Dynamic characteristics of a re-reinforced steel reinforced concrete building based on seismic observation—using observed first natural period and predominant period

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DYNAMIC CHARACTERISTICS OF A RE-REINFORCED STEEL REINFORCED CONCRETE BUILDING BASED ON SEISMIC OBSERVATION--USING OBSERVED FIRST NATURAL PERIOD AND PREDOMINANT PERIOD

Ota Toshinari, Watanabe Hiroshi

KGU, Yokohama, Japan

Abstract--Seismic observation has been carried out on a re-reinforced medium- rise steel reinforced concrete building (Kawamoto-ind. Building), located at Naka-ku, Yokohama, Japan, since 20 February, 2015. The purpose is to contribute to monitoring the structural deterioration with age and the structural changes by experiencing various levels of earthquakes and also giving grounds for discussing the usability of the building if it should experience a massive earthquake. The first natural period and the predominant period of horizontal vibration of the building were considered by using the records observed since 23 February, 2015. The first natural periods measured during 23 February, 2015-5 January, 2017 were 0.67-0.79 seconds in the longitudinal direction and 0.71-0.85 seconds in the transverse direction. The first natural period in each direction temporarily increased in the 30 May, 2015 earthquake. The first natural period in the transverse direction reached the maximum value in the 28 December, 2016 earthquake, and reached the minimum value in the 5 January, 2017 earthquake.

Keywords: seismic retrofit; natural period; predominant period; structural health monitoring; dynamic characteristics.

steel reinforced concrete building seismic observation deterioration

1. Introduction

Seismic observation has been carried out on a re-reinforced medium-rise steel reinforced concrete building (Kawamoto-ind. Building), located at Nakaku, Yokohama, Japan, since 20 February, 2015. The purpose is to contribute to monitoring the structural deterioration with age and the structural changes by experiencing various levels of earthquakes and also giving grounds for discussing the usability of the building if it should experience a massive earthquake. The first natural period and the predominant period of horizontal vibration of the building were considered by using the records observed since 23 February, 2015.

2. Target Building and Observation System

Table 1 shows specifications of the building. Fig. 1 shows the target building. The building was completed in 1973. In 2014, 38 steel framed braces were set in the 1st to 7th floors, and 21 columns were bound with steel plates in the 1st to 3rd floor as seismic retrofitting for the existing building. T The first horizontal natural period on the re-rein- forced building was calculated by using the following two methods:

a) by using abbreviated calculation equation:

Ті = 0.01Я, (1)

b) by using eigenvalue analysis concerning the natural periods in the longitudinal direction, Ти, and those in the transverse direction, Tit, which were obtained based on the lateral stiffness and weights of each story, taking into consideration the secondary walls in the allowable stress design.

Table 1. Specification of the building

Fig. 1. Target building

Fig. 2 shows the layout of the seismometers. Six IT seismometers, which were set at 100 Hz in three directions, were installed on B2F, IF, 3F, 5F, 7F, and RF. Another seismometer was installed on the 7th floor to measure the eccentricity of the building from 23 February, 2015, through 15 April, 2016. Table 2 shows specification of the seismometers.

Fig. 2: Layout of seismometers in the building

Table 2. Specification of the seismometers

3. Earthquake specifications

Table 3 shows specifications of earthquakes with measured seismic intensity 3 or larger on the building. The occurrence time of the northwest Chiba Pref. Earthquake was 5:00:9.6 p. m. JST 23 February, 2015, and its epicenter was Chiba Pref., northwest 35 deg 33.8' N, 140° 08.2' E and depth 68 km, A/j = 4.5. JMA (Japan Meteorological Agency) seismic intensity 3 was observed at Kamisugeta-cho, Hodogaya ward, Yokohama, Japan. Maximum measured seismic intensity on the building was 2.53. Fig. 3 shows observed acceleration waveforms. A record is missing on 23 February 2015 because of technical trouble with an instrument on the 7th floor.

Fig. 4 shows the distribution of maximum acceleration. The response acceleration increased on upper stories in both directions. The observed maximum accelerations in the longitudinal and transverse directions were smaller than half of the calculated horizontal accelerations which gave inertia force to the horizontal story shear coefficient Co = 0.2 in seismic calculation.

Table 3. Specifications of earthquake

* indicates the earthquake of seismic intensity 2 described later.

Fig. 3: Observed acceleration waveforms (2015.2.23 EQ. and 2015.5.30 EQ.)

4. Fourier spectra of accelerations

Fourier spectra were calculated from the time history data of accelerations. These spectra smoothed with Parzen window at a band width of 0.2Hz in the FFT method. Recorded data were examined from 20.49 to 120 sec. Transfer functions were calculated by using Fourier spectrum ratios based on the first floor. Fig. 5 shows Fourier spectrum ratios on RF/1F, 7F/1F, 5F/1F, 3F/1F and B2F/1F. The first mode in the longitudinal direction and the 3rd mode in the transverse direction were predominant respectively.

Fig. 6 shows relations between the horizontal first natural period and predominant period, and the measured seismic intensity. Fig.6 also shows approximation straight lines for the first natural period and the predominant period, which were obtained by using least squares method as broken lines. The actual measurement was 0.67-0.79 seconds against T\ = 0.61 seconds and Til = 0.72 seconds. The actual measurement was 71-0.85 second s against T\ = 0.61 seconds and 7ir= 0.82 seconds. The actual measurement on predominant period in the longitudinal direction was 0.43-0.79 seconds. The actual measurement on predominant period in the transverse direction was 0.38-0.54 seconds. The coefficients of variation on approximation straight line were 3.2 % in the longitudinal direction and 2.7 % in the transverse directions, which were obtained by using least squares method on the first natural periods. The coefficient of variation on approximation straight line were 8.3 % in the longitudinal direction and 3.6 % in the transverse directions, which were obtained by using the least squares method on the predominant periods. By comparing coefficients of variation, it was found out that the variation of first natural period was smaller than that of predominant period.

The value of the first natural period in the transverse direction was 1.12 times larger than the obtained value from the approximation straight line in the 28 December, 2016 earthquake, with a measured seismic intensity 3. The value of the first natural period in the transverse direction was smaller than the obtained value from the approximation straight line in the 5 January, 2017 earthquake, with a measured seismic intensity 2. Fig. 7 shows the change over time in the natural period and in the predominant period. The first natural period in each direction temporarily increased in the 30 May, 2015 earthquake. The first natural period in the transverse direction reached the maximum value in the 28 December, 2016 earthquake, and reached the minimum value of 5 January, 2017 earthquake.

5. Conclusion

The first natural periods measured during 23 February, 2015-5 January, 2017 were 0.67-0.79 seconds in the longitudinal direction and 0.71-0.85 seconds in the transverse direction.

The predominant periods measured during 23 February, 2015-5 January, 2017 were 0.43-0.79 seconds in the longitudinal direction and 0.38-0.54 seconds in the transverse direction. The first natural period in each direction temporarily increased in the 30 May, 2015 earthquake. The first natural period in the transverse direction reached the maximum value in the 28 December, 2016 earthquake, and reached the minimum value in the 5 January, 2017 earthquake. Further earthquake response analysis in a target building will be examined, taking into consideration aging changes, influences of torsion and other factors in deterioration. It will help keep examining the condition of the building by accumulating more data.

Acknowledgement

We will pay our respects and deep appreciation to the Kawamoto Ind. Corp. for their cooperation to this publication by providing us with a seismograph system and others. Advice and comments given by Assoc. Prof. Kouichi Kusunoki, the Earthquake Research Institute, the University of Tokyo, and Prof. Emeritus Norio Abeki, Kanto Gakuin University, on the occasion of seismometer installation, have been a great help in this study.

References:

1. http://www.jma.go.jp/jma/index.html

2. Muraoka, N., Maruyama, Y. and Yamazaki F.; Estimation of response spectrum and JMA seismic intensity of main shock using Fourier spectrum ratios of aftershocks and microtremors. J. Struct. Constr. Eng., AIJ, 2006, 603, 176-186 (in Japanese with English abstract).

3. Ota, T. and Watanabe, H.; Dynamic Characteristics of a Re-reinforced Steel Reinforced Concrete Building Based on Seismic Observation, 11th IWSMRR, Granada Spain, 6pages, 2016.

Ота Тосинари, Ватанабэ Хироси УКГ, Йокогама, Япония

Изучение динамических характеристик железобетонного здания после усиления конструкций на основе сейсмического мониторинга. Использование первого и преобладающего периода собственных колебаний

Абстракт -- С 20 февраля 2015 года проводился сейсмический мониторинг железобетонного здания средней этажности (Kawamoto-ind. Building), расположенного по адресу Нака-ку, Йокогама, Япония. Предварительно было осуществлено усиление его конструкций. Целью данной работы стало наблюдение за особенностями разрушения здания в результате длительной эксплуатации и за изменением его расчетной схемы в результате воздействия землетрясений различной интенсивности. Также необходимо было сделать заключение о возможности безопасной эксплуатации здания в том случае, если произойдет сильное землетрясение.

Первый и преобладающий период собственных колебаний здания в горизонтальном направлении был определен с использованием записей, которые велись с 23 февраля 2015 года. Первые периоды собственных колебаний измерены с 23.02.2015 по 05.01 2017. Их значения составили 0,67-0,79 секунды в продольном направлении и 0,71-0,85 секунды в поперечном направлении. Первый период собственных колебаний по каждому направлению временно увеличился во время землетрясения 30 мая 2015 года. Первый период собственных колебаний в поперечном направлении достиг максимального значения во время землетрясения 28 декабря 2016 года, и минимального значения во время землетрясения 5 января 2017 года.

Ключевые слова: усиление конструкций; период собственных колебаний; преобладающий период; мониторинг здания; динамические характеристики.

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