1 INTRODUCTION

With the expansion of the urban environment of many cities, the old industrial areas have been included inside residential areas. Many of the chimneys masonry industrials have stayed as ornamentals elements. These chimneys were constructed during the period of the industrial revolution, towards the end of the 19th Century and the beginning of the 20th Century until the 1950’s, and became obsolete when electric power substituted this production system based on steam power. These chimneys were designed to withstand mainly self-weight and wind, but not special loads as seismic movements. Industrial chimneys made in masonry currently in existence in many European areas are now considered in many places as part of the cultural heritage and often protected by laws.

In the city of Valencia (Spain) an investigation project has been developed with the purpose of evaluating the possible behaviour of these structures in front of the seismic action. As previous step to this analysis 10 masonry chimneys of this city and their environment have been monitored. Different geometries are selected to analyze their dynamic characteristics, with the purpose of being able to extrapolate the obtained results. With the obtained results a numerical model of each tower has been developed to evaluate the effect of the possible earthquakes in the environment of the city of Valencia, and to extrapolate the results to the other chimneys in the city.

Modelling masonry structures demands a deep knowledge of the distinctive features that masonry exhibits as a construction material and its behaviour in the structure where it is used. Many different strategies can be adopted to model masonry, from micro-modelizations to macro-modelizations, through homogenization techniques [1]; sometimes half-way approaches using continuum models representing the bricks and interface models for mortar are used to analyze masonry.

In this work the results of the experimental tests obtained on two chimneys of the same time, structural typology, material, technique constructive and different geometry are presented. Both chimneys are located in the city of Valencia, one in Gaspar's square Aguilar and the other one in the square of the Palace of Congresses.

Monitoring masonry chimneys with Operational Modal Analysis

Francisco J. Pallarés Technical University of Valencia, ICITECH, Valencia, Spain

Salvador Ivorra Universidad de Alicante, Departamento de Ingeniería de la Construcción, Alicante, Spain

José M. Adam Technical University of Valencia, ICITECH, Valencia, Spain

ABSTRACT: The present paper presents the test set up carry out in two industrial masonry chimneys located in the downtown of the Valencia city (Spain). Four accelerometers were installed on the structures in different directions and heights in order to obtain natural frequencies and vibration modes. Both ambient vibrations and impulse loads using a calibrated hammer were used to produce accelerations in the structures. Finite element models calibrated with the experimental results are also described. Furthermore, the automatic procedure of modal identification and investigations considering the influence of the soil in the modal parameters obtained, are also presented.

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2 THE STRUCTURE AND THE MATERIAL

Industrial chimneys usually have three well differentiated parts: it bases or access of the smoke, central area and coronation. The two selected chimneys for this work have very different rigidities, the first of them-Fig. 1a- it is located in Gaspar’s Aguilar square, in the south area of the Valencia city, it possesses to total height of 32.8 m and to variable section from the 3.06 m of width in the bases up to 1.5 m in coronation. The section of the central part of the chimney is octagonal.

The second studied chimney is located in the vicinities of the Palace of Congresses of Valencia-Fig. 1b - it reaches a height of 20.28 m, and has a variable section from the 2.37 m in the base until the 1.28 m in coronation.

Figure 1. Gaspar Aguilar chimney. Congress Palace Chimney Both chimneys are built in masonry, using bricks and lime mortar. Since no experimental

tests were done in the material, usual values [2] for the main mechanic parameters were used to obtain a numerical model:

(A)

(B)

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• uniaxial compressive strength: fc= 637.500 N/m2 • uniaxial tensile strength: ft= 196.200 N/m2 • elastic modulus: E= 5.886e9 N/m2 • Poisson coefficient ν=0.2 • density: γ=1600 kg/m3

Fig. 2 shows a detail of masonry coronation of Congress Palace chimney, brick masonry can be observed.

Figure 2. Brick masonry coronation of chimney situated near of Congress Palace.

3 EXPERIMENTAL TEST

In order to know the dynamic response of this type of construction when different loads are having effects on these chimneys, mainly earthquakes, measurements were taken for each particular chimney commented, sited in the city of Valencia (Spain).

The chimney was subjected to a series of non-destructive tests in order to characterise its dynamic structural response, damping capacity and natural vibration frequencies. The results were used to calibrate a numerical model that was subsequently used to calculate the chimney’s resistance to seismic movements [2, 3]. Fig. 3 shows the location of the accelerometers on the chimney (on the crown and at two-thirds of its height).

The working range of these accelerometers varies between 0.5 and 2000 Hz, with a conversion factor equal to 1000 mV/g. According to the arrangement commented, two different situations are registered:

- ambient vibrations in the E-W and N-S directions could be determined. - excitation of the tower by means of a hammer impact. The dynamic data obtained from the ambient vibration have been registered by a Kyowa

PCD-320 equipment with a sample rate of 50 Hz. Temporary acceleration movements have been analysed with DAS-100a Kyowa software to obtain frequency results.

Fig. 4, shows the accelerations registered in W-D at 21.5 height when an impact is introduced on the structure at the coronation by means of an hammer impact.

The ambient vibrations are registered during 30 minutes at a sampling rate of 50 H. Fig. 5 shows an time interval of ten minutes of this time registered at the four accelerometers installed on the structure.

Processing the accelerations recorded in this phase, a Fourier analysis was carried out on the output signals to obtain the natural frequencies of the first vibration modes. The filtered result can be seen in Fig. 6, with frequency values for the first and second mode of 1.07 and 3.32 Hz respectively.

404 IOMAC'09 – 3rd International Operational Modal Analysis Conference

2H/3

H

(c)

(a)

(b)

(d)

Figure 3. Gaspar Aguilar chimney. (a) Installing the accelerometers. (b)-(d) Sensor location. Accelerometer PCB 39600 installed.

Figure 4: Accelerations recorded at a height of 21.5 m showing response to impact hammer.

Chimney section

Accelerom

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(a) (d)

(b) (c)

Figure 5: Ambient vibrations registered on Gaspar Aguilar chimney. (a) E-W direction, 32 m height. (b) N-S direction, 32 m. (c) N-S direction, 21.5 m height. (d) E-W direction, 21.5 m height.

Figure 6: Spectral Analysis. Accelerometer in E-W and h=32 m.

The damping ratio has been determined starting from the logarithmic decrement of

underdamped vibrations. The calculations are made from the accelerometers registered in the experimental phase with the hammer impact. (6). This value, for the case of the masonry, is very variable and it depends on numerous factors, highlighting the humidity and the temperature (7). The obtained value: 1.5%, it has been compared with the mean values provided by diverse authors. Given the characteristics of the masonry have not been possible to determine a damping ratio for each vibration mode, admitting an identical mean value for all the vibration modes.

Starting from the results of the previous table, it is possibly to conclude that the first two first modes experimentally determined in the N-S are bending modes, the same as the first two modes in E-W are also bending modes. This chimney is much more rigid in E-W than N-S. The same procedure is used to determine the dynamic characteristics of the chimney situated near of the Congress Palace.

406 IOMAC'09 – 3rd International Operational Modal Analysis Conference

Table 1. Results from FFT from Gaspar Aguilar Chimney. First Natural freq.

(Hz) Second natural freq.

(Hz) Acc. N-S direction, 21.5 m height 1.09 3.41 Acc. E-W direction, 21.5 m height 1.35 3.52 Acc. N-S direction, 32 m height. 1.07 3.32

Acc. E-W direction, 32 m 1.34 3.63

Table 2. Results from FFT from chimney situated near form Congress Palace. First Natural freq.

(Hz) Second natural freq.

(Hz) Acc. N-S direction, 13 m height 1.52 4.41 Acc. E-W direction, 13 m height 1.45 4.78 Acc. N-S direction, 13 m height. 1.59 4.32 Acc. E-W direction, 20 m height 1.48 4.73

Starting from the results of the previous table, it is possibly to conclude that the first two first

modes experimentally determined in the N-S are bending modes, the same as the first two modes in E-W are also bending modes. This chimney is more rigid in E-W than N-S.

4 NUMERICAL MODEL

All the previous data were used here to validate a numerical model based on a modal analysis, adjusting the elastic modulus as the main parameter [5].

The finite element method is applied to the masonry structure so a spatial discretization of the structure is performed. Many are the models that can be used to reproduce the behaviour of this particular masonry structure [1] but, in this work, a 3D model has been used to reproduce the actual geometry of the chimney accurately (Fig. 6). This geometry and the subsequent numerical application were made employing the commercial software SAP2000 [4].

3D-M.E.F. model

First mode

Bending. N-S

Second mode Bending E-W

Third mode

Bending N-S

Fourth mode Bending E-W

Figure 6: Numeric model for the Gaspar Aguilar Chimney.

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A macro-model with a homogeneous material was used to show the results presented here. The main characteristics of the masonry material were those previously commented.. The adjusted numerical model presented the following natural frequencies, compared to the experimental ones:

Table 3. Results of modal analysis for the Gaspar Aguilar chimney

Natural frequencies 3D numerical model (Hz) Experimental frequency (Hz) 1 1.12 1.08 2 1.40 1.35 3 3.5 3.41 4 3.56 3.52

The elastic modulus adjusted for this chimney is: 4.35 e9 N/m2 A same procedure has been developed for the Palace of congresses chimney, the modal

analysis results are presented in table 4.

Table 4. Results of modal analysis for the Palace of Congresses chimney Natural frequencies 3D numerical model (Hz) Experimental frequency (Hz)

1 1.48 1.45 2 1.55 1.52 3 4.38 4.41 4 4.78 4.75

The elastic modulus adjusted for this chimney is: 4.75 e9 N/m2

5 CONCLUSIONS

The realized analysis allows to conclude which is of the two studied towers more rigid. It has been possible to determine the dynamic characteristics of these structures, this is a previous step for the calibration of a numeric model and to be able to study their seismic behaviour. The described tests present the natural frequencies and the damping ratio of each one of the two masonry chimneys studied.

REFERENCES

1. Lourenço P.B., Computational strategies for masonry structures, PhD. Thesis, Civil Engineering Department, Delft University, The Netherlands, (1996).

2. SAP2000. Theory Reference Manual. CSI-Berkeley. 2008 3. IVORRA S., and PALLARÉS F.J. Dynamic investigations on a masonry bell tower. Engineering

Structures, 2006, 28, No. 5, 660-667. 4. IVORRA S., PALLARÉS F.J., and ADAM J.M. Dynamic behaviour of a modern bell tower – A case

study. Engineering Structures, 2008 (Article in press - Accepted for publication). 5. Ivorra, Ph.D Thesis,” Dynamic actions in bell towers by swinging bells”, (in Spanish), Polytechnical

University of Valencia, 2002. 6. Paz, M. Leigh, W. (2006) Structural Dynamics: Theory and Computation. Springer 7. Luis Ramos, Laurent Mevel, PauloB. Lourenço, Guido De Roeck, Dynamic monitoring of historical

masonry structures for damage identification, in Proceedings of the 26th International Modal Analysis Conference (IMAC-XXVI), Orlando, Fl, US, February 2008