Analysis of the seismic performance of the Cultural … of the seismic performance of the Cultural and Congress Center of Angra do Heroísmo Miguel Ramos Ávila This archipelago is

Analysis of the seismic performance of the Cultural and Congress Center of Angra do

Heroísmo

Miguel Ramos Avila

Civil Engineering Department, Instituto Superior Técnico, University of Lisbon, Portugal

ABSTRACT

The present dissertation focused on the analysis of an irregular reinforced concrete structure. The

structure belongs to a building located on Terceira Island, Azores, where seismic activity is a major

concern that has to be taken into consideration in the design of structures.

Within the modeling process, the irregular structure of the building motivated the search for the

best modeling framework, with the help of automatic calculation software and different structural

elements. This way, an investigation of the best methods for the manipulation of the structural elements

characteristics with the purpose of simulating their real behavior is performed.

The safety analysis of the building subject to seismic actions was carried out. The quantification

of the seismic action was made according to the provisions of Eurocode 8, and the behavior factor was

determined according to the structural characteristics of the building.

The resistant stresses have proved not to be sufficient to sustain the horizontal loads to which the

case study building is subjected. This was mainly due to the fact that this structure has been designed

based on legislation previous to EC 8.

1 – INTRODUCTION

1.1 – OBJECTIVES

The first part of this dissertation aims to present a brief historical background regarding the

seismic activity of the Azores archipelago, the region where the building is located. The second part

involves analyzing a relatively recent case study which consists of a frame structure system built in

reinforced concrete. The structural complexity of this case study motivates to explore different modeling

approaches of the structural elements of the building.

1.1 – Azores seismic activity

Since the beginning of its existence, humanity had to learn to cope with seismic activity.

Earthquakes have caused countless deaths over the past few centuries mainly due to the collapse of

buildings caused by the seismic induced accelerations that result from the tectonic movement.

The Portuguese mainland has been center of intense seismic activity of which there are historical

records, unlike the Azores Island, where few historical events have been recorded (Borges et al., 2008).

Analysis of the seismic performance of the Cultural and Congress Center of Angra do Heroísmo

Miguel Ramos Ávila

This archipelago is located in the junction area of three major lithospheric plates, shown in

Figure 1 (Machado, 1992). Over the years, several tectonic models have been proposed as an attempt to

explain the seismic activity in this region.

Figure 1 - Schematic model of the Azores tectonic plates (in Madeira et al., 1992)

Over the past few years, the analysis of seismic events in the archipelago islands has enabled the

researchers to classify them according to different degrees of seismic intensity, with the islands of São

Miguel, Faial and Terceira falling within the higher intensity group, with levels exceeding class V

(Nunes, 2008).

Over the past few centuries, some major seismic events have been recorded in the archipelago.

For instance, the earthquake that occurred on January 1, 1980 on Terceira Island should be highlighted, in

light of the case that it is the event on which this case study is based.

Angra do Heroísmo was the most affected city (Nunes, 2008), where it recorded a magnitude of

7 and intensity VII on the Modified Mercalli scale, as depicted in Figure 2 (Oliveira, 1992).

Figure 2 - Isoseismic lines of the earthquake that occurred in January 1, 1980 (in Oliveira, 2008)


Miguel Ramos Ávila

1.2 – TERCEIRA ISLAND BUILDING ARCHITECTURE

The predominant traditional building architecture of the Terceira Island did not have a

satisfactory behavior during the 1980 earthquake. It is estimated that, in total, the earthquake damaged

fifteen hundred houses, such as the cases illustrated in Figure 3.

Figure 3 - Angra do Heroísmo devastated by 1980 earthquake (in Guedes et al., 1992)

This construction, as evidenced by the picture above, was characterized by a poor quality

masonry, and it can be observed that most of the damage occurred in the upper floors, contrary to what

generally happens in reinforced concrete building structures, where most damages are commonly located

at the bottom floors (Guedes, 1983).

The 1980 earthquake brought significant changes to the city from an architectural point of view.

Many houses have been reconstructed, this time using concrete blocks, and built upon foundation beams

and peripheral straps that support the floor slabs and reinforced concrete cover. The design and

construction of new buildings, started to incorporate seismically resilient features such as reinforced

concrete beams and columns (Guedes, 1983).

Nearly all buildings constructed over the past fifty years were built using reinforced concrete,

following the standards set by the RSA (Regulation of Safety for Actions in Buildings and in Bridges)

and the REBAB (Regulation of Reinforced and Pre-stressed Concrete structures).

2 – CASE STUDY – CULTURAL AND CONGRESS CENTRE OF ANGRA DO

HEROISMO


Miguel Ramos Ávila

2.1 Case study

The building under consideration is a reinforced concrete construction built in 2003 and located

in the city center of Angra do Heroísmo (Figure 4). The building is used as a support for multiple cultural

activities and local authority services.

Figure 4- Location of the Cultural and Congress Center of the Angra do Heroísmo

2.2 Geometrical characteristics

The building under consideration is developed in plan with a circumference of 42.87 meters in

diameter and has a height of 18.5 meters.

In the peripheral zone of the building, beams were placed in conjunction with walls and cores, so

that they have better resistance to seismic actions.

Regarding the slab spans, the largest ones measure approximately 8 x 11 meters, which

corresponds to the lightened slab sections. With respect the solid slabs, the dimensions are approximately

7 x 4.5 meters.

The beams and slabs that can be observed in the following figures contain a span ranging

between 4 and 7 meters in average and its section varies between 0.38 and 0.6 wide and 0.55 to 1.46 in

height. However, some of the beams located in the bottom slab area, on the right hand side of the

building (Figure 25), have 14 meter spans, and therefore the sections measure approximately 60 x 80

centimeters at these locations.


Miguel Ramos Ávila

Figure 5 - Floor Plan 2 (beams with 14 meters to go red)

With respect to foundations, these were placed on a very uneven terrain due to the fact that the

structure was located on a hillside, which demands for a solution of foundations built up over different

levels. The building consists of support walls with heights ranging between 3.7 and 7.3 meters and

thickness between 0.44 and 0.46 meters along its periphery, following the slope where the structure is

located.

Figure 6 – Foundations plant of level -1 (left) and level 0 (right)

2.3 Calibrations

The modeling of the building was carried out using the commercial program SAP2000

(Computers and Structures Inc., 2008) and took advantage of the high structural complexity of the

building to explore multiple alternatives for modeling some of its structural elements.

The modeling of the flat slab using frame elements proved to be more advantages than the use of

shell elements. However, some calibrations are still necessary in order to simulate the real behavior of the

slab.

For calibration purposes an isolated model of a flat slab, discretized using shell elements was

created. In addition, a force was applied in both directions (Figure 5).


Miguel Ramos Ávila

Figure 7 - Illustration of a flat slab model (Sap2000)

Afterwards, a model of the same slab is created, but now built with the help of frame elements,

wherein the same force is applied at the same point. Table 1 summarizes the values of the percentage

errors for different multiplication factors of Inertia Iyy and shear area.

Inertia Iyy

Direction 0.5 Error 1 Error 2 Error 3 Error

Sh

ear

area

0.5 Dx (m) -0.07 -326.90 -0.04 -144.31 -0.03 -53.29 -0.02 -22.75

Dy (m) -0.03 -93.89 -0.02 -27.48 -0.01 6.11 -0.01 17.56

1 Dx (m) -0.07 -310.20 -0.04 -127.54 -0.02 -36.53 -0.02 -5.99

Dy (m) -0.02 -76.34 -0.01 -9.92 -0.01 23.66 -0.01 35.11

2 Dx (m) -0.07 -301.80 -0.04 -119.76 -0.02 -28.14 -0.02 2.40

Dy (m) -0.02 -67.94 -0.01 -0.76 -0.01 32.82 -0.01 43.51

3 Dx (m) -0.07 -299.40 -0.04 -116.77 -0.02 -25.75 -0.02 4.79

Dy (m) -0.02 -64.89 -0.01 2.29 -0.01 35.11 -0.01 46.56

Table Erro! Não foi especificada nenhuma sequência. - Displacements and respective percentage error for

different values of multiplicative factors

The support walls were also subjected to analysis within the course of the modeling exercise

with the purpose of creating frame elements that can properly simulate the stiffness of the structure. With

this, using the unit load method, the most suitable height and section values were determined so that the

frame element has the same bending and shear stiffness of a retaining wall.

The case study building includes a set of beams with curvilinear longitudinal axis (figure 6),

whose modeling was also subject of optimization and calibration.


Miguel Ramos Ávila

Figure 8 - Location of curved beams

These curves are modeled by several rectilinear elements distributed along the longitudinal

section of the beam. In this thesis, we tried to establish a minimum number of rectilinear sections that

would accurately simulate the beam alignments.

Similarly to what was done in the flat slab case, five models with different discretizations

(Figure 7) were created and two different load cases were applied (vertical and horizontal), so that the

percentage errors between each model could be determined.

Figure 9 - Model alignments AL1 e AL2 with increasing discretization (left to right)

The analysis of the percentage error regarding the displacements under horizontal forces in the

reference points, show the difference of the results for the a uniformly distributed load case, as well as the

displacement for the case of a horizontal load, it is concluded that the differences between models 4 and 5

are minimal and the results increasingly tend to converge with increasing number of rectilinear portions.

At the end of all these calibrations, and after all loads and structural masses were properly

inserted, a modal analysis was performed with the purpose of eliminating some of local vibrational

modes that originated from defective connections between the adjacent model elements. The final model

of the structure is displayed in figure 8.


Miguel Ramos Ávila

Figure 10 - Final model of the structure

2.4 Behavior factor

In order to choose the best alternative for the linear structural analysis of a building subject to

seismic action, Eurocode 8 provides a set of rules that aims to define the regularity status of the building,

both in height and plant. The choice of linear analysis as well as the type of model to use depends on

these regularly conditions. The behavior coefficient to adopt also depends on these same conditions.

From the structural analysis, it was concluded that the case study is irregular in both height and

plant. Therefore, the adopted behavior factor must be reduced by 20%.

Taking into account that the case study building is torsionally flexible, the base value for the

behavior factor is 2. Applying the reduction that has been previously mentioned, a behavior factor of 1.6

is assumed for the linear elastic analysis of the case study. The structure was considered to be of class of

importance 3, yielding and importance factor of 1.45.

3- DISCUSSION OF THE RESULTS

3.1 Dynamic characteristics of the structure

The characteristics of the first twelve modes of vibration of the structure are presented in Table

2.


Miguel Ramos Ávila

Mass Participation Factors

Mode Period (s) Frequency (cycles/s) ux uy ∑𝑢𝑥 ∑𝑢𝑦

1 0.431 2.319 0.011 0.223 0.011 0.223

2 0.301 3.317 0.004 0.000 0.015 0.224

3 0.238 4.196 0.056 0.150 0.071 0.373

4 0.177 5.649 0.465 0.000 0.536 0.373

5 0.173 5.797 0.000 0.007 0.536 0.381

6 0.164 6.111 0.000 0.005 0.536 0.385

7 0.150 6.679 0.004 0.001 0.540 0.386

8 0.146 6.856 0.002 0.002 0.541 0.388

9 0.145 6.877 0.003 0.001 0.545 0.389

10 0.135 7.387 0.000 0.181 0.545 0.570

11 0.129 7.724 0.001 0.002 0.546 0.571

12 0.123 8.148 0.037 0.009 0.583 0.581

Table 2 - Characteristics of the first twelve modes of vibration

As can be observed in the above table, for the first modes of the structure the mass participation

factors do not exceed, in both directions, 58%. One of the reasons for this is that a significant amount of

the structural mass is located next to the supports that simulate the ground. Consequently, vibration

doesn't occur for low frequency modes.

The first vibration mode of the structure is purely translational in the y direction, and a higher

flexibility is noticeable on the upper floors, wherein the stiffness of the building is lower.

The second vibration mode is a local vibration mode at the top beam, while the third mode is a torsional

mode of vibration. Pure translation along the x direction can only be perceived in the fourth vibration

mode

3.2 Safety verification

The safety verification of the structure for the seismic combination of actions was made based on

the internal foces obtained using the computer software where the model was built. Prior to the safety

verification, it is important to address the level of knowledge regarding the structure under analysis.


Miguel Ramos Ávila

Eurocode 8 - Part 3 (EC8, 2004) considers three levels of knowledge of an existing structure which are

determined based on the amount of available information. Information for structural assessment includes,

for instance, building inspections and in situ material testing. These levels are shown in Table 3.

For each type of structural element:

Level of knowledge Inspections Material testing confidence factor analysis

limited 20% of elements One sample per floor 1.35 linear

normal 50% of elements Two samples per floor 1.2 all

total 80% of elements Three samples per

floor 1 all

Table 3 - different levels of knowledge for an existing structure

There is no reason to doubt that the material properties are the ones described in the project

proposal or that the work has not been performed according to what was previously planned.

However, inspections and testing have not been performed, to confirm the quality of the

materials and the correct execution of the project. Under these conditions, it was conservatively decided

to consider the characteristic values of the resistance of the materials.

Three structure columns and beams with a high level of internal efforts (M, V, and N) were

chosen. Their resistance to cross-section loads and biaxial bending was demonstrated. After an analysis of

acting and resistant internal efforts, it was found that security was not fulfilled for the combination of

seismic actions. The same occurs for the footings in which the safety was assessed. Its reinforcement

level is not sufficient to sustain the effects during an earthquake.

With regard to damage limitation, Eurocode 8 - Part 1 (EC8, 2003) stipulates limit values for

relative displacements between floors.

The following equation must be checked:

𝜕1. 𝜐 ≤ 0.005. ℎ (1)

Where:

𝜕1 - is the design interstorey drift

𝜐 - is the reduction factor which takes into account the lower return period of the seismic action

associated with the damage limitation requirement

ℎ - is the storey height


Miguel Ramos Ávila

It was observed that in certain areas of the structure on which the values of displacements

between floors were evaluated, the condition for damage limitation was not verified.

The failure to meet the safety criteria of the sections when subjected to horizontal loads was

expected for various reasons. Internal efforts in the structure were derived from a linear elastic analysis,

and therefore, improperly values may have been considered, since (i) Its distribution is considered to be

proportional to the stiffness of the elements and (ii) non-linearity (considered simplified from the

behavior factor) is evenly distributed across all elements. Another very important aspect to consider is the

difference between the regulations currently in force, which were considered for modeling the structure,

and the regulation in force at the time of design that was used in its design.

Although it is out of the scope of this dissertation to refer in detail all the differences that might

exist between the Eurocodes and the previous regulations, it is essential to note that these differences do

exist and that the Eurocode has defined different seismic zones and introduced different approaches to

deal with seismic action and its effect on the structures, resulting in this cases, in a much stronger seismic

action.

4 – CONCLUSIONS

The Azores archipelago is located in a very seismically active area. Since its settlement, there

have been reports of several seismic events, many of which resulted in significant damage to the buildings

and population.

After the 1980 earthquake, on Terceira Island, this region has experienced a rapid expansion of

its construction sector, this time adopting techniques and materials with better performance when

subjected to horizontal loads.

The building of our case study has a high stiffness, decreasing upwards due to the fact that it is

enclosed by retaining walls at lower levels. Due to its setbacks and the sudden change of mass and

stiffness along its height, this structure presents a type of behavior classified as torsionally flexible in EC

8, which can be observed from the third mode of vibration obtained through a modal analysis.

Regarding structural elements, it is concluded that, from the modeling point of view, it is

possible to replace elements that could only be modeled using heavy computational finite elements and

several degrees of freedom, by lighter ones such as frame elements, commonly used to model columns

and beams. This can be achieved through the manipulation of certain factors, such as the inertia, the

lengths and shear areas.

The dynamic characterization of the structure allowed observing that many models may show

local vibration modes that are related not only with its characteristics but also to the modeling approach

used for certain elements of the structure.

The occurrence of local vibration modes, together with the fact that the structure contains a large

amount of mass close to the ground, leads to accumulated mass participation values of about 90% only

for very high vibration modes.

The fact that some structural elements did not meet the safety verifications defined in standards

when subjected to shear forces and biaxial bending can be explained by several reasons. On a linear


Miguel Ramos Ávila

elastic analysis, the stresses are proportionally distributed along the vertical elements according to their

stiffness. For this reason, internal forces were evaluated for some particular element, even though they

would in fact be lower, due to the reduction of its stiffness in the case of splitting cracks.

For design of structures, all these aspects must be taken into account, especially for the case of

irregular structures, as the study case. A nonlinear elastic analysis would be the better option so that the

nonlinear behavior of reinforced concrete elements could be estimated more adequately.

5 – BIBLIOGRAPHY

(Borges et al., 2008)

(EC8, 2003)

Borges, J. F., Caldeira, B., Bezzeghoud, M., Buforn, E. “Sismicidade e

Sismotectónica dos Açores: implicações geodinâmicas”, In Oliveira,

C.S., Costa, A., Nunes, J.C. (ed.) Sismo 1998 – Açores. Uma década

depois, Portugal, 2008

European Committee for Standardization. “Eurocode 8: Design of

structures for earthquake resistance” - Part 1: “General rules, seismic

actions and rules for buildings”, Bruxelas, 2003

(EC8, 2004) CEN, European Committee for Standardization – “Eurocode 8: Design

of structures for earthquake resistance – Part 3: Assessment and

retrofiting of buildings”, EN 1998-3, 2004

(Computers and Structures

Inc., 2008)

Computers and Structures Inc., SAPv15, Analysis Reference Manual,

Berkeley, California, USA, 2008

(Guedes, 1983) Guedes, J.H.S.C. “Novas experiências, técnicas e materiais nas acções

de reconstrução”, In Problemática da Reconstrução, Angra do

Heroísmo, Instituto Açoriano de Cultura, 1983

(Lucas, Oliveira & Guedes,

1992)

Lucas, A., Oliveira, C.S., Guedes, J.H.S.C. “Quantificação dos danos

observados no parque habitacional e do processo de reconstrução”,

In Oliveira, C.S., Lucas, A.R. & Correia-Guedes J. (Eds.), Monografia,

10 anos após o sismo dos Açores de Jan. 1980, Lisboa, Laboratório

Nacional de Engenharia Civil, 2: 667-741, 1992


Miguel Ramos Ávila

(Machado, 1992) Machado, F. “Modelos Tectónicos dos Açores”, In Oliveira, C.S.,

Lucas, A.R. & Correia-Guedes J. (Eds.), Monografia, 10 anos após o

sismo dos Açores de Jan. 1980, Lisboa, Laboratório Nacional de

Engenharia Civil, 1: 175-179, 1992.

(Nunes, 2008) Nunes, J.C. “Caracterização Sumária da Sismicidade da Região dos

Açores”, in Oliveira, C.S., Costa, A., Nunes, J.C. (Eds.), Sismo 1998 –

Açores. Uma década depois, 2008

(Oliveira, 1992) Oliveira, C.S. “Algumas considerações sobre o comportamento das

edificações com elementos em betão armado.”, In Oliveira, C.S.,

Lucas, A.R. & Correia-Guedes J. (Eds.), Monografia, 10 anos após o

sismo dos Açores de Jan. 1980, Lisboa, Laboratório Nacional de

Engenharia Civil, 2: 461-480, 1992

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Analysis of the seismic performance of the Cultural … of the seismic performance of the Cultural and Congress Center of Angra do Heroísmo Miguel Ramos Ávila This archipelago is