structure and the random loads, including the loads
due to seismic events. In case that an isolation device gets
distorted more on vertical direction (or even break), a large
concentration of stress may occur in the supra-structure and
lead to local damage or loss of its stability.
With oscillating isolation systems type the isolation device
need to have a very small stiffness on horizontal so that the
first eigen vibration period of the supra-structure supported on
the isolation devices may be much greater (preferably over 3
times) than the dominant repetition period of the seismic
movement (period Tc of the ground response spectrum if no
information is available) and then the first eigen vibration
period of the supra-structure considered to be embedded at the
level of the isolation surface. It is only this situation when the
seismic energy build-up in the oscillating system made-up of
the supra-structure and the isolation system is small and a
small amplification of the seismic acceleration is resulting.
Since with the oscillating isolation systems the supra-structure
relative movement is anti-phase as to the ground seismic
movement (the infrastructure bearing movement), then in
order to limit the relative displacements between the
infrastructure and the supra-structure (relative displacements
governing the isolation system safety) an additional damping
(usually by means of hydraulic devices) is installed.
The increase of the oscillating isolation system damping
capacity will also lead to the increase of the seismic
accelerations transferred to the supra-structure and to the
increase of the seismic loads. For that reason a maximum 30%
critical damping ratio is recommended [1-3].
Moreover, for the seismic isolation devices be also applied to
tall buildings where during an earthquake or wind loads a turnup moment greater than the stability moment (given by the
dead-weight) may occur, at the surface of separation between
the infrastructure and the supra-structure the isolation devices
should also overtake the tensile forces in order to provide the
isolated supra-structure stability against turning-up.
The existing oscillating isolation systems do not provide the
overtaking of some tensile forces in order to assure the
building stability.
The seismic qualification of the building as per Alternative 5
consists in the selection of an oscillating isolation system so
that the eigen vibration period of the isolated supra-structure
should be greater than 3 times Tc. Also, the eigen vibration
period should be 3 times greater than the eigen vibration
period of the supra-structure embedded at the level of the
isolation surface. The isolation system should be conservative
(have more devices) to withstand the yielding of the
translation movement of the isolators due to the shear and
bending overloading.
2.6 Alternative 6. The building seismic isolation by nonoscillating devices (innovative solution).
In this case, dynamically, the building is situated on the right
of the isolation area (the eigen vibration period is infinite
because the building supra-structure is not oscillating and it is
semi-borne by the infrastructure).
Alternative 6 is different from alternative 5 due to the fact that
the isolation devices do not develop elastic forces on
horizontal and do not built-up a potential energy for a relative
displacement pre-set around the position of balance (relative
displacement which depends on the maximum ground seismic
movement- phase 1). In order to limit the maximum horizontal
displacements (accidental) of the supra-structure versus the
infrastructure and the elimination of some possible shock
effects, the non-oscillating isolation system has to develop a
non-elastic force (phase 2) after having consumed the relative
displacement pre-set around the position of balance from
phase 1.
The non-oscillating isolation devices can achieve a gliding
movement in phase 1 with small friction around the position of
balance in the pre-set area of values. Within that interval the
isolated supra-structure is no longer developing an oscillation
movement during an earthquake because there is not the
possibility of a potential energy built-up in the system. The
supra-structure shall be “semi-borne” around the position of
balance, by the infra-structure which is embedded in the
ground, with displacements smaller than the ground seismic
displacements. By gliding the isolation system is actually
totally “cutting-off” the transfer of the seismic action from the
infra-structure borne by the ground, to the building suprastructure. The supra-structure with the isolation system is no
longer making-up an oscillating system which is building-up
the kinetic and potential energy and which generate an
amplification of the seismic movement in accelerations and
displacements (like the case of alternative 1-5).
The relative seismic displacement between the supra-structure
isolated with the non-oscillating systems (alternative 6) and
the infra-structure embedded in the ground is much smaller
than in the case of employing the oscillating isolation systems
(alternative 5) and also smaller than the maximum ground
seismic displacement. The seismic acceleration transferred to
the isolated supra-structure in fractions of gravitational
acceleration,”g”, is equal to the friction-gliding rate,”μ”, of the
non-oscillating isolation devices. For that reason it is
desirable to have a relative small friction rate which could
assure the required stability of the supra-structure and also
could allow a slight displacement between the infra-structure
and the supra-structure, with very small accelerations. The
minimum rates for gliding that can be reached today without
“rolling” or with active friction control systems, are
encompassed between 0.03 – 0.05, which lead to a maximum
supra-structure acceleration of 0.05 g. In case of using some
ball gliding systems (rolling), the friction ratio is reduced to
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values below 0.01, which is not sufficient to provide a stability
of the isolated supra-structure.
In order to provide the stability of the supra-structure, the
isolation systems have to achieve a shock-free limitation of the
accidental displacements between the supra-structure and the
infra-structure. For that reason the ideal solution is the
following: after a pre-set relative displacement - phase 1(in
which non-oscillating seismic movements are developing and
the force-distortion characteristic is horizontal) - the isolation
system may achieve a non-linear increase of the force with the
displacement - phase 2 - in order to shock-free limit the
possible accidental displacements.
Compared with the oscillating isolation system, the nonoscillating isolation system with friction-gliding there is no
danger of the insulator break because the components are
subjected to very low shear forces (loads) and the damage of
the contact surface due to compression cannot occur because
of the specific geometry of the isolators.
By now, 3 types of non-oscillating systems have been
developed in this domain.
Type 1 - The isolation system developed in Japan consists of a
PTFE (polytetrafluoro-thylene) end anchored in a steel and
rubber structure which is gliding on a horizontal stainless steel
plate. The system cannot limit the relative displacement
between the supra-structure and the infra-structure to pre-set
values and neither is it capable to overtake tensile forces. For
that reason the isolation system can be used only by installing,
in parallel, some elastic devices and damping devices (LRB –
Lead Rubber Bearings, HDRB – High Damping Rubber
Bearings) between the infra-structure and the supra-structure,
and which are arranged in the horizontal separation plane. In
the literature, the system is referred to as “Flat surface slider
with dampers”. The system was applied in Japan and Italy,
e.g. NATO HEADQUARTERS in Napoli in 2007 -2008.
Type 2 - The isolation system developed by THK company in
Japan starting with 1996, is a system according to which the
movement in horizontal plane is produced by the combination
of two perpendicular movements produced on profiled bars
and balls. The system, called “Linear Motion guide with
Caged Ball”, has a friction coefficient, μ < 0.01, which may
provide a seismic acceleration transferred to the suprastructure, smaller than 0.01g. Since the system does not
provide stability upon small horizontal actions and a minimum
force for reverting to the initial position, it can be used only by
the in-parallel installation of some oscillating isolators in the
usable ratio of 2/3 THK / “Elastomeric isolators”. By this
combination, THK non-oscillating isolation system is
practically transformed into an oscillating isolation system
with large vibration period. In point of safety, the system may
break at great accidental loads due to the fact that the
displacement in two perpendicular directions generate the
occurrence of some great moments in the guiding-sliding
system and oscillating isolators may break.
Type 3 - SERB type non-oscillating isolation system
developed in Romania starting with the year 2000, is a
capsulated system with non-linear limitation of the relative
displacements in the isolation surface , to a pre-set value, after
performing a pre-set relative displacement. For this type of
isolation system it is no longer mandatory to install
mechanical devices to control, limit and damp the relative
displacements between the infra-structure and the suprastructure because the limitation is achieved inside the isolation
device as per a pre-set law. With SERB devices the relative
movement in horizontal plane is produced by gliding with
small friction coefficient between several surfaces (minimum
5) which operate in parallel and are located in two stiff semibodies, with the possibility of a controlled relative
displacement among them. On horizontal direction the
devices have a force-displacement characteristic parallel with
the displacement axes upto a pre-set value and next a nonlinear elastic increase of the force is occurring in order to
avoid shocks at large accidental displacements. With these
devices there is not the possibility to lose the isolation capacity
or to break or get destroyed. On vertical direction, SERB
device has a stiff behavior, small distortions (1-2mm) also
having a hysteresis loop. SERB isolation devices can also
overtake tensile loads upto 60% of the maximum compression
load.
The maximum acceleration of a building supra-structure
isolated by non-oscillating isolation devices, SERB type, is
maximum 0.05g and the maximum relative seismic
displacement between the infra-structure and the suprastructure is smaller than the maximum seismic displacement of
the ground and of about 3 times smaller than the relative
displacement of the oscillating isolation systems ( alternative 5).
SERB devices are the only devices existing today, which can
provide the desired correlation between the maximum
acceleration transferred to the structure and the maximum
relative displacement between the infra-structure and the
supra-structure so that an advanced isolation of the suprastructure is assured without overloading the technological
connections between the infra-structure and the supra-structure
(see figure below).
By now this alternative was applied in Romania. Starting with
2003, semi-oscillating and non-oscillating isolation systems
were applied to some heavy equipment have been developed
[7, 9, 10]. Case studies including nuclear objectives have also
been developed [8]. There were also achievements in the
production of 1:1 scale building isolation devices and they
were experimentally tested with the most developed
installations in Romania. The experimental results are the
closest to the ideal isolation solution in the sense that the
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hysteresis diagrams show a constant force transferred to the
isolated supra-structure equivalent to a 0.03g seismic
acceleration for a pre-set displacement area, followed by a
non-linear increase with a big hysteresis loop.
The seismic qualification of a building as per Alternative 6
depends very much on the type of isolation devices used. For
types 1 and 2, the seismic qualification is similar with
alternative 5, except that in these cases it is much easier to
obtain a high eigen vibration period of the supra-structure. For
type 3 of non-oscillating devices, the seismic qualification is
easier in the sense that the isolation is automatically obtained
( infinite vibration period) and the only condition to be
satisfied is the relatively uniform overtaking of the suprastructure loading by the isolation devices.
Ratio =
1.61
Figure 3.2. Amplitude of kinetic and potential energy of the system
ENERGY TRANSFER FROM THE SEISMIC
MOVEMENT TO THE BUILDING AND ITS
BUILDING-UP IN OSCILLATING SYSTEM
In order to substantiate the seismic qualification alternatives,
function of the dynamic characteristics of the buildings and of
the seismic action kinetics (expressed by the ratio between the
building dominant vibration period and the dominant period in
the seismic movement T/Ts), Figures 3.1 - 3.4 illustrate the
variations of the amplitude of power transferred from the
excitation to the oscillating system Pe and of the power
dissipated by the oscillating system Pa, as well as the variation
of the kinetic energy amplitude Ekin and potential energy Wpot
built-up in the oscillating system versus the amplitude of the
power and excitation energy for a mass unit of the oscillating
system, for a 5% and 20% damping of the oscillating system
[10, 13, 14].
Analyzing the diagrams, it results that the most efficient
solution for the building protection against seismic events is
the building isolation by non-oscillating systems (infinite
vibration period). In this case, the acceleration of the isolated
supra-structure is limited to the value of the isolation device
friction rate and the relative displacement between the isolated
supra-structure and the infra-structure embedded in the
ground, is smaller than the maximum ground displacement.
versus the excitation amplitude, function of
T TS ,   5%
Variation of Damping and Seismic ground power amplitude versus period
ratio. Critical damping ratio = 20%
Pd
Pe
100
Damping and Seismic ground power
3.
10
1
0.1
0.01
0.001
0.01
0.1
1
10
100
ratio = T/Ts
Figure 3.3. Amplitude of power transferred from the excitation, to
the oscillating system and the power dissipated in the system versus
the excitation power function of
T Ts ,   20%
Variation of Damping and Seismic ground power amplitude.
versus period ratio. Critical damping ratio = 5%
Pd
Pe
Damping and Seismic ground power
100
10
1
0.1
Figure 3.4. Amplitude of kinetic and potential energy of the system
0.01
versus the excitation amplitude, function of
0.001
0.01
0.1
1
10
100
ratio = T/Ts
Figure 3.1. Amplitude of power transferred from the excitation, to
the oscillating system and the power dissipated in the system versus
the excitation power function of
T Ts ,   5 %
T TS   20%
,
4. SEISMIC ISOLATION OF THE DETRITIATION
BUILDING ON ROMANIA CERNAVODA NPP SITE
CTRF Building on Cernavoda NPP site is situated within Unit
1 enclosure, in the vicinity of HPECC Building and D2O
Tower Building. The building infra-structure is sectioned
(isolated) at 5 cm above the ground area at El. + 15.80 dnMB.
That is the elevation where the gliding plane between the
ground embedded infra-structure and the supra-structure
7
Copyright © 2009 by ASME
minim 15m inaltime
isolated by 28 non-oscillating SERB 960x960x220 devices
(see Fig 4.1) are located.
The sealing between the ground embedded infra-structure and
the isolated supra-structure is made by an elastic-gliding
system with SERB 300x15xX devices which provides the
satisfaction of sealing requirements in any condition.
The maximum seismic acceleration transferred to the isolated
supra-structure on horizontal is 0.05g and equal at all the
building elevations because the supra-structure installed on
SERB isolation devices are behaving like a stiff body. The
maximum seismic displacement between the ground
embedded infra-structure and the isolated supra-structure is
estimated at about 14cm, as per the preliminary analyses. This
result shall be verified by experimental testing conducted with
a 1:1 scale isolation device in installation conditions identical
with the conditions on site.The isolation devices can overtake
3000KN compression loads and 1800KN tensile loads and it
has about 23cm maximum displacement in horizontal plane.
Figures 4.2–4 illustrate the experimental determinations and
the obtained results with a SERB prototype loaded on a JICA
rig and for which the maximum possible displacement on
horizontal plane is 160 mm.
+28,0
Detail
B
-0,01
Figure 4.3. Non-oscillating capsulated SERB isolation device 960 x
960 x 230 prototype.
SERB
isollation
devices
1100
459
25
10
55
-0,596
Incinta pentru ventilatoare si putul de
montaj al tubulaturii de legatura
1000
596
541
500
325
200
+24,50
400
-0,956
+20,0
300
4500
700
700
700
300
300
3500
400
200
350
Et 4
+15,50
700
Et 3
Detail
A
SERB
sealing
devices
700
300
350
300
200
+12,0
4000
200
-0,01
400
Et 2
500
200
+8,00
300
400
4000
-0,956
Et 1
300
4000
200
+4,00
Parter
325
325
400
-0,956
440
400
20
=
=
400
B
700
20
A
400
Ø37
65
225
3500
800
220
4500
700
225
400
300
440
541
-0,596
10
459
596
200
-0,01
-0,596
-0,956
=
-4,550
65
500
A
225
=
220
225
65
800
4700
4700
4900
B
C
4700
D
Detail - fixing column
4900
E
F
Figure 4.4. Non-oscillating capsulated SERB isolation device 960 x
960 x 230 prototype. The experimental hysteresis horizontal load
diagram for +/- 150 mm. Preload 1200KN and 1800KN
Analyzing the hysteresis diagrams it results that on horizontal
direction, SERB devices do not build-up potential energy for
a pre-set relative displacement because they have a constant
load-displacement characteristics, followed by a non-linear
increase of load with the displacement and a large energy
dissipation on the hysteresis loop.
On vertical, SERB devices are practically stiff but with high
damping capacity.
Figure 4.1. Cross–section view of the Detritiation Installation
Building.
Figure 4.2. Non-oscillating capsulated SERB isolation device 960 x
960 x 230 prototype.
Figure 4.5. Non-oscillating capsulated SERB isolation device 960 x
960 x 230 prototype. The experimental hysteresis vertical load
diagram. Preload 600KN.
8
Copyright © 2009 by ASME
5. RESULTS
Consider that the isolating device allows a limited friction
movement on horizontal plane. The movement law is given by
  g  sign (x ) , where: x - the
the equation: x  u
 - the ground
relative acceleration; x - the relative velocity, u
acceleration and  - the friction coefficient.
The input used for this analysis is a synthetic accelerogram
with PGA = 0.2g and corner period, TC = 0.7s. The timehistory ground acceleration is compatible with the ground
response spectra on Romania Cernavoda NPP site.
The calculations were made for two isolation periods: T = 3TC
= 2.1s and the maximum isolation period (allowed by the
Eurocode8, [1] and Romanian code, [2]), which is T = 3s.
The effective damping ratio was chosen as 5% (only for
comparisons) and the maximum value (allowed by [1] and
[2]), which is β = 30%.
For a comparative analysis of the isolation system efficiency,
herein below a calculation was run to determine the seismic
response of a structure for an oscillating and non-oscillating
isolation system employing the same input conditions.
To compute the relative displacement for the base isolation
oscillating solution, a linear time-history integration method of
a SDOF with 5% and 30% viscous damping ratio was used.
Three ways of calculation were employed: 1) SAP2000
computer code; 2) hand calculation using the Romanian
seismic code, /2/, and, 3) SITON-numerical method.
Tables 5.1 and 5.2 present a comparison between the relative
displacements obtained by two base isolation solutions: an
oscillating isolation solution and a non-oscillating friction
sliding solution (the case of Type3 with no viscous damper),
with μ = 0.05 and 0.1.
isolation, the relative displacement value obtained by the
oscillating isolation gives a higher value than the friction
sliding solution, within a broad range of friction coefficients.
The time-history absolute acceleration for an oscillating
system with T = 3s, β = 5% and β = 30% (the standard method
today) shows a maximum value of 1.3m/s2 and 0.94m/s2,
respectively, while the sliding non-oscillating system gives a
maximum value of 0.5m/s2 for μ = 0.05 and a maximum
value of 1m/s2 for μ = 0.1.
The results of the analysis point-out the following:
i) The increase of the oscillating isolation system damping
leads to the decrease of the relative displacements between the
infra-structure & supra-structure but the isolation effect is
getting lower due to the increase of the supra-structure
accelerations. This phenomenon is developed at the nonoscillating systems as well, by the increase of the friction
coefficient, but the extent is smaller;
ii) The evaluation of the maxim relative displacements with
the oscillating systems may be affected by errors because of
the calculation procedures employed in the current computer
programs and also of the limits imposed by the design
prescription.
β = 30%
μ = 0.1
Table 5.1. The relative displacement for the oscillating solution.
Relative displacement, [cm]. SAP2000
β(%)
T = 2.1s
T = 3.0s
5
22.5
29.5
30
9.8
16.1
Relative displacement, [cm]. P100/2006
β(%)
T = 2.1s
T = 3.0s
5
20.5
29.3
30
11.3
16.1
Relative displacement, [cm]. SITON
β(%)
T = 2.1s
T = 3.0s
5
22.6
29.6
30
9.8
16.3
μ = 0.05
β = 5%
Figure 5.1. Time-history relative displacement for oscillating
system with T = 3s, β = 5% & β = 30%, and for sliding nonoscillating system with μ = 0.05 & μ = 0.1.
β = 5%
Table 5.2. The relative displacement for the friction sliding solution.
Relative displacement, [cm]
μ = 0.05
μ = 0.1
13.9
3.5
All the three methods used in the calculation of the relative
displacement considering the base isolation oscillating
solution, give close results. In case of the non-oscillating
friction sliding solution, depending on the friction coefficient,
the relative displacement shows values lower than with the
oscillating solution. In case of a 3s period of the oscillating
β = 30%
μ = 0.05
Figure 5.2. Time-history Absolute acceleration for
oscillating system with T = 3s, β = 5% & β = 30%, and for
sliding non-oscillating system with μ = 0.05.
9
Copyright © 2009 by ASME
In our opinion, from the isolator calculation view point, the
computing of the oscillating system employing generated
accelerograms rises a significant problem.
The farther the isolation system is from the amplification area
(TB – TC period range), the larger the relative displacement is.
In other words, if the isolation period had not been limited at
3s, the relative displacement would have been larger and
larger. We think that this fact appears because the generated
time-history acceleration includes long period components
inside. In the other hand, the initial velocity, V0, is not equal to
zero, (like many computer programs uses) and a component
proportional with V0 remains unbalanced in the solution of
relative displacement, [11-12].
6. CONCLUSIONS
The paper is a brief analysis of the possible solutions needed
to be applied in order to make a good anti-seismic protection
of all kinds of buildings.
A number of 6 types of alternative solutions are described,
pointing out the advantages and weak points specific to any
solution:
 Elastic behavior of the building (classic solution);
 Plastic hinges in the building (classic solution);
 The control, limitation and damping of relative level
displacements by hydraulic dampers or mechanical devices
with damage (modern solution);
 The control, limitation and damping of relative level
displacements by telescopic devices with non-linear
strengthening and large damping (innovative solution);
 The seismic isolation of the building by oscillating devices
(modern solution);
 The building seismic isolation by non-oscillating devices
(innovative solution);
The actual modern and innovative solutions applied in many
countries is the base isolation method that cuts-off the seismic
ground motion transfer to the buildings, accomodating small
relative deflection of the structure and very low accelerations.
Within this seismic protection method the two isolation
solutions are presented: with oscillating devices and with nonoscillating devices.
This paper presents the new non-oscillating & capsulated
seismic isolator systems - SERB 960x960x220 devices – and
an isolation solution for the detritiation building at Romania
Cernavoda NPP while maintaining the pressure vessel capacity
beneath the isolation surface.
The paper demonstrates by calculation that the relative
displacement and the absolute acceleration for non-oscillating
systems have lower values than with oscillating systems with
friction coefficient in the range of 0.05-0.1.
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