Development of a low cost system for modal parameters identification under ambient vibration: A low rise building as a case study

Highlights

• A low cost acquisition and processing system has been developed to characterize the response of the structure.

• A good agreement between the dynamic parameters determined by the different methods of operational modal analysis was obtained.

• A good agreement between the experimental results and the finite element modeling was performed.

Abstract

The dynamic behavior analysis of the existing buildings is a subject of great importance for seismic risk and vulnerability evaluation. The difficulty consists in the accurate determination of the parameters characterizing the main modes of a structure. Recently, research studies have been oriented towards the use of ambient vibrations to extract such parameters. Indeed, this method allows extracting modal parameters such as natural frequencies, damping and mode shapes without using any artificial excitation. Based on this approach, acquisition and processing systems can be found on the market. However, these systems remain relatively expensive and are more adapted to high-rise structures. In the case of low-rise building, such systems might not be an optimal solution. In this work, we have developed a low-cost acquisition and processing system which is more appropriate to low-rise buildings. This system can measure the response of this kind of structure under ambient solicitations and extract its modal parameters with high precision. Dynamic parameters are extracted using different operational modal analysis (OMA) based methods and finite element modelling. The experimental results show good agreement with the numerical data. The error percentages for the 1^st, 2^nd, 3^rd, 4^th and 5^th modes are 0%, 2%, 1.7%, 4% and 0.1%, respectively. Consequently, it was concluded that ambient vibration is a confident approach for identifying the modal parameters of structures.

Introduction

Modal analysis under ambient vibration, also called Operational Modal Analysis (OMA), is an efficient way to study and monitor the stability of civil engineering structures. This analysis characterizes the dynamic behavior of a structure under ambient solicitations by identifying its modal parameters such as damping coefficients, modal frequencies, and mode shapes. This technique presents the advantage of using the ambient solicitations, which can be generated by several sources such as ocean swell, wind, etc., without the necessity of any artificial excitation. This technique, called Output-only, is often preferable because it avoids the drawbacks of forced solicitations. Among these drawbacks, it is very difficult to provide adequate excitation for high-rise building structures. In addition, artificial loading is generally expensive and risks damaging the structure [1].

Several output-only system identification methods in the literature may be classed into two classifications: methods in the frequency-domain and time-domain. In the first category, there are mainly methods that are based on the estimation of the spectral density function, such as the Peak Picking (PP) method [2], the Frequency Domain Decomposition (FDD), and the Enhanced Frequency Domain Decomposition (EFDD) [3]. On the other hand, among the temporal-domain based methods, we find the Random Decrement Technique (RDT) [4], Multimode Random Decrement Technique (MRDT) [5], the Eigensystem Realization Algorithm (ERA) [6].

In addition, several research works on the modal identification of structures have been performed. Among these studies: Michel et al. [7] have shown the usefulness and effectiveness of OMA methods for understanding the dynamic behavior of structures. Antonio et al. [8], studied the structural identification of high-rise structures using the model update approach. Foti et al. [9] applied operational modal analysis (OMA) techniques to identify the mode shapes and modal frequencies of the construction.

In recent years, the development of operational modal analysis methods remains the most appropriate for structural health monitoring. Indeed, they allow characterizing the state of integrity of structures or materials without damaging them (contrary to destructive methods). In addition, many studies have used the frequency change as an indicator of damage. This approach is based on the assumption that natural frequencies are sensitive indicators of structural integrity. It has indeed been observed that changes in structural properties cause changes in the structure's natural frequencies. Among the research works that show the usefulness of modal identification in structural health monitoring: Kaloni et al. [10] showed damage detection using non-parametric methods. Mishra et al. [11] presented several case studies on real structures and laboratory research for structural health monitoring of civil engineering constructions. Nicoletti et al. [12] used ambient vibration testing at different construction phases to monitor the evolution of building modal properties during major construction phases.

Consequently, identifying the modal parameters (frequency, damping, and mode shapes) is essential because it can be used as a basis for a structural health monitoring method.

Currently, several 'black box' systems exist on the market for structure modal parameter identification. These systems are mainly developed to receive data from many sensors that can be installed in high-rise structures. However, these systems are expensive and not considered an optimal solution for low-rise structures where the number of required sensors is significantly reduced. Furthermore, they are inaccessible, black-box and thus inappropriate for many scientists and engineers all over the world. The main motivation behind this work is to develop a low cost system to identify modal parameters of low-rise structures. The developed system is explained in details so it can be replicated by others. In addition, several operational modal analysis methods are explained and implemented. This system allows us applying OMA to low-rise structures in Morocco. To the knowledge of the authors, this has never been performed in the past. The comparison between the obtained results using OMA methods and finite element are performed in order to help the reader choosing the appropriate method and in the same way demonstrate the reliability of the system.

In this paper, we present a description of the acquisition chain developed to record the response of a low-rise building. The extraction of the modal parameters (frequency and damping) was performed using both the frequency domain methods (PP, FDD, and EFDD) and the time domain methods (RDT and MRDT). To validate the obtained experimental measurement results, the Finite Element Method (FEM) was used.