The AMBIENT VIBRATION measurement is a kind of output data-only dynamic TESTING where the traffics and winds are used as agents responsible for natural or environmental excitation. Therefore an experimental modal analysis procedure for AMBIENT VIBRATION TESTING needs to base itself on output-only data. The modal analysis involving output-only measurements present a challenge that requires the use of special modal identification technique, which can deal with very small magnitude of AMBIENT VIBRATION contaminated by noise. Two complementary modal analysis methods are implemented. They are rather simple peak picking (PP) method in frequency domain and more advanced stochastic subspace identification (SSI) method in time domain. According to these methods Graphical User Interface (GUI) software developed in MATLAB. This program is called SIP and can be used in modal parameter identification of large civil structures such as bridges and towers. This paper presents the application of AMBIENT VIBRATION TESTING and experimental modal analysis on large civil engineering structures and also gives brief review of program. Also modal parameters of the Babolsar arch steel bridge are extracted using SIP and AMBIENT VIBRATION data from dynamic test carried out on this bridge.

Infrastructures such as bridges, buildings, pipelines, marine structures, etc., play an important role in human life. Since major disasters in these structures, such as the collapse of bridges or buildings, often result in many casualties, damages, and social and economic problems, most industrialized countries allocate significant funds to monitor their health. Failure detection strategies and continuous monitoring of the structure's condition, especially after natural and manufactured disasters, make necessary measures to be taken in the early stages of failure and can reduce the cost of maintenance and the possibility of collapse. Structural health monitoring methods often provide an opportunity to reduce maintenance, repair, and retrofit costs during the structure's life cycle. Most of the structural health monitoring methods proposed and implemented to identify possible damages depend on the structure's dynamic characteristics. One of the most practical methods, which uses the results of time domain system identification to detect failure, is the damage locating vector (DLV) method. The DLV method aims to identify load combinations that result in zero strain fields for damaged members in both healthy and damaged structures. To accomplish this, we find a vector in the null space of the difference between the plasticity matrices of the two structures. The singular value analysis method is used on the plasticity difference matrix to calculate this space. The method involves applying the space vectors to the healthy structure and recording the internal stresses of the members, which are then converted into weighted normal stress (WSI) using statistical tools. The member with a lower WSI is more likely to be damaged. Since truss structures are usually used in bridges, long-span structures, as well, as a wide range of steel buildings with simple and braced frames, this research uses the covariance-based random subspace optimal method in identifying the modal characteristics, which is very efficient in low excitations, has been taken into consideration to check and monitor health during operation. To investigate the capability of the DLV method in the damage detection of these structures, a 5-story residential building with a simple steel frame was subjected to the Centro earthquake. According to the desired damage scenario, the second and fifth floors were introduced as the damaged floors in this earthquake by applying a 30 and 50% reduction in the cross-section. To account for uncertainty in the data collection, we included the mean root square of the second sensor's data in the results for sensors 3 and 5. As a result of this uncertainty, the damping error between 5 and 10% has been shown in the damaged and healthy structure. Using the method (SSI_ORT), it was observed that two DLV vectors were extracted. Further, with the increasing uncertainty of the random VIBRATION test results, it was observed that the extraction DLVs could extract the possible damaged elements with high accuracy. Next, the effect of input and output noises on the results obtained from the DLV method was investigated. This study found that by increasing the SNR of the outputs by 15% while increasing the error of the extracted modal characteristics, the extracted DlVs also lose sufficient accuracy in diagnosing structural damage.

Condition monitoring of old railway bridges is a very essential and important subject.Dynamic TESTING is carried out to determine characteristics of the bridge. Although, dynamic TESTING with shaker or drop weight is expensive, but dynamic TESTING with AMBIENT VIBRATION is very economical. The method of analyzing the AMBIENT signals is still under study. In this study, the AMBIENT VIBRATION TESTING of Akbar Abad railway bridge was carried out. The acceleration signals of the bridge were processed by stochastic subspace identification method. The state space model of the bridge was identified and the modal characteristics of the system were calculated.

In stochastic subspace methods, the most important factor influencing the dynamic specifications is the dimensions of the Hankel matrix include the number of rows and columns. Using small matrix dimensions is unlikely to identify existing poles, and selecting very large dimensions not only increases the likelihood of virtual and bias poles but also increases computational costs. In this study, it is intended that the optimal dimensions of the Hankel matrix in the balanced stochastic subspace method be calculated in such a way that in addition to covering the existing poles, it also has a minimum computational cost. For this purpose, the condition number of the Hankel Matrix and Energy Indicator is used in two steps. The steps are as follows: First, calculate the optimal order of each cycle, and then use the optimal order to draw the condition number of the system matrix for different dimensions and calculate the desired dimension from its convergence. To verify the accuracy of the proposed method, the AMBIENT VIBRATION test of the Namin Entrance Bridge has been used. This bridge is located at the entrance of Namin city, 25 km from the center of Ardabil province, Iran, which includes two spans of 27. 10m with a concrete deck. The deck of the bridge is located on beams with I sections, which are 2. 5m away from each other, and the whole set of beams and deck is located on a system of foundations and piles with a diameter of 120cm. This bridge being the only entrance to the city and is exposed to various traffic loads, it was necessary to monitor the dynamic characteristics of the bridge as modal frequencies and damping ratios to evaluate the performance and ensure the health of the bridge structure. According to the numerical analysis and the length of the data (12000), the minimum order and the maximum number of cycles are 22 and 55, respectively. By diverging the curvature of the energy indicator graph, the optimal order is determined in the initial 5-12% of the singular values of cycles. For example, the maximum order of the 6th cycles was obtained, 28-62. Also, from the convergence of the maximum condition number of cycles from the 8th cycle, the optimal dimension was selected 352. In a general summary, it can be said that the use of the energy indicator concept in finding the effective order of the stability diagram has a significant effect on reducing the uncertainty of the extracted results. So that from the three identified stable poles, two poles have been extracted in the effective-order area. Also, using the concept of conditional number to find the optimal dimension of the system was effective, so that by drawing a stability diagram for the 15th cycle, it was found that the identified modal characteristics were not significantly different from the results of the optimal cycle (8th). Finally, the extracted modal properties have an acceptable agreement with the numerical model and frequency domain decomposition method (FDD). The modal frequencies of both methods (FDD & B-SSI) have a good correlation but the damping ratios were very different. In frequency domain methods the damping ratios being very sensitive to the quality of data collection, one can expect that the results of the subspace method are closer to reality.

The VIBRATIONs applied to bridges during their normal operation conditions can be defined as AMBIENT VIBRATIONs. These VIBRATIONs include the effects of traffic, wind and other low excitations imposed to the structure. Detection and evaluation of the location and dimensions of the damages are accounted as one of the most important stages for structural maintenance. These damages may often occur due to ignorance of standard clauses during structural design, neglecting proper construction regulations, high age of the structure and improper structural maintenance. Using a simple and classic method for damage detection in bridge piers can be significantly efficient. Nowadays, metaheuristic algorithms are widely used in structures. Although in this study, it is aimed to use only mathematical and classic methods of optimization, which imply better physical understanding, however in this paper, a new method based on inverse problem, is proposed to detect AMBIENT damages in bridge piers. In this proposed method, firstly the damage percentage and its location are assumed based on a typical damage scenario. Then, assuming lack of sufficient VIBRATION data, the variable (stiffness degradation percentage) is defined using optimization method and introducing objective function for axial flexural member of the bridge pier. Assuming linear damage up to spalling of the concrete, the errors at three modes using linear optimization simplex method are revealed 0.7, 0.1 and 1.45 percent (mostly three equations). The obtained results showed good agreement with the initial values. Also, results of an experiment are used to validate the models. Evaluation of the experimental and analytical results showed that the proposed method for damage detection of RC bridge piers can be precisely applied.

Different approaches are presented for systems identification in the literature. Benchmark problems for the system identification and damage detection of civil engineering structures are established, and different methods are illustrated by international participants. In this paper, the dynamic characteristics of a three story shear frame, subjected to nonstationary white noise excitation are identified by the use of Natural Excitation Technique (NExT), Wavelet and Hilbert transforms. Because the AMBIENT VIBRATION imposed on the system is nonstationary, the response acceleration of the system is also nonstationary. Therefore, a method is used to turn nonstationary signals into stationary ones. Natural Excitation Technique is applied to extract free VIBRATION responses of the system from the available stationary signals. Continuous Wavelet Transform (CWT) of free VIBRATION decay decomposes the signals to a set of sub-signals corresponding to natural VIBRATION modes. The mother wavelet used is modified complex morlet wavelet. Analytical complex signals are extracted from the mentioned sub-signals using Hilbert Transform. The Hilbert transform is applied to each modal response to obtain the instantaneous phase angle and amplitude as functions of time t. Then, a linear least-square fit algorithm is used to fit the instantaneous phase angle and the log of instantaneous amplitude. From the slopes of these linear least-square lines, the natural frequency and damping ratio of each mode can be identified. Based on a single measurement of the free VIBRATION time history at a proper location of the MDOF linear system, all natural frequencies and damping ratios can be identified. When the responses at all degrees of freedom are measured, the complete system dynamic characteristics can all be identified, including the mode shapes, damping and stiffness matrices. The applications of the proposed method are illustrated in detail using a linear three degrees of freedom shear frame. Simulation results show that the accuracy of the method in identifying the system characteristics is remarkable.

AMBIENT VIBRATION test is an effective and economical means for identification of dynamic properties of structures such as dams. Mathematical models generally are developed for the design purpose. Structural and material parameters are assumed from similar projects or limited material tests. It was found desirable to verify the results obtained from mathematical model used regularly in the Iranian dam design practice by comparing with the behavior of the actual as - built structures. Indeed this was done by dynamic tests and good correlation was found. In this paper a new combination of different techniques is employed to achieve highest possible precision using the minimum available hardware.

Abstract: There is a worldwide interest in the proper design of embankment darns to ‎resist earthquake loadings. For the first time in Iran, a complete AMBIENT VIBRATION ‎survey due to low-level loads such as wind, machinery activities, low level tectonic ‎activities, and water exit from bottom outlet was performed on Marun embankment ‎dam. These kinds of AMBIENT VIBRATION tests are suitable for manifesting the lower ‎VIBRATION modes of the dam body. Using different signal processing methods such as ‎Power Spectra Density, the results of in-situ tests have been used to evaluate the ‎natural frequencies, mode shapes and modal damping of the dam body. Besides ‎AMBIENT VIBRATION tests, the 3-D modal analysis of the dam body was performed using ‎ANSYS software. The foundation and abutment flexibility effects on dynamic ‎characteristics of the dam body were investigated and the dynamic soil properties ‎were used from Engineer s report and some empirical relations. Also initial shear ‎modulus of the dam body and foundation materials was evaluated by refraction ‎survey. In this paper, the test procedures, related signal processing results, numerical ‎analysis results and its comparison with the dynamic characteristics of the dam body ‎obtained from the full-scale dynamic tests will be presented Finally, calibrating ‎procedures of the numerical model (based on increasing the accuracy of dam body ‎geometry, soil and rock material parameters and foundation and abutment flexibility) ‎will be discussed‏.‏