Debonding Damage Detection and Assessment in a CFST Composite Column Using Modal Dynamic Data

Structural damage detection is one of the primary goals of structural health monitoring. Minimum safety can be provided upon timely identification of the damaged elements and appropriate decisions (repairing or replacing the damaged elements). Today, the use of concrete-filled steel tube composite columns in the construction industry, especially high-rise buildings, is increasing. In these columns, the concrete core debonding from the steel tube is considered a prevalent type of damage. This study discusses the impact of such debonding on dynamic modal properties (natural frequencies and vibration mode shapes) and the detection of debonding damage area based on wavelet analysis. Debonding to a depth of 3 mm is defined as reduction of concrete stiffness in connection with the steel tube, and the column was subjected to frequency analysis. Modal information, including frequency values and vibration mode shapes, were extracted. Differences in frequency values and Modal Assurance Criterion (MAC) smaller than one were observed between primary and secondary shapes of vibrational modes due to the presence of debonded areas. The results showed that with the addition of a new debonding damaged area, the rate of reduction of frequency values increased. The damage index was proposed based on the detail coefficients obtained from discrete wavelet analysis of primary and secondary shapes of vibration modes to identify the area of detachment damage. The results demonstrated that the relative minimum and maximum values of the damage index for all modes occurred in debonding damaged areas. Moreover, the damage index values for different damaged areas were independent of each other. Indeed, the damage index values for other debonding damage situations did not change when a new debonding damaged area was added. This is a positive point in the damage detection process with multiple debonded areas because in this case, the inability to detect a debonding damaged area cannot affect the detection of other debonding damage situations

Estimation of the peak response of nonlinear systems under the effect of near-field design spectrum using equivalent linearization methods

One of the important issues in performance-based earthquake engineering is the accurate estimation of the response statistics of nonlinear systems under seismic excitation. In this study, the near-field effects contained in the building design regulations against earthquakes (Standard No. 2800, fourth edition) are investigated on the nonlinear response of single degree of freedom systems with bilinear hysteretic restoring force characteristics. Near-field effects are presented in the Standard No. 2800 as spectral correction factor “N” in different seismicity regions. In this study, the site is located in a very high-seismicity zone and the ground type is considered as type II. Second- and third-order linearization methods are used to estimate the response of nonlinear systems. These methods are applied to derive equivalent linear properties. To achieve research goals, it is necessary to represent the input excitation as a quasi-stationary stochastic process compatible with target elastic design spectrum. In this regard, an effective numerical method in the field of random vibrations was used to determine the response spectrum compatible power spectra. After calculating the equivalent parameters related to linearization methods, random vibration theory and time history analysis approaches within the linear range were used to calculate the response of equivalent linear systems. In order to validate the results, the response values obtained from equivalent linearization methods for both approaches were compared with the results of nonlinear time history analysis (NTHA). For this purpose, 250 artificial non-stationary records compatible with two modes of the target spectrum were used to provide reasonable estimates of the peak response of the bilinear hysteretic systems. Artificial records were generated using simulation methods in the field of random vibrations by considering specified envelope functions and power spectral density. According to the results of this study, the second-order linearization method has insufficient accuracy in estimating the response of nonlinear systems under severe excitation. As the natural period of the systems increases, the discrepancy between the results of the equivalent linear system and the results of NTHA increases. Moreover, the good agreement between the third-order linearization results and the results of NTHA indicates the high efficiency of this method in estimating the response of nonlinear systems under the near field excitation

Reliability assessment of wind load combinations based on Iranian National Building Code

The new generation of design codes is established by reliability-based calibration methods. The overall aims of these methods are to achieve consistent levels of safety or structural reliability under different type of uncertainties. According to these methods, the acceptable reliability level of structures is obtained based on statistical descriptions of loads and resistance and also consideration of different types of uncertainties such as the physical uncertainty, the statistical uncertainty and the model uncertainty. In the last decades, based on reliability-based calibration approaches, load and resistance factor design (LRFD) method has been developed for steel buildings design. In this method desired level of safety is obtained by a set of partial load and resistance factor. The design load combinations for steel structures in Iranian National Building Code (INBC), Part 6, are generally based on other codes such as ASCE/SEI 7-10 standard and National Building Code of Canada (for wind load factors), while the effect of Iranian statistical data for load and resistance has not been considered. In comparison to other loads, such as gravity loads, wind load has a high degree of uncertainty and also it is completely site dependent. Therefore it is important to estimate a suitable statistical model for wind load and also investigate the reliability level of structures subjected to wind load combinations. This paper is a parametric study to assessment the reliability level of wind load combinations for steel beams based on INBC. For this purpose, wind load statistical data are provided for whole of Iran by the climatology data of wind speeds. Based on the FOSM method explicit formulation for reliability index of beams is calculated. The reliability indices for a range of practical load ratios are obtained and compared to the target reliability index. The results indicated that reliability level of wind load combinations in INBC is lower than target reliability index. One of the main reasons for the low level of reliability index for wind load combinations is related to underestimation of reference speed. The results show that by considering the reference speeds based on statistical data, reliability index approaches to target reliability index