نشریه مهندسی عمران و محیط زیست دانشگاه تبریز
سال چهل و نهم شماره 2 (پیاپی 95، تابستان 1398)

It is well established that the seismic ground response of surface topographies may differ from those of free field motion during earthquakes. Although the topography effect on ground response can be crucial when the wavelength is comparable to irregularity dimensions, only a few building codes have considered this issue. This is because of the complex nature of seismic wave scattering by topographical structures, which can only be solved accurately and economically using advanced numerical methods under realistic conditions. Among the numerical methods, the BEM is a very effective tool for dynamic analysis of linear elastic bounded and unbounded media. In this research, a three-dimensional model of the Pacoima dam site is prepared. Time domain 3D boundary element method is used to apply the non-uniform excitation at the dam supports. This model is subjected to the vertically propagating incident S waves. Time delay can be characterized by calculating the value of time delay for which the cross-correlation between two records is maximized. Finally, to obtain the time delay in topographic site, a function considering effective parameters such as the height from the canyon base, wave velocity and predominant frequency, is presented. Furthermore, a code was developed for generating the spatial variation of seismic ground motions.

Urmia lake is located in a closed basin between 37° 04’ N and 38° 17’ N latitude and 45° E and 46° E longitude in the northwest of Iran and the second extremely-largest saline lake in the world (Sima and Tajrishy, 2013). It has been recently much regarded due mainly to its dramatic area shrinkage and desiccation. The lake, because of its unique natural and ecological importance, has been considering as a National Park (Abbaspour and Nazaridoust, 2007), Ramsar Site (https://rsis.ramsar.org) and UNESCO Biosphere Reserve (http://www.unesco.org). Following the continuous drying process of Urmia Lake as well as considering its geographical and political location, livelihood, identity and historical dependency of the regional people to it, its restoration and prevention from drying up have become highlighted so far. According to several universal experiences connected with partial restoration approach and rehabilitation of the lakes coping with dryness (e.g. Micklin, 2007), Urmia Lake partial restoration is also discussed among Urmia Lake Restoration Program (http://ulrp.sharif.ir) experts. To this end, this study aims at investigating the partial restoration-related scenarios arisen through ULRP specialists using a 2D MOHID Water hydrodynamic system.

Forecasting rainfall situation has paramount importance for water resources engineering. In this field, linear time series models are widely used in hydrology. Models of ARMA family have always had weaknesses which can be solved by integration of ARMA linear time series models with nonlinear time series models such as ARCH. In economy, the sequence of error variance in one model of errors’ size in previous time periods is the main assumption for Autoregressive conditional heteroscedastic models (Engle, 1982). At first, models of ARCH family for time series were used in financial and economic problems. In problems related to hydrology, ARCH models have been studied less.

This study focuses on an approach to improve the behavior of low-rise shear walls with a rectangular cross-section and a height-to-length ratio of unity. The significance of this study lies within the need for a better understanding of the complex shear mechanism, not to mention the shear-flexure interaction in low-rise shear walls which results in different failure patterns under seismic loading. Shear forces may cause diagonal tensile or compressive failures. Moreover, cracks at the foot of the wall during elastic deformation could cause the shear slip failure which could limit the ductility of the wall. To this end, validation studies were conducted on a shear wall specimen chosen from the literature (i.e., the specimen tested by the nuclear power engineering corporation of Japan (NUPEC) (1996).

2.1. Finite element modeling

Numerical analyses were initially carried out in a nonlinear finite element software specifically developed for concrete structures, namely ATENA. “3D nonlinear cementitious 2” material, a fracture-plastic model that combines models for fracture and plastic behavior was used. The classic orthotropic smeared crack formulation and crack band model are used in this model. In this model, the tensile softening branch is defined by two parameters, fracture energy, and the maximum crack width. Moreover, the post-peak branch of the compressive behavior is characterized by the ultimate strain. Bi-linear strain hardening stress-strain curve was used for reinforcements. Eight-node isoparametric brick elements and 2-node truss elements were used to define concrete and reinforcements, respectively. The “Newton-Raphson” solution method with an updated stiffness matrix in each iteration was used to solve the nonlinear system of equations. In order to investigate the behavior of low-rise concrete shear walls, a short shear wall with dimensions of 0.2 m (thickness), 3.5 m (height), and 3.5 m (length) with a rectangular cross-section resting on a 9-m length strip foundation proposed by the International Institute of Earthquake Engineering and Seismology was used. To investigate the influential parameters governing the behavior of the wall, the so-called wall was numerically modeled and was subjected to lateral increasing loads; details of the wall were according to ref Iiees (2011).

3.1. Validation studies

Comparison of experimental and numerical results in terms of shear strength and displacement values showed that the values are in very good agreement with one another and therefore the methodology was verified
3.2. Effect of slots Results obtained from the analysis for the specimen (as shown in Fig. 2) shows that the failure pattern of the wall is mainly shear-dominant with a sudden decrease in shear strength value after the peak load. As for the effect of openings, it can be observed in Fig. 3, when compared to the wall without slots, the post-peak ductility of the wall increases noticeably which could act as a warning prior to its failure. Moreover, according to Fig. 4, it can be seen that the tensile cracks have propagated throughout the height of the wall which helps to avoid stress concentration and make efficient use of the tensile capacity of the concrete.

Results showed that walls with slots have an overall improved ductility in comparison to models without slot. Additionally, creating slot in the wall helps in the redistribution of internal forces, prevents stress concentration. Furthermore, the presence of slots changes the failure pattern of the wall from shear-dominant to shear-flexure dominant. Finally, for a wall with a slot ratio of 0.15, displacement corresponding to the peak load increases by 138% compared to its counterpart with no slots.

Stone columns are one of effective methods of confronting liquefaction in saturated sands. The main objectives of this method is to increase soil strength and pore water drainage rate in surrounding soils, considering higher permeability of stone column materials. Different researchers have studied the effects of using stone columns on reducing liquefaction potential. Generally, these studies can be classified as numerical and experimental studies. Considering the fact that interactions between stone column and soil are tridimensional, the aim of present study is numerical investigation of application of current method in decreasing liquefaction potential.