بررسی اثر عوارض توپوگرافی های هم جوار بر پاسخ لرزه ای ساختگاه

نوع مقاله : Articles

نویسندگان

دانشگاه تربیت دبیر شهید رجایی

چکیده

اثرات توپوگرافی می تواند طبیعت لرزش زمین را در طول زلزله تحت تأثیر قرار دهد. اگرچه خوشبختانه اثر توپوگرافی ساختگاه بر پاسخ لرزه‌ای زمین در سال های اخیر بیشتر مورد توجه قرار گرفته و در دستور کار راهنما و آیین نامه های لرزه ای گنجانده شده، ولی همچنان ابعادی از تأثیر این عامل پیچیده بر پاسخ لرزه ای ساختگاه، نیاز به مطالعه و بررسی دقیق تر دارد تا کمتر شاهد خسارات پیش‌بینی‌نشده‌ی ناشی از عامل توپوگرافی در زلزله ها و یا هرگونه حرکات لرزه ای زمین باشیم. در یک حرکت لرزه ای، توپوگرافی محیط اطراف سبب گیر افتادن و انعکاس امواج لرزه ای در بین عوارض توپوگرافی می شود و این پدیـده موجب تشدید بیشتر حـرکت لرزه ای در بین نامنظمی های توپوگرافـی می شود. در این مقاله به‌منظور بررسی چگونگی اثر عامل توپوگرافی محیط اطراف بر پاسخ لرزه ای ساختگاه، چندین مطالعه ی عددی المان  محدود با به‌کارگیری برنامه ی آباکوس (ABAQUS) انجام شده  است. نتایج نشان داد که وجود عوارض توپوگرافی بیشتر بین منبع لرزه زا و ساختگاه، سبب تشدید بیشتر حرکت لرزه ای می شود و این امر برای تپه های دورتر از منبع و نقاط خط‌الرأس توپوگرافی ها، محسوس تر است. همچنین شتاب ساختگاه بیش از سرعت و جابه جایی تحت تأثیر عامل توپوگرافی محیط اطراف (اثر متقابل توپوگرافی ها) قرار می گیرد.Effect of the Adjacent Topographies on Seismic Site ResponseSaeed Ghaffarpour Jahromi and Sama KarkhanehTopography can have significant effects on the seismic ground response during an earthquake. Fortunately, the topography effect on the seismic motion is considered more in recent years and seismic design regulations and guides include this factor; however, study about many aspects of this complicated factor is essential to reduce unforeseen damages caused by earthquakes or any seismic motion in topographical areas. A single topographic irregularity (a canyon or a hill) has been studied by many researchers, and many realistic applications are also found. Nevertheless, to the best of our knowledge, few studies deal with the interaction of topographic irregularities. The aim of this study is to determine how a series of irregularities interact as compared to a single irregularity. When a seismic motion happens in a topographical area, seismic waves are trapped and reflected between the topographic features because of the surrounding topography. Therefore, the interaction between topographies can amplify the seismic ground response. In order to reveal how the interaction between topographies influence on seismic response, several numerical finite element studies have been performed by ABAQUS program, the results of which are presented in the form of the time history and dimensionless graphs. Due to the evaluate effect of geometry type, height, length and angle of slope on the seismic response; rectangular, trapezoidal and triangular topographies are studied with different heights (20-100 m) and different angles (15-75 degrees). As the result of these simulations, although seismic ground motion amplification of the various geometries (rectangular, trapezoidal and triangular topography) is different, the total trend is similar. It means that increasing slope height, length and angle of all types of geometry has a greater effect on seismic response amplification. In order to study the interaction of topographic irregularities, several models with different numbers of topographies are evaluated. Models are series of triangular topographies with the height of 100m and slope angle of 30°. Distance between each of the topographies is 10 m, and the total length of topographic irregularities is about 1079.2 m. Shear Wave Velocity (V_S) is 560 m/s. In this study, different numbers of topography in different distance of seismic source are evaluated. Applied Seismic motion is the record of Manjil earthquake in horizontal direction. Besides, models are two-dimensional and flexible. In many studies, different kinds of energy absorbing boundaries have been investigated and results show that the best boundary conditions is infinite element method. ABAQUS infinite elements can be used to define infinite boundaries in the dynamic problems. These elements have Elastic behavior and absorb the wave energy so that they act as absorbent boundaries. Researchers such as Nielsen, Preisig and Jeremic Have examined the performance of these elements. Modal analysis in Abaqus software has been done to determine the natural frequencies of the soil site. Soil damping is related to strain and differs in the diverse strain levels. Therefore, average of the soil critical damping ratio is assumed 5%. Application of the critical damping ratio is depreciation of the seismic waves reflected from the model boundaries. When the natural frequencies of the soil site are determined, α_R and β_R (Riley damping coefficients) can be calculated. The results show that site seismic response is very different when there is no topographic irregularity between the seismic source and the site; in comparison with several topographic irregularities exist between them. Further topographic features between the seismic source and the site would cause further seismic motion amplification and is so tangible for the hills far away from the source and the ridges. Importance of this issue is because the greater number of topographic irregularities are caused the greater value of seismic motion amplification and in reality, there is a series of topographic irregularities together. Although changes rate of the acceleration dimensionless graphs is greater than velocity and both are greater than displacement dimensionless graphs, their trend changes are similar to each other. It means that surroundings topography (topographies interaction) have effects on site acceleration more than site velocity and displacement. Finally, it is concluded that topographies interaction factor (surrounding topography) should be considered as an effective and independent parameter of a single topography, while the seismic regulations has not paid enough attention to this problem.Keywords: Topography Effects, Surrounding Topography, Site Seismic Response, ABAQUS Program, PLAXIS Program. 

کلیدواژه‌ها


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