EMD-based output-only identification of mode shapes of linear structures

Omid Bahar,Soheil Ramezani 국제구조공학회 2015 Smart Structures and Systems, An International Jou Vol.16 No.5

The Hilbert-Huang transform (HHT) consists of empirical mode decomposition (EMD) and Hilbert spectral analysis. EMD has been successfully applied for identification of mode shapes of structures based on input-output approaches. This paper aims to extend application of EMD for output-only identification of mode shapes of linear structures. In this regard, a new simple and efficient method based on band-pass filtering and EMD is proposed. Having rather accurate estimates of modal frequencies from measured responses, the proposed method is capable to extract the corresponding mode shapes. In order to evaluate the accuracy and performance of the proposed identification method, two case studies are considered. In the first case, the performance of the method is validated through the analysis of simulated responses obtained from an analytical structural model with known dynamical properties. The low-amplitude responses recorded from the UCLA Factor Building during the 2004 Parkfield earthquake are used in the second case to identify the first three mode shapes of the building in three different directions. The results demonstrate the remarkable ability of the proposed method in correct estimation of mode shapes of the linear structures based on rather accurate modal frequencies.

Hybrid Coupled Building Control for Similar Adjacent Buildings

Farshid Fathi,Omid Bahar 대한토목학회 2017 KSCE JOURNAL OF CIVIL ENGINEERING Vol.21 No.1

The concept of connecting two adjacent buildings has been effectively used to mitigate the structural responses of such buildings due to strong ground motions. Lots of analytical and experimental studies have shown the efficiency of this conventional strategy for dissimilar coupled buildings. However, it is absolutely inefficient for the similar connected buildings. The authors, in their previous works, have provided a new Hybrid Coupled Building Control method (HCBC method) which is applicable to all adjacent buildings. This control strategy reduces the structural responses, and thus reduces the internal forces, of the connected buildings more efficiently than the conventional Coupled Building Control method. In order to apply this strategy, one of the buildings is equipped with a base isolation system, and an active actuator link connects the upper floor of the shorter building to its corresponding floor of the other building. In this paper, the HCBC method is developed for the similar coupled buildings. Herein, this structural control method is applied to two similar connected moment-resisting frame buildings. The best performance of the system is obtained, by designing the control system, so that the two connected buildings stay within their elastic range due to the ground motions. The performances of the HCBC models using active and passive strategies are also compared in details.

Dynamic identification of soil-structure system designed by direct displacement-based method for different site conditions

Vahidreza Mahmoudabadi,Omid Bahar,Mohammad Kazem Jafari,Amir Safiey 국제구조공학회 2019 Structural Engineering and Mechanics, An Int'l Jou Vol.71 No.4

This study mainly aims to assess the performance of soil-structure systems designed by direct displacement-based method coupled with strong column-weak beam design concept through various system identification techniques under strong ground motions. To this end, various system identification methods are employed to evaluate the dynamic characteristics of a structure (i.e., modal frequency, system damping, mode shapes, and plastic hinge formation pattern) under a strong seismic excitation considering soil-structure interaction for different site conditions as specified by ASCE 7-10. The scope of the study narrowed down to the code-complying low- to high-rise steel moment resisting frames with various heights (4, 8, 12, 16-story). The comparison of the result of soil-structure systems with fix-based support condition indicates that the modal frequencies of these systems are highly influenced by the structure heights, specifically for the softer soils. This trend is more significant for higher modes of the system which can considerably dominate the response of structures in which the higher modes have more contribution in dynamic response. Amongst all studied modes of the vibration, the damping ratio estimated for the first mode is relatively the closet to the initial assumed damping ratios. Moreover, it was found that fewer plastic hinges are developed in the structure of soil-structure systems with a softer soil which contradicts the general expectation of higher damageability of such structural systems.

Structural damage identification for elements and connections using an improved genetic algorithm

Meysam Ramezani,Omid Bahar 국제구조공학회 2021 Smart Structures and Systems, An International Jou Vol.28 No.5

All structures are exposed to damage during their lifetime. Timely detection of the damages can prevent the reduction of stiffness/resistance of structures. The large number of elements in structures in comparison to the number of measurable data can limit the performance of the closed-form methods. This study presents a new damage detection method determining damage severity and location in the elements and connections via Improved Genetic Algorithm (IGA) based on limited number of mode shapes. This study describes how damage can be accurately identified based on the least number of modes. In this approach, healthy elements are identified by the IGA algorithm and removed from the search space. In this way, the damaged elements are examined more carefully and the severity of the damage is estimated more accurately. In this study, to evaluate the performance of the proposed method, two numerical examples are used. The numerical study includes a 2D truss structure under 4 damage scenarios and a 3D structure with a much larger number of elements under 6 different damage scenarios. Moreover, the performance of this algorithm in presence of noise in modal information is also examined. The results show that the proposed method can accurately detect damage to elements and connections, even in the presence of noise, by using only one mode in the 2D truss and two modes in the 3D structure. In order to evaluate the efficiency of this method in determining the damage of connections, a cantilever beam has been modeled and experimentally tested. The connection stiffness of the beam has been computed using both IGA and load-deformation measurement methods. In the IGA method only the first mode shape of the beam is employed to determine the connection stiffness. To derive the mode shapes in rotational degrees of freedoms which contain valuable information on connection stiffness, a novel, straightforward, and practical approach has been proposed. The results also indicate the high performance of this method to accurately estimate the connection stiffness.

Damage detection using both energy and displacement damage index on the ASCE benchmark problem

Mohammad Javad Khosraviani,Omid Bahar,Seyed Hooman Ghasemi 국제구조공학회 2021 Structural Engineering and Mechanics, An Int'l Jou Vol.77 No.2

This paper aims to present a novelty damage detection method to identify damage locations by the simultaneous use of both the energy and displacement damage indices. Using this novelty method, the damaged location and even the damaged floor are accurately detected. As a first method, a combination of the instantaneous frequency energy index (EDI) and the structural acceleration responses are used. To evaluate the first method and also present a rapid assessment method, the Displacement Damage Index (DDI), which consists of the error reliability (β) and Normal Probability Density Function (NPDF) indices, are introduced. The innovation of this method is the simultaneous use of displacement-acceleration responses during one process, which is more effective in the rapid evaluation of damage patterns with velocity vectors. In order to evaluate the effectiveness of the proposed method, various damage scenarios of the ASCE benchmark problem, and the effects of measurement noise were studied numerically. Extensive analyses show that the rapid proposed method is capable of accurately detecting the location of sparse damages through the building. Finally, the proposed method was validated by experimental studies of a six‐story steel building structure with single and multiple damage cases.

Global and Local Damage Detection in Continuous Bridge Decks Using Instantaneous Amplitude Energy and Cross-correlation Function Methods

Mohammad Javad Khosraviani,Omid Bahar,Seyed Hooman Ghasemi 대한토목학회 2021 KSCE JOURNAL OF CIVIL ENGINEERING Vol.25 No.2

This paper attempts to detect damage in the length of continuous deck bridges by providing new methods based on acceleration responses and their Instantaneous Amplitudes (IAs). The DD in this paper has two steps: firstly, determining the vicinity of damage in global damage detection; secondly, determining the location of damage in local damage detection. Then by acceleration signals, the IA values of healthy and damaged structural responses extracted using Hilbert-Huang Transform (HHT). Further, with the impression of the IA energy damage index (EDA), damage location is detected. Moreover, for the accurate evaluation of the proposed method, damage locations are determined using the cross-correlation damage index (CCDI). To assess the feasibility and reliability of the proposed methods, several analytical models of concrete bridges of one to three-spans, as well as experimental models of a simply supported steel beam, slab-on-girder composite bridge, and the UFC benchmark bridge are investigated. To consider noise pollution during data acquisition, a certain amount of noise is added to the response. The comparison of the obtained analytical results with the experimental ones showed that the proposed methods can determine the damage locations with appropriate accuracy for different damage scenarios. Also, the results of the CCDI index concluded to the acceptable exact results associated with a rapid estimation.

Investigation of the LS Level Hysteretic Damping Capacity of Steel MR Frames Needs for the Direct Displacement-Based Design Method

Reza Esmaeil Abadi,Omid Bahar 대한토목학회 2018 KSCE JOURNAL OF CIVIL ENGINEERING Vol.22 No.4

Proper modeling of the Hysteretic Damping (HD) capacity of structural models at a desired performance level is the keyrequirement for reliable estimation of design base shears in Direct Displacement-based Design (DDBD) method. Priestley and hiscolleagues on the basis of their research, have proposed a formula to predict the HD capacity of Steel Moment-resisting Frames(SMRFs) which is presented in DBD12. The current study examined this relation and proposes a reliable equation for design ofSMRFs using DDBD method at the Life Safety (LS) performance level and the Equivalent Viscous Damping (EVD) hypothesis. Forthis purpose, a wide range of SMRFs were analysed using linear/nonlinear static/dynamic time history methods under differentloading conditions. The damping ratios of all models were calculated using Jacobsen’s and Jennings’s formulas and the procedureproposed by FEMA-440. The results show an exponential trend that diverges from the empirical formula presented in DBD12. Twonew relations are proposed for hysteretic damping based on ductility and the ratios of the initial and equivalent periods.

Approximate Methods to Estimate Residual Drift Demands in Steel Structures with Viscous Dampers Designed by the DDBD Approach

Seyed Behdad Alehojjat,Masood Yakhchalian,Omid Bahar 한국강구조학회 2023 International Journal of Steel Structures Vol.23 No.3

Nowadays, using performance-based design procedures are widely applied by structural engineers. The direct displacement-based design (DDBD) approach is also one of the most efficient and popular procedures in these frameworks. This paper investigates the residual inter-story drift ratio (RIDR) demands in the mid-rise steel structures equipped with fluid viscous dampers (FVDs) designed by the modified DDBD approach. For this purpose, the capability of three approximate methods, i.e., the FEMA P-58, Erochko et al. and coefficients methods for estimating RIDR demands is studied. These methods have been proposed for moment-resisting frames (MRFs) without dampers. In order to evaluate the estimated RIDR demands in the approximate methods, three mid-rise steel MRFs with different velocity exponents for FVDs are designed according to the modified DDBD approach. Nonlinear time-history analyses are carried out by a set of spectrum-matched records at two seismic hazard levels including the designed earthquake (DE) and maximum considered earthquake (MCE). The results show that in most cases the Erochko et al. method overestimates the mean of maximum RIDRs at both the DE and MCE hazard levels. On the other hand, although the FEMA P-58 method overestimates the median of maximum RIDRs, when compared with the corresponding values obtained from analyses at the DE level, it underestimates the values at the MCE level. Also, the coefficients method is calibrated for the structures studied. Finally, a modified equation for a more precise estimation of RIDR demands is presented, by implementing the particle swarm optimization (PSO) algorithm.