Journal of Seismology and Earthquake Engineering
Volume:17 Issue: 3, Autumn 2015

Seismic control strategy of cable-stayed bridges is usually performed by implementing bearing devices in the connection point of deck and pylon. In this case, owners usually refuse to use superior seismic strategy because of its cost. In Mashhad cable-stayed bridge as a case study, Pot Bearing device has been used probably because of the lower costs, while it is not very effective in seismic behavior. However, Elastomeric Bearing Pads or Lead Rubber Bearings are more effective in absorbing earthquake's energy due to higher damping. Therefore, in this paper, we are going to thoroughly solve a double criterion problem about selecting bearing devices of Mashhad bridge considering the construction costs and earthquake losses. Indeed, if economically justified, this paper tries to improve the passive seismic control device of the Mashhad bridge from its current Pot Bearing to another type. The economic justification is studied using seismic risk assessment process alongside the simultaneous analysis of costs and losses. To achieve this goal, it is necessary to design and control the bridge for seismic behavior with three aforementioned different bearing devices. Then, the seismic risk assessment process is performed for each case. The final results of seismic risk assessment process are achieved as total loss ratio curves. Then, the proposed Cost-Loss-Benefit (CLB) method will compare the three cases by defining Benefit Ratio (BR) as a profitability measure. The final results indicate that both of the alternative cases increase the costs and decrease the losses compared to the existing Pot Bearings. However, simultaneously considering the costs and losses, the BR coefficient reveals the profitability of the use of Lead Rubber Bearings in Mashhad cable-stayed bridge.

Rapid growth of performance-based earthquake engineering has caused increasing interest in Nonlinear Time History Analysis (NLTHA) as an effective tool for the estimation of dynamic structural demand and capacity. Considering the preparation of a set of ground motions as the key step for NLTHA; many ground motion selection and modification approaches have been proposed to ensure reliable analysis results by reducing possible bias due to the random selection of ground motions. Apart from the existing differences among these methods, there is a common aspect in almost all of them, which can be considered as a limitation: They are constructed on the basis of simplifying assumptions that are not necessarily valid for irregular or complex structural systems. This paper evaluates the efficiency of a recently proposed structure-specific record selection scheme in terms of collapse simulation of vertically irregular frames. Utilization of the method is assessed by case studies on which different strength, stiffness and combined irregularity patterns are applied. The influence of proposed reduction in the number of used records on the estimated collapse capacities is evaluated by statistical tools. The results confirm the ability of the method in estimating median collapse capacity with 82% reduction in computational cost and maximum observed error of 16%.

In general, the analysis and design of buildings must satisfy two criteria. First, under frequently occurring low to moderate earthquakes, the structure should have sufficient strength and stiffness to control deflection and to prevent any structural damage. Second, under rare, severe earthquakes, the structure must have sufficient ductility to prevent collapse. In this case, significant damage to the structure and nonstructural elements is acceptable. In this paper, the performance of an innovative, eccentric and knee bracing system called Eccentrically Knee Brace (EKB) is discussed and the behavior is investigated. A combination of eccentrically braced steel frames and knee braced steel frames has been assessed, and the concepts of the design of defined schemes are reviewed. As the structural fuse of the frame, the knee element will yield first during a moderate earthquake. In large earthquakes, both of them contribute in dissipating energy. Two half-scale EKBs were tested using the SAC loading protocol and an innovative loading protocol. The performance evaluation procedure includes laboratory and computer simulations. Cyclic loading tests were conducted to study the behavior of EKB.

The problem of non-classical dynamic analysis of structures resting on flexible bases is studied in this paper. Because of the presence of the underlying soil in the dynamic model of structure that acts like an energy sink, the damping matrix is not proportional to structural mass and stiffness, and theoretically a non-classical approach should be followed in modal analysis. Considering one to twenty-story buildings, two types of soils, and several suits of ground motions each containing 10 earthquake records specifically selected for each building, the seismic responses are calculated using a time history modal analysis in this paper. Three cases are considered: fixed-base buildings with classical analysis, flexible-base buildings with classical and non-classical analysis. Using the non-classical analysis, it is shown that soil-structure interaction effects cannot be recommended to be taken into account for moment frame buildings with the fundamental fixed-base periods smaller than one second. Cases for which the base flexibility should be considered for the higher modes are distinguished too. Finally, it is made clear that on each soil type, when the actual non-classical nature of the SSI system must be accounted for.

The main target of seismic microzonation is to reduce the seismic risk vulnerability. The application of seismic microzonation in urban planning is an effective step towards decreasing hazards and damages of earthquakes. In this research, the risk of earthquake occurrence in Kermanshah has been taken into account in the form of maximum acceleration and spectral acceleration values for the three return periods of 475, 975 and 2475 years. It was found that ground accelerations at bedrock level increase from north to south of the city. Next, geophysical studies have been done to produce the fundamental frequency map of the ground. They show that the alluvium layer in most parts of Kermanshah is rather thin but at the central extend of the city. The fundamental frequency map is also used to assess the determination of land usage in Kermanshah for future, based on avoiding resonant hazard for the buildings as built according to national regulations.

The response spectrum of an oscillator with bilinear stiffness excited by band-limited Gaussian white noise is considered. The response is obtained by integrating over all energy levels weighting each with the stationary probability density of the energy. The procedure presented leads to estimates of linear and nonlinear response spectra in frequency domain and agrees well with those obtained by direct numerical simulation. Development of stochastic-based response spectra based on the frequency information concerning ground motions is important in engineering. Approximation of non-stationary ground motions by band-limited white noise is shown to be adequate for systems at the structural periods of engineering interest. Formulating the nonlinear response based on the excitation frequency information opens a door for wider use of seismological theory for regions with scarcely available recorded ground motion data. Despite simplicity and computational efficiency of the method, it provides an accurate prediction of the observed nonlinear response spectra on average.