بررسی رفتار غیرخطی عوارض توپوگرافی تپه‌ای‌شکل در اثر انتشار امواج SV

نوع مقاله : Articles

نویسندگان

گروه مهندسی عمران، واحد زنجان، دانشگاه آزاد اسلامی، زنجان، ایران

چکیده

در این تحقیق، رفتار غیرخطی عوارض توپوگرافی تپه‌ای­شکل در برابر هجوم امواج برشی SV با استفاده از برنامه اجزای محدود آباکوس1 و در قالب مطالعات پارامتریک مورد مطالعه قرار گرفته است. مدل رفتاری استفاده شده در مدل‌سازی عددی بر اساس مدل غیرخطی سخت‌شونده کینماتیک با ضابطه گسیختگی وون- مایسز2 می‌باشد. در این راستا، ابتدا تپه ذوزنقه‌ای‌شکل3 با چهار ضریب شکل (SR) برابر 1/0، 3/0، 5/0، 7/0 و با زاویه شیب 45 درجه، مبنای مطالعات پارامتریک مد نظر قرار گرفته شده است. سپس جهت بررسی تأثیر شکل توپوگرافی تپه، علاوه بر تپه ذوزنقه‌ای، تپه‌ها با شکل‌های نیم‌سینوسی4 و نیم‌بیضی5 و با چهار ضریب شکل یاد شده نیز مورد بررسی قرار گرفته است. نتایج به‌دست‌آمده نشان می‌دهند که رفتار غیرخطی خاک منجر به برآوردهای کمتر بزرگ‌نمایی امواج نسبت به رفتار خطی شده است. حداکثر مقادیر بزرگ‌نمایی تپه‌ها در ضرایب شکل مختلف در دو رفتار خطی و غیرخطی در تاج تپه‌ها ظاهر می‌شوند. همچنین مقادیر بزرگ‌نمایی غیرخطی در تپه‌های ذوزنقه‌ای با ضریب شکل 7/0 در فرکانس‌های پایین با اختلافی در حدود 15 درصد نسبت به مدل خطی، کاهش می‌یابند. با فاصله گرفتن از اطراف تپه به طرفین، پاسخ‌ها به سمت میدان آزاد بدون حضور عارضه تپه تمایل می‌یابند. همچنین در این تحقیق مقادیر PGA در عمق و نسبت امپدانس مصالح داخل تپه به بسترطبیعی مورد بررسی قرار گرفته است.

کلیدواژه‌ها


عنوان مقاله [English]

Assessment of Seismic Non-linear Behavior of Hill-Type Topographies Subjected to propagating SV Waves

نویسندگان [English]

  • Mohammad Majd Pezeshki
  • Hamid Alielahi
Department of Civil Engineering, Zanjan Branch, Islamic Azad University, Zanjan, Iran
چکیده [English]

IntroductionGenerally, topographic effects are mostly introduced by hills, canyons, basins, and slopes. Besides, the topographic features produce significant seismic site-effects and can apply a crucial influence on the severity of building damages and its spatial distribution during earthquakes. Some experience observation such as Tarzana hill in the 1994 Northridge earthquake and the Kushiro Meteorological Observatory in the 1993 Kushiro-Oki earthquake, revealed the effective role of surface topographies such as hilland ridges on the seismic damage on the crest and hillsides. Reviewing the technical literature can deduce that the most studies about the effects of topography and the amplification patterns are limited to the linear behavior of the medium.However, theuse of linear models to estimate amplification patterns of waves can lead tomisleading resultsthanthe actual behavior ofthe soil, especially in the soft soils. The non-linear seismic behavior of the hill topographic features unknown in comparison withother types of topographic irregularities, e.g. basins or alluvial valleys. Hence, in this study, the effects of non-linear behavior of the hilltypeof topographies due to propagating SV waves using ABAQUS program in the form of parametric studies are discussed.In this regard, the trapezoidal-shaped hill has been taken into account for parametric study with four shape ratio (SR=0.1, 0.3, 0.5 and 0.7). Besides, in order to evaluate the effect of topography geometry, the semi-sine and semi-elliptical shapes of the hills have been studied. The constitutive model used herein is based on non-linear Kinematic Hardening model with Von-Mises failure criterion.Parametric StudiesIn this research, the finite element method (ABAQUS software) is used to evaluate the hill-typeTopographiesbehavior due to the vertically in-plane propagating incident SV waves. The hill-typeTopographieshas been excited vertically by Ricker-type pulse excitations. In this regard, two center frequencies (fp) of low (i.e. 1.4 Hz) and high (i.e. 4.3 Hz) have been considered to cover all frequency responses with a maximum acceleration equal to 0.3 g. The constitutive model used herein is based on non-linear Kinematic Hardening model, which is suitable for claymaterials. The results are presented as horizontal component amplifications (direct component) or vertical (indirect component).Spectral amplifications have been defined with respect to the free-fieldmotion, based on the maximum Fourier amplitude of the horizontal or vertical component, to the maximum Fourier amplitude of the Free-field motion. In this research, the horizontal and vertical component amplifications are shown, with AHand AVabbreviations, respectively. Moreover, the obtained results provided for the dimensionless distance (X/L) in which X is points from the center of the hill and L is half the width of the hill. All results have been compared in two linear and non-linear soil behaviors.Concluding RemarksThe obtainedresults indicated that considering the non-linear soil behavior can reduce seismic response of topographic effects in comparison with linear behavior. Furthermore, the maximum amplification appears on the crest of the hill in both linearandnon-linear behaviors.The non-linearamplification values reducedabout 15 percent at the lowerfrequencies for the trapezoidal-shaped hill with shape ratio (SR) of 0.7 compared with linear behavior. The seismic response of topographic irregularities tends towards free-field ground motion with far from surrounding hills. Besides, in this research the PGA values versus depth and the impedance ratio between inside of hill and its base materials have been studied. The results of this research can be used in seismic hazard and microzonation studies of various urban areas and to point out this the hills with softer materials than the bed, where soil behavior can be non-linearand usinglinear models, can lead tomisleading resultsthanthe actual behavior ofthe soil.

کلیدواژه‌ها [English]

  • Non-linear behavior
  • Hill-TypeTopographies
  • Amplification
  • Numerical method
  • SV Waves
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