Floor response spectra for base-isolated multi-storey structures     

Fa-Gung Fan  Goodarz Ahmadi 《地震工程与结构动力学》1990,19(3):377-388

A study of floor response spectra for a base-isolated multi-storey structure under sinusoidal and seismic ground excitations is carried out. Several base isolation systems including the laminated rubber bearing, the pure-friction, the resilient-friction, the Électricité de France and the sliding resilient-friction systems are considered. A sinusoidal ground acceleration and several earthquake accelerograms (including those of El Centro 1940, Pacoima Dam 1971 and Mexico City 1985) are used to evaluate the floor response spectra. The characteristics of the spectra generated by different base isolation systems are studied, and the results are compared with those for the fixed-base structure. It is shown that the structural contents can be protected against earthquakes by the use of properly designed base isolation systems. In particular, the laminated rubber bearing system appears to be remarkably effective in protecting the secondary systems under a variety of conditions.  相似文献   

Failure Process after Peak Strength of Artificial Joints by Fractal Dimension   总被引：2，自引：0，他引：2  

M. Askari  M. Ahmadi 《Geotechnical and Geological Engineering》2007,25(6):631-637

Because lack of information when the joints exhibit strain softening behavior, the transition from peak to residual values is assumed to decrease either linearly or exponentially. Also, displacement of slide side after peak is much larger than the peak displacement and the stress approaches the residual state, thus studying failure process after peak strength is very important. In this research, three types of artificial joints with tooth-shaped asperity under repeated direct-shear were tested. Continuously movement of slide side after peak strength was monitored during shearing test. Reduction of shear parameters was examined according to two failure criteria (Barton and Patton). JRC value (Joint Roughness Coefficient) for a given profile was estimated by fractal dimension. One of the results of this study is that Barton’s criterion predicts a good estimation of residual strength and the second result is when the amount of fractal dimension of a joint surface increases, the JRC amount also increases, and with having value of h (height average) and l (base average) of a tooth-shaped asperity can determine the JRC of joint surface. M. Askari is a M.Sc Student of Engineering Faculty, Mining Engineering Department, Tarbiat Modares University, Tehran.  相似文献   

Two-domain description of solute transport in heterogeneous porous media: Comparison between theoretical predictions and numerical experiments     

F. Cherblanc  A. Ahmadi  M. Quintard 《Advances in water resources》2007

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Large-scale replenishment of groundwater depletion by long-lived extreme rainstorms in arid regions: case study of the 2019 floods in Iran     

Golian  Mohsen  Katibeh  Homayoon  Najafi  Zeinab  Saadat  Habib  Saffarzadeh  Ali  Ahmadi  Aboozar  Khazaei  Mahdi  Sametzadeh  Ershad 《Hydrogeology Journal》2021,29(6):2171-2186

The Dena rainstorm in Iran in March and April 2019 caused about US$ 8.3?×?10⁹ damage in the country; however, it resulted in the replenishment of half of the dam reservoirs and 35% of ponds and lakes. Also, it increased the volume of groundwater stored in aquifers by 3.6?×?10⁹ m³. In arid and semiarid regions such as most parts of Iran, which usually face water scarcity, getting water from rainstorms is essential for replenishing water resources. This research aims to quantify the direct and indirect effects of the Dena rainstorm on the replenishment of Iran’s groundwater storage using the groundwater balance method and water-table fluctuation method. Studies showed that the main mechanisms for replenishment of groundwater storage due to the rainstorm included increases in precipitation recharge, surface runoff recharge, and artificial recharge, and reductions in irrigation withdrawal and evapotranspiration, while the contribution of each factor is estimated to be about 23, 28, 2, 15, and 32%, respectively.

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Simplified dynamic analysis to evaluate liquefaction-induced lateral deformation of earth slopes: a computational fluid dynamics approach     

Yaser Jafarian  Ali Ghorbani  Omid Ahmadi 《地震工程与工程振动(英文版)》2014,13(3):555-568

Lateral deformation of liquefiable soil is a cause of much damage during earthquakes, reportedly more than other forms of liquefaction-induced ground failures. Researchers have presented studies in which the liquefied soil is considered as viscous fluid. In this manner, the liquefied soil behaves as non-Newtonian fluid, whose viscosity decreases as the shear strain rate increases. The current study incorporates computational fluid dynamics to propose a simplified dynamic analysis for the liquefaction-induced lateral deformation of earth slopes. The numerical procedure involves a quasi-linear elastic model for small to moderate strains and a Bingham fluid model for large strain states during liquefaction. An iterative procedure is considered to estimate the strain-compatible shear stiffness of soil. The post-liquefaction residual strength of soil is considered as the initial Bingham viscosity. Performance of the numerical procedure is examined by using the results of centrifuge model and shaking table tests together with some field observations of lateral ground deformation. The results demonstrate that the proposed procedure predicts the time history of lateral ground deformation with a reasonable degree of precision.  相似文献   

Collapse capacity of soil‐structure systems under pulse‐like earthquakes          下载免费PDF全文

Faramarz Khoshnoudian  Ehsan Ahmadi  Mahdi Kiani  Mohammad Hadikhan Tehrani 《地震工程与结构动力学》2015,44(3):481-490

In this paper, a comprehensive study is carried out to examine the possibility of dynamic instability produced in soil‐structure systems using an ensemble of 50 pulse‐like records. A number of structural models with various vibration periods varying from 0.1 to 2 s are used in this study. The superstructure is simulated as a non‐linear SDOF oscillator with a two‐segment backbone curve having negative post‐yield stiffness. The soil is idealized based on the cone model concept widely used for practical purposes. The results of this investigation demonstrate that as the pulse period increases, the collapse relative lateral strength ratio decreases and probability of dynamic instability enhances. Moreover, soil flexibility makes the system dynamically more unstable, and as the non‐dimensional frequency increases, the collapse relative lateral strength ratio highly reduces. Additionally, the aspect ratio has insignificant effects on the collapse relative lateral strength ratio. Furthermore, comparison of the collapse relative lateral strength ratios resulting from pulse‐like motions with those obtained from studies under non‐pulse‐like motions (Miranda and Akkar; FEMA 440) for fixed‐base conditions shows that high‐velocity pulses exacerbate the dynamic instability problem and decrease the collapse relative lateral strength ratio. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

A parametric study on the axial behaviour of elastomeric isolators in multi-span bridges subjected to horizontal seismic excitations     

E. Tubaldi  S. A. Mitoulis  H. Ahmadi  A. Muhr 《Bulletin of Earthquake Engineering》2016,14(4):1285-1310

This paper investigates the potential tensile loads and buckling effects on rubber-steel laminated bearings on bridges. These isolation bearings are typically used to support the deck on the piers and the abutments and reduce the effects of seismic loads and thermal effects on bridges. When positive means of fixing of the bearings to the deck and substructures are provided using bolts, the isolators are exposed to the possibility of tensile loads that may not meet the code limits. The uplift potential is increased when the bearings are placed eccentrically with respect to the pier axis such as in multi-span simply supported bridge decks. This particular isolator configuration may also result in excessive compressive loads, leading to bearing buckling or in the attainment of other unfavourable limit states for the bearings. In this paper, an extended computer-aided study is conducted on typical isolated bridge systems with multi-span simply-supported deck spans, showing that elastomeric bearings might undergo tensile stresses or exhibit buckling effects under certain design situations. It is shown that these unfavourable conditions can be avoided with the rational design of the bearing properties and in particular of the shape factor, which is the geometrical parameter controlling the axial bearing stiffness and capacity for a given shear stiffness. Alternatively, the unfavourable conditions could be reduced by reducing the flexural stiffness of the continuity slab.  相似文献   

Comparison of the performance of power law and probability distributions in the frequency analysis of flood in Dez Basin,Iran     

Farshad Ahmadi  Feridon Radmaneh  Gholam Ali Parham  Rasoul Mirabbasi 《Natural Hazards》2017,87(3):1313-1331

Determination of the return period of design flood depends on the nature of the project and the consequences of the flood and is based on economic criteria, human casualties, and hydrological factors. Underestimation of flood might result in casualties and economic damages, while the overestimation leads to capital waste. Therefore, in this research, the flood frequency analysis of Dez Basin, Iran was conducted within the period of 1956–2012 using power law approach together with ordinary distributions, including normal, log normal, Pearson type III, exponential, gamma, generalized extreme value, Nakagami, Rayleigh, logistic, generalized logistic, generalized Pareto, and Weibull distributions. The power law comes from the fractal nature of earth science phenomena such as precipitation and runoff. Accordingly, in this research the partial duration flood series of five hydrometric stations in Dez Basin were extracted using power law with the intervals of 7, 14, 30, and 60 days and then compared with the annual maxima. The results indicated that the annual maxima were not suitable for frequency analysis of the flood in Dez Basin, and the 30-day partial duration series obtained from the power law has a better correspondence with the flow and properties of the Dez Basin. The independence and stationarity of the 30-day partial duration series were examined by Wald–Wolfowitz test, confirming the independence of the considered series. Next, the power distribution and the typical statistical distributions were fitted onto the data of the flood in Dez Basin, with the performance of each distribution being investigated using normalized root-mean-square error and Nash–Sutcliffe criteria. The results revealed that in the SDZ and TPB stations, power distribution had a better performance than other considered distributions. Moreover, in the SDS, TPS, and TZ stations the power distribution stood in the second rank in terms of the best distribution. As the performance of power distribution in the estimation of the flood in Dez Basin has been very satisfactory and calculation of its parameters and its application is easier than ordinary probability distributions, thus it can be suggested as the superior distribution for flood frequency analysis in Dez Basin.  相似文献   

On the seismic behavior of cylindrical base-isolated liquid storage tanks excited by long-period ground motions     

M.R. Shekari  N. KhajiM.T. Ahmadi 《Soil Dynamics and Earthquake Engineering》2010

A common effective method to reduce the seismic response of liquid storage tanks is to isolate them at base using base-isolation systems. It has been observed that in many earthquakes, the foregoing systems significantly affect on the whole system response reduction. However, in exceptional cases of excitation by long-period shaking, the base-isolation systems could have adverse effects. Such earthquakes could cause tank damage due to excessive liquid sloshing. Therefore, the numerical seismic response of liquid storage tanks isolated by bilinear hysteretic bearing elements is investigated under long-period ground motions in this research. For this purpose, finite shell elements for the tank structure and boundary elements for the liquid region are employed. Subsequently, fluid–structure equations of motion are coupled with governing equation of base-isolation system, to represent the whole system behavior. The governing equations of motion of the whole system are solved by an iterative and step-by-step algorithm to evaluate the response of the whole system to the horizontal component of three ground motions. The variations of seismic shear forces, liquid sloshing heights, and tank wall radial displacements are plotted under various system parameters such as the tank geometry aspect ratio (height to radius), and the flexibility of the isolation system, to critically examine the effects of various system parameters on the effectiveness of the base-isolation systems against long-period ground motions. From these analyses, it may be concluded that with the installation of this type of base-isolation system in liquid tanks, the dynamic response of tanks during seismic ground motions can be considerably reduced. Moreover, in the special case of long-period ground motions, the seismic response of base-isolated tanks may be controlled by the isolation system only at particular conditions of slender and broad tanks. For the case of medium tanks, remarkable attentions would be required to be devoted to the design of base-isolation systems expected to experience long-period ground motions.  相似文献   

Numerical modeling of MDOF structures with sliding supports using rigid‐plastic link     

A. Vafai  M. Hamidi  G. Ahmadi 《地震工程与结构动力学》2001,30(1):27-42

In this paper, the responses of multi‐degree‐of‐freedom (MDOF) structures on sliding supports subjected to harmonic or random base motions are investigated. Modeling of the friction force under the foundation raft is accomplished by using a fictitious rigid link which has a rigid–perfectly plastic material. This will result in identical equations of motion for the sliding structure, both in the sliding and non‐sliding (stick) phases which greatly simplifies the implementation of the method into a numerical algorithm. In this model the phase transition times are determined with high accuracy. This has two advantages: first, it prevents the so‐called high‐frequency oscillation of the relative velocity at the end of the sliding phase, and second, the time steps can be selected so that each falls exactly within one phase of motion. In this case, the stiffness matrix of the structure remains constant throughout each phase and thus any method for solving the non‐linear differential equations of motion (e.g. Newmark method) can be used without iteration. The proposed method, besides its simplicity, is numerically very efficient and considerably reduces the required analysis time compared with most of the other methods. Copyright © 2001 John Wiley & Sons, Ltd.  相似文献