João Pedro Guilherme Alves. Instituto Superior Técnico, Department of Civil Engineering

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1 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, 1

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

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. 2

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

3 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. SOUTH EAST WEST NORTH NORTH 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 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 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;

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. E m [GPa] γ [KN/m 3 ] 2,0 9,07 Figure 3.4 General view of the model; comparison with the real structure. 4

5 3.3. DYNAMIC CHARACTERISTICS OF THE STRUCTURE Only the first vibration modes obtained are important to characterize the behaviour and 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 translate in frequencies associated to the two very similar 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 walls. Table 3.2 Periods, Frequencies and participation mass rations of the first three modes. Mode Period [s] Frequency [f] Ux Uy ΣUx ΣUy Rz ΣRz 1 0,422 2,37 2 0,389 2,57 3 0,251 3,99 6,49% 80,07% 6,49% 80,07% 16,41% 32,47% 67,82% 9,06% 74,31% 89,13% 32,47% 48,88% 14,41% 0,16% 88,73% 89,29% 39,09% 87,97% 3.4. DAMAGE SUFFERED 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 misalignment clearly noticeable (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 [Oliveira, 2008]. Figure 3.5 Diagonal cracks in the short columns. According to Eurocode 2 ( EC2 ) [CEN, 2003], the shear resistance for particular 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 Elastic Response Spectrum- EC8 vs RSA EC8 Response Spectrum Type II 1,5x RSA Response Spectrum Tipe 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: 6

7 P9.2 -Vy P9.2 -Vy [m] Structure with masonry walls [EC8] Structure without masonry walls [EC8] Structure without masonry walls [RSA] [KN] [m] Figure 3.7 Efforts analysis for P9.2. RSA EC [KN] 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 postearthquake 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] 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 7

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]. Base shear force (KN) 1,20 Modal -X 1,05 Uniforme -X Top Displacement(m) Base shear force(kn) 1,20 Modal -Y 1,05 Uniforme -Y Top Displacement(m) 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: 8

9 Analysis Case 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) dx (m) Fx Modal Uniform - 0,0613-0, ,0536-0,0804-0,1012 0,1046 Fy Modal 0,0840-0,1260-0,105 Uniform 0,0802-0,1203-0, DESIGN AND DIMENSIONING OF REINFORCED CONCRETE STRUCTURES ACCORDING TO EUROCODE 8 This section intends to put into practice the concepts of analysis and design according to EC8 in the present case of study. The structure will be dimensioned in accordance to Capacity Design methodology for DCM structures, expressed in a detailed way in the EC8. The objective is to check 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 Torsional System, with a reference behaviour factor q0 of 2,0. Knowing that this value should be reduced by 20%, because of the vertical irregularity of the building, the final value for the behaviour factor is: Design Response Spectrum - ζ=5% EC8 Response Spectrum Type II 1,5x RSA Response Spectrum Tipe Figure 3.9 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. In fact, the application of the criteria strong column/ weak beam, which requires a lower resistance for

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]. (A) 5 Ø25 + 4Ø20 5Ø20 (B) 5Ø12 (C) Y X Ø16 2Ø Ø Figure 3.10 Comparison between the different types of analysis: (A) Capacity Design; (B) Direct Dimensioning and (C) Real section 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. 10

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: 3,50 3,00 Uniforme - X [Real Structure] Uniforme - X [Strengthened Structure] Base shear force [kn] 2,50 2,00 1,50 1, Top Displacement [m] 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. 11

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. 12

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, Costa, A.A.; Arêde, A.; Costa, A.; Oliveira, C.S. [2008]. Estudo experimental in-situ de paredes em alvenaria do Faial, Açores. Sismo Açores. Uma década depois. Edição C.S. Oliveira et al., Governo dos Açores/SPRHI, S.A. Eurocode 8 [2004] EN : Eurocode 8: Design of structures for earthquake resistance Part 1: General rules, seismic action and rules for buildings, CEN, Brussels, Belgium, 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, SAP2000 [CSI, 2003], V11.0.2, Berkeley, California, USA; July

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