D. S. Li’s research while affiliated with Dalian University of Technology and other places

Structure frequency, which is always regarded as damage detection feature for its more convenient testability and higher measurement precision, is also susceptible to environmental and operational variations. The variations lead to frequency sequence non-stationary and make much disturbances for actual structure health monitoring and damage detection. So cointegration method in econometrics is introduced to deal with the non-stationary problem by the linear combination of non-stationary variables, thus the non-stationary problem for structure damage detection feature caused by environmental factor variation is transformed to stationary problem. At first, ADF (augmented Dickey-Fuller) test and EG (Engle-Granger) test are detailedly introduced to check non-stationary order of the feature sequence and calculate the cointegration coefficients. Next, the cointegration relationship between frequency sequences is verified using a simple supported beam and online damage detection procedure based on cointegration of frequency under the influence of environmental temperature is introduced. At last, the validity and robustness of the proposed method are illustrated through a prestressed concrete beam and a simple supported steel bridge.

Fiber Bragg grating (FBG) sensors demonstrate great potentials for structural health monitoring of civil structures to ensure their structural integrity, durability and reliability. The advantages of applying fiber optic sensors to a tall building include their immunity of electromagnetic interference and multiplexing ability to transfer optical signals over a long distance. In the work, FBG sensors, including strain and temperature sensors, are applied to the construction monitoring of an 18-floor tall building starting from its construction date. The main purposes of the project are: 1) monitoring the temperature evolution history within the concrete during the pouring process; 2) measuring the variations of the main column strains on the underground floor while upper 18 floors were subsequently added on; and 3) monitoring the relative displacements between two foundation blocks. The FBG sensors have been installed and interrogated continuously for more than five months. Monitoring results of temperature and strains during the period are presented in the paper. Furthermore, the lag behavior between the concrete temperature and its surrounding air temperature is investigated.

The nature of time delay of earthquake excitation in structures from the perspective of wave propagation is addressed, and then the time delay is used to modify conventional structural dynamics equation, which assumes implicitly the simultaneity of earthquake initial excitations at all degrees of freedom of a structure. A simple and practical method is then developed to compute structural responses considering the time delay effects of earthquake excitations in the structure. Through the computation of floor displacement and top floor maximum displacement according to current standards on seismic design of buildings, it was shown that conventional method neglecting the time delay effects of wave propagation in structures would not provide accurate structural responses, and should be modified. The introduction of the time delay effects of wave propagation in structures is thus of great importance for the safety of more and more supertall buildings to be constructed. The method proposed in the paper deals with this problem and provides a practical solution by incorporating time delay effects on analyzing structural responses for high-rise buildings subjected to earthquake excitations.

Fiber Bragg grating (FBG) sensors demonstrate great potentials for structural health monitoring of civil structures to ensure their structural integrity, durability and reliability. The advantages of applying fiber optic sensors to a tall building include their immunity of electromagnetic interference and multiplexing ability to transfer optical signals over a long distance. In the work, FBG sensors, including strain and temperature sensors, are applied to the construction monitoring of an 18-floor tall building starting from its construction date. The main purposes of the project are: 1) monitoring the temperature evolution history within the concrete during the pouring process; 2) measuring the variations of the main column strains on the underground floor while upper 18 floors were subsequently added on; and 3) monitoring the relative displacements between two foundation blocks. The FBG sensors have been installed and interrogated continuously for more than five months. Monitoring results of temperature and strains during the period are presented in the paper. Furthermore, the lag behavior between the concrete temperature and its surrounding air temperature is investigated.

A novel sensor placement method is proposed for structural health monitoring. The aim of the method is to select sensor locations from a set of possible candidate positions for achieving the best identification of modal frequencies and mode shapes. The proposed method depends on both the characteristics and the actual loading situations of a structure. It selects the sensor positions with the best subspace approximation of the vibration responses from the linear space spanned by the mode shapes. We search the best subspace approximation with a relative small subset through our recently proposed representative least squares method to approximate the original least squares estimator with the whole data set. The selected data subset is required to be representative and sufficiently approaching to portrait the scenario defined by the original full data set. The basic ideas of the representative least squares method are illustrated by a simple linear regression example. The algorithm for finding the representative subset for subspace approximation is described, and is applied to the I-40 Bridge.

The comparison and inherent relationship between two influencing sensor placement methods, i.e. modal kinetic energy (MKE) and effective independence (EI), are addressed in this paper. The problem is of primary concern for dynamic testing, damage identification and structural health monitoring. By analyzing the sensor placement problem with EI method from the perspective of a new reduced system, the connection of MKE with EI method is revealed. The latter is an iterated version of the former, and the reduced mode shapes are ortho-normalized repeatedly during iterations of the latter. Two alternative forms for efficient computation of the iterative EI method are presented. Finally, both methods are applied to the I-40 Bridge located over the Rio Grande in Albuquerque, New Mexico, and the derived relationship is verified.

The ordinary least squares method to estimate parameters is indefinitely affected by outliers. Regression diagnostics and robust regression methods are proposed to tackle this problem. There are cases in which the entire data set for regression estimation is completely reliable, however, only a small subset is allowed to be actually fitted. The original whole data set may come from past experiments, or from a simulation model, and will only be partially feasible in future estimations. We propose a representative least squares method to deal with such situations. The objective of this approach is to obtain an optimal estimator with a small subset to approximate the original least squares estimator with the whole data set. The selected subset is, therefore, required to be representative and sufficiently approaching to portrait the scenario defined by the original full data set. The basic ideas of the representative least squares method are illustrated by a simple linear regression example. The algorithm for finding the representative subset is described, and finally the properties of the representative least squares estimator are discussed. An initiative excitation-based sensor placement algorithm in structural health monitoring is presented. The proposed method can be used for sensor placement for achieving the best identification of modal frequencies and mode shapes in structural health monitoring. We will discuss its applications in further works.

The comparison and inherent relationship between two influencing sensor placement methods, i.e. Modal Kinetic Energy and Effective Independence, are addressed in this paper. The problem is of primary concern for dynamic testing, damage identification and structural health monitoring. By analyzing the sensor placement problem with Effective Independence method from the perspective of a new reduced system, the connection of Modal Kinetic Energy with Effective Independence method is revealed. The latter is an iterated version of the former, and the reduced mode shapes are ortho-normalized repeatedly during iterations of the latter. Two alternative forms for efficient computation of the iterative Effective Independence method are presented. Finally, both methods are applied to the I-40 Bridge, and the relationship is verified.

This paper discusses of the monitoring of the temperature distribution by an optical fiber Bragg grating sensor system. The sensors are embedded in a pipe underground. Ground-source heat pump system utilizes the thermal energy of the underground soil to heat or cool a building. Optical fiber Bragg grating sensors are used to measure the temperature distribution of the soil and the thermal difference of the circulating water system. The sensors were designed and packaged delicately to eliminate the influence of strain as well as to improve the thermal sensitivity. Three Bragg grating sensors were placed at intervals of five meters in a deep well with a length of 25 meters. Data were collected prior to the installation of the monitoring system and immediately after the installation of the water pipes using a commercial FBG interrogation system. The ultimate purposes of these tests are to measure the water temperature fluctuation along the pipe at the places where the sensors were installed during the operation of the ground source heat pump system, and evaluate the reliability of FBG sensor system for a long period.