Comparison of techniques for modal analysis of concrete structures

doi:10.1016/S0141-0296(99)00054-1

Engineering Structures

Volume 22, Issue 9, September 2000, Pages 1159-1166

https://doi.org/10.1016/S0141-0296(99)00054-1 Get rights and content

Abstract

This paper describes different experimental techniques for obtaining modal parameters of structures. Attention is focused on those techniques that may be applicable to in situ concrete structures (e.g. bridges). In a first stage, experiments are made on reinforced concrete beams of 6 meters length. The beams are excited using three types of excitation methods: impact hammer excitation and two different electromagnetic shaker signals: pseudo-random and swept-sine signals. The modal parameters are determined either by performing curve-fitting procedures on series of measured frequency response functions or by applying the stochastic subspace identification technique to the time response signals of the structure. The influence of the non-linear behaviour of the concrete beams is investigated by performing measurements at different excitation amplitudes.

It appears that modal parameter estimates are affected by excitation techniques, data acquisition parameters and processing methods. The main cause of this is the non-linear behaviour which is observed even at very low vibration amplitudes. However, the influence on resonant frequencies and mode shapes is negligible. This is not the case for the modal damping ratios, so that the estimation of these parameters is unreliable.

Introduction

The comparison of techniques used for the dynamic analysis of concrete structures is a part of a large experimental program. The aim of this program is to investigate how dynamic characteristics can be used to detect and evaluate damage in concrete structures, such as bridges.

In concrete structures, damage is associated with structural modifications, which can be observed through changes in modal parameters: resonant frequencies, modal damping ratios, mode shapes and their derivatives. Modal testing techniques are used to determine these modal parameters, by performing curve fitting procedures either on series of measured frequency response functions (FRFs), or on time histories of the structures dynamic response. Several researchers have already investigated changes in dynamic characteristics with simulated damage, using both model and real structures. A literature review can be found in [1]. One of the main problems encountered is that modal parameter estimates are affected by different variables which are sometimes difficult to evaluate and control, such as environmental conditions, excitation techniques, data acquisition parameters, data processing methods and human factors. If the variation of the modal parameters due to these experimental parameters is of the same order than the variations due to damage, wrong conclusions could be drawn concerning the state of health of the structure.

The importance of the dynamic loading effect due to excitation devices is discussed in [2]. It is shown that the resonant frequencies shift with different excitation devices and the user has to be careful in the interpretation of the results obtained.

In this paper, the results obtained from different excitation techniques, data acquisition parameters and data processing techniques are compared and discussed.

Section snippets

Modal parameter estimation methods: theoretical aspects

In this research, two methods are used to extract the modal parameters from the dynamic measurements: the Frequency Domain direct Parameter Identification (FDPI) method implemented in the CADA-X system [3] and the Stochastic Subspace Identification method (SSI). The first method is a frequency domain technique, where the modal parameters are derived from series of measured frequency response functions. The second method is a time domain technique, and the dynamic characteristics are derived

Experimental work

A series of dynamic measurements are performed on reinforced concrete beams using different excitation techniques. The two previously described data processing methods are applied on the measured data to determine the dynamic characteristics of the beams. These measurements permit to evaluate the influence of excitation techniques and data processing methods on the estimated dynamic characteristics of the beams.

Results and discussions

Only the results obtained for one of the three beams are presented here and commented. Those obtained for the other two beams confirmed the observations.

Modes shapes

Both the previously modal parameter estimation methods discussed permit to clearly identify mode shapes and nodal line patterns. Fig. 7 represents the 6 first eigenmodes of one of the three reinforced concrete beams.

The smoothness of the mode shapes is a result of the high quality of the measurements.

Conclusions

Three reinforced concrete beams were subjected to vibration testing in free boundary conditions. Complete experimental modal analyses were performed using different excitation methods. The modal parameters were determined using the FDPI and SSI methods.

It appears from these experiments that:

•
the modal parameter estimates are affected by data acquisition and processing techniques, more for modal damping ratios and less for resonant frequencies. The modal damping ratios are very sensitive and

Acknowledgements

This work is a part of a research project G-0243.96 sponsored by the Flanders Fund for Scientific Research. Its financial support is gratefully acknowledged.

References (10)

Doebling SW, Farrar CF, Prime MB, Shevitz DW. Damage identification and health monitoring of structural and mechanical...
Olbrechts T, Vandepite D, Sas P, Heylen W. Influence of excitation systems on the dynamic behaviour of test structures....
CADA-X. Modal Analysis: user manual, Rev. 3.4. Belgium: LMS International,...
Peeters B, Wahab MA, De Roeck G, De Visscher J, De Wilde WP, Ndambi JM, Vantomme J. Evaluation of structural damage by...
Bencat J. Statical and Dynamical Testing of Structures. In: Proc. of 2nd RILEM Int. Conference on Diagnosis of concrete...

There are more references available in the full text version of this article.

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