ABSTRACT: The relating small Lorca 2011 earthquake, occurred on May 11th with epicentral distance of 5.5 km from the city centre, moment magnitude 5.2 and shallow hypocentre (4.6 km deep), caused in Lorca an outwardly disproportionate impact: 9 casualties, near 300 injured, one RC building and some masonry constructions collapsed, 1018 buildings (15.8 %) officially declared with structural damage (329 of them had to be demolished), and 1328 buildings (20.7 %) with slight structural damage and/or moderate non-structural damage. In order to estimate variations of fundamental period for swaying motion and damping factor (h) of a large set of existing RC-building structures and its relation with damage level suffered, ambient vibration measurements were performed on the top floor pre- and post- Lorca 2011 earthquake. Before the earthquake sequence, the total number of measured buildings was 59, with a number of storeys range from 2 to 12. After the earthquake 34 buildings were measured, of these, 23 were measured pre- and post-earthquake. The relationship between T and the number of stories (N) obtained before the earthquake is T = (0.054±0.002) N. Measurements performed in 34 damaged buildings after the quake show a period elongation according to T* = (0.075±0.002) N expression. This period elongation caused by the quake reveals a stiffness degradation of the structures. The average h value obtained ex-ante is 3.2% with standard deviation (SD) 2.6% and for and ex-post is h* is 2.2 % (SD =1.6 %). Comparing the results obtained from pre- and post-earthquake measurements on the same 23 RC-buildings in Lorca, a clear increasing in the natural period values of damaged buildings with their damage degree (EMS scale) has been observed.

KEY WORDS: Dynamic behaviour of RC buildings; period elongation; damping factor; damage to buildings.

1 INTRODUCTION

Lorca town is located in Murcia province (SE Spain), belonging to the eastern part of the Betic Cordillera (Figure 1). This is the most hazardous seismic region of Spain, characterized by frequent earthquakes of small and moderate magnitude (generally smaller than 5.5). Recently, two shallow quakes occurred on May 11th, 2011

with epicentres near Lorca town (Mw = 4.6 and 5.2, respectively). The mainshock of the Lorca seismic series occurred on May 11th at 16:47 UTC (18:47 local time), with epicentral distance of 5.5 km from the city centre and shallow hypocentre (4.6 km deep). Peak ground acceleration (PGA) of 0.37 g and peak ground velocity (PGV) of 35.4 cm/s were observed at the Instituto Geográfico Nacional (IGN, 2011; www.ign.es) Lorca strong-motion station. The maximum macroseismic intensity was initially estimated as VII (EMS scale) by the IGN and the Instituto Andaluz de Geofísica (IAG, 2011; www.ugr.es/~iag), highlighting again the influence of the earthquake ground response on building behaviour during the shaking. There were damaged buildings in all districts of the town (Foto 1), but the levels and percentages were fundamentally different depending on building vulnerability and ground motion characteristics. The most severe damage appears in La Viña (southwestern zone), La Alberca and La Alameda districts (close to the Guadalentin River) as shown in Figure 2. The seismic response of a building depends on its dynamic

characteristics (fundamental period, T, damping ratio, h, and modal shape) and on the input ground motion. Small-

amplitude vibrations are commonly used to estimate dynamic parameters assuming a linear behaviour of the building structure. Along this line, ambient noise analysis is a quick, efficient and inexpensive technique, mostly because only a short duration of time series data may be needed to obtain stable results [1]. Many studies use ambient noise analysis (e.g.:[2-12]) because of the results obtained don’t differ essentially from earthquake record or forced vibration techniques.

Figure 1. Location of Lorca town within the Betic region.

Omori [13] was the first to relate the increase of the

fundamental period in buildings with damage (and also the decrease with structural retrofitting). The damaging process in buildings during earthquakes produces a permanent loss of

Changes in dynamic characteristics of RC buildings determined from ambient

vibration measurements performed pre- and post- the Lorca 2011 earthquake.

M. Navarro1,2, F. Vidal2, C. Aranda2, T. Enomoto3 & G. Alguacil2 1Departamento de Química y Física, Universidad de Almería, La Cañada, 04120, Almería (España)

2Instituto Andaluz de Geofísica, Universidad de Granada, 18071, Granada (España) 3Department of Building Engineering, University of Kanagawa, Yokohama (Japón)

mnavarro@ual.es, fvidal@ugr.es, ca.arquitect@gmail.com, enomot01@kanagawa-u.ac.jp, alguacil@ugr.es

Proceedings of the 9th International Conference on Structural Dynamics, EURODYN 2014Porto, Portugal, 30 June - 2 July 2014

A. Cunha, E. Caetano, P. Ribeiro, G. Müller (eds.)ISSN: 2311-9020; ISBN: 978-972-752-165-4

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structural stiffness and then a permanent increase of the fundamental period. During the past two decades, an abundant scientific literature (see [10, 14-17] and references therein) deals with this period elongation (stiffness degradation) after damaging earthquakes. The use of natural period as a diagnostic parameter has its

basis on the assumption that natural frequencies are sensitive indicators of structural integrity [18] and are directly related to strength building. Many damage detection techniques are based on the assumption that damage causes nonlinear behaviour in structures [19] due to development of cracks. In order to detect damage, a comparison of the undamaged and damaged states in a structure is required [20].

Figure 2. Damage distribution in Lorca town due to the May 11th, 2011 Lorca earthquake. Squares represent the

districts with higher damage.

In contrast to period and mode shapes, damping ratio is not an intrinsic parameter of the building, and is not typically used as an indicator of damage in structures. Damping ratio depends of many factors as structure and soil characteristics as well as soil-structure interaction. Thus its determination is an extremely complex problem because it is influenced not only by the amplitude of motion but also by the time variations of the mentioned factors [21], [22].

Foto 1. Examples of damage observed on RC, masonry and cultural heritage structures in Lorca town due to May 11th,

2011 earthquake.

The two relevant goals of this work are: i) to calculate the fundamental translational periods as well as damping ratios of a large set of existing RC-framed buildings in Lorca and Mula towns and to estimate period-height empirical relationship; ii) to appraise variations of these dynamic parameters obtained pre- and post- Lorca 2011 earthquake sequence at the same buildings, mainly period elongation after strong ground shaking and its relation with damage level suffered by this subset of structures.

2 DATA ANALYSIS

The measurements of ambient vibration were performed in Lorca and Mula towns in March 2005 and complemented in June 2011 to evaluate the dynamic characteristics (funda-mental period, T, and damping ratio, h) of RC-framed existing buildings and to estimate variations of these dynamic parameters comparing pre- and post-earthquake measurements at the same buildings. Before the earthquake sequence, the total number of measured buildings was 59, with a number of storeys ranging from 2 to 12, 44 of them located in Lorca town. After the earthquake, 34 of them were measured again using the same methodology. For all buildings we determined the fundamental period and damping ratio on the two orthogonal components (Table 1).

2.1 Natural Period

The ambient vibration measurements were performed at the geometrical centre of plan on the roof floor of the building using a data acquisition system composed by three short period sensors and a digitizer. These devices provide an acceptable response for frequencies ranging from 0.25 to 70Hz. The sensors were oriented firstly to the longitudinal direction, second to the transverse direction and third to the vertical direction, respectively. A time history 300 s long of

La Viña

La Alameda La Alberca

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ambient noise signal was recorded, sampled at a rate of 100 samples per second. The fast Fourier transform (FFT) was applied to every

record in order to calculate the spectral characteristics of displacement response. The Fourier amplitude spectrum show a pronounced peak, centred at the fundamental mode (Figure 3). This peak is more pronounced in the case of longer periods or higher buildings. In the cases of smaller buildings the peaks might be more difficult to identify. Figure 3. Examples of amplitude spectra of the transverse

component for buildings with 3, 6, 9 and 12 storeys, respectively.

The results obtained from the measurements performed

before May 11th, 2011 Lorca destructive earthquake show that the longitudinal and transversal natural period of buildings increases with the number of storeys. The lowest and highest values of average fundamental period (T) are 0.11 ± 0.02 s, and 0.61 ± 0.08 s, respectively. These values correspond to RC buildings with height between 2 and 12 storeys (Table 2). The empirical relationship between the natural period of fundamental mode (T) and the number storeys (N) obtained before May 11th, 2011 Lorca destructive earthquake (Figure 4) was:

T = (0.054±0.002) N (1)

This result is similar to those obtained in other European cities by using ambient vibrations (e.g.: [3, 5, 10, 11, 22, 23]). After May 11th, 2011 Lorca destructive earthquake, 34 RC

buildings with different damage degree were measured in Lorca town. The Fourier amplitude spectrum shows changes in the natural period of damaged buildings measured before and after Lorca earthquake (Figure 4). The results show again that the natural period of damaged

buildings (T*) increases with the number of storeys (N) and EMS’s damage level. The best linear fit (R2 = 0.985, SD = 0.002) between T* and N for Lorca damaged RC buildings (Figure 5) gives the following relationship:

T* = (0.075±0.002) N (2)

This increase in modal period reveals a reduction of stiffness in the damaged structures.

Figure 4. Example of noticeable changes in the natural period of the longitudinal and the transverse directions measured

before (TL, TT) and after (TL*, TT*) Lorca earthquake for a building of 10 storeys and damage grade 2 (LRC21, Table 1) Figure 5. Relationship between the average natural period (T) and the number of storeys (N) for RC buildings of Mula and Lorca towns obtained from ambient noise analysis. Symbols and fit lines in red and blue colours

correspond to buildings measured before [T(N)] and after [T*(N)] the 2011 earthquake, respectively. Vertical bars

represent standard deviations. To know the period–height relationships for different level

of damage is of great interest. Even though we work with a low number of measured damaged buildings and it is well known that the statistical significance falls down when the number of buildings in each damage class is much smaller, we attempt to relate T* and N for different degrees of damage. The best T*-N linear fits corresponding to measured buildings with damage grades 1 (G1), 2 (G2) and 3–4 (G3 and G4) (EMS scale) are plotted in Figure 6.

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Table 1. Parameters of the buildings measured in Lorca town pre- and post-earthquake.

BLDG

CODE NS DL (m) DT (m) TL (s) hL (%) TT (s) hT (%) D TL* (s) hL* (%) TT* (s) hT* (%) ∆TL (%) ∆TT (%)

LRC01 7 39.60 21.50 0.33 1.90 0.30 3.90 1 0.45 3.10 0.44 3.70 34.85 48.00 LRC02 5 38.30 25.70 0.34 5.60 0.34 1.30 3 0.59 2.20 0.68 0.90 72.35 100.00 LRC03 9 34.20 15.00 0.46 1.40 0.46 2.40 - - - - - - - LRC04 6 55.50 14.50 0.46 1.10 0.41 3.50 - - - - - - - LRC05 4 29.40 8.30 - - 0.22 5.70 1 0.22 6.00 0.25 3.50 - 12.59 LRC06 8 27.10 19.50 0.40 1.20 0.47 1.30 2 0.59 1.20 0.64 0.60 46.25 36.60 LRC07 11 38.50 20.80 0.55 5.20 0.56 4.10 1 0.66 1.50 0.73 2.30 20.00 30.54 LRC08 12 12.00 9.00 0.69 1.10 0.71 3.60 1 0.89 1.20 0.79 0.50 29.13 10.85 LRC09 2 18.30 7.50 0.10 3.00 0.15 10.00 2 0.19 2.40 - - 92.40 - LRC10 4 18.00 15.00 0.16 2.70 0.16 1.30 3 0.20 1.00 0.19 1.60 22.06 21.75 LRC11 5 22.70 15.50 0.24 6.80 0.25 5.40 3 0.43 2.50 0.28 3.10 77.50 13.60 LRC12 5 12.90 10.10 0.19 4.40 0.22 2.20 1 0.22 2.50 0.25 1.90 16.84 14.55 LRC13 5 26.30 15.30 0.27 1.20 0.37 1.00 2 0.45 1.50 0.52 1.10 64.81 41.62 LRC14 5 13.50 9.60 0.23 1.20 0.26 1.20 2 0.26 3.80 0.31 1.00 11.30 19.23 LRC15 6 20.30 19.30 0.32 1.60 0.32 1.80 2 0.55 2.80 0.45 1.90 72.81 39.06 LRC16 3 13.40 11.60 0.20 6.00 0.25 3.10 4 0.35 1.10 0.41 3.30 73.50 63.60 LRC17 2 22.00 9.70 0.10 15.40 0.11 13.20 2 0.10 0.16 4.40 0.00 48.91 LRC18 2 17.60 13.15 0.14 3.10 0.09 7.80 - - - - - - - LRC19 4 18.10 14.60 0.21 6.70 0.18 6.40 2 0.28 4.30 0.28 5.80 33.33 57.78 LRC20 11 23.70 19.90 0.48 2.40 0.68 1.50 2 0.76 1.80 0.82 0.90 58.46 20.74 LRC21 10 24.70 18.30 0.65 1.20 0.52 1.20 2 0.82 1.30 0.79 0.80 25.85 51.35 LRC22 11 25.50 18.00 0.53 1.00 0.48 1.00 - - - - - - - LRC23 6 37.20 24.60 0.36 2.90 0.34 1.50 4 0.62 2.40 0.54 1.50 72.22 58.53 LRC24 12 23.30 22.10 0.51 0.90 0.56 1.70 1 0.76 0.90 0.63 1.70 48.63 12.14 LRC25 8 17.80 17.70 0.41 2.30 0.45 1.20 - - - - - - - LRC26 3 17.00 16.00 0.16 5.00 0.21 1.30 1 0.19 3.50 - - 16.56 - LRC27 10 22.00 20.00 0.57 1.20 0.63 3.10 1 0.71 1.40 0.76 1.60 23.86 20.33 LRC28 7 19.00 12.00 0.42 1.60 0.42 1.90 1 0.57 1.20 0.50 1.10 35.45 18.93 LRC29 3 21.00 13.00 0.20 4.20 0.21 1.90 - - - - - - - LRC30 8 35.50 28.10 0.38 1.80 0.38 1.20 - - - - - - - LRC31 9 19.20 13.20 0.57 0.90 0.57 1.10 - - - - - - - LRC32 9 25.00 8.50 0.42 4.10 0.37 4.50 - - - - - - - LRC33 12 34.00 32.00 0.57 2.90 0.60 2.60 - - - - - - - LRC34 5 - - - - - - 2 0.36 2.00 0.36 1.30 - - LRC35 10 - - - - - - 2 0.59 0.70 0.82 1.00 - - LRC36 13 - - - - - - 2 0.85 2.60 1.20 0.70 - - LRC37 13 - - - - - - 2 0.76 0.80 1.20 2.80 * * LRC38 9 - - - - - - 2 0.76 1.10 0.82 0.70 * * LRC39 5 - - - - - - 3 0.29 1.20 0.43 1.00 * * LRC40 2 - - - - - - 2 0.17 2.00 0.13 8.10 * * LRC41 2 - - - - - - 2 0.14 7.50 0.11 3.10 * * LRC42 8 - - - - - - 2 0.56 1.10 0.58 4.10 * * LRC43 5 - - - - - - 3 0.40 2.10 0.44 2.00 * * LRC44 5 - - - - - - 3 0.40 4.90 0.49 1.70 * *

NS = number of storey DL (m) = longitudinal dimension DT (m) = transverse dimension TL , TL* = longitudinal period TT , TT* = transverse period hL , hL* = longitudinal damping hT , hT* = transverse damping D = damage grade (EMS) * = post-earthquake ∆TL(%) = percentage increase of TL ∆TT(%) = percentage increase of TT

The equations for the 34 RC damaged buildings are:

T*(G1) = (0.065±0.002) N (3)

T*(G2) = (0.077 ± 0.002) N (4)

T*(G3 - G4) = (0.089 ± 0.008) N (5)

with correlation coefficients R2 of 0.987, 0.992 and 0.932, respectively. For the statistical reasons already mentioned, these equations are only indicative.

The relative variations of natural periods increase as the damage degree increases, as deduced when comparing the results obtained for the same 23 buildings measured in Lorca before and after the earthquake (Table 2). Results show that normalized period variations in both main directions are very similar, showing an average difference of about 9 %, being the longitudinal period relative increment (∆TL) larger than the transversal one (∆TT) (Figure 7). The observed trend in the average increase of T values

indicates that we can expect ∆T greater than a 10% in some

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buildings visually classified as undamaged (14% for ∆TL and 10% for ∆TT).

Table 2. Average values of natural periods and damping factors corresponding to each number of storey, obtained pre-

and post-earthquake (*) for all building set.

NB = number of buildings. Other symbols as in Table 1.

Figure 6. Period–height relationships for different degrees of damage and compared with the pre-event ones. Fit lines in different colours correspond to buildings measured after the 2011 earthquake with EMS damage grade (undamaged G1,

G2, G3 and G4, respectively).

2.2 Damping factor

In order to evaluate directly the damping factor (h) of existing RC building structures in Lorca and Mula towns, RANDOMDEC technique was applied using ambient noise measurements. This technique was first developed by Cole [24] as a structural monitoring technique for space craft, and it has been proved as an effective crack detection method. The RANDOMDEC technique has been shown to be applicable to determine the damping of dynamic systems subject to unknown random excitation, such as ambient vibration, and it has been applied to detect damages in structures [25], [26], to determine damping factor of soil [27] and RC buildings (e.g.

[4, 10, 11, 28, 29]). The RANDOMDEC analysis requires only the measured output of the dynamic response of a structure, and not the random excitation input.

Figure 7. Mean increase (∆, in %) of natural periods (longitudinal TL, transversal TT, and average T) versus grade of damage for 23 Lorca buildings measured pre- and post-

earthquake. The solution of the differential equation of motion of a

damped single-degree-of-freedom (SDOF) system under linear behaviour depends on its initial conditions (displace-ment and velocity) and forcing loading. The RANDOMDEC analysis allows to obtain the response due to initial displacement (Randomdec signature), representative of free vibration decay curve of the system [26]. Then, we obtain the damped free vibration response of the structure (Figure 8).

Figure 8. Examples of random decrement signatures for several RC buildings measured after the 2011 earthquake with different storey number. a) LRC16; b) LRC15; c) LRC21; d)

LRC07. See parameters of the buildings in Table 1. The results obtained from the measurements performed pre-

earthquake in Lorca and Mula towns show that h values for longitudinal and transverse components ranging from 0.9 to 15.4%, (higher values corresponding to the lower buildings), being the average h value 3.2% with standard deviation 2.6 %. Lorca buildings measured after the quake had an average damping ratio h* of 2.2 % (SD = 1.6 %).

NS NB T(s) h (%) NB* T * (s) h * (%)

2 5 0.11±0.02 7.19±4.53 4 0.14 ± 0.03 4.58 ± 2.63

3 6 0.19±0.03 4.83±2.22 2 0.31 ± 0.11 2.63 ± 1.33

4 6 0.22±0.05 3.51±1.96 3 0.24 ± 0.04 3.46 ± 2.43

5 8 0.29±0.07 2.95±1.88 9 0.40 ± 0.12 2.04 ± 1.05

6 3 0.37±0.06 2.07±0.93 2 0.54 ± 0.07 2.15 ± 0.57

7 4 0.39±0.08 1.96±0.81 2 0.49 ± 0.06 2.28 ± 1.32

8 5 0.42±0.04 1.66±0.69 2 0.59 ± 0.04 1.75 ± 1.59

9 3 0.43±0.06 2.40±1.56 1 0.79 ± 0.04 0.90 ± 0.28

10 2 0.59±0.06 1.68±0.95 3 0.75 ± 0.09 1.13 ± 0.36

11 3 0.55±0.07 2.53±1.75 2 0.74 ± 0.07 1.63 ± 0.59

12 3 0.61±0.08 2.13±1.07 2 0.77 ± 0.11 1.08 ± 0.51

13 - - - 2 1.00 ± 0.23 1.73 ± 1.13

(a) (b)

(c) (d)

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In Table 2 we present the damping ratio values for buildings measured both before (h) and after (h*) the seismic sequence regarding to the number of floors. As expected, h generally decreases with number of storeys, but in the case of buildings measured before the quake, the lowest average h value is 1.66±0.7%, that corresponds to buildings of 8 floors.

Damping factor data and best power estimated fits corresponding to buildings pre- and post-earthquake are graphed in Figure 9. In general, our results show damping factor slightly decrease when structures have been damaged, being mainly for the low rise buildings (N ≤ 5 floors) which have a greater decrease (Table 3).

Table 3. Average values of natural periods and damping factors corresponding to each number of storey, obtained pre- and post-earthquake for 23 damaged buildings of Lorca.

AD = average damage; ∆ h = damping variation. Other symbols as previous Tables.

Figure 9. Relationship between the average damping factor (h) and the number of floors (N) for RC buildings of Lorca town obtained from ambient noise measurements. Symbols and fit lines in gray and black correspond to buildings measured

before and after the 2011 earthquake, respectively. According to previous researches (e.g. [2, 7, 11, 29, 30, 31,

32]) the empirical relationship between the two variables (h, T) has been investigated, considering one typical formulation h T = constant) generally used to tackle this problem. The relationship between the damping factor h and the

natural period T for swaying motion has been estimated (Figure 10) as:

hT = 0.75 ± 0.04 % sec (6)

from data obtained before 2011 earthquake, and for after that:

h*T* = 0.80 ± 0.05 % sec (7)

As expected, both results are similar, because hT value can be considered almost constant for buildings located on soils with same typology, being hT value larger for buildings on soft ground than on hard ground. However, these hT values are different compared with the results obtained in other cities by using ambient vibrations (e.g.: [2, 4, 6, 7, 11]). These differences could be caused by the different structural typologies and different soil conditions in each region.

Figure 10. Relationship between damping factor (h) and

natural period (T) for swaying motion of Mula and Lorca RC buildings measured before (in red) and after (in blue) the 2011

earthquake.

NS NB T(s) h (%) AD T* (s) h * (%) ∆T (%) ∆h

2 2 0.12 ± 0.02 10.40 ± 5.41 2.0 0.15 ± 0.05 3.40 ± 1.41 26.72 -7.00

3 2 0.21 ± 0.04 3.85 ± 2.08 3.0 0.31 ± 0.11 2.63 ± 1.33 49.60 -1.22

4 3 0.19 ± 0.03 4.56 ± 2.42 2.3 0.24 ± 0.04 3.70 ± 2.09 24.40 -0.86

5 5 0.27 ± 0.06 3.26 ± 2.28 2.2 0.40 ± 0.16 2.05 ± 0.96 47.59 -1.21

6 2 0.34 ± 0.02 1.95 ± 0.65 3.0 0.54 ± 0.07 2.15 ± 0.57 58.60 0.20

7 2 0.37 ± 0.06 2.33 ± 1.06 1.0 0.49 ± 0.06 2.28 ± 1.32 32.26 -0.05

8 1 0.44 ± 0.05 1.25 ± 0.07 2.0 0.61 ± 0.04 0.90 ± 0.42 39.43 -0.35

10 2 0.59 ± 0.06 1.68 ± 0.95 1.5 0.77 ± 0.05 1.28 ± 0.34 30.05 -0.40

11 2 0.57 ± 0.08 3.30 ± 1.66 1.5 0.74 ± 0.07 1.63 ± 0.59 40.22 -2.28

12 2 0.62 ± 0.10 1.83 ± 1.23 1.0 0.77 ± 0.11 1.08 ± 0.51 23.55 -0.75

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3 DICUSSION AND CONCLUSIONS

Previous research works have shown that damage distribution in Lorca town is specially concentrated in areas where predominant period of ground is between 0.3 and 0.5 s [33]. The significant short duration of the May 11th shaking (around 1.0 s) probably reduced the resonance effect. Nevertheless, the energy input spectra of ground motion (Figure 11) obtained convolving the transfer functions derived from SPAC 1-D models with the record of Lorca IGN station [34] clearly show that most of the energy supplied by the ground is in the range of 0.3-0.6 s. In this period range, a pseudo-velocity level of 100 cm/s was exceeded [34]. Figure 11. Relative energy input spectra for the simulation points (SP) and for the LOR station record (rock). For

reference, the expected energy input spectra for European earthquakes records where local intensities were VII and VIII

are also plotted [35] The relationship between average period of fundamental

mode (T) and the number of storeys (N) here obtained for horizontal motion with measurements performed in buildings of Lorca and Mula towns before the earthquake is T = (0.054 ± 0.002) N. This expression is very similar to other empirical period–height relationships obtained for RC structures in the European built environment, but quite different from seismic code easy-to-apply rules. Due to the known influence of the “infill walls” on the

increase of the building frequency relatively to that corresponding to the structural frame, the use of the calculated formula have to take into account it was estimated from small-amplitude vibrations assuming linear behaviour of the structure. Thus, we can know the period with which the building initially responds to the earthquake, but in the case of a strong shaking this period will be higher than predicted by this formula, especially when a substantial loss of the building stiffness occurs due to degradation by damage of masonry infill and the structural frame. This growth of period during a strong shaking has been tested experimentally (e.g. [35, 36, 37]). Hence, the strong influence of masonry infill and of cracking on the fundamental period values of RC buildings is here highlighted.

The results obtained from 34 RC buildings with different damage degree measured in Lorca town after the 2011 earthquake gives the following period–height expression: T* = (0.075 ± 0.002) N. This period elongation after the quake reveals a relevant stiffness degradation of the structures. The increasing in the fundamental period of damaged

buildings with their damage degree (shown in Figures 6 and 7) is a relevant finding. This empirical result was also tested from ambient noise measured pre- and post-earthquake on the same 23 RC buildings in Lorca. Also should be noted that trending in the average increase of T values suggests that we can expect more than a 10% of this ∆T in some buildings visually classified as undamaged. This result is in agreement with the loss of natural frequencies of about 10% obtained by Zembatya et al. [38] in apparent intact state of RC frames before the appearance of visible cracks. In contrast to natural frequency, estimated damping ratio

does not has shown up a significant variation with earthquake damage degree. Variations of this parameter here calculated are quite small and sometimes the same order as the measurement errors. That result points out that damping parameter as was obtained here is a bad indicator of damage in structures. The product of damping coefficient and the natural period

for swaying motion remain near constant when we compare hT values obtained before and after 2011 earthquake, 0.75 ± 0.04% and 0.80 ± 0.05%s, respectively. This result suggests the most effective factor dominating hT value could be the soil condition of each site.

ACKNOWLEDGMENTS

The authors wish to express their sincere gratitude to all those who helped them during the two building measurements surveys, especially to local Civil Protection people of Lorca town. This research was carried out within the framework of research coordinated project CGL2011-30187-C02-01-02 funded by the Spanish Ministry of Science and Innovation.

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