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STUDY OF THE DYNAMIC BEHAVIOUR OF THE FIRE DEPARTMENT IN LAJES, PICO
(AZORES)
BASED ON THE JULY 9, 1998 EARTHQUAKE
João Pedro Guilherme Alves
Instituto Superior Técnico, Department of Civil Engineering
Abstract: The main goal of this work is to study the behaviour of a building located in a region
where the seismic hazard is very high, more specifically, the Fire Department in Lajes, Pico
(Azores). This building suffered some local damages during the July 9, 1998 earthquake and it is
important to understand what caused that kind of damage. It will be analysed, using the static and
dynamic analysis and Eurocode 8 [CEN, 2004], to see if the structure has other kinds of problems
that could compromise the response in future seismic activities. Some sort of interventions, with the
purpose of repairing and improving the response of the structure, will be suggested.
Keywords: Infilled Walls; Short Column; Capacity Design; Pushover Analysis; Seismic Retrofit of
Reinforcement Concrete Structures.
1. INTRODUCTION
The catastrophic consequences of recent seismic activities have began to alert general population,
leading to the research and development of analysis methods and techniques that could minimize
the economic and human damage due to seismic events. One of these techniques can be the
study of damages that structures have suffered due to a seismic action. This study allowed the
establishment of a group of principles that provide a good behaviour for structures against
horizontal forces. If a structure has a large capacity of energy dissipation, through post elastic
deformations of their members, it can be dimensioned for inertia forces under those that would
develop if all the structure remained in the elastic range.
The Fire Department in Lajes, Pico (Azores) is a one-storey building with an apparent structural
simplicity but it is located in a region where the seismic hazard is very high. During the July 9, 1998
earthquake the structure suffered some damages in two outside columns (diagonal cracks).
Apparently, the reason of these damages was the use of secondary elements in undesirable
places, more specifically the partial filling of masonry walls near the reinforcement concrete
elements, causing local and negative effects.
With the aim of understanding the behaviour of the whole structure in response to ground shaking
and discover what might have caused that damage, an analytical model was defined in the
computer program SAP2000 [CSI, 2006]. In order to evaluate the performance of the building,
2
several analyses were made, namely the linear response spectrum and time-history analysis and
also the static nonlinear pushover analysis.
2. SEISMIC CONTEXT
The July 9, 1998 earthquake caused great damage in the areas close to the epicenter where
intensity evaluation attained VIII in EMS-98 scale. Attenuation of waves is not regular with distance
with places of anomalous higher intensities such as in Almagreira close to Lajes do Pico where
intensity was about 2 degrees above the average in the region. The earthquake shaking was
recorded at Observatório Príncipe de Mónaco in Horta (Figure 2.1) with a PGA of 0,4 g. This
location, a pyroclastic hill, shows an important site effect.
Figure 2.1 – Distribution of damages of the July 9, 1998 earthquake [Oliveira et al., 2008].
3. CASE OF STUDY - FIRE DEPARTMENT IN LAJES, PICO (AZORES)
3.1. DESCRIPTION OF THE STRUCTURE
As seen in Figure 3.1, the structure is aligned almost in the NS and EW directions, and is located in
opposite side, in relation to the strong motion (sm) record site, of the possible fault trace.
Figure 3.1 – Location of the Fire Department (black circle); the star indicates the epicenter.
The building in study is a special structure, very similar to the current fire department buildings in
this region, with two distinct spaces: one for the offices and rooms and the other reserved to keep
the vehicles. These two parts are separated by a dilatation joint. The garage, with approximately 10
meters high, was the object of the study (Figure 3.2). It is oriented north and is symmetrical
regarding y axis, presenting a big open space, with no intermediate floors.
Figure 3.2 – General view of the garage. Figure 3.3 – plant and orientation of the façades.
The North façade shows a big opening, giving the chance to park the vehicles while the South
façade is mostly close, showing only some openings for windows on the top. The West and East
façades are practically equal, differing in the disposal of the masonry walls (the West façade has
no openings), which are going to influence directly the response of the structure.
The materials of the structure are concrete B.20.1 and steel A400NR. The masonry panels are
composed by blocks of concrete masonry with mechanical properties determined by experimental
shear tests as referred in the next section.
3.2. MODELING
The building was modeled with relative simplicity because the structure does not present many
difficulties and all the elements are regular and simple. Although, and due to the origin of the
cracks, masonry walls were modeled to translate the real behaviour of the structure in terms of the
seismic response.
3.2.1. MODULATION OF THE MASONRY WALLS
There are several proposals to modulate the walls, present in the past by researchers and that can
be divided into three groups [Proença et al., 2007]:
• Trough diagonal rods connecting the panel;
• Trough springs;
SOUTH
NORTH
EAST WEST
NORTH
4
• Trough continuous models or finite elements.
The first method is the most commonly used. The width of the equivalent frame a, presented by
Mainstone (1971), can be determined by the following equation:
� � 0,175 � � ��,� � (3.1)
Where the H is the height of the columns between beams, D is length of the diagonal and λ it’s a
non-dimensional parameter, calculated trough the expression:
� � ��� � sin 2�4 � � � (3.2)
Where:
Em – Modulus of elasticity of the masonry panels;
t – thick of the panel;
θ – angle of the diagonal rod;
E – Modulus of elasticity of the pier;
I – Inertia of the pier;
h – height of the wall.
In the present work two rods per panel will be utilized, where the width of each rod is a/2. The
mechanical properties of masonry walls are presented in
Table 3.1 – Mechanical properties of masonry walls comonly used in Azores.
Em [GPa] γ [KN/m3]
2,0 9,07
Figure 3.4 – General view of the model; comparison with the real structure.
3.3. DYNAMIC CHARACTERISTICS OF TH
Only the first vibration modes obtained are important to characterize the
dynamic response of the structure, because of the non concentration of mass at the center of the
roof. The lack of vertical elements with great stiffness should tra
the two very similar main translation modes. It is
should present a relevant torsional component because of the asymmetrical distribution of the
walls.
Table 3.2 – Periods, Frequencies and participation mass rations of the first three modes
Mode Period
[s]
Frequency
[f]
1 0,422 2,37
2 0,389 2,57
3 0,251 3,99
3.4. DAMAGE SUFFERED BY
As already referred, the garage structure suffered moderate damage at the two exterior columns of
the West façade with a diagonal cracking descending to the South in entire length of the short
column. A close view of the crack points out to a shear crack 0
misalignment clearly noticeable
shear pulse with orientation as indicated in
the main ground pulse was towards Nor
Island. This information is of most relevance to explain the mechanism of fault rupture
2008].
Figure
According to Eurocode 2 (“EC2”) [CEN, 2003],
HARACTERISTICS OF THE STRUCTURE
Only the first vibration modes obtained are important to characterize the behaviour
structure, because of the non concentration of mass at the center of the
roof. The lack of vertical elements with great stiffness should translate in frequencies associated to
main translation modes. It is also important to refer that these two modes
should present a relevant torsional component because of the asymmetrical distribution of the
Periods, Frequencies and participation mass rations of the first three modes
Frequency Ux Uy ΣUx ΣUy Rz
6,49% 80,07% 6,49% 80,07% 16,41%
67,82% 9,06% 74,31% 89,13% 32,47%
14,41% 0,16% 88,73% 89,29% 39,09%
BY THE STRUCTURE
As already referred, the garage structure suffered moderate damage at the two exterior columns of
the West façade with a diagonal cracking descending to the South in entire length of the short
column. A close view of the crack points out to a shear crack 0.5 cm width causing a horizontal
(Figure 3.5). The orientation of the crack points out to a larger
shear pulse with orientation as indicated in the second figure below with red arrows. It means that
the main ground pulse was towards North, the same direction as reported in various places in Faial
Island. This information is of most relevance to explain the mechanism of fault rupture
Figure 3.5 – Diagonal cracks in the short columns.
(“EC2”) [CEN, 2003], the shear resistance for particular element is 114kN.
behaviour and the
structure, because of the non concentration of mass at the center of the
in frequencies associated to
these two modes
should present a relevant torsional component because of the asymmetrical distribution of the
Periods, Frequencies and participation mass rations of the first three modes.
Rz ΣRz
16,41% 32,47%
32,47% 48,88%
39,09% 87,97%
As already referred, the garage structure suffered moderate damage at the two exterior columns of
the West façade with a diagonal cracking descending to the South in entire length of the short
.5 cm width causing a horizontal
. The orientation of the crack points out to a larger
with red arrows. It means that
th, the same direction as reported in various places in Faial
Island. This information is of most relevance to explain the mechanism of fault rupture [Oliveira,
element is 114kN.
6
3.5. DYNAMIC ELASTIC ANALISYS
For the definition of the elastic response spectrum it was necessary to consult the Nation Annex
regarding the Azores archipelago in order to obtain the necessary parameters for its
characterization. In this region it is only necessary to consider the Type 2 Seismic Action for the
analysis. According to the Annex, the case of study is located in zone 1, a zone where the peak
ground acceleration takes higher values. It is also considered the worst ground condition (for
Azores, it’s the type C), which is associated with a factor of soil S = 1,5.
In what concerns the importance class of the building, the structure was included in the group of
facilities that should remain intact and operational after a seismic event. Therefore, the building
was defined as a Class IV (γ1 = 1,4) structure. This factor will directly influence the definition of the
elastic response spectrum.
Figure 3.6 – Comparison of the two elastic response spectrums.
It is easy to conclude that, for this area and kind of structure, the seismic action proposed by the
EC8/National Annex induces bigger spectral accelerations than the RSA.
The influence of masonry walls in the primary elements will be carefully analysed. The main
differences between the efforts generated by both EC8 and RSA project actions will also be
compared. To observe the development of the efforts, a linear elastic analysis was performed
without consideration of any factor of behaviour.
The next figures show the shear diagrams VY, for the response spectrum analysis, related to the
diverse considerations that were made:
0.0
2.0
4.0
6.0
8.0
10.0
12.0
14.0
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Elastic Response Spectrum- EC8 vs RSA
EC8 Response Spectrum Type II
1,5x RSA Response Spectrum Tipe 1
7
Figure 3.7 – Efforts analysis for P9.2.
Some conclusions can be drawn with this analysis:
• When the masonry walls are not considered directly in the model and a RSA response
spectrum analysis is made, the value of the shear design effort, though lower, is very close
to the resistance value;
• On the other hand, when the same model (without walls) is submitted to a EC8 seismic
project action, the shear effort rises considerably, as a result of a more severely response
spectrum, causing the failures in the elements;
• When the walls are introduced in the model, the element shows, as expected, greater
efforts, both for the spectrum of the RSA as to the action of EC8. These efforts will be
higher than the shear resistance of the element, so that there is evidence of the
fundamental need of accounting directly or indirectly of such elements in the design.
Another kind of dynamic analysis can be performed. The time-history analysis is not normally used
for current design of structures but they can be very useful to study the effect on the affected post-
earthquake structures. In the case of study two different real records obtained during the 1998
action in Horta and the GZCAH, will be applied. The results of this analysis can be consulted in
[Alves, 2009].
3.6. PUSHOVER ANALYSIS
In current dimensioning, the nonlinear behaviour of a structure is considered by means of
behaviour factors. The problem is that this type of analysis cannot predict the development of the
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
-750 -600 -450 -300 -150 0 150 300 450 600 750
[m]
[KN]
P9.2 - Vy
Structure with masonry walls [EC8]
Structure without masonry walls [EC8]
Structure without masonry walls [RSA]
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
-750 -600 -450 -300 -150 0 150 300 450 600 750
[m]
[KN]
P9.2 - Vy
RSA EC8
8
damages in structures. As an alternative to the linear methods, the EC8 prescribes a non linear
static method that takes directly in account the nonlinearly response of the building, extremely
useful to evaluate the structural performance of both new and existing buildings. More information
about this methodology can be seen in other works like [Bento, R. et al., 2000].
In the present case of study, the nonlinear behaviour was directly accounted in the model, by
defining a moment-curvature relationship for every section of the building. After that, it was
important to obtain the capacity curves of the structure for the two main directions.
The capacity curves (Figure 3.8) represents the relation between base shear force and control
node displacement. The top displacement comes from the application of different distributions of
horizontal loads until a certain rupture criterion is met. The adopted criteria for the two directions
and the description of the rest of the methodology are presented in [Alves, 2009].
Figure 3.8 – Capacity curves for the different load patterns adopted.
The application of the method recommended in the EC8 results in target displacements for both
loads and directions, which should be compared with the maximum obtained by the capacity
curves. These results are presented in the following table:
0.0
150.0
300.0
450.0
600.0
750.0
900.0
1,050.0
1,200.0
0.000 0.025 0.050 0.075 0.100 0.125 0.150
Ba
se s
he
ar
forc
e (
KN
)
Top Displacement(m)
Modal - X
Uniforme - X
0.0
150.0
300.0
450.0
600.0
750.0
900.0
1,050.0
1,200.0
0.000 0.025 0.050 0.075 0.100 0.125 0.150
Ba
se s
he
ar
forc
e(K
N)
Top Displacement(m)
Modal - Y
Uniforme - Y
Table
Analysis Case
Target Displacement
dy (m)
Fx
Modal
Uniform
Fy
Modal 0,0840
Uniform 0,0802
3.7. DESIGN AND DIMENSIONING OF REINF
EUROCODE 8
This section intends to put into practice the
the present case of study. The structure will be dimensioned
methodology for DCM structures,
if it’s possible to accomplish all the requirements of the new code maintaining the current
conceptual design of the structure.
It was considered that the structure can be assimilated to a Flexible Torsion
reference behaviour factor q0 of 2
of the vertical irregularity of the building,
Figure 3.9 – Comparison of the two
It was very difficult to apply all the prescriptions of the EC8 for the current design of the structure.
fact, the application of the criteria
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
0 0.5 1
Table 3.3 – Final results of the pushover analysis.
Target Displacement 150% x Target Displacement
C.C. Displacement
dy (m) dx (m) dy (m) dx (m) dy (m)
- 0,0613 - 0,0920 -
- 0,0536 - 0,0804 -
0,0840 - 0,1260 - 0,105
0,0802 - 0,1203 - 0,109
IMENSIONING OF REINFORCED CONCRETE STRUCTURES ACCORDING TO
This section intends to put into practice the concepts of analysis and design according to E
the present case of study. The structure will be dimensioned in accordance to Capacity Design
structures, expressed in a detailed way in the EC8. The objective is to check
ible to accomplish all the requirements of the new code maintaining the current
conceptual design of the structure.
It was considered that the structure can be assimilated to a Flexible Torsional System, with a
of 2,0. Knowing that this value should be reduced by 20%,
of the vertical irregularity of the building, the final value for the behaviour factor is:
Comparison of the two design response spectrums.
It was very difficult to apply all the prescriptions of the EC8 for the current design of the structure.
criteria strong column/ weak beam, which requires a lower resistance
1 1.5 2 2.5 3
Design Response Spectrum - ζ=5%
EC8 Response Spectrum Type II
1,5x RSA Response Spectrum Tipe
C.C. Displacement
dy (m) dx (m)
0,1012
0,1046
-
-
TURES ACCORDING TO
concepts of analysis and design according to EC8 in
Capacity Design
objective is to check
ible to accomplish all the requirements of the new code maintaining the current
System, with a
0. Knowing that this value should be reduced by 20%, because
It was very difficult to apply all the prescriptions of the EC8 for the current design of the structure. In
a lower resistance for
3.5 4
Response Spectrum Type II
x RSA Response Spectrum Tipe 1
10
the extreme sections of the beams that converge on a particular node to the columns that converge
in the same node, resulted in columns heavily reinforced (more than the reinforcement allowed in
accordance to the codes). This criteria is much easier to apply when the dimensions involved
(height and width of the beam and column, respectively) are the same; which was not the case in
all the nodes of the building.
The next figure shows the huge difference between the sections of the column P9.2 in: A)
Application of the Capacity Design philosophy; B) Direct Dimensioning with the EC8 Design
Response Spectrum; and C) section of the real column. More results of this analysis can be found
in [Alve., 2009].
0.6
0
0 .30
0.6
0
0 .30
(A)
X
Y
5 Ø25 + 4Ø20
2Ø16
3 Ø12
0.6
0
0 .30
5Ø20
2Ø16
(B) (C)
5Ø12
Figure 3.10 – Comparison between the different types of analysis: (A) – Capacity Design; (B) – Direct
Dimensioning and (C) – Real section.
3.8. RETROFIT OF REINFORCED CONCRETE STRUCTURES
There are several interventions that could solve the local problems that the structure presents. The
following techniques can be used to rehabilitate this kind of damage:
• Total filling of the masonry walls near the cracked columns;
• Separate the walls of structural elements: the introduction of a joint could improve a better
behaviour of the structure.
• Give more bending and shear resistance to the damaged elements with more
reinforcement.
Due to the lack of strength and stiffness of the building, observed in the pushover analysis that was
made, an intervention that could minimize the global problems that the structure presents was
studied. It will only be considered a solution for the X direction, with the introduction of reinforced
concrete walls, oriented precisely in this direction.
11
Figure 3.11 – Global view of the strengthened structure.
The next figure shows the difference between the capacity curves of the real structure and the new
one for uniform load:
Figure 3.12 – Comparison between the capacity curves of the real and new structure for uniform load.
The increase of stiffness of the structure in this direction is obvious: for similar values of top
displacement, the strengthened structure presents a base shear about two times larger than the
real structure.
The application of the N2 methodology in the new structure resulted in the following target
displacement:
�� ! � 0,03#
4. CONCLUSIONS
The aim of this work was to study the dynamic behaviour of the Fire Department in Lajes, Pico in
response to a seismic action. The several analyses that were employed allowed the identification of
phenomenon that caused the damages observed in the structure after the 9 of July, 1998 event.
0.0
500.0
1,000.0
1,500.0
2,000.0
2,500.0
3,000.0
3,500.0
0.000 0.050 0.100 0.150 0.200 0.250
Ba
se s
he
ar
forc
e [
kN
]
Top Displacement [m]
Uniforme - X [Real Structure]
Uniforme - X [Strengthened Structure]
12
For that, an analytical model was made in SAP2000 containing both structural and non structural
elements. The modulation of the last ones was very important to obtain coherent results.
The main conclusion was that this structure presents several local and global problems. The
building shows not only a great structural irregularity but also some singularities that can be
associated to fragile rupture mechanisms, more specifically the partial filling of the masonry walls
right next to vertical concrete structural elements. It was possible to observe clearly this interaction
on the elastic analyses that were performed.
In what concerns the pushover analysis, we can conclude that the structure does not present the
adequate resistance and stiffness in the two main directions: for the Y direction, the target
displacement for the seismic action obtained by the application of the N2 methodology was bigger
than the maximum displacement supported for that direction; in the X direction, despite of the
target displacement for this direction being smaller than the one that the building can support, it is
greater than the gap that exists between the case of study and the building next to it.
The next step of this work was to evaluate the new prescriptions of the EC8 when directly applied
in the case of study. The main conclusion that can be taken from this analysis is that the current
design of the building presents some flaws that avoid the full application of the methodology
recommended in EC8, known as Capacity Design. However, it is important to remember that the
structure was designed according to the requirements of the old national codes. So, it was not
expected that the current conceptual design could provide a good response, with logical and
coherent reinforcement in the concrete elements, specially the vertical ones.
In the last chapter of this work, potential interventions that could be made to the structure, to
minimize the problems that were found in the previous analysis, were studied. For the local
problems, some measures, that could avoid the negative interaction between the walls and the
primary elements, were proposed. In what concerns the global behaviour, the introduction of
concrete walls proved to be a good intervention, reducing substantially the displacements and
damages for a design seismic action.
13
REFERENCES
Alves, J. [2009] – “Estudo do Comportamento de Estruturas durante o Sismo dos Açores de
1998:Edifício dos Bombeiros nas Lajes” Dissertação para Obtenção do grau de Mestrado em
Engenharia Civil, Instituto Superior Técnico, Lisboa.
Bento, R., Lopes, M., [2000] “Modelação Fisicamente Não Linear de Estruturas de Betão Armado”,
Disciplina de Modelação e Análise Estrutural, Instituto Superior Técnico, 1999-2000.
Costa, A.A.; Arêde, A.; Costa, A.; Oliveira, C.S. [2008]. “Estudo experimental in-situ de paredes em
alvenaria do Faial, Açores. Sismo 1998 - Açores. Uma década depois”. Edição C.S. Oliveira et al.,
Governo dos Açores/SPRHI, S.A.
Eurocode 8 [2004] “EN 1998-1: Eurocode 8: Design of structures for earthquake resistance – Part
1: General rules, seismic action and rules for buildings”, CEN, Brussels, Belgium, 2004.
Proença, J., Manso, J., Guerreiro, L. and Oliveira, C. S. [2007] “Contributo das paredes de
alvenaria para o comportamento sísmico de estruturas de betão armado. Pesquisa e
recomendações para modelação e análise”, 7º Congresso de sismologia e engenharia sísmica,
Lisbon, Portugal.
RSA [1983] “Regulamento de Segurança e Acções para Estruturas de Edifícios e Pontes”,
Decreto-Lei nº 235/83, de 31 de Maio, Porto Editora, 2005.
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