THE IMPROVING OF THE SEISMIC PERFORMANCE OF EXISTING OLD PUBLIC UNREINFORCED MASONRY BUILDINGS

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THE IMPROVING OF THE SEISMIC PERFORMANCE OF
EXISTINGOLD PUBLIC UNREINFORCED MASONRY
BUILDINGS
Ion VLAD Professor, Romanian National Center for
Earthquake Engineering and Vibrations,
Technical University of Civil Engineering,
Bucharest, Romania
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CONTENT

• MAIN FEATURES OF STRONG EARTHQUAKES IN ROMANIA
• MASONRY BUILDINGS IN ROMANIA. A SHORT HISTORY.
• CASE STUDY. TECHNICAL ASSESSMENT.
• 3.1 Short presentation
• 3.2 Architectural description of the
  building
• 3.3 Structural description
• 3.4 Elastic base shear force
• 3.5 Mode of failure of the masonry walls
• 3.5.1 Nominal value of the medium
  compressive stress
• 3.5.2 Demands for the seismic action
• 3.6 The conclusions of the technical
  assessment
• STRUCTURAL STRENGTHENING SOLUTION
• 4.1 The introduction of the
  spectral position concept
• 4.2 Structural concepts of the
  strengthening solution
• 4.3 Structural analysis of the
  strengthening solution
• 4.4 Quantitative results
• SEISMIC ISOLATION
• CONCLUSIONS

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1. MAIN FEATURES OF STRONG EARTHQUAKES IN
ROMANIA
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1.1 Seismotectonics and seismicity of Romania
Romania is one of Europe's most seismically
active regions, together with other Balkan and
Mediterranean countries (Bulgaria, Turkey,
Greece, former Yugoslavia and Italy). The
seismic activity of Romania is considerable.
There are (conventionally) nine distinct foci
seismic regions. Among these, the most
important are Vrancea, Fagaras, Banat and
Dobrogea. Vrancea is by far the most seismically
active region of Romania, placed around the
curvature of the Carpathian Mountains.
5
Recent strong earthquakes and the seismic
zonation map in force (in terms of MSK
intensities).
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• The seismic hazard of Romania consists of two
  types of earthquakes
• subcrustal (intermediate) earthquakes of
  moderate to large magnitudes (6lt MW lt7.5), with
  depth of focus ranging between 60...170 km
  (seismic region Vrancea)
• crustal earthquakes less active and less
  intensive with MW66.5 (the other eight distinct
  seismic regions).
• The upper limit of magnitudes for Vrancea
  earthquakes is considered to be MW ? 8.0.
• Focal depths of 90...150 km are particular for
  Romania, thats why not too much international
  interest exist for this type of earthquakes.

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March 4, 1977 Vrancea earthquake

• MG-R 7.2
• the first strong motion recorded in Romania was
  the triaxial accelerogram obtained on a SMAC-B
  type strong motion accelerograph the peak ground
  acceleration values were
• N-S 0.20 g E-W 0.16 g V 0.10 g
• a glance at the record shows that the long period
  components were predominant, aspect that
  surprised the engineering community of Romania
  the highest values of periods occurred in the
  range of 1.0...1.6 s for the N-S component, and
  of 0.7...1.2 s for the E-W component
• untill the 1977 seismic event, the design was
  based on the elastic spectra shape that had been
  imported from the Soviet code SN-8-57 (?0 3.0
  TC 0.3 s), which, at its turn, corresponded to
  the 1940 El Centro earthquake spectra.

8

• The difference in shape of the response spectra
  between far-field (Bucharest) and near-field (El
  Centro), as well as the shift of the spectral
  maxima is obvious.
• It is to be expected that the damage should occur
  especially for the flexible buildings, having
  fundamental eigen-periods of vibration greater
  than 1 sec.

9
August 30/31, 1986 Vrancea earthquake

• MG-R 6.9
• the maximum peak ground acceleration value was
  close to 0.3 g (recorded in Focsani, near the
  instrumental epicenter)
• PGA values in Bucharest ranged between 0.06 g and
  0.16 g (for the N-S component) and between 0.04 g
  and 0.11 g (for the E-W component)
• the values of observed periods ranged between
  0.7...1.1 s
• the 1986 accelerogram recorded at the same
  location as in 1977 (INCERC, Bucharest) had PGA
  values of 0.10 g (E-W component) and 0.09 g (N-S
  component), with periods of about 1.1 s
• this fact supports the idea that intermediate
  depth earthquakes tend to produce motions
  characterized by longer periods when their
  magnitude increases.

10
May 30 and 31, 1990 Vrancea earthquakes.

• MG-R 6.7 for May, 30th, 1990
• MG-R 6.1 for May, 31st, 1990
• 5 stations recorded PGA values larger than 0.20g
  
• PGA values in Bucharest ranged between 0.07 g and
  0.14 g (many records of the main shock on the E-W
  direction were stronger than on the N-S
  components - opposite to the previous two seismic
  events)
• the values of the observed periods have been much
  shorter this time.

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2. MASONRY BUILDINGS IN ROMANIA. A SHORT
HISTORY.
As in most countries of the world, buildings
with structural systems masonry walls type,
were the most frequent ones used in the past.
For many centuries masonry buildings have been
designed by using some practical rules derived
from well defined ratios among the dimensions of
the main structural elements, based on experience
acquired over the years. The beginning of the
XX century marked the introduction of new
materials in the structural systems of masonry
buildings, such as reinforced concrete and steel.
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• Before 1880 most of the buildings were of
  traditional shape, with load-bearing brick walls,
  and floors and roofs made of timber joists and
  wooden planks, without any provisions for
  horizontal forces.
• Between 1880?1910 a series of heritage
  buildings were built in Romania, with storey
  heights of 4.56 m, having wider spans covered by
  brick vaultlets supported by steel beams
• Between 1910?1920 the use of reinforced
  concrete in floors and frames was initiated the
  effect of wind was, sometimes, taken into
  consideration.
• Between 1920?1940 an important number of
  reinforced concrete residential buildings were
  achieved by applying the German technical
  legislation (DIN 1045/1932, 1937).
• Between 1940?1954 reinforced concrete members
  replaced gradually wooden, steel, or brick
  components of floors and lintels, so that after
  1950 these disappeared almost entirely.

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2.1 Short on Romanian seismic legislation
The year of birth of Romanian earthquake
engineering is considered the year of 1940.
Following the 1940 seismic event (MG-R 7.4),
preliminary instructions regarding the earthquake
resistant design of the reinforced concrete and
masonry buildings were published by the Ministry
of Public Works and Communication (1941). After
the Second World War, the same institution
developed new technical guidelines titled
Instructions for preventing the damage of
buildings located in seismic zones (1945).
Later, in 1963, the first Romanian seismic code
was published. This code underwent successive
modifications in 1970, 1978, 1981, 1990, 1992 and
2006.
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3. CASE STUDY. TECHNICAL ASSESSMENT.

• General view of the National History and
  Archeology Museum
• in Constanta

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Original "drawings"
The building was designed by a Romanian
architect in 1911, and the project consisted only
of a few architectural drawings. The founding
stone of the building was set in 1911 and the
building was built in several stages (because of
the First World War), being completed between
1919 and 1921.
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3.2 Architectural description
The shape in plane of the building can be
inscribed in a rectangle having its sides equal
to 35 m and 45 m respectively. Its configuration
consists of four wings, which realize an in-plane
tubular shape, generating a central perimeter
(16m 16m) of interior courtyard type. The
building has a general basement of about 68 m
height, a ground floor of about 6.0 m height, a
partial mezzanine occupancy of about 30 of the
ground floor space, two more storeys of 5.0 m
height, respectively 4.0 m, and an attic of 3.0 m
height. The attic-storey can be found only on
three of the four sides of the building (N, E,
W).
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The dominant architectural element consists of
the main façade, marked by a slight withdrawal of
the entrance area in regard with the façades
plane, but also by a vertical detachment of the
central volume, ended with an octagonal tower
with a clock, of open turret type. The cupola
of the tower is sustained by eight arches
supported by eight reinforced concrete pillars.
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3.3 Structural description
The overall structural system of this building
consists of the superstructure, the
substructure, the structure of foundation and the
foundation medium.
The superstructure comprises the storeys
situated above the ground-floor ground-floor,
partial mezzanine, first floor, second floor,
attic and the tower.
The vertical component of the structural system
of the superstructure consists of structural
masonry walls, disposed along four axes on the
longitudinal direction (axes 1, 4, 7 and
10) on the transversal direction (axes A,
C, E and G).

• The main structural deficiencies of the vertical
• components of the structural system are
• irregularities in disposing door and window
  openings,
• together with the variability of the
  dimensions of these
• openings
• the fact that the structural wall horizontal
  section areas
• differ on the two main directions of the
  building
• there are also irregularities of the structural
  walls
• horizontal sections, at each storey, on the
  vertical
• direction.

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The horizontal component of the structural system
of the superstructure consists of four floor
structures with steel girders and
reinforced-concrete plates. The floor of the
mezzanine is a reinforced-concrete one, with a
small area. The floor above the first level is
incomplete (an area of about 150 m2 situated
between axes F-G and 4-7, in the Adrian
Radulescu Hall zone is missing).

• The main structural deficiencies of the
  horizontal
• components of the structural system are
• the limited floor area of the mezzanine storey
  (a
• later structural modification) represents a
  local zone of
• irregularity which affects the structural
  walls stiffness
• and contributes to an eccentric distribution
  of masses
• the lack of a floor area at the first storey
  created by the
• existence of the Adrian Radulescu Hall can
  lead to
• important damage in this part of the building
  during an
• earthquake
• the lack of a RC floor at the attic storey.

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The substructure of the building is 6 to 8 m high
and consists of stone masonry walls, constituting
the general basement. The structure of the
foundation consists of continuous stone cyclopean
concrete walls type of approximately 10m height,
beneath all the substructure walls (this
information was taken from the National Archive
documents of Constanta and from the press at that
time, and was confirmed in 2008 by performing a
geotechnical study).
22
Instrumental investigations of the National
History and Archeology Museum Constanta
overall stiffness of the building
Location of sensors
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Time domain and corresponding amplitude Fourier
spectra (ambient vibrations).
24
Amplitude Fourier spectra (vertical direction)
and auto-correlation functions (transversal
direction) ambient vibrations.
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After performing the entire program of
instrumental investigations the following results
have been obtained

• the fundamental eigenperiod on the longitudinal
  direction of the building was T1,L 0.32 s,
  while the fundamental eigenperiod on the
  transversal direction was T1,T 0.35 s
• on the basis of auto-correlation functions of the
  recorded signals, it turned out that the values
  of the fraction of critical damping pertain to
  the interval 34 (being quite low compared with
  those obtained for similar buildings of brick
  masonry)
• the shape in plane of the building also led to
  rotational motions and modal coupling
  (T1,TORSION 0.26 s).

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3.4 The elastic base shear force
was estimated according to the Romanian
P100-1/2006 code(an EUROCODE 8 version).

• The seismic characteristics of the Constanta city
  area are
• the peak ground acceleration value for a
  reference period of 100
• years
• ag 0,16 g m/s2
• the corner period for structural systems with
  behavior in the
• elastic range
• TC 0,7 s
• the dynamic amplification factor
• ?0 2,75

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• The coefficient of the base shear force has
  resulted
• The elastic base shear force has resulted

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• Nominal value of the medium compressive stress
• The nominal value of the medium compressive
  stress ?0 due to gravity loads was obtained

• Demands for the seismic action
• The nominal medium tangential stress values
  ?0,demanded, taking into account
  QB,CODE,elastic, have resulted
• - for the longitudinal direction (Amasonry100
  m2)

- for the transversal direction (Amasonry 65
m2)
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• Expected mode of failure of the masonry walls
• By structural analysis, the following tangential
  stress values (at the first floor) were obtained
• ?0,resistant 0.25 N/mm2

Comparing this value with the ?0,demanded it has
resulted - on the longitudinal direction
?0,resistant(0.25 N/mm2) lt ?0,demanded
(0.58 N/mm2) - on the transversal direction
?0,resistant (0.25 N/mm2) lt ?0,demanded
(0.87 N/mm2).

• CONCLUSIONS

• the structural system of the building doesnt
  resist in the elastic
• range of behavior to the shear force
  established according to the
• seismic code in force
• the mode of failure of all structural elements
  is of brittle type.

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• Nominal degree of seismic assurance R

For the computation of the nominal degree of
seismic assurance R the following relations are
used - on the longitudinal direction
- on the transversal direction
These values of R, smaller than R 0.5,
mainly on the transversal direction, justify and
impose the strengthening of the building, on both
directions.
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4. STRUCTURAL STRENGTHENING SOLUTION. THE
SPECTRAL POSITION CONCEPT

• By spectral position it is understood the pair
  of values represented by the fundamental
  eigenperiod (Tn,1) and the base shear force
  coefficient (cB,y), corresponding to the maximum
  strength capacity offered by the structural
  system, considering the associated mechanism of
  plastification.
• This new concept was conceived by the Romanian
  designer, eng. Emilian Titaru.
• For the museum building the spectral positions
  correspond to the following characteristics
• on the longitudinal direction Tn,1 0.4
  s cB,y 0.20
• on the transversal direction Tn,1 0.4 s cB,y
  0.13.
• The cB,y values correspond to the brittle mode of
  failure of the existing building.

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The pairs of values Tn,1 and cB,y placed the
structural system of the building in unfavorable
spectral positions of the inelastic response
spectra. For a period of vibration Tn,10.4 s
and for the two values of cB,y (0.20 and 0.13),
large values of displacements can be
observed. These unfavorable spectral
positions, on both directions, led to
exaggerated values for the required ductility
factors.
Inelastic displacement response spectra
33
Displacement response spectra (Constanta city
May 30th, 1990)
34
Energy input response spectra (Constanta city
May 30th, 1990)
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4.2 Structural concepts of the strengthening
solution

• The spectral principle of the strengthening
• The spectral principle of the strengthening
  solution can be expressed, as follows for
  improving the safety of the building to strong
  future seismic actions, its unfavorable
  spectral position must be changed to a
  favorable spectral one.

• Structural concepts of the strengthening
  solution
• The design strengthening solution consists of the
  introduction of a subsystem of coupled reinforced
  concrete walls disposed along the perimeter of
  the existing building interior courtyard.

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• The strengthening subsystem of reinforced
  concrete walls, by the interaction with the
  masonry structural walls of the existing
  superstructure, will assure
• by its stiffness it will increase the overall
  structural stiffness of the building, thus
  obtaining a shortening of the fundamental period
  of vibration, Tn,1
• by its strength capacity it will increase the
  value of the indicator of the strength capacity
  of the overall superstructure cB,y
• the new composed structural elements will have
  enough strength, stiffness and ductility, so that
  damage during a future strong earthquake be
  avoided.

strengthened walls
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• Structural models of analysis

Transversal direction
Longitudinal direction
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• 4.3 Quantitative results
• The introduction of the strengthening
  subsystem of coupled reinforced concrete walls
  will have the following two main effects
• the shortening of the eigenperiod of vibration of
  the strengthened building in comparison with its
  value before strengthening, as follows
• on the longitudinal direction (direction parallel
  to axes 4 and 7)
• Tn,1 0.26 s (Tn,1,measured 0.4 s)
• on the transversal direction (direction parallel
  to axes C and E)
• Tn,1 0.30 s (Tn,1,measured 0.4 s).

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• the decrease of the values of the base shear
  forces in the initial superstructure, as follows
• on the longitudinal direction the base shear
  force will be reduced to 35.5, compared to its
  value before strengthening
• on the transversal direction the base shear force
  will be reduced to 54, compared to its value
  before strengthening.

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It was arrived to a value of the indicator of
the strength capacity of the overall
superstructure cB,y 0.25, and thus to
acceptable values of displacements.
Inelastic displacement response spectra
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5. SEISMIC ISOLATION
5.1 Seismic surprises

• November 10, 1940 Vrancea earthquake

• Two aspects surprised the engineers and the
  seismologists of that time
• the seismic intensity of the ground motion in the
  Bucharest area
• the damage of the high buildings of flats with
  8...12 levels the fact that only one of them
  collapsed was considered as an accident and not
  a rule.

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• March 4, 1977 Vrancea earthquake

• Two aspects should be emphasized
• the specialists in the field of constructions and
  the seismologists stated that they were
  surprised by the fact that the seismic motion
  was so strong
• after the processing of the only accelerographic
  record obtained during the earthquake, the
  foreign researchers were surprised by the
  configuration of the Romanian earthquake seismic
  spectra.

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5.2 Seismic isolation in Romania

• The base isolation technique is still in its
  infancy in Romania.
• The author of this paper wishes that, at the
  occurrence of the next strong seismic motion in
  Romania, the behavior of the buildings base
  isolated would not represent another
  surprise.

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5.3 Risks considering the safety of the buildings
to whom base isolation methods will be applied

• These risks are generated by
• insufficient knowledge of some of the
  characteristics of intermediate earthquakes that
  occur in Romania
• insufficient knowledge of seismic isolation
  devices related to the earthquake peculiarities
  in Romania.

• How to avoid these risks
• compensatory measures in the design process, by
  passing from the Muto principle enough strength
  and high ductility to the new one, specific for
  seismic isolation little strength and very long
  fundamental period
• supplementary safety measures for seismic
  isolation devices.

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5.4 Peculiarities of Vrancea strong motion versus
seismic isolation in Romania

• A first characteristic that differentiates
  earthquakes occurring within the Romania
  territory from earthquakes occurring in other
  parts of the world is the focal mechanism (the
  focal depth, the frequency content of the seismic
  motion, the seismic waves directivity, the
  occurrence rate of the strong motions, the
  returning period etc.).
• The second characteristic specific to the Romania
  territory is given by the depth of the
  sedimentary layer. In Bucharest the sedimentary
  deposit is of about 1.5 km, while in the Vrancea
  region it is over 6.5 km.
• The third very important characteristic of the
  Romania earthquakes consists in the persistence
  of the Vrancea foci position, a characteristic
  that pertains exclusively to this seismogenic
  zone.

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In the next slide are presented, comparatively,
the input energy spectra for the Vrancea type
earthquake (Bucharest, March 4, 1977) and for the
Imperial Valley (El Centro, May 18, 1940)
earthquake. It can be noticed that the maximum
value of the input energy for the Vrancea
earthquake (T1.6s) is powerfully put to
evidence, being of about 2.4 times bigger than
the maximum value of the input energy
corresponding to the El Centro earthquake (T0.4
s T0.9 s T 2.8 s).
47
Input energy spectra for the Vrancea type
earthquake (1977) and El Centro (1940)
48
The input energy spectrum corresponding to the
accelerogram recorded in the Galati town (1990)
shows the existence of several values, but its
maximum value, well focused, corresponds to a
period value equal to 3.2 s.
Input energy spectra (Galati, 1990)
49
Comparison of the SD spectra (1977, Vrancea and
1940, El Centro earthquakes)
One can notice that the displacements in case of
Vrancea earthquake computed for the city of
Bucharest start being large and very large for
values of periods T longer than 1 s. Instead, the
SD spectrum computed for the El Centro earthquake
has reduced values for periods in the range 1?2
s, which start to grow after the period value
equal to 2 s.
50
Comparison of the SA spectra (1977, Vrancea and
1940, El Centro earthquakes
One can notice that the accelerations in case of
Vrancea earthquake, computed for the city of
Bucharest, have large values in the range of
periods up to 2.5 s. Instead, the SA spectrum
computed for the El Centro earthquake shows that
the acceleration values strongly diminish for
periods longer than 1.2 s.
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The goal of base isolation is to reduce the
seismic forces that are exerted by an earthquake
on a building structure. Thats why the building
which is going to be seismic isolated must be
placed in a zone of the SA spectra of specific
locations with convenient periods. At the same
time, values of horizontal displacements that the
isolators must undergo should be taken into
consideration. At the design of a seismic
isolation for El Centro type earthquakes, the
seismic forces can be reduced by placing the
building in the period range of 1.2?2 s. At the
same time, for this period range, the horizontal
displacements that the isolators must undergo are
reduced, of about 10...12 cm. In contrast with
the above presented case, the design of a
seismic isolation in Bucharest, for Vrancea
type earthquakes, the seismic forces can only be
reduced by placing the building in the period
range over 2.5 s. The straight consequence of
being obliged to place the building in the zone
of very long periods consists in the fact that
the isolators that are to be used must assure
horizontal displacements of the order 40...45 cm.
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5.5 Some conclusions on seismic isolation

• The seismic isolation of buildings in Romania is
  strongly dependent on the seismic peculiarities
  of the seismic action generated by Vrancea
  earthquakes. The crucial problem in performing
  base isolation seems to be that of isolators. We
  consider as primary objective to create a
  prototype of an isolator to perform well to large
  displacements that are characteristic for Vrancea
  earthquakes.
• The countries where isolating systems of the base
  have been developed are countries with particular
  accelerograms of El Centro type. In order to be
  able to use the concept of seismic isolation in
  Romania, particularly in Bucharest, I consider
  that isolators that allow horizontal
  displacements of about 45 cm are necessary.
• The elaboration of a design code for seismic
  isolation of buildings, under the peculiar
  seismic conditions of Romania

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6. FINAL CONCLUSIONS

• I considered that the strengthening of this
  monumental old unreinforced masonry building is
  engineering in its purest form.
• The relationships and responsibilities of the
  structural engineer, in comparison with other
  participants in the strengthening and
  rehabilitation process, are unique.
• It was found out that the building has the
  tendency of localizing damage at the first level,
  with the development of a soft and weak first
  level effect (situation which corresponds to a
  possible general progressive collapse).
• The strengthening subsystem of coupled reinforced
  concrete walls will lead to the results that have
  been already presented.

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• The links between the existing brick masonry
  walls and the reinforced concrete structural
  walls, together with those with the existing
  floors of the building, will be assured by a
  proper adherence, by using chemical anchors
  bonded in drilled holes with polymer adhesives,
  masonry injection and carbon fiber cords
  (depending on the actual situation which will be
  revealed in situ).
• On the basis of the experimental and analytical
  investigations carried out so far, one can
  conclude that the problem of seismic resistance
  of old masonry buildings can be handled by means
  of adequate technical methods (traditional ones,
  modern ones using seismic isolation, or both).
• Due to the special status of the building, the
  strengthening solution had to be chosen so that
  its character of historical and architectural
  monument should not be affected.

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Thank you for your attention!