Sharif Digital Repository

Probabilistic seismic analysis of structures with incremental dynamic analysis (IDA) is a widely used method to offer comprehensive evaluation of the seismic performance of structures. Although IDA is a powerful computer-intensive method, it is really a time-consuming procedure. Accordingly, in this study, for coping with this problem, significant motion duration is used instead of total motion duration. This truncation can significantly reduce the computational effort and time. In order to determine the influence of truncation, fragility curves and their mean annual frequencies (MAF) in each limit state are used with two different damage indices, namely modified Park & Ang and maximum... 

In this paper, the dynamic characteristics of skewed bridges are explored analytically. Closed form solutions for translational and torsional periods of free vibration and mode shapes are given for slab-girder skewed bridges. Moreover, the seismic displacement of the deck of skewed bridges is calculated using the response spectrum method and its skew term is compared with the requirement of AASHTO. The effects of seismic force resisting elements, such as elastomeric bearings and end diaphragms are included. It is shown that the skew term in AASHTO's equation can underestimate the seat width requirement for some bridges. A new skew term for the bridge seat width requirement is suggested  

A practical optimization method is applied to design nonlinear truss-like structures subjected to seismic excitation. To achieve minimum weight design, inefficient material is gradually shifted from strong parts to weak parts of a structure until a state of uniform deformation prevails. By considering different truss structures, effects of seismic excitation, target ductility and buckling of the compression members on optimum topology are investigated. It is shown that the proposed method could lead to 60% less structural weight compared to optimization methods based on elastic behavior and equivalent static loads, and is efficient at controlling performance parameters under a design... 

The presented studies have been conducted on the scope and magnitude of damages inflicted by earthquakes over the years. Many RC buildings, designed and constructed in the 1960s and 1970s, have serious structural deficiencies, especially in their beam-column joints. Such deficiencies result from inadequate joint shear strength or confinement of column reinforcement. The joints require elaborate retrofit measures to ensure the performances comparable to those of new designed structures. In this research, a new seismic retrofitting method is presented for beam-column joints based on the two-dimensional enlargement of the joint by using steel plate, angles, and posttensioning rods. The seismic... 

Numerous buildings have been damaged or destroyed in previous earthquakes by developing soft storey. Almost all the seismic codes have provisions to prevent soft storey in structures, most of them have recommended the ratio of stiffness between adjacent storeys, but none of them has proposed the method to calculate the storey stiffness. On the other hand a great number of previous researches on stiffness have been focused on approximate methods and accurate methods by using analytical softwares have been almost neglected. In this study, six accurate methods for calculating the storey stiffness have been studied on 246 two-bay reinforced concrete frames. It is shown with the results of the... 

Dual-pipe damper (DPD) is a metallic yielding device for passive control of structures, introduced recently by the authors. The objective of the current study is to provide guidelines for implementing DPDs in actual steel buildings, evaluate and compare their performance against other metallic dampers. In this study, a representative load-displacement model for DPDs is proposed for the first time, and assessed based on previous experimental cyclic tests. Guidelines for the design of DPD devices are also presented. Three steel moment resisting frames of 5, 10 and 20 stories are designed and then equipped with DPDs of various properties. The responses of the frames to seven earthquake... 

During an earthquake, pounding of bridge superstructure into abutments is potentially destructive to structural elements and may cause unseating failure and collapse. By using energy dissipative devices such as passive dampers, the harmful pounding force may be prevented. One of the most economical passive dampers for structural applications is the pipe damper, recently introduced by the authors. In this paper, computer models of a two-span 60-m long conventional overpass bridge are equipped with different lengths of two types of pipe dampers, namely the dual-pipe (DPD) and infilled-pipe (IPD) dampers. A simplified lumped-mass model for the bridge considering soil stiffness is proposed and... 

A simple Dynamic-based Pushover analysis for Plan Asymmetric buildings (DPPA) is proposed with the aim of properly considering the effects of torsional behavior as well as the higher modes in the applied lateral load pattern. According to the proposed method, the peak story drifts obtained from the response spectrum analysis (RSA) can be resolved into their translational and rotational components, and the related equivalent static lateral forces and torsional moments can be calculated. Consequently, for accurate estimation of the maximum drift demands of plan-asymmetric buildings, it is proposed that the drift responses obtained from the RSA for the stiff and flexible edges of the building... 

We report on a stochastic analysis of Earth's vertical velocity time series by using methods originally developed for complex hierarchical systems and, in particular, for turbulent flows. Analysis of the fluctuations of the detrended increments of the series reveals a pronounced transition in their probability density function from Gaussian to non-Gaussian. The transition occurs 5-10 hours prior to a moderate or large earthquake, hence representing a new and reliable precursor for detecting such earthquakes. © 2009 The American Physical Society  

In this paper, a comprehensive accelerogram database of the Iranian plateau containing 3585 data with all three components is gathered. The raw data are processed by the wavelet-based denoising method, and results are compared with the contaminated data. All the data are classified into mainshock and aftershock categories using the time and spatial window method. Afterward, the data are categorized into the pulse-like and non-pulse-like events based on the detection of velocity pulse in any of horizontal and/or vertical directions. Eventually, among 3585 data, the ones with an average shear wave velocity of top 30 m of subsurface soil profile are selected and their important ground motion... 

A smeared crack approach has been proposed to model the static and dynamic behavior of mass concrete in three-dimensional space. The proposed model simulates the tensile fracture on the mass concrete and contains pre-softening behavior, softening initiation, fracture energy conservation and strain rate effects under dynamic loads. The validity of the proposed model has been checked using the available experimental results under static and dynamic loads. The direct and indirect displacement control algorithms have been employed under incremental increasing static loads. It was found that the proposed model gives excellent results and crack profiles when compared with the available data under... 

The current performance-based seismic assessment procedure can be computationally intensive as it requires a large number of time history analyses (THA) each requiring time intensive post-processing of results. This study proposes the endurance time analysis (ETA) method as an alternative method to THA and incremental dynamic analysis (IDA). ETA is a time history based dynamic pushover procedure that applies a set of gradually intensifying acceleration functions to the structure and monitors the performance of the building accordingly. In this paper, the application of ETA in the seismic assessment of multistory steel concentrically braced frames is compared with THA and IDA methods.... 

Employing dampers to control wave-induced and seismic vibrations of offshore jacket platforms is an attractive method in order to mitigate fatigue and seismic damage. However, adjustable parameters of a damper are designed by considering only one type of environmental loads; either normal-condition load or extreme-condition load. So, it is important to investigate effectiveness of damping system, for both of two main categories of environmental loads. Also it is ideal for the system to have an acceptable performance in both normal and extreme conditions. The idea investigated in the current study is to use a friction damper device (FDD) and a tuned mass damper (TMD) simultaneously in... 

In this paper, the seismic response of anchored cylindrical steel tanks with various dimensional parameters has been investigated by endurance time method considering fluid-structure interaction effects. Various response quantities, such as stresses and displacements, have been evaluated by subjecting tanks to specially designed intensifying accelerograms and their performance is judged based on their response at predefined level(s) of dynamic excitation. It is shown that ET analysis can reliably predict the result of ground motion time history analysis in all seven tank models studied. In four of the tanks in which height of the shell and the level of fluid is the same, the variation of... 

In this paper a new approach to risk based seismic assessment of structures is proposed based on acceptable risk levels. In this approach, the probabilistic seismic vulnerability, predicted lifetime of structure, importance and knowledge factors are considered in the evaluation process. Park & Ang damage index is used to quantify damage states of structure. By performing several incremental dynamic analyses, all damage states of structure from minor to heavy and collapse are considered under all possible ground motions. The structural risk level and the necessity of seismic retrofit measures are specified by seismic risk index of structure. This probabilistic performance based method is... 

In this paper completely closed RBS (CC-RBS) is proposed as a function of a new ribbed bracing system (RBS). RBS is an innovative system which has capacity of performing variable stiffness that can be used for controlling structural deformations and frequency shifting to compensate seismic energy. The overstrength, ductility, and response modification factors of steel moment frames equipped with CC-RBS was evaluated by performing pushover analysis of model structures with various stories and compared with those which Iranian seismic code of practice, Standard No. 2800 is supposed for steel moment frame equipped with ordinary brace.The results were compared with those from nonlinear... 

Performance-based optimal seismic design of structures requires repetitive and massive dynamic analyses. In the Endurance Time (ET) method, structures are subjected to specially designed intensifying acceleration functions and their response at various excitation levels is estimated by each single response-history analysis, thus, significantly reducing the required computational demand. In this paper, the ET method is utilized to acquire optimal placement of viscous dampers in short steel frames with the target of obtaining the desirable performance in two hazard levels simultaneously. In the optimization procedure, the damping coefficients of the added dampers are treated as design... 

Abstract Codes require that the span-to-depth ratio of the beams in moment resisting frames (MRF) should be limited to guarantee the development of plastic hinges with a sufficient length at beam ends. This requirement limits the application of MRFs mostly in framed-tube structures. In the current design procedure of steel MRFs, the strength and stiffness design of the structure are coupled often leading to overdesigned structures and foundations. To overcome the abovementioned limits, in this paper a new replaceable shear structural fuse is introduced for MRFs. The fuse is a shear link designed as a sacrificial component by weakening the shear strength of a segment at the middle of the beam... 

The endurance time method (ET) is a dynamic analysis procedure in which intensifying dynamic excitations are used as the loading function. Seismic performance is evaluated over the entire range of equivalent seismic intensities of interest in each response history analysis, considerably reducing computational demand. In this paper, a general methodology for performance-based retrof itting of typical steel frames by the ET method is discussed. A baseline steel momentresisting frame is considered, and its seismic performance is evaluated by the ET method, considering viscous dampers and shear wall panels as strengthening alternatives. Each retrof itting option includes several design...