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Claudio CECCOLI, Pier Paolo DIOTALLEVI, Marco SAVOIA, Claudio MAZZOTTI
and Tomaso TROMBETTI1 6800$5<
In the present paper, a seismic isolation system with HDRB is designed for a monumental
building, the ex Caserma Zucchi in Reggio Emilia. A seismic hazard analysis is first performed, in
order to estimate the peak ground accelerations for given return periods. A FEM analysis is then
performed, using artificial earthquake accelerograms generated from EC8 elastic response
spectrum as well as national and international acceleration records. For the rubber bearings,
equivalent linear idealization in the preliminary design stages of the isolation system and an
endochronic hysteretic model for non linear analyses are used. The base isolation system is shown
to be adequate for all the seismic inputs considered, drastically reducing in the masonry
superstructure both accelerations and torsional effects due to non regular stiffness distributions.
1
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Seismic isolation with rubber devices like HDRB (high damping rubber bearing) systems has been successfully
used in civil engineering in the last two decades. Initially adopted for bridge and viaduct isolation, it has been
more recently used for new civil constructions of strategic importance, such as public buildings, hospitals, etc. In
fact, peak accelerations on the constructions are strongly reduced when a correctly designed base-isolation
system is used, so avoiding, or at least strongly reducing, damage in structural elements, architectural
components and contained goods. Torsional oscillations can be also strongly reduced, because the center of
stiffness of base isolation system can be made close as possible to the projection of the center of mass, so that
base isolation can be recommended also for structures which are irregular in elevation or plane distribution.
Such benefits (reduction of accelerations in the superstructure and of torsional effects, effectiveness for irregular
buildings) make base isolation particularly attractive for retrofit of existing buildings. Nevertheless, very few
applications of base isolation can be found at present for ancient or monumental buildings or, at least, with
historical-artistic value, for three reasons: 1) The design of the isolation system requires particular care in a very
detailed relief of existing resisting elements in order to design the under-foundation system which must assure
indeformability of the isolator plane; for instance, the presence of contiguous but not connected portions of the
building or of foundations at different levels must be carefully considered; 2) Details of the executive design, of
intervention phases and of procedures for isolator maintenance are very important in order to assure no damage
in superstructure during base isolation retrofit execution; 3) The limitations by national authorities for
preservation of buildings with artistic value. Therefore, base-isolation retrofit of monumental buildings, for the
reduced number of realisations until now and for their specificity, always constitutes a specific case.
Nevertheless, to conduct a correct base-isolation design, a series of procedures of design and verification must be
followed that, in their general lines, can be considered common to every proposal of realisation of base-isolation
interventions.
In the present paper the design procedures of a base-isolation system for monumental buildings are illustrated
with reference to the ex Caserma Zucchi of Reggio Emilia (Italy). This building represents a typical case of
building where base-isolation can be considered advantageous. Such building has, in fact, a remarkable
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Point 1
Point 4
Point 5
Ground floor
y
x
Point 2
Point 3
First floor
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importance in the city of Reggio Emilia because the building is isolated from other constructions and its future
destination demands a greater seismic protection, since it will house the forthcoming Faculty of Engineering.
Besides, it does not contain artistical or historical goods (frescos, floorings, etc.) of such relevance to constitute
insurmountable ties from the point of view of the maintenance.
In the preliminary stages of analysis, an hazard study of the site has been performed, in order to define the peak
ground accelerations corresponding to given return periods. Then, five artificial earthquake accelerograms have
been generated matching the elastic response spectrum given by EuroCode 8. Some national and international
accelerograms have been also considered in the analysis.
Starting from detailed geometric and structural reliefs and from in situ experimental tests conducted before the
intervention of structural recovery (presently in progress), a finite element modelling of the building has been
performed. Both base-isolated and not isolated structures have been studied. For the HDRB isolators, two
different idealizations have been used. In the preliminary design stage, in order to optimize the disposition of
isolators, an equivalent linear behaviour has been considered. In the verification phase, where accelerograms
with the higher peak ground accelerations have been used, a non linear endocronic model has been used for the
isolators, whereas linear finite elements have been considered for the superstructure. This choice is motivated by
the circumstance that, when a building is correctly isolated, accelerations in the superstructure must be small.
The designed isolation system has been shown to be effective for all the categories of earthquakes considered in
numerical analyses.
Sect. A – A'
Sect. C – C'
Sect. B – B'
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The ex Caserma Zucchi is situated very close to the historical centre of Reggio Emilia, near of the old city wall.
The building is placed inside a small park, with no adjacent buildings. It has a C-shaped plan and three floor
levels. The construction of the central portion of the building began in 1845 and it was finished in 1853.
Originally, in the basement of the central body few continuous walls where present because arcades where
present to allow the market of the livestock. After 1877 the cattle market was transformed definitively in
barracks. The consequent most remarkable intervention, subsequent to the original construction of the building,
has been the filling of arcades of the ground floor with masonry, and dividing walls in correspondence of the
(D)
12
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10
[m / s2]
t=100 years
anni
t=200 years
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t=475 years
anni
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0.8
0.5
8
6
Elastic
spectrum
Spettroresponse
di risposta
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0.4
4
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value
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dei 5 spettri
2
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0
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0.33 0.4
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1
0
1
2
3
4
5
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side wings were also built. Besides, the recent construction of a reinforced concrete staircase, inside the preexisting staircases on the west side of the building (on the right in Figure 1), has substantially modified the
dynamic behaviour of the building. In the last few months the structural restoration began and the building will
house the new Faculty of Engineering.
The principal body and the lateral wings have plane dimensions, respectively, of 121 m × 22 m and 22 m × 22
m; the total surface of the building is about 3630 m2. The vertical structure is constituted by masonry column
(with circular section those in the central part of the building and rectangular section those in the perimeter of the
structure) positioned to the vertexes of a rectangular mesh of dimensions 7.2 m × 5.5 m for the central part of the
principal body and for the side bodies of 7.2 m × 7.2 m in the intersection of the three principal part of the
building (Figure 1). The first floor reaches 7.7 m from the ground and is sustained by cross vaults with circular
arcades 7.2 m wide (Figure 2). The second floor, placed at 13.6 m, and the coverage, placed at 19.0 m, have a
wooden carrying structure. The overall dimensions of walls, columns and vaults are those typical of buildings of
that century.
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Many accelerograms have been considered, with different characteristics, to investigate possible anomalous
behaviours of the isolated structure. The seismic inputs adopted in numerical analyses can be summarised in the
following categories:
1. Artificial earthquakes defined according to Euro Code 8 prescriptions;
2. Earthquakes recorded in the past near the building area;
3. Earthquakes recorded in Italy and characterised by significant intensity;
4. Earthquakes of international reference (earthquakes broadly used in literature);
5. Near source earthquakes, characterised by low frequencies energy content.
Such choice follows the usual precedures adopted in the actual practice of design of relevant interest. As an
example, seismic verification of tall buildings in Japan is performed using record of accelerations of earthquakes
of international reference (El Centro, Taft and Parkfield are the most used earthquakes), local strong earthquakes
(for instance Tokyo 1956), near source earthquakes (e.g. Hachiunohe 1968) and artificial earthquakes obtained
from the analysis of historical data about past local seismic activities (Omika and Sugano, 1982).
The following criteria have been followed to select acceleration records from earthquakes defined according to
the previously defined categories:
1. $UWLILFLDOHDUWKTXDNHV
Five artificial earthquakes, generated by the program SimQKE (Gasparini HW DO, 1976), have been used in the
preliminary stages of design of base isolation system. The peak ground acceleration (PGA) to be adopted has
been evaluated making use of statistical methods. In recent studies conducted by some of the authors on the
recorded historical local earthquakes (Ceccoli HW DO, 1999a), the cumulative density functions of probability
(CDF) of the PGA for the city of Reggio Emilia corresponding to different periods of observation have been
obtained (see Figure 3a). For instance, using as period of observation of 475 years (in agreement with EC8
provisions) and considering the 50 percentile of the corresponding CDF, a value of the maximum ground
acceleration of 0.33 g is obtained (see Figure 3a). The artificial earthquakes, characterised by a PGA equal to
0.33 g and called SimQKE033 in the following, have been therefore produced starting from indications given by
the National Seismic Service (SSN, 1993) and using the elastic spectra provided by the EC8. The main
prescriptions are the following: the maximum amplification of the elastic spectra equal to 2.5, the duration 7 of
the stationary part of the accelerograms produced is a function of the peak ground acceleration (equal to 21.5 s in
the present case), the average value of the elastic spectra obtained from the artificial earthquakes must be greater,
in the range of periods characterised by the maximum amplification, to the value given by Code provisions
(Figure 3b).
2. (DUWKTXDNHVUHFRUGHGQHDU5HJJLR(PLOLD
In the area of Reggio Emilia some seismic events characterised by peak ground acceleration of modest entity
(around 0.15-0.20 g) have been recorded in the last two decades. Such events must be considered as result of the
normal sismogenetic activity of this area. In view of the high probability that the Ex-Caserma Zucchi, during its
own useful life, can be interested from analogous seismic events, the seismic input related to the earthquake
recorded in Reggio Emilia in 1996 (Decanini et al, 1997) has been included in the analysis. Such local
accelerogram is used for verifying that no damage occur with seismic events that can be considered frequent.
3. 1DWLRQDOHDUWKTXDNHV
To investigate the structural behaviour under seismic inputs of considerable intensity (and therefore with respect
to the limit conditions), the recordings of earthquakes of high magnitudo in Italy (or in the immediate proximity)
have been used. In particular, Tolmezzo (1976) and Montenegro Bar (1979) earthquakes have been considered.
4. ,QWHUQDWLRQDOUHIHUHQFHHDUWKTXDNHV
In order to allow a comparison between the seismic behaviour of the Ex-Caserma Zucchi with that of other
structures retrofitted by base isolation, the analysis has been carried out also considering records of earthquakes
often used in international literature. In particular, seismic events of Parkfield (1966) and El Centro (1940) have
been considered.
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Seismic base isolation can result poorly effective if the isolated structure is subjected to seismic input
characterised by high low frequencies energy content. This aspect has been studied in several analysis because of
its importance in a correct seismic design (see for instance Inaudi and Kelly, 1993; Ceccoli HW DO, 1999b).
Earthquake accelerograms with high low frequencies energy content can be recorded mainly in two conditions:
(D) in correspondence of particular geological conditions (alluvial deposits of elevated thickness and weakly
compacted, as in the case of the Mexico City earthquake in 1985); (E) in proximity of the seismic epicentre (the
so-called near field earthquakes). Although the geological features of Reggio Emilia area are different from
those described above, near field earthquakes have been considered for completeness of analysis. Among the
near-field earthquakes, the accelerogram recorded at Lucern Valley (Northridge earthquake, 1992) has been
used.
4
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The seismic behaviour of the structure, both in the base isolated and in the original condition, has been studied
using the FEM code SAP2000 (1998). A commercial code has been used to develop analysis that can also be
performed within the normal design activity. Such program not only allows for linear analysis, but also analysis
where the non linear behaviour is restricted to a limited number of degrees of freedom, with evident advantages
from the computational point of view (Wilson, 1998). In the present case the non linear behaviour is
concentrated mainly in the isolation devices, simulated using endocronic laws (Wen, 1976).
To describe the global behaviour of the building without requiring a strong increase in the computational effort,
every single structural element (beams, columns, building walls, floors, etc.) has been modelled with only one
linear finite element, for a total of 220 beam elements and 750 4-nodes plate-elements (Figure 4). The walls with
openings have been modelled with equivalent continuous plate-elements with the same global volume and an
equivalent thickness. The presence of stairs inside the staircases has been considered by imposing, at each floor
level, no distortion of the staircase cross-section. Similar considerations have been made to simplify the wooden
floor structure. Finally, all external and internal walls have been considered like carrying walls, due to their
important stiffening effect towards the horizontal actions.
The results of several experimental in-situ measurements, carried on the structure before the intervention of
structural recovery, have been used to characterise the mechanical properties of the different materials. These
properties are summarised in Table 1.
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Materials
Mass density (kg/m3)
Density
(kN/m3)
Elastic Modulus (MPa)
Poisson’s coefficient
Bricks
200
20
5060
0.20
5
Wood
80
8
10000
0.30
Concrete
250
25
25300
0.20
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Figure 4 shows the finite element model and the first three modes of the fixed-base structure with the
correspondent periods. Such values fall in the central part of the EC8 design spectra where the dynamic
amplification reaches its maximum value. The first mode shows that the building length in the [-direction and
the consequent slenderness of its central body, together with the elevated deformability of wooden floors, causes
horizontal displacements in the central body much bigger than those in the lateral side wings of the structure.
The artificial accelerograms SimQKE033 have been applied to the structure in the longitudinal direction (the [direction in Figure 1). Analytical results show that the peak acceleration at the end of two side bodies of the
building (Point 2) is twice the peak acceleration obtained in the lateral staircases (for instance Point 4). The same
earthquakes applied in the orthogonal direction (\-direction) produce the maximum peak acceleration in the
central part of the building. For instance, at the roof level (Point 5) the displacement is more than 7 mm and the
correspondent peak acceleration is very high, as can be observed from results reported in Table 2.
Significant displacements, transverse to the earthquake direction, have been registered during dynamic analyses
with international reference earthquakes (described in Paragraph 3) acting in \-direction. In these cases
transverse displacements can reach 15-20% of longitudinal displacements.
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The fundamental objective of base isolation systems with HDRB is the attainment of a fundamental period of the
building close to 2 seconds or more, thus limiting the dynamic amplification factor. In a preliminary phase of
analysis the isolation devices have been considered having a pure shear deformation with linear elastic
behaviour. In this phase, the optimal position of isolation devices and their geometrical and mechanical (in the
linear range) properties have been defined.
As a first isolation proposal, HDRB isolators with equal stiffness properties have been placed under all the
columns and the intersections of the masonry walls. The total stiffness of the complex of isolators under the
building has been obtained, at this design stage, prescribing a suitable value for the vibration period to the
isolated structure, considering the superstructure as a simple d.o.f. system with the total mass of the building.
Following the SSN (National Seismic Service) Guidelines (1993), the vibration period of the isolated structure
has been set equal to 2 seconds. The damping index for the isolators has been assumed equal to 10-15 per cent, a
typical value for this kind of isolation devices. The lateral stiffness required to the set of base isolators has been
found equal to 1300 kN/m.
The linear modal analysis underlined a significant torsional-translational coupling, due to the eccentricity of the
stiffness center with respect to the center of mass, because placing isolators with equal stiffnesses under all the
wall intersections results in a too high number of isolators under the staircase. Hence the first proposal has been
modified by attributing to isolators under the staircase a reduced stiffness, equal to 75 per cent of that of those
prescribed for the main body of the building.
Figure 5 shows the first three vibration modes and periods of the isolated structure. The first mode is a torsional
mode, while the second and third ones are substantially translational modes. The vibration periods are slightly
higher than 2 seconds originally prescribed in the rigid body scheme due to the (small) deformation of the
superstructure. The isolation system can be considered effective, since it increases the first period of the structure
from 0.18 seconds (fixed-base) to about 2 seconds in the isolated case.
6.1
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The seismic behavior of the isolated structure has been studied, using the artificial accelerograms described in
Paragraph 3. For earthquakes acting along [-direction, the motion of the isolated building is almost purely
translational, due to the small eccentricity between stiffness and mass centers. The average of accelerations
obtained adopting the five artificial SimQKE033 accelerograms are reported in Table 2 and compared with
analogous results obtained for the fixed-base structure. It can be observed that base isolation is very effective,
strongly reducing accelerations in the superstructure. For instance, considering accelerograms acting along [direction, accelerations are more than 16 m/s2 in some nodes of the fixed-base structure and reduce to 1.5 m/s2
due to base-isolation. Moreover, maximum accelerations are approximately equal in all significant positions of
the structure, so confirming the rigid body behavior of the superstructure. Analogous considerations can be
drawn for earthquakes acting along \-direction. Base-isolation is then able to solve the problem of high
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flexibility of the central body, which caused high acceleration amplification in the non isolated structure.
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-direction (m/s2)
Fixed-Base
Base-Isolated
structure
structure
8.17
1.55
16.31
1.61
11.52
1.57
7.14
1.53
7.97
1.52
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Fixed-Base
Base-Isolated
structure
structure
7.58
1.43
7.73
1.54
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1.52
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1.32
16.65
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2
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HDRB isolators are characterised by a typical strongly non linear shear-displacement relation, which depends on
their dimensions and aspect ratio. In the current practice, bilinear model are often adopted to describe the
hysteretic shear-displacement relation (Figure 6), where different stiffnesses are introduced to model small and
large shear strains. Stiffnesses depend on both mechanical characteristics and vertical load applied to isolation
devices; for instance, for high compressive loads the stiffness of the second linear branch is usually smaller, thus
modifying the original hysteresis loop. In the present analysis, an endochronic model (Wen, 1976) has been
adopted to define the hysteretic behaviour of isolation devices; endochronic models require the assignement of
two stiffness values (for small and large strains) and some parameters to describe the transition between the two
different behaviors (see Figure 6).
The parameters of the endochronic model have been defined in order to have stiffness comparable with that
defined in the linear analysis. For this purpose, an equivalence method between linear and bilinear behaviour has
been defined. In fact, the usual criterion of adopting the secant stiffness of hysteresis loops at the maximum
displacement that will be attained during earthquake motion underestimates the device stiffness. Some numerical
investigations (see for instance Ceccoli HWDO, 1999c) states a different equivalence method, which has been tested
using several international accelerograms and different isolation systems. This criterion suggests the equivalence
of a linear and a bilinear model having the same force at 50 per cent of maximum displacement and a 15 per cent
of equivalent damping ratio. Of course, this criterion requires a preliminary evaluation of the expected maximum
displacement of the isolation devices.
Following this criterion, the endochronic model has been chosen by setting an initial stiffness equal to 5750
kN/m and the stiffness of the seconds branch equal to 1/10 of the initial stiffness. The intersection point of the
two branches corresponds to a 57 kN shear force and a 10 mm displacement.
V Shear
Taglio
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xmax
Spostamento
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The non linear analysis of isolated and non isolated structures have been performed adopting the accelerograms
described in section 3.
As an example, Figure 7 shows displacements and accelerations at node 2 (see Figure 1) adopting one of
artificial accelerograms SimQKE033. The behaviors of base isolated and fixed base buildings are compared. In
Figure 7b the horizontal line represents the maximum earthquake peak acceleration; base isolation system
provides for a drastic reduction of the accelerations in the superstructure.
In Figure 8 the ratios between the peak acceleration at the roof level (node 2 and 5) and the correspondent peak
input acceleration of earthquakes are reported. With reference to artificial earthquakes, a strong reduction of
peak acceleration can be observed, with maximum accelerations in the superstructure about 5 per cent of that
corresponding to fixed base condition. The effectiveness of base isolation system is smaller for near source
(Parkfield) earthquake or Mexico City earthquake, characterised by high low-frequency components. Tolmezzo
earthquake, characterised by high accelerations, short duration and an high high-frequency energy contents,
causes very high accelerations at the non isolated structure. In this case, base isolation system is very effective
strongly reducing accelerations in the superstructure.
Further information can be obtained by observing the distribution of maximum accelerations and displacements
along the building height, obtained from one of the five SimQKE033 accelerograms. Two nodes in the central
body of the building (node 5) and in the staircase (node 4) have been considered. The reduction of both drift
displacements and accelerations due to base isolation is evident. Moreover, the slenderness of the central body of
the building is the cause of the difference of displacements in the two nodes for the fixed base case. On the
contrary, in the isolated structure the accelerations are almost constant along the building height, thus confirming
the accuracy of the hypothesis, used in the preliminary design phase, that the superstructure behaves as a rigid
body over the isolation devices. Moreover, accelerations reach about the same values the whole structure,
because a correct base isolation systems is able to strongly reduce the torsional effects.
8
&21&/86,216
0.010
0.009
0.008
0.007
0.006
0.005
0.004
0.003
0.002
0.001
0.000
-0.001
-0.002
-0.003
-0.004
-0.005
-0.006
-0.007
-0.008
-0.009
-0.010
Fixed base
Edificio
incastrato
Edificio isolato
Isolated
2
Accelerazione [m/s ]
Acceleration
Spostamento [m]
Displacement
In the present paper the effectiveness seismic retrofit with base isolation with HRDB of a monumental building
(the ex-caserma Zucchi in Reggio Emilia) has been discussed. The adopted criteria for preliminary design and
verifications can be summarised in five main stages: 1) Definition of the seismic hazard in the construction site
and evaluation of the peak ground acceleration (PGA) related to a given earthquake return period and selection
of input earthquake accelerograms; 2) Preliminary design of base isolation system, considering linear elastic
behaviour of isolation devices, in order to define a correct isolator distribution under the building and the
corresponding (linear equivalent) stiffness in order to reach a prescribed value for the period of the isolated
structure; 3) Structural analysis considering linear behavior for the superstructure and a non-linear endocronic
model for isolation devices whose stiffness characteristics are defined according to equivalence criteria; 4)
Computation of stress resultant in superstructure structural elements and verifications; 5) Design of executive
details and procedures to realize from the constructive point of view the isolation system as well as to allows for
a periodic maintenance.
0
5
10
15
20
25
30
35
18
16
14
12
10
8
6
4
2
0
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-4
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-8
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-12
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Fixed
Edificiobase
incastrato
Edificio isolato
Isolated
0
5
10
15
(D)
20
25
30
Tempo
Time [s]
Tempo
Time [s]
(E)
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5 .0 0
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Isolated
building
4 .0 0
3 .0 0
2 .0 0
1 .0 0
Lucern Valley
Mexico City
Parkfield
El Centro
Bar
Montenegro-
(D)
Tolmezzo
SimQKE033
Reggio Emilia
0 .0 0
6.00
Fixed
base
building
Edificio
incastrato
5.00
Isolated
Edificio building
isolato
4.00
3.00
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spostamento
displacement
spostamento
displacement
accelerazione
acceleration
Node
4
Nodo
4
Fixed
baseincastrato
building
Edificio
Edificiobuilding
isolato
Isolated
Node
5
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accelerazione
acceleration
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The last aspect, not illustrated in the present paper, is mainly related to technological problems; operations
needed to insert isolation devices under monumental buildings can be quite expensive and complex in some
cases, but they are analogous to procedures for foundation repairs of masonry buildings. In fact, retrofit
operations with base isolation require, in a transition phase, bearing elements (walls and columns) to be
supported by means of temporary foundations. Standard but effective techniques can be adopted for continuum
elements (walls): partial excavation must be carried out by assuring the self-bearing of portion of masonry walls.
Columns present much more difficulties because the vertical loading must be temporarily transferred from the
old foundation to provisional adjacent foundations. The new foundations must be designed to carry all the
vertical loads (with supports constituted by isolation devices) and to assure high inplane stiffness, for instance
with a grid of beams joining continuous foundations under masonry walls and discrete column foundations.
ACKNOWLEDGEMENT
The financial contribution of MURST-40% grant (Title "Protezione sismica dell’edilizia esistente e di nuova
edificazione attraverso sistemi innovativi", Prot. 9708186721_008) is gratefully acknowledged.
REFERENCE
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