فهرست مطالب

علوم و مهندسی زلزله - سال هشتم شماره 4 (پیاپی 29، زمستان 1400)

مجله علوم و مهندسی زلزله
سال هشتم شماره 4 (پیاپی 29، زمستان 1400)

  • تاریخ انتشار: 1401/01/29
  • تعداد عناوین: 8
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  • مریم صدقی*، مهدی زارع، آرزو درستیان صفحات 1-17

    زلزله های نزدیک گسل نسبت به زلزله های دور از گسل دارای خصوصیات متفاوتی می باشند. در سال های اخیر در آیین نامه های مختلف اثرات ناشی از زلزله های دور و نزدیک به صورت جداگانه بررسی شده است. مطالعه حاضر نتایج بررسی 214 شابنگاشت بدست آمده از زمین لرزه های کم عمق پوسته ای رخ داده با فاصله کانونی کمتر از 80 کیلومتر از گسل مسبب، جهت تعیین طیف طرح در ایران و مقایسه آن با طیف طرح آیین نامه 2800 را نشان می دهد. این شتابنگاشت ها از زمین لرزه های بزرگ ایران، آمریکا و اروپا جمع آوری و پردازش شده اند. پس از محاسبه طیف پاسخ، طیف طراحی از میانگین طیف پاسخ شتابنگاشت ها برای هر چهار نوع خاک (I، II، III، IV) ارایه شده در آیین نامه 2800 محاسبه شد. برای مقایسه نتایج این مطالعه با ایین نامه رکوردهایی با بزرگای بیشتر از 5.5 و فاصله رو مرکزی کمتر از 80 کیلومتر انتخاب گردید و همچنین پارامتر میرایی 5% مورد استفاده قرار گرفت. نتایج حاصل نشان می دهد که طیف طراحی هر دو مولفه خاک نوع یک تا سه در این مطالعه تا پریود 0.39 ثانیه با طیف طراحی آیین نامه همخوانی دارد درحالیکه در پریودهای بالاتر از 0.39 ثانیه طیف طرح آیین نامه بالاتر از طیف طرح این مطالعه قرار می گیرد. از طرفی طیف طرح مولفه قایم خاک نوع IV بالاتر از طیف طرح آیین نامه قرار گرفته است که این می تواند به دلیل تعداد کم رکوردهای این نوع خاک باشد.

    کلیدواژگان: شتابنگاشت، طیف طراحی، فاصله کانونی، زمینلرزه های نزدیک گسل
  • هادی طبسی*، سمیه بهاالدینی صفحات 17-27

    حوزه درون قاره ای شمال قم در ایران مرکزی جای گرفته است. راستای عمومی گسلهای مهم این پهنه شامل گسل قم، البرز و کوشک نصرت، شمال باختر- جنوب خاور بوده و همگی با درازای بیش از 50 کیلومتر تحت تاثیر همگرایی صفحه عربی- اوراسیا، در لرزه خیزی و دگرشکلی گستره قم نقش مهمی داشته اند. یکی از روش های ارزیابی زمین ساختی، بهره گیری از شاخص های زمین ریخت سنجی است. در این میان، با توجه به ویژگیهای سنگ شناختی و نحوه قرارگیری و پراکندگی حوضه ها، شاخص های نسبت عرض به ارتفاع دره، هیپسومتری و گرادیان طولی رودخانه برای ارزیابی انتخاب شده و نتایج تحلیل شده اند. این داده ها، در کنار داده های هندسه گسلها و جهت یابی تنش توانسته برخی ویژگی های زمین ساختی گستره را روشن کند. مطالعات زمین ریخت سنجی، گستره های اطراف گسل ساوه و گسل کوشک نصرت در بخش شمال ساوه و هم چنین مناطقی از جنوب شهر قم را دارای توان بالاآمدگی نشان می دهد. سایر پهنه ها بالاآمدگی قابل توجهی ندارند. پایین بودن جنبایی قایم در برآوردهای زمین ریخت سنجی دلالت بر نبود جنبایی نیست بلکه می تواند کارکرد همزمان مولفه های فشاری و برشی یا همان دگرشکلی خاص ایران مرکزی را نشان دهد. این نوع حرکت با دانستن جهت یابی تنش به سادگی قابل تفسیر خواهد بود. دگرشکلی برشی- فشاری گستره قم، ناشی از حرکت رو به شمال خاوری ورقه عربی و محدود شدگی ناشی از پوسته سخت خزر است. حرکت های راستالغز همزمان با جابجایی معکوس در طول گسلها باعث شکل گیری چین خوردگی هایی نیز شده است. بدون شک، گستره ی قم محدوده ای است که از نظر مطالعات زمین شناسی لرزه ای مورد توجه محققین با دیدگاه های مطالعاتی متفاوت بوده که مقایسه نتایج این پژوهش ها به کشف مجهولات جنبایی گستره کمک کرده است.

    کلیدواژگان: قم، گسلهای قم، البرز و کوشک نصرت، جنبایی، دگرریختی
  • علی عسگری*، فرامرز رنجبر، حبیب اکبرزاده بنگر صفحات 29-49

    سترش جانبی ناشی از روانگرایی خاک، اغلب باعث ایجاد خرابی های قابل توجهی در پی های عمیق پل ها، بنادر، سازه های دریایی و ساختمان ها می-شود. با وجود انجام مطالعات مختلف آزمایشگاهی، عددی و صحرایی توسط محققان پیشین، همچنان رویکرد جامعی برای ارزیابی اثرات گسترش جانبی بر شمع ها وجود ندارد. شبیه سازی های عددی ابزای مهم و اقتصادی برای بررسی اثرات گسترش جانبی بر شمع ها هستند. در این پژوهش ، به بررسی اثر شیب زمین بر روی رفتار گروه شمع های 1×1، 2×2 و 3×3 به صورت پارامتریک و با استفاده از شبیه سازی سه بعدی عددی پرداخته شده است. بدین منظور، ابتدا مدل عددی ساخته شده با نرم افزار اجزای محدود اپنسیس، با نتایج یک آزمایش میز لرزان بزرگ مقیاس، صحت سنجی شده و سپس به ازای شیب های مختلف زمین ، پارامترهایی نظیر: تغییرات فشار آب حفره ای، تغییرات جابجایی جانبی شمع و خاک، لنگر خمشی در شمع ها و تغییرات شتاب شمع و خاک تحت زلزله ی السنترو مورد بررسی قرار گرفته است. نتایج نشان می دهد که با افزایش شیب زمین، فشار آب حفره ای کمتری تولید شده و زایل شدن اضافه فشار آب حفره ای زودتر آغاز می شود اما جابجایی جانبی شمع و خاک افزایش می یابد. از طرفی دو عامل، در روند تغییرات و میزان جابجایی ها نقش دارد، عامل اول تنش برشی استاتیکی در راستای شیب که اثر افزایشی بر میزان جابجایی دارد و عامل دوم رفتار اتساعی خاک ناشی از تغییر شکل است که اثر کاهشی بر روی روند جابجایی دارد.

    کلیدواژگان: شیب زمین، گسترش جانبی گروه شمع، خاک روانگرا، شبیه سازی سه بعدی عددی، اپنسیس
  • شیما سادات حسینی، علی قنبری*، محمد داودی صفحات 51-65

    در سازه های عظیم و طویل مانند پل ها فرض گیرداری پی و تحریک یکنواخت لرزه ای در تمام تکیه گاه های سازه موجب انحراف پاسخ ها از واقعیت می شود. هدف از انجام این مطالعه ارزیابی پاسخ لرزه ای یک پل طویل تحت اثر اندرکنش خاک- سازه و تحریک غیر یکنواخت می باشد که در آن یک مدل تحلیلی جدید جهت مدلسازی اثر اندرکنش خاک-سازه ارایه شده است. همچنین تحریکات غیر یکنواخت در محل تکیه گاه های مختلف محدوده ی مطالعه ی پل صدر به عنوان یک پل طویل شبیه سازی شد و بر مدل اعمال گردید. مقایسه پاسخ های دینامیکی سازه در شرایط اندرکنش و تحریک غیر یکنواخت با حالت پایه گیردار و تحریک یکنواخت حاکی از تاثیر بسزای هر دو عامل اندرکنش و تحریک غیر یکنواخت در تحلیل و طراحی پل های طویل بوده و نشان داد که عدم در نظرگیری این عوامل ممکن است منجر به پاسخ های دور از واقعیت شود. نتایج بیانگر این مطلب است که در نظر گیری اثر اندرکنش و تحریک غیر یکنواخت می تواند تا 275 و 176 درصد باعث افزایش تغییر مکان نسبی عرشه در راستای طولی و عرضی شود. همچنین لحاظ نمودن اثر اندرکنش به طور میانگین کاهش 67 و 75 درصدی برش و لنگر پایه و در نظر گیری تحریک غیر یکنواخت افزایش 37 و 29 درصدی آن مقادیر را نشان می دهد.

    کلیدواژگان: اندرکنش خاک سازه، تحریک غیر یکنواخت زمین، پل های طویل، پاسخ لرزه ای، پل صدر
  • سید بهرام بهشتی اول*، سید علی حسینی صفحات 67-83

    با توجه به ابعاد مقطع و طول تیرهای همبند در ساختمان های بتنی با شیوه اجرایی قالب تونلی، تعبیه میلگردهای برشی قطری در آنها عملا دشوار بوده و در اکثر موارد، ضمن اجرا از این تسلیح صرف نظر می شود. بدین سبب بررسی ها نشان داده است که این المان ها به عنوان فیوز خرابی، در اتلاف انرژی ورودی زلزله عملکردی ضعیف خواهند داشت. در این مطالعه، طرح جایگزینی تیرهمبند بتنی با نوع فولادی برشی تعویض پذیر پیشنهاد شده است. ابتدا روابط مربوط به تعیین ظرفیت برشی این تیر ارایه شده و سپس با استفاده از نرم افزار اجزاء محدود ABAQUS  به بررسی صحت این روابط پرداخته شده است. در ادامه رفتار لرزه ای ساختمان های قالب تونلی طراحی شده با تیرهای همبند فولادی پیشنهادی و بتنی، در محدوده غیرخطی مورد ارزیابی و مقایسه قرار گرفته است. نتایج نشان می دهد استفاده از تیرهمبند فولادی پیشنهادی  اگرچه به سبب کاهش سختی در سیستم و بالتبع افزایش تغییرمکان های نسبی طبقات،  احتمال رسیدن دیوارها به اولین سطوح خرابی را به میزان جزیی افزایش می دهد اما، تحت زلزله طرح به هیچ وجه سبب کاهش بازه قابلیت اعتماد ساختمان ها در حصول به عملکردهای از پیش تعیین شده نمی گردد. همچنین استفاده از این نوع تیر همبند فلزی، باعث افزایش شکل پذیری ساختمان های قالب تونلی می شود. اجرای آسان و سهولت در تعمیر یا تعویض این نوع تیر همبند فلزی بعد از یک زلزله مخرب از مزایای دیگر آن در مقایسه با نوع بتنی می باشد.

    کلیدواژگان: سیستم قالب تونلی، تیر همبند تعویض پذیر، قابلیت اعتمادلرزه ای، منحنی شکنندگی
  • مهیار نوبخت، مسعود عامل سخی*، فاطمه شش پری صفحات 85-92

    این پژوهش مطالعه ای است روی رفتار لرزه ای تپه های نیم سینوسی شکل که در معرض امواج برشی قایم ریکر قرار گرفته اند. برای مدلسازی از نرم افزاری که برمبنای روش تفاضل محدود می باشد، استفاده گردیده است. چون تمرکز بر روی تاثیرات توپوگرافی است، از پارامتر ضریب شکل (نسبت ارتفاع به نیم پهنای تپه) و پارامترهای زمین به عنوان متغیر استفاده شده است. رفتار محیط مدلسازی الاستیک خطی در نظر گرفته شده است. در نتیجه تپه های شبه سینوسی با ضریب شکل های متفاوت (1/0، 4/0 و 8/0)، در  زمین با جنس های متفاوت تیپI ، II و III (که تفاوت آنها طبق استاندارد 2800 در چگالی و سرعت موج برشی در محیط پیوسته است) مدل شده اند که هرکدام از آنها تحت موج ریکر با فرکانس ثابت قرار گرفته اند. در این تحقیق جابجایی های افقی و عمودی برای نقاط مختلف روی تپه محاسبه شده اند. همچنین بزرگنمایی جابجایی با توجه به مدل میدان آزاد بدست آمده است. نتایج نشان دهنده این مطلب است که با افزایش ضریب شکل، هم جابجایی افقی و هم جابجایی قایم و درنتیجه بزرگنمایی در تمام موارد افزایش می یابد. همچنین از نمودارهای حاصله می توان دریافت که با تغییر جنس زمین از تیپ I به تیپ III جابجایی ها کاهش می یابد. این درحالی است که بزرگنمایی تغییری نکرده و در تمامی موارد تقریبا ثابت است.

    کلیدواژگان: تپه های نیم سینوسی، موج ریکر، اثرات توپوگرافی، بزرگنمایی
  • سید ناصر هاشمی* صفحات 93-103

    در این تحقیق، الگوی مهاجرت زلزله های متوالی[i] رخ داده در ناحیه زاگرس، در بازه زمانی 2019-1976 و برای زلزله های با بزرگای مساوی و بیش از 5/4 ریشتر مورد مطالعه قرار گرفته است. به این منظور زمان بین رخداد[ii]، فاصله مهاجرت رومرکزی و راستای مهاجرتی زلزله های متوالی، برای زلزله های با بزرگی مساوی و بیش از 5/4، 0/5 و 5/5 ریشتر محاسبه شده و توزیع آماری این داده ها مورد تحلیل و مدل سازی آماری قرار گرفته است. بررسی توزیع آماری زمان بین رخدادی زلزله ها حاکی از تطابق خوب این داده ها با توزیع آماری گاما[iii] و ویبول[iv] است. داده های فاصله مهاجرتی زلزله های متوالی نیز به خوبی الگوی کاهشی، مشابه با توزیع زمان بین رخدادی زلزله ها را نشان می دهد. همچنین، داده های روند مهاجرت زلزله های متوالی نیز الگویی کاملا هم راستا با روند کلی گسل های فعال ناحیه زاگرس را نشان می دهد که تایید کننده این نظر است که فعال شدن قطعات مجزای سیستم های گسلی در این ناحیه نقش اصلی را در توالی زمانی و مکانی رخداد زلزله ها ایفا می کنند. نتایج حاصل از این تحقیق می تواند گامی موثر برای شناخت بهتر الگوی زمانی-مکانی لرزه خیزی در ناحیه زاگرس و تلاشی برای دستیابی به پیش بینی زلزله در این ناحیه و در مقیاس ناحیه ای محسوب شود.

    کلیدواژگان: مدل سازی آماری، زمان بین رخدادی زلزله ها، توزیع آماری زلزله ها، مهاجرت زلزله های متوالی، لرزه خیزی
  • رسول مظلوم*، احسان معانی میاندوآب صفحات 105-113

    زلزله یکی از خطرناک ترین بلایای طبیعی عصر حاضر به شمار می رود که همواره اهمیت خود را به طور عینی نمایان کرده است. زلزله سانحه ای طبیعی است که بر اساس میزان بزرگی خود می تواند در مدت کوتاهی فجایعی عظیم به وجود می آورد. هدف این مقاله، استخراج و و ارایه مدل برای تغییرات سرعت امواج زلزله(() با استفاده از الگوریتم ژنتیک می باشد. داده های مورد استفاده در این تحقیق از مرکز لرزه نگاری کشوری مربوط به موسسه ژیوفیزیک دانشگاه تهران دریافت شده است. در این تحقیق، سه استان کرمانشاه، آذربایجان شرقی و کرمان انتخاب شدند. به منظور استفاده از الگوریتم ژنتیک، ابتدا داده های دریافت شده از مرکز لرزه نگاری کشوری برای این سه استان با هم ادغام شده که بالغ بر تعداد 1863 رخداد زلزله برآورد گردید. پس از استخراج داده های مربوطه، نسبت به محاسبه سرعت امواج زلزله ()  اقدام شد. سپس با نادیده گرفتن حدود 25% از این داده ها، نسبت به استخراج مدل ریاضی برای  سرعت امواج زلزله، اقدام شد. در انتها، فرمول به دست آمده در مورد داده های چشم پوشی شده اولیه(25%) اعمال گردید، که نتایج مشابهی بدست آمد. در این مقاله به بررسی سرعت متوسط انتشار امواج طولی زلزله بر حسب فاصله از محل وقوع زلزله پرداخته شد. برای این منظور سه زلزله مختلف در سه نقطه متفاوت ایران در در نظر گرفته شده و زمان رسیدن این امواج و ثبت آن ها در ایستگاه های مختلف از اطلاعات موسسه ژیوفیزیک استخراج و با داشتن فاصله ایستگاه ها و اختلاف زمان انتقال موج، سرعت متوسط امواج محاسبه گردید. در همه موارد رفتار کلی یکسانی مشاهده شد و آن افزایش سرعت امواج زلزله با انتشار و طی مسافت و میل کردن آن به مقدار 7.74 کیلومتر بر ثانیه برای هر سه مورد تست می باشد.

    کلیدواژگان: الگوریتم ژنتیک، مدلسازی، سرعت امواج زلزله، زلزله
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  • Maryam Sedghi *, Mehdi Zare, Arezoo Dorostian Pages 1-17

    Near fault earthquakes have different characteristics comparing to far field recorded events, they often contain strong coherent dynamic long period pulses and permanent ground displacements.In the recent years, the effects of near field and far field earthquakes have been studied separately in various building codes.Building codes have been revised and updated depending on the improvements in the representation of ground motions, soils and structures. These revisions have been more frequently seen in recent years. One of the key changes in earthquake codes has been performed on the design spectra.Design spectra are used in seismic analysis methods such as equivalent static lateral force analysis, dynamic spectral analysis and time history dynamic analysisVarious seismological and geophysical parameters affect the shape of design spectra. Ambraseys et al. (1996) and Bommer and Acevedo (2004) presented and discussed the effects of earthquake magnitude, source-to-site distance, site classification, and style-of-faulting on the strong-motion accelerograms and consequently design spectra. Present study shows the results of 214 accelerograms, obtained from shallow crustal earthquakes with epicenral distance less than 80 km from causative fault, to determine the design spectrum in Iran. A comparison is the performed with the design spectrum of Iranian building code (Standard No.2800). So far, due to the lack of information and registered accelerogras in Iran, as well as the unclear ground conditions at the place of record registration; no effective action has been taken to determine the design spectrum in Iran. The site conditions have been classified into different categories in earthquake codes. These categories are named ground types, soil profile types, local site classes or subsoil classes.In Iranian Seismic Building Code, four types of soils I, II, III, IV have been determined, Site Class I: Rock, or hard Rock , or thin alluvium on bed rock with Vs30 more than 750 m / s, Site Class II, Very dense soil or soft rock with high thickness on rock bed with Vs30 between 375 to 750 m / s, Site Class III Stiff soi or soils with medium density or hardness with Vs30 of 175 to 375 meters per second and Site Class IV ; soft soils with a Vs30 of less than 175 meters per secondThe accelerograms, obtained from major earthquakes in Iran, United States and Europe have been collected, then processed.. After calculating the response spectra, the design spectra was plotted from the average response spectra values for all soil types, (I, II, III, IV) presented in the Standard No.2800. To compare the results of this study with Spectra presented in Iranian Seismic Building Code, records obtained from event having magnitudes greater than 5.5 and recorded in the epicentral distances less than 80 km were selected and 5% damping was used for calculations. The results show that design spectrum for soil classes I, II, and III , were consistent with standard No.2800 for the spectral values of Periods 0.0 to 0.39 seconds, while in the periods longer than 0.39 seconds, the design spectrum of the Standard No.2800 is more conservative. than present design spectra . Meanwhile, the spectral ordinates for design spectra of vertical component for soil class IV , were representative for higher values for present spectra , comparing to that presented in Standard No.2800. This mighe be related to epistemic uncertainty imposed by still few available records for soft soil site (class IV).

    Keywords: Accelerogram, Design spectra, Epicentral distance, Near-Fault Earthquake
  • Hadi Tabassi *, Somayeh Bahauddini Pages 17-27

    The placement of the Iranian crust at the junction of the Arabian, Indian and Eurasian plates has caused tectonic disruption. The creation of numerous faults and many folds along with seismicity in the Iranian crust are signs of this disruption. The continental basin of northern Qom is located in central Iran. The general trend of the important faults in this area includes Qom, Alborz and Kushk-e Nosrat faults, is northwest-southeast. This faults with a length of more than 50 km affected by Arab-Eurasian plate convergence, and have played an important role in seismicity and deformation of Qom area.Morphological evidence and the existence of numerous fault fragments that have cut Quaternary sediments indicate the activity of these structures around the city of Qom. Despite the valuable studies that have been done on the faults of Qom, Alborz and Kushk-e Nosrat, still there are ambiguous opinions and ambiguities about how these faults move and their seismic potential. Qom fault with a length of more than 50 km, has a northwest-southeast trend that continues from the mountains southeast of Qom to the south of Zefreh in the northeast of Isfahan. During this fault, Oligo-Miocene limestone and marl are driven on Quaternary sediments. Alborz fault also has a northwest-southeast trend with a dip to the southwest that has cut the northern limb of the Alborz anticline. Alborz fault has also driven Oligo-Miocene units on Quaternary units. Kushk-e Nosrat fault is part of the fault system. It starts from the southeast of Hoz-e Soltan Lake and continues to the Avaj fault. The general direction of Kushk-e Nosrat fault is 280 to 290 degrees and in most cases, it has a dip more than 80 degrees to the south. One of the objectives of this study is to identify how these faults displacement and activity.One of the methods of tectonic assessment is the use of geometric indices. Among these, according to the lithological characteristics and the location and distribution of the basins, the indices of Vf, hypsometry and SL have been selected and the results have been analyzed. Results of morphometric indices with fault geometry and stress orientation data, have been able to elucidate some tectonic features of the area. Morphometric studies show the areas around Saveh fault and Kushk-e Nosrat fault in the northern part of Saveh as well as areas in the south of Qom have the potential to rise. Other zones do not have significant uplift.The geometry of the faults and orientation of the maximum compressive stress show that the maximum compressive stress direction is N30 to N330 and according to the direction of the fault, the fault mechanism is estimated to be Dextral. The stress inferred from the seismic event of 2007/06/18 also confirms the right-lateral shear motion along with the reverse motion in the faults.The low vertical uplifts does not indicate the absence of activities, but can indicate the compressive and shear components or the specific deformation of Central Iran. This type of movement can be easily interpreted by knowing stress orientation. The shear-compressive deformation of the Qom area is due to the northeastern movement of the Arabian plate and the limitation due to the Caspian hard crust. Undoubtedly, the Qom is an area that different seismic researchers has been considered and studies with different views and study styles. Comparing the results of these studies has helped to discover the Unknown activity zone.In general, the northern part of Qom is an area with high seismicity that the mechanism of earthquakes is mostly strike-slip with thrust. Therefore, moderate to large earthquakes are expected to occur in the future.

    Keywords: Qom, Alborz, Kushk-e Nosrat Faults, Activity, deformation
  • Ali Asgari *, Faramarz Ranjbar, Habib Akbarzadeh Bengar Pages 29-49

    Lateral spreading-induced soil liquefaction has imposed significant damage to the deep foundations of bridges, ports, offshore structures, and buildings. Despite various experimental, numerical, and field studies by previous researchers, there is also no comprehensive approach to assessing the effects of lateral spreading on pile groups. Numerical simulations are an economical tool for investigating and a means of representing the seismic performance of the pile groups at sites with liquefaction-induced lateral spreading.In this paper, the effects of various pile groups (e.g, of 1×1, 2×2, and 3×3) on reducing the potential for liquefaction during the earthquake are investigated parametrically, applying three-dimensional finite element (FE) simulations using OpenSees software. To examine the ground inclination angle and array of pile group effects, different models have been subjected to the El Centro (1940) earthquake. This study evaluates the effect of each of these factors on soil acceleration, lateral displacement, excess pore pressure and piles bending moment. The numerical model has been verified and calibrated in the literature through the analysis of well documented a large-scale (1-g) shake-table test.The results are shown that with increasing the ground slope angle, in a specified point of the soil, less pore water pressure is produced and the dissipation of pore water pressure starts earlier, but the variations of pore water pressure increase. In mildly sloping ground, with began of lateral deformations, soil heave occurs in the upslope and soil subsidence occurs in the downslope of the piles, and with increasing ground slope angle, the amount of these subsidence and heave increase. On the other hand, the ground slope angle is a very effective parameter in the lateral and vertical displacement of piles that should be considered in design code.

    Keywords: Ground slope angle, Lateral spreading of Pile groups, Three-dimensional finite element (FE) simulations, Liquefaction, OpenSees
  • Shima Sadat Hoseini, Ali Ghanbari *, Mohammad Davoodi Pages 51-65

    An analytical study of the effect of non-uniform excitation on the seismic response of Sadr BridgeShima Sadat Hoseini, Ph.D candidate of Civil [email protected] University, Department of Civil Engineering, Moffateh Avenue, Tehran, Iran.Ali Ghanbari, Full [email protected] University, Department of Civil Engineering, Moffateh Avenue, Tehran, Iran.Mohammad Davoodi, Associate [email protected] Institute of Earthquake Engineering and Seismology, Department of Geotechnical Earthquake Engineering, Tehran, Iran.Extended Abstract:IntroductionLarge-dimensional structures, such as long-span bridges, receive different ground motions at different supports in earthquake events. Seismic wave propagation and local site conditions cause spatial variation of ground motion. It may result in pounding or even collapse of adjacent bridge decks owing to the out of phase response. In addition, dynamic Soil-Structure Interaction (SSI) resulting from the interaction of the bridge with the surrounding soil also affects the dynamic bridge response. In most bridges, shallow foundations are not appropriate. Because they do not provide the required capacity or may experience excessive settlements or deformations. In such structures, pile groups are used as foundation systems. Pile foundations have to be designed to support not only vertical loads, but also lateral loads due to earthquake, wind and vehicle impact loads. Therefore, soil-pile interaction is added to above factors in dynamic behavior of long-span bridges. From the above reasons, it is very important to consider both Soil-Pile-Structure interaction (SPSI) and Spatially Varying Earthquake Ground Motions (SVEGM) effects in evaluation of the seismic response of long-span bridges. Material and methodsThis paper presents a study about the spatial variability effects of ground motion and Soil-Pile-Structure Interaction (SPSI) on the dynamic response of a long bridge. Two decks of the considered bridge with length d1=100 m and d2=150 m are supported by four isolation bearings which are connected to three elastic piers standing on the pile foundations. The structure of the bridge continues from both sides. The decks are considered as lumped mass model with the total mass of m1=1.2106 kg and m2=1.8106 kg. All of the bearings have the same dynamic properties with an effective stiffness Kb1 and equivalent viscous damping Cb1 for the left span, and Kb2 and Cb2 for the right span. The concrete piers with heights of h1=14 m, h2=16 m and h3=15 m are modelled as elastic columns with lumped mass m3=m4=m5=2105 kg at the top of each pier. The lateral stiffness of the piers are Kp3=2108 N/m, Kp4=108 N/m and Kp5=3108 N/m. To simplify the analysis, a constant damping ratio of 5% is used for bearings and piers. The most widely used model to perform the analysis of piles under lateral loads, consists in modeling the pile as a series of beam elements and representing the soil as a group of unconnected-concentrated springs perpendicular to the pile which is known as Discrete Winkler Model. The Spatially Varying Earthquake Ground Motion (SVEGM) is simulated by SIMQKE-II record generator. Target response spectrum and power spectral density function used in the simulation are determined depending on the January 17, 1994, Northridge earthquake. To evaluate the effect of SPSI, the soil surrounding the pile foundation is modelled by frequency- independent springs and dashpots in the horizontal and rotational directions. The effect of soil-pile mass is considered by lumped-mass soil-pile model. A new analytical model is proposed to study the effect of both SVEGM and SPSI on dynamic response of long bridges. Results and discussionThe results indicate that considering the effect of non-uniform excitation and soil-structure interaction can increase the relative displacement of the deck in the longitudinal and transverse directions by 275% and 176%. Also, considering the interaction effect, on average, shows a reduction of 67% and 75% of the base shear and moment considering non-uniform excitation, shows an increase of 37% and 29%, respectively. ConclusionThe main conclusions drawn from this study can be written as:1- Based on the results obtained from the proposed analytical model, SVEGM affects the seismic behavior of long-span bridges. Influence of SVEGM on decks displacements and maximum shear forces in piers is more significant in softer soil types. It means soil condition as an important factor affects the dynamic response of long bridges.2- The importance of the SPSI effect on the dynamic response of the bridge is also investigated in comparison with fixed-base case. It is observed that the results obtained from the SPSI case are usually amplified in comparison with the fixed-base case. This effect is more significant in softer soil types. It means that the variation of the soil conditions where the bridge supports are located on, has important effect on the bridge dynamic response. Since the proposed model is very similar to real soil-pile-structure systems, suggested equation derived from it, can be used to simulate the seismic behavior of long-span bridges.3- If the effects of SPSI and SVEGM are considered simultaneously, it should be noticed that the results will not be as same as which obtained from the addition of the response determined from these effects separately. The effects of SPSI and SVEGM amplify each other especially in softer soil conditions. It is also observed that considering the effect of SPSI with respect to the SVEGM can change the dynamic response of long-span bridges in comparison with the cases in which one of these factors or none of them is considered.4- In general, the recommendation of fixed-base case with uniform ground excitation in dynamic design regulations of bridges is valid only if SPSI and SVEGM effects are negligible. These assumption can be used in seismic design of bridges on very stiff soil conditions and not very long bridges. Otherwise, this recommendation leads to usually underestimating the dynamic response or even bridge structure damage.

    Keywords: Spatially Varying Earthquake Ground Motions, Soil-Structure Interaction, Long Bridges, Dynamic response, Analytical Model
  • S. Bahram Beheshti Aval*, S. Ali Hussaini Pages 67-83

    The coupled-shear wall system has been proven to be an outstanding lateral-load resisting system in the medium- to high-rise buildings. In this system, several individual shear walls are coupled with coupling beams. Thus, the walls show an integrated performance in resisting the earthquake loads by providing control over lateral displacements. In tunnel form buildings, the strength and the lateral stiffness are influenced by the coupling beam. In this system, the coupling beams act as a fuse in this system; they are the first elements to undergo inelastic deformations. The proper design of the concrete coupling beams (CCBs) leads to the use of diagonal reinforcements and high-level of detailing. There is a problem in construction of RC tunnel form buildings related to inserting the diagonal reinforcing shear bars, which is practically difficult and in most cases, this reinforcement is ignored. Therefore, these elements as the structural fuses have a minor contribution in dissipation of seismic input energy. Recent researches showed that the steel coupling beams (SCBs) exhibit better performance than the CCBs in terms of ductility and energy absorption during cyclic loadings. The pinching effect was not observed in the hysteretic loops for SCBs, which implied a more stable post-elastic behavior and a higher energy dissipation capacity than the CCBs. Also, the energy dissipation capacity of SCBs was more than three times larger than that of the CCBs. In this study, a regular symmetric plan seven-story tunnel form building was used. The coupling beams with a length of 1 m and a depth of 0.7 m above the openings were inserted. The building was assumed to be residential and located in Tehran. The story height was 3 m, and the soil was considered type B based on the regulatory seismic code of Iran. The building was designed according to ACI 318-14 by using ETABS software. The thickness of the walls and the slabs were 20 cm and 15 cm, respectively. No. 8 rebar with spacing of 20 cm was designed for the vertical and horizontal reinforcements in two layers. In the first two stories, only the vertical reinforcements were of No. 12 rebar. The compressive strength of concrete and the yield strength of reinforcements for structural members were 25 MPa and 400 MPa, respectively. The diagonal reinforcements of the CCBs were designed to provide the ductility and the improvement of the shear strength. In this research, the CCBs were substituted by SCBs with circular holes. The shear strength of the beam was improved with diagonal stiffeners. The holes in the web of the beam were used to convey installations and to avoid perforating shear walls in tunnel form buildings. The diagonal stiffeners prevented the buckling of the beam’s web. Also, the shear strength and the tension field action were improved. Therefore, the seismic performance of the coupling beam was overall enhanced. As the main design equipment the SCB, the relations related to determining the shear capacity of the steel beam were exploited. Then, the seismic behavior of tunnel-form structures with regular RC coupling beams against the same structures designed with the proposed steel beams has been evaluated and compared in a nonlinear range. Finite element modeling and nonlinear analyses were conducted in PERFORM-3D. The incremental dynamic analysis regarding the probable ground motions was performed on the buildings to consider the effects of amplitude variation, frequency content, and the duration of ground motions on the response. The results showed that the use of the proposed SCB reduces the system stiffness and thus increases story drifts. In addition to decreasing the probability of the walls to attain the first levels of failure slightly, under the design earthquake and the maximum probable earthquake, the reliability of buildings in achieving predetermined performances was increased. Indeed, the use of this proposed coupling beam also increases the ductility of the tunnel form structures. Easy to implement and easy to repair or replacement of this steel coupling beam after a destructive earthquake are its other advantages compared to RC type.

    Keywords: Tunnel Form System, Replaceable Coupling Beam, Seismic Reliability, Fragility Curve
  • Mahyar Nobakht, Masood AmelSakhi*, Fatemeh Sheshpari Pages 85-92

    This study presents the results of a numerical study on seismic behavior of two-dimensional semi-sine shaped hills that were subjected to vertically propagating incident SV wave of the Ricker wavelet. The finite difference software is used to model and analyze the different sizes of the hills.Earthquakes are natural events that can have considerable economic and social injuries and have effects on people life and their environment. Geotechnical earthquake engineering has been noticed particularly in recent decays. Soil mass and soil structure response analysis against earthquake movements is one of the most important practical concepts in geotechnical earthquake engineering. In the case of 2D response analysis of hills, researchers studied seismic response of triangular shaped hills under vertically propagating SH waves. He used finite difference method and represented that topography has specific effect on acceleration distribution in different points of roughness. Concentration is on topographic effects, so parameters such as shape factor (the ratio of the height to half width of the hill) and the type of ground took into account in this research. The medium was assumed to have a linear elastic constitutive behavior. So we modeled semi-sine hills with different shape factors (0.1, 0.4 and 0.8) where subjected to Ricker wavelet with constant frequency on grounds with different properties that differ from each other in density and shear velocity (three types based on 2800 standard). In this research the horizontal and vertical displacements of different points on hills calculated and the results are compared with each other in different shape factors.The results indicates results in different parts that shows horizontal displacement and vertical displacement for types of field under Ricker wavelet and compared with Trinidad earthquake wave.Amplification factor calculated based on the ratio of horizontal components of motion to displacements of free-field model. The finite difference software used to run the numerical analyses was Flac 2D. The aim of this project was to investigate the response of topography effects on semi-sine shaped hills under Ricker wavelet which is the second derivative of Gauss function.The obtained results show that both horizontal and vertical displacements increased with change in shape factor. It can be seen that changing geotechnical properties of the ground type from one to three; the displacements reduced.

    Keywords: semi-sine hills, Ricker wavelet, Topography, Amplification
  • Seyed Naser Hashemi * Pages 93-103
    Introduction

    Most of the seismic energy (or tectonic loading) accumulated in lithosphere of active regions is released through the occurrence of large earthquakes that usually show complex spatio-temporal patterns. Hence, the study of the spatial and temporal pattern of these occurrences is very important for revealing the seismotectonic nature of these regions. Over the past decades, the statistics of the waiting times between consecutive earthquakes (so-called inter-event times) have become the focus of research. Statistical analysis of inter-event times of earthquakes allows the derivation of useful information that can allow the development of earthquake forecasting strategies, and, inter-event time statistics for moderate to small events may be used to extrapolate inter-event time behaviour at larger scales. Assuming that the release of seismic energy (by occurring earthquakes) is stationary in the whole of the Zagros region, in this research, the spatio-temporal relationships of the occurrences of large earthquakes in this region have been studied.2-

    Methodology

    In this study, the migration pattern of successive earthquakes in the Zagros region during the period 1976-2019 has been studied for earthquakes with magnitude 4.5 Richter and greater. In order to carry out this work, the earthquake data of the examined region with M ≥ 4.5 (1976-2019) have been obtained from the USGS catalog. Then, the inter-event time, migration distance, and migration trend of successive earthquakes with different lower magnitude thresholds of 4.5, 0.5 and 5.5 Richter were calculated and the statistical distribution of these data was analyzed and modeled.3-

    Results and Discussion

    Statistical analysis of the inter-event times between consecutive earthquakes in the Zagros region shows that among the different models used in statistical modeling of data, Weibull and Gamma models show the best agreement with the statistical distribution of inter-event time data. In addition, it is observed that larger earthquakes are less compatible with these models. Furthermore, migration distance data from successive earthquakes also shows a decreasing pattern, similar to the inter-event time distribution data. In addition, the study of the relationship between the two variables of migration distance and time interval between events shows that especially for earthquakes of smaller magnitude, it is not possible to find a significant relationship between these two variables, but for larger earthquakes, it seems that a positive correlation between these two variables exists. This finding indicates that earthquakes with more inter-event times are expected to occur farther apart from each other, which could be a reason for seismicity migration behavior of earthquakes in this region. Also, the directional pattern of earthquakes migration data shows a pattern consistent with the general trend of active faults in the Zagros region, which confirms the idea that the activation of discrete segments of fault systems in this region plays a key role in the temporal and temporal pattern of seismicity. Based on the results of this study, it is expected that earthquakes with magnitude 5.5 and greater, tend to occur with an average migration distance of about 418 km and an average waiting time of 198 days, and in the dominant directional azimuth of N62W or S62E, compared to their previous events.4-

    Conclusions

    The results of this study can be considered as an effective step to better understanding the temporal-spatial pattern of seismicity in the Zagros region and also as an attempt to achieve earthquake prediction in a regional scale. It is expected that in the future, with the possibility of access to more accurate data and the use of other new methods such as neural network modeling and artificial intelligence, it will be possible to better understand the temporal and spatial pattern of earthquakes, which undoubtedly is an important and effective step to achieve earthquake prediction on a regional scale.

    Keywords: Inter-event time distribution, Statistical distribution of earthquakes, Earthquake directional migration pattern, Seismicity
  • Rasool Mazloom *, Ehsan Maani Miyandoab Pages 105-113

    Earthquake is one of the most dangerous natural disasters of the present age, which has always shown its importance objectively. An earthquake is a natural disaster that, depending on its magnitude, can cause massive catastrophes in a short time. The purpose of this paper is to extract and present a model for earthquake wave velocity changes (V_(P_n )) using Genetic Algorithm(GA). The data used in this study were received from the National Seismological Center of the Institute of Geophysics, University of Tehran. In this study, three provinces of Kermanshah, East Azerbaijan and Kerman were selected. Earthquake event characteristics of each of these three provinces in the period between 2006 to the end of 2018 and with a focal depth of up to 30 km and magnitude between 4 to 8 were selected.In order to use the Genetic Algorithm(GA), first the data received from the National Seismological Center for these three provinces were merged, which was estimated at 1863 earthquake events. After extracting the relevant data, the earthquake wave velocity (V_(P_n )) was calculated. Then, ignoring about 25% of this data, a mathematical model for earthquake velocity was extracted. Finally, the obtained formula was applied to the initial ignored data (25%), which had similar results. To model the changes in wave velocity according to distance changes, a mathematical relation was considered as an exponential function and the unknown parameters of the model were determined using a Genetic Algorithm(GA). To find a suitable model between distance and speed, the following relation is considered for it.V(x)=a+be^(-kx)In this regard, a, b and k are constant coefficients and x is the distance from the earthquake site in terms of one thousand kilometers. In the above equation, the coefficients must be determined so that the output of this model with the recorded data has the least amount of error. For this purpose, the Genetic Algorithm(GA) optimization method is used. For this purpose, the error between the model output and the actual data was considered as the objective function of the optimization problem and the optimization variables were determined with the aim of minimizing this objective function.The output of the model for training and test data showed that the proposed model has acceptable accuracy for modeling the velocity of longitudinal waves. This model can be used to determine the arrival time of waves of an earthquake to different points. It is also possible to estimate the location of the earthquake by recording the occurrence of the earthquake at several different points and using the provided relationship. In this paper, the average speed of propagation of longitudinal waves of the earthquake in terms of distance from the location of the earthquake was investigated. For this purpose, three different earthquakes in three different parts of Iran are considered And the time of arrival of these waves and their recording in different stations were extracted from the information of the Institute of Geophysics, University of Tehran And having the distance of stations and the difference of wave transmission time, the average speed of the waves was calculated. The same general behavior was observed in all cases and that is to increase the speed of earthquake waves by propagating and traveling at a distance of 7.74 km / s for all three test cases. To model changes in wave velocity in terms of distance changes, A mathematical relation is considered as an exponential function and unknown parameters of the model were determined using genetic algorithm.

    Keywords: Genethic Algorithm (GA), Modeling, Wave Velocity, Earthquake