Keywords: structural analysis and design, dynamic analysis, finite element analysis, structural modelling, FEM in design, damage identification.

The ability to assess structural damage of existing constructions has been interest to engineers for a long time ago. The most common forms of deterioration are concrete degradation, corrosion and fatigue related damage. The assessment of the structural resistance of existing constructions such as buildings, bridges, dams etc. uses often mathematical models whose parameters can hardly be precisely estimated by analytical procedure only. Over the years, a range of techniques has been developed for the inspection of structures. Among others, conventional, though efficient, methods such as visual inspection and non-destructive testing are widely used to diagnostic and to evaluate damaged constructions. The information obtained can be very valuable for both damage identification and material properties updating in finite element modelling. In many cases, however, the overall performance may not be adequately estimated solely by these methods.

The vibration data is an attractive way to extract some global characteristics such as the natural frequencies, damping ratios and mode shapes. Particularly, the ambient vibration testing which is appropriate for experimental modal analysis has been used for a long time in investigating the dynamic behaviour of structures at low vibration amplitudes. Recently, many techniques have been developed for detection, location and characterisation of structural damage by dynamic testing taking into account some environmental effects such as the temperature and the excitation sources [1,2,3]. Some of these methods were successfully applied in simulation examples and very controlled experiments [4,5,6]. Powerful identification techniques that are able to locate damage based on realistic measured data sets still seem a long way from being achieved [7].

This paper propose a generalised procedure for practical use of ambient vibration testing in conjunction with analytical modelling for assessment of the global structural condition of a large civil engineering construction which is the structure of the new Algiers airport.

In the first instance, several finite element models were elaborated and the results of non-destructive and destructive tests on the concrete were put into the equations. Preliminary analyses were carried out to determine an optimum number of measurement points. Then series of ambient vibration tests were conducted and modal analyses were performed to validate the mathematical models.

In the light of the results obtained in this particular case, the following observations are noteworthy:

• The preliminary modal analysis is essential to optimize the sensor locations.
• The ambient vibration testing sensitivity to material characteristics is acceptable for ordinary diagnostic purposes of such civil engineering constructions.
• Local flexibilities and imperfect seismic joints can be detected using the ambient vibration tests together with simple FE models.
• The analytical and experimental correlation gives an indication of the overall integrity of the structure in accordance with the hypotheses of the finite element models.

1

S.W. Doebling, C.R. Ferrar, M.B. Prime and D.W. Shevitz, "Damage identification and health monitoring of structures and mechanical systems from changes in their vibration characteristics: a literature review", Los Alamos Report : LA-13070-MS, 1996.

2

A. Felber and R. Cantieni, "Advances in ambient vibration testing: Ganter Bridge, Switzerland", Structural Engineering International, 6(3), 187-190, 1996. doi:10.2749/101686696780495671

3

B. Peeters, J. Maeck and G. De Roeck, "Vibration-based damage detection in civil engineering: excitation sources and temperature effects", Smart Materials and Structures, 10 (2001), 518-527, 2001. doi:10.1088/0964-1726/10/3/314

4

P. Cawley and R.D. Adams, "The location of defeats in structures from measurements of natural frequencies", Journal of strain analysis, 14(2), 49-57, 1979. doi:10.1243/03093247V142049

5

C.H.J. Fox, "The location of effects in structures: a comparison of the use of natural frequency and mode shape data", Proceedings of the Tenth International Modal Analysis Conference, 522-528, 1992.

6

A. Masuda and A. Sone, "Identification of structural degradation by time- frequency system analysis", Proceedings of the 12 WCEE, paper 1010, 2000.

7

M. Friswell and J.E.T. Penny, "The practical limits of damage detection and location using vibration data", Proceedings of the 11VPI&SU symposium on structural dynamics and control, 1-10, 1997.

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