فهرست مطالب
مجله مهندسی عمران شریف
سال سی‌ام شماره 1 (بهار 1393)
- تاریخ انتشار: 1393/03/20
- تعداد عناوین: 13
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صفحات 3-11در این نوشتار، تاثیر کلسیت در کائولینیت بر فرآیند نگهداری آلاینده در خاک و رفع آلودگی از خاک توسط EDTA مطالعه شده است. EDTA در نسبت های EDTA/Pb برابر 0٫2 تا 1 به کائولینیت دارای درصدهای کربنات و غلظت های مختلف آلاینده افزوده و سپس مقدار P b و C a آزادشده توسط EDTA اندازه گیری شد. در این فرآیند رفع آلودگی به واسطه ی کاهش pH خاک، بخشی از کربنات حل می شود و در نتیجه، ظرفیت بافرینگ خاک کاهش می یابد. اگر خاک مورد رفع آلودگی، مجددا در معرض آلودگی قرار گیرد؛ این موضوع اهمیت ویژه یی خواهد داشت. با انحلال کلسیت و آزادشدن یون کلسیم، رقابت بین کاتیون های سرب و کلسیم برای تشکیل کمپلکس با EDTA رخ می دهد، که باعث کاهش بازده ی رفع آلودگی می شود. استفاده از محلول EDTA با غلظت کم و انجام فرآیند شستشو در چند مرحله باعث کاهش انحلال کلسیم می شود و افزایش بازده ی رفع آلودگی توسط EDTA را سبب خواهد شد.
کلیدواژگان: کائولینیت، کربنات، ژئوتکنیک زیست محیطی، فلز سنگین سرب، E D T A -
صفحات 13-20در این پژوهش عملکرد لرزه یی، قاب های فولادی با مهاربندهای کمانش تاب به روش تحلیل دینامیکی غیرخطی مقیاس شده که به تازگی معرفی شده و همین طور تحلیل استاتیکی غیرخطی مورد بررسی قرار گرفته است.
برای این کار تعدادی قاب مهاربندی شده با پیکربندی های مهاربندی قطری، ضربدری دوبخشی، و شورون طراحی شده اند. مقادیر پاسخ غیرکشسان سازه ها شامل جابجایی، جابجایی نسبی طبقات و بام، و برش طبقات حاصل از تحلیل ها با یکدیگر مقایسه شده اند. در نهایت، اثر تعداد رکوردهای زلزله در تحلیل دینامیکی غیرخطی مقیاس شده در پراکندگی نتایج مورد بررسی قرار گرفته است. نتایج نشان می دهند که روش تحلیل استاتیکی غیرخطی در برخی مواقع مقادیر پاسخ ساختمان را دست بالا تخمین می زند. ضمنا افزایش تعداد رکوردهای زلزله، تاثیر چندانی در بهبود پراکندگی نتایج در روش دینامیکی مقیاس شده ندارد.
کلیدواژگان: مهاربند کمانش تاب، تحلیل استاتیکی غیرخطی، تحلیل دینامیکی غیرخطی مقیاس شده -
صفحات 21-31در این نوشتار، اثرات انعطاف پذیری پی در نحوه ی اعمال ضریب اصلاح تقاضای عضو در دستورالعمل بهسازی لرزه یی ساختمان های موجود بررسی شده است. بدین منظور از روش ساده و کارآمد و در عین حال با دقت مناسب تیر مستقر بر خاک غیرخطی وینکلر استفاده شده است. در این راستا، قاب های 3، 6، 10 و 15 طبقه ی بتنی مستقر بر خاک های نرم، متوسط، و سخت طراحی و نتایج تحلیل های غیرخطی تاریخچه ی زمانی هر یک در حالت های پایه ی صلب و انعطاف پذیر با نیروهای پیشنهادی روش استاتیکی خطی دستورالعمل مقایسه شده است. نتایج نشانگر آن است که ترکیبات بارگذاری دستورالعمل برای اجزاء کنترل شونده توسط تغییرشکل با بارهای ثقلی قابل توجه (برای نمونه ی تیرها) می تواند به برآورد غیرواقع گرایانه ی تقاضای لرزه یی منجر شود. در انتها، یک ترکیب بارگذاری جدید، به منظور رفع این نقیصه، پیشنهاد شده است.
کلیدواژگان: اندرکنش خاک، پی، سازه، مدل سازی، وینکلر، انعطاف پذیری، حرکت زمین -
صفحات 33-41در این نوشتار، با استفاده از توابع پتانسیل تغییرمکان، حل دقیق ارتعاش آزاد صفحات مستطیلی ایزوتروپ همگن با ضخامت دلخواه ولی ثابت بر روی چهار لبه ی ساده ارائه شده و تابع پتانسیل تغییرمکان و به کمک آن معادله ی مشخصه ی ارتعاش آزاد صفحه با حل دو معادله ی دیفرانسیل پاره یی حاکم از مرتبه ی 4 و 2 به روش جداسازی متغیرها و اعمال شرایط مرزی به دست آمده است. ویژگی عمده ی این پژوهش، عدم وجود محدودیت ضخامت و اعتبار آن برای صفحات نازک، ضخیم نسبی و ضخیم است. به منظور اعتبارسنجی، نتایج حاصل از این پژوهش با نتایج به دست آمده از سایر کارهای تحلیلی، که عمدتا مربوط به صفحات ضخیم نسبی متکی بر نظریه های تغییرشکل برشی مرتبه ی اول و بالاتر بوده مقایسه شده است. نتایج نشان می دهد که با افزایش ضخامت بسامد بدون بعد کاهش که مقدار این کاهش در مودهای بالاتر ارتعاشی با شدت بیشتری همراه است.
کلیدواژگان: حل دقیق، صفحات مستطیلی ضخیم، تابع پتانسیل تغییرمکان، ارتعاش آزاد، مسئله ی سه بعدی کشسانی -
صفحات 43-51به منظور ارتقاء ضوابط آیین نامه یی در مورد ساختمان های آجری کلافدار، مطالعات بیشتر تحلیلی و آزمایشگاهی بر روی این سازه ها لازم به نظر می رسد. در مورد مدل سازی اجزای محدود ساختمان های آجری کلافدار، مطالعات محدودی در ادبیات فنی موجود است. در این نوشتار، پارامترهای مناسب برای مدل سازی اجزای محدود دو دیوار آجری کلافدار، با و بدون بازشو، ارائه می شود. این پارامترها را نگارندگان از نتایج آزمایش های انجام شده استخراج و ارائه کرده اند. نتایج تحلیل نیز با نتایج آزمایش های انجام شده قایسه و صحت سنجی شده است. نتایج نشان می دهد که مدل اجزای محدود ارائه شده، در عین سادگی تخمین خوبی از رفتار دیوار آجری کلافدار ارائه می کند؛ و همچنین پارامترهای مدل سازی برای دیوارهای ساخته شده بر طبق استاندارد 2800 و با مصالح مرسوم در ایران، اختلاف قابل توجهی با پیش فرض های معمول در سایر نقاط دنیا دارند.
کلیدواژگان: ساختمان های آجری کلافدار، مدل اجزای محدود، بار جانبی، استاندارد، 2800 زلزله ایران -
صفحات 53-61امروزه فوم ها به منزله ی نسل جدیدی از مصالح سبک در صنایع خودروسازی و هوافضا مورد توجه قرار گرفته اند. در این پژوهش، اثر نقص در هندسه ی ریزساختار در رفتار فوم سلول باز مورد مطالعه قرار گرفت. به این منظور، از روش همگن سازی، به عنوان راهکاری مناسب برای ارتباط ویژگی های مکانیکی مواد در مقیاس های بزرگ (جسم) و کوچک (ریزساختار) بهره گرفته شد. به این ترتیب که ابتدا با بازتولید یک مدل همگن سازی موجود، رفتار تک محوری نمونه ی فوم با ساختار منظم محاسبه شد. در ادامه، با اعمال آشفتگی در هندسه ی ریزساختار، تغییرات خصوصیات تک محوری فوم نسبت به حالت منظم مورد بررسی قرار گرفت. نتایج به دست آمده، محدوده ی تغییر کمیت های تک محوری فوم را با افزایش آشفتگی در ریزساختار مشخص کرد. همچنین، حالت هایی از تغییر هندسه ی ریزساختار، که سبب بیشترین تغییرات در کمیت های تک محوری فوم می شوند، شناسایی شدند.
کلیدواژگان: مدل رفتاری، روش همگن سازی، فوم های سلول باز، نقص هندسی -
صفحات 63-72در این پژوهش، برای بررسی مقاومت پیوستگی بتن فوق توانمند، 36 آرماتور فولادی از بلوک های بتنی بیرون کشیده شدند. علاوه بر این، تعداد 9 تیر آزمایشگاهی که هر یک از اتصال دو تیر خمشی پیش ساخته ی مجزا تشکیل شده است و همچنین یک نمونه ی بدون وصله در مقیاس واقعی ساخته و آزمایش شدند، و به کمک نتایج آزمایش بیرون کشیدگی اثر پارامترهای مختلف در پیوستگی بررسی شدند. مقایسه ی شاخص شکل پذیری نمونه های تیری وصله دار نشان می دهد که استفاده از بتن فوق توانمند در محل اتصال قطعات پیش ساخته ی اعضای خمشی باعث ساده سازی جزئیات اتصال شده است. در این نوشتار، رابطه ی طراحی برای تعیین طول وصله و مقدار آرماتور جانبی مورد نیاز وصله ارائه شده است. مقدار میانگین نسبت مقاومت پیوستگی آزمایشگاهی به مقاومت پیوستگی حاصل از رابطه ی پیشنهادی در نمونه های ساخته شده از بتن معمولی 1٫10 با انحراف معیار 0٫05 و در نمونه های ساخته شده از بتن فوق توانمند 1٫27 با انحراف معیار 0٫10 به دست آمده است.
کلیدواژگان: اتصال، بتن فوق توانمند (U H P C)، شکل پذیری، طول وصله، قطعات پیش ساخته، مقاومت پیوستگی -
صفحات 73-80آیین نامه های معتبر ساختمانی در یکی دو دهه ی اخیر، روابطی را برای محاسبه ی نیروی وارد بر اجزای غیرسازه یی ساختمان ها ارائه کرده اند. در این مطالعه، روابط پیشنهادی آیین نامه ی یوروکد با مقادیر حاصل از تحلیل های تاریخچه ی زمانی قاب های فولادی 3، 5 و 7 طبقه ی خمشی، مهاربندی هم محور و برون محور با رفتار کشسان و خمیری تحت رکوردهای حوزه ی دور و نزدیک قایسه و نشان داده شده است که مقادیر حاصل از این روابط در بسیاری از موارد، غیرمحافظ کارانه و کمتر از حد لازم است. بر این اساس و با استفاده از نتایج تحلیل های ذکرشده برای اصلاح مقادیر این آیین نامه، اصلاحاتی پیشنهاد می شود. این اصلاحات عمدتا مرتبط با اثر محل قرارگیری عضو غیرسازه یی در ارتفاع سازه و نیز ضریب تشدید ارائه شده در آیین نامه ی مذکور است. در این نوشتار، همچنین اثر زلزله های حوزه ی نزدیک و شدت زلزله در نیروی وارد بر اعضای غیرسازه یی نشان داده شده است.
کلیدواژگان: اجزای غیرسازه یی، شدت زلزله، قاب برشی، زلزله ی حوزه ی نزدیک، طیف پاسخ طبقه، ضریب تشدید، بیشینه ی شتاب طبقه -
صفحات 81-93هدف این پژوهش، ارائه ی فرمول بندی مناسب به منظور بررسی اثر هم زمان مولفه های انتقالی و دورانی حرکت زمین در پاسخ سدهای بتنی وزنی با احتساب اندرکنش سد و مخزن به روش اجزاء محدود است. بدین منظور، مولفه های دورانی حرکت زمین با استفاده از مولفه های انتقالی آن با درنظرگرفتن سرعت امواج وابسته به بسامد تولید می شوند. سپس این مولفه ها به روش های مناسب به مدل اجزاء محدود سیستم سد مخزن اعمال و پاسخ دینامیکی آن با استفاده از روش نیومارک و با درنظرگیری تغییرمکان به عنوان متغیر مجهول در گره های شبکه ی اجزاء محدود کل سیستم که به روش لاگرانژی لاگرانژی معروف است، به دست می آید. نتایج به دست آمده نشان می دهد که با توجه به بیشینه ی مقدار مولفه های دورانی و نیز طیف محتوای بسامدی آنها، ممکن است به طور قابل توجهی، تحت شرایط خاص موثر باشند.
کلیدواژگان: مولفه های دورانی حرکت زمین، سرعت امواج وابسته به بسامد، اندرکنش سد و مخزن، روش لاگرانژی، لاگرانژی -
صفحات 95-106پایگاه داده یی از نتایج محفظه ی کالیبراسیون آزمایش مقاومت نوک مخروط جمع آوری شده است. از این پایگاه داده استفاده شده و اثر پارامترها و شرایط خاک (شامل: چگالی نسبی، تنش قائم، تنش افقی، مدول محدودشده و نسبت بیش تحکیمی) در مقاومت نوک مخروط مورد بررسی قرار گرفته است. با استفاده از این بررسی ها، چگالی نسبی و تنش افقی به عنوان پارامترهای اصلی انتخاب و رابطه یی بین آنها و مقاومت نوک مخروط پیشنهاد شده است. این رابطه با محاسبه ی معکوس می تواند جهت تعیین تنش افقی خاک بر اساس مقاومت نوک مخروط مورد استفاده قرار گیرد. میزان دقت معادله با استفاده از پایگاه داده ی آزمایش های کالیبراسیون مخروط صحت سنجی می شود. همچنین کارایی آن با دو معادله ی موجود پیشنهادشده ی سایر پژوهشگران مورد مقایسه قرار می گیرد. همچنین علت کارابودن و دلایل انتخاب پارامترهای مورد استفاده بررسی و تحلیل می شود.
کلیدواژگان: آزمون C P T، آزمایش محفظه ی کالیبراسیون، مقاومت نوک مخروط، چگالی نسبی، تنش افقی -
صفحات 117-124تبدیل موجک یک ابزار ریاضی است که تغییرات زمانی بسامد را در یک سیگنال نشان می دهد. از طرفی دیگر، زمان دوام یک روش پوش آور دینامیکی است؛ که عملکرد آن بر اساس زمان رسیدن به شاخص خرابی موردنظر است. در این نوشتار، با استفاده از تبدیل موجک و الگوریتم های بهینه سازی به تولید توابع شتاب سازگار با طیف آیین نامه ی 2800 پرداخته شده است. روش به کار گرفته شده، در واقع، یک نوع اصلاح داده ها در حوزه ی زمان و بسامد است. برای این منظور کارایی الگوریتم های ابتکاری، کلاسیک، و ترکیبی از آنها برای تابع شتاب ساده شده (5 ثانیه یی)ارزیابی می شود. الگوریتمی که در طی تکرار مشخص، کمترین میزان خطا را داشته باشد؛ به منزله ی بهترین روش بهینه سازی انتخاب می شود. سپس با استفاده از این روش، سه تابع شتاب اصلی (20 ثانیه یی)تولید می شود. در نهایت نیز مقادیر خطای تابع شتاب تولیدشده با توابع شتاب a20E T A مقایسه شده است، که نشان دهنده ی کارایی این روش نسبت به روش مرسوم است.
کلیدواژگان: تابع شتاب، زمان دوام، تبدیل موجک، روش های بهینه سازی -
صفحات 125-135استفاده از الیاف پلیمری F R P یکی از کارآمدترین روش های کاهش اثرات خوردگی بر سازه هاست. از آنجایی که تاثیر خوردگی بر خاموت های فلزی به علت قطر پایین آنها و همچنین نزدیکی آنها به سطح بیش از میلگردهای طولی است، در این نوشتار، یک پژوهش آزمایشگاهی بر روی اثر جایگزینی خاموت های ساخته شده از الیاف F R P به جای خاموت های فلزی تحت بارگذاری چرخه یی صورت گرفته است. به این منظور، سه اتصال بتنی ساخته آزمایش شد. از 3 اتصال ساخته شده، یک اتصال به عنوان اتصال مرجع دارای میلگرد طولی و خاموت فلزی بود. میلگردهای طولی کلیه ی نمونه ها، فلزی و خاموت های دو نمونه ی باقی مانده از جنس F R P ساخته شده در محل بودند. سه نمونه ی اتصال تحت بارگذاری چرخه یی قرار گرفتند. مطابق نتایج به دست آمده ملاحظه شد که خاموت های F R P دارای عملکرد مناسبی در مقایسه با خاموت های فلزی بودند.
کلیدواژگان: اتصال بتنی، خاموت F R P، چشمه ی اتصال، شکل پذیری -
صفحات 137-146اتصالات متداول در قاب های ساده، کاملا مفصلی عمل نمی کنند و مقداری از لنگر تیر را به ستون منتقل می کنند؛ بنابراین فرض رفتار مفصلی ایده آل برای قاب های ساده گرچه محافظه کارانه به نظر می رسد، ممکن است در برخی حالات به نتایج دور از واقعیت منجر شود. هدف از این نوشتار، بررسی و تحلیل رفتار واقعی اتصالات به کمک نبشی های جان و نشیمن است که از انواع اتصالات کاربردی در قاب های ساده هستند. برای این منظور، بعد از مرور برخی مفاهیم مرتبط با اتصالات نیمه صلب، رفتار غیرخطی قاب های ساده ی فولادی 3، 5 و 8 طبقه مورد ارزیابی قرار گرفته است. طبق نتایج حاصل، درجات گیرداری اتصالات مذکور کمتر از 10٪ است.در نظرگرفتن رفتار واقعی اتصالات در مقایسه با رفتار مفصلی ایده آل باعث بهبود وضعیت تیرها بین 4 الی 18 درصد می شود، در حالی که وضعیت ستون ها وخیم تر می شود. افزایش مقادیر طراحی ستون ها، در ستون های کناری، مخصوصا در طبقات فوقانی، قابل توجه است.
کلیدواژگان: قاب ساده ی فولادی، اتصال تیر به ستون، گیرداری جزئی، نبشی جان، نبشی نشیمن
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Pages 3-11In this paper, the influence of calcium carbonate (calcite) in kaolinite on the buffering capacity of clayey soils and soil contaminant remediation with EDTA is investigated. For this purpose, several mixtures of kaolinite and different percentages of carbonate were prepared and were laboratory contaminated with different concentrations of $Pb(NO_{3})_{2}$. The kaolinite sample initially had 4% natural carbonate. To investigate the impact of carbonate on the soil interaction process with EDTA, 6, 11, 16, 21 and 26 percentages of calcite was added to soil samples. Therefore, different calcareous soil samples had 10, 15, 20, 25, and 30 percentage of calcite, respectively. Then, different concentrations of EDTA were applied to these samples in the EDTA/Pb ratios of 0.2 to 1. This range of EDTA is proposed by prior research. After the mixing period, the samples were centrifuged. Then, the concentration of Pb and Ca was measured in supernatant. The results show that in this method of soil remediation, due to the solubility of a part of carbonate in low pH, the buffering capacity of soil decreases. With the solubility of calcium carbonate and the release of calcium ions, both calcium and Pb ions will tend to interact with EDTA and form a Ca-EDTA or Pb-EDTA complex. This, in turn, will reduce the efficiency of soil remediation with EDTA. The results of this paper show that the use of EDTA, at low concentrations, with the application of several soil washing will decrease the solubility of carbonate and consequently increase the efficiency of contaminant removal by EDTA from calcareous soils. In addition, according to the results of this paper, several soil washing with low concentrations of EDTA will not significantly decrease the soil pH. Therefore, the soil buffering capacity will remain constant in this method of soil remediation. In fact, in such a case, the remediated soil will keep its retention capability after the remediation process.Keywords: Kaolinite, carbonate, geo, environmental engineering, heavy metal, Pb, EDTA
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Pages 13-20Strong earthquakes during the last two decades and also post-earthquake field investigations have revealed that steel braced frames exhibit non-ductile behavior due to the buckling of braces during severe ground motion. Recently, modified braces, namely; ``buckling restrained braces'' (BRB), have been proposed, in which the buckling of braces due to compression forces is prevented. Thus, such braces exhibit the same behavior in tension and compression. Since these braces have recently been proposed, more investigations into their seismic performance are needed. Nowadays, the computational effort of computers has facilitated the implementation of nonlinear analysis procedures in structural engineering. Nonlinear dynamic analysis is the most exact method for seismic evaluation of structures. However, it is time-consuming and complicated to be utilized in 5engineering practice. In contrast, static pushover analysis (SPO) is more practical, but, it has several limitations. Recently, a scaled nonlinear dynamic procedure (scaled NDP) has been proposed. In this procedure, dispersion of the responses of nonlinear dynamic analysis is reduced by scaling a few ground motion records. Ground motion records are scaled to a value equal to roof target displacements, which are obtained by a static nonlinear procedure.Therefore, this procedure is suitable for cases in which a sufficient number of ground motion records are not available. Recently, scaled NDP has been recognized by FEMA440 as an effective tool for the seismic evaluation of structures since it utilizes the benefits of both nonlinear dynamic and nonlinear static procedures. In this study, the seismic behavior of buckling restrained braced frames is investigated using scaled NDP and a nonlinear static procedure. Two groups of BRBF structures including 3-story and 12-story frames with diagonal, x-split and chevron bracing configurations are designed, and seismic responses, including displacements, story drifts and story shears, are compared by scaled NDP and SPO analyses. To evaluate the accuracy of the scaled NDP procedure, the effect of the number of earthquake ground motion records on the dispersion of the results is also investigated. Results show that nonlinear static analysis is able to estimate relatively good results, although it overestimates the responses in some cases. Also, an increase in the number of earthquake ground motions does not considerably improve the dispersion of results.Keywords: BRBF, nonlinear static procedure, nonlinear dynamic procedure
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Pages 21-31Soil-Foundation-Structure Interaction (SFSI) in a structural modelling procedure can change seismic structural response. However, SFSI effects are mostly ignored in the analysis procedure of structures, due to a general engineering belief regarding its conservative effects. This conservativeness is not always the case, although the period and the damping of the structure changes by considering the SFSI effects and, consequently, seismic demand decreases. Consideration of SFSI effects still contains some level of difficulty due to the needed in dvance modelling procedures, but this issue can be solved by employing advanced finite element modelling programs, e.g; OpenSees software. The aim of the current paper is to evaluate the influence of SFSI effects on the component demand modifier factor, based on Iranian instructions for the seismic rehabilitation of existing buildings. For this purpose, the beam on the nonlinear Winkler foundation approach is used, which is a simple and efficient method. First, a collection of 3, 6, 10 and 15 storey concrete moment-resisting frames, founded on soft, medium and hard soil, are designed and analysed for the case of fixed-base and flexible-base assumptions. Eight ground motion records were chosen in order to estimate the median response of frames for a pre-defined seismic scenario. A comparison has been made between the results of the nonlinear response history analysis of each frame under flexible-base and fixed-base conditions, with the response based on the equivalent linear static approach. The results show that the equivalent linear static approach load combinations for the elements with significant gravity loads that are controlled by deformation actions (e.g. beams) can lead to non-conservative prediction of the seismic demand. Finally, a new load combination has been proposed in order to limit the influence of the demand modifier factor only on the seismic loads. This new proposed load combination can be used to improve the equivalent linear static approach in the instructions for seismic rehabilitation of existing buildings.Keywords: Soil, foundation, structure interaction, modelling, Winkler, flexibility, ground motion
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Pages 33-41In this paper, using displacement potential functions, the exact solution of a three dimension elasticity problem is presented for free vibration of rectangular isotropic plates. It is assumed that the materials of the plate are homogeneous, linearly elastic, arbitrary, but with a constant thickness, and all four edges of the plate are on simple supports. The governing equations in terms of displacement potential functions are two differential equations of fourth and second orders. Assuming harmonic motion and using a separation of variables, the solution of the governing differential equations for displacement potential functions results in exponential and trigonometric expressions along the plate thickness and the other two lengths, respectively. The boundary condition are zero vertical displacement and zero bending moments on all four edges, and all components of stresses, including normal and shear stresses on the top and bottom of the plates, are zero. Applying these boundary conditions result in a characteristic equation of the free vibration of the plate, with which solution, the plate frequency vibration can be calculated.In order to verify the solutions, the obtained results are compared with other analytical work that are largely based on first and higher order deformations theories for moderately thick plates. The most important characteristic of the method presented in this paper is that there is no limitation for thickness in determining free vibration frequency and its validation for thin, moderately thick and thick plates. The investigations have been done for a wide range of aspect ratio (length to width) and thickness to length. The obtained results show that increasing plate thickness decreases nondimensional frequency, and that this decrease intensifies at higher modes of vibration. In addition, investigations show that the Poisson's ratio has little effect on nondimensional frequency increasing the thickness of the plate. This phenomenon is negligible when the thickness of the plate is decreased.Keywords: Exact solution, thick rectangular plates, displacement potential function, free vibration, three dimension elasticity problem
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Pages 43-51Confined masonry walls are made up of masonry walls and confining elements on their four sides. This system is a conventional form of housing construction, as well as a good alternative for post-disaster reconstruction of seismically damaged and/or collapsed buildings in many countries, including Iran. Seismic behavior of confined masonry buildings rigorously depends on material properties, construction methods and worker skill levels. There are few studies that consider the effects of the local constructing parameters of Iran, such as common materials and labour skills, on the modeling of confined masonry walls. In this paper, a computer program, DIANA, is used for the finite element modeling of confined masonry walls. Modeling parameters are estimated by material tests conducted in the structural laboratory of the International Institute of Earthquake Engineering and Seismology (IIEES). The models are validated with the results of tests performed on two one-story one-bay confined masonry walls performed in the same laboratory. The walls are designed according to the Iranian seismic code and are subject to lateral cyclic loading; the first being solid and the second with a central window opening. The scale of 1:2 is assigned to all parts of the walls including bricks. Two types of mortar with different sand to cement ratio are used to make the masonry walls. Proper modeling parameters for concrete ties and the macro modeling of masonry panels, which are estimated by material tests, are introduced for both specimens. The results obtained from the models match the experimental results quite well. Results show that the elasticity module of such walls are considerably smaller than that proposed in previous studies in foreign counties. Also, results indicate that changing the sand to cement ratio of mortar can affect the lateral resistance of walls significantly. So, it can be concluded that this ratio should be addressed in the Iranian codes.Keywords: Finite element modeling, confined masonry walls, Iranian standard no.2800
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Pages 53-61Solid foams are novel materials with outstanding characteristics that have found numerous applications in the construction of light, stiff structures during the past two decades. At the micro-level, solid foams benefit from a cellular structure, which is composed of either closed or open cells. Open-cell foams, which were investigated in this study, are described as a network of inter-connected struts (or ligaments) with beam-like action. The mechanical design of foams requires their mechanical properties to meet certain criteria.These criteria can be satisfied by careful selection of the base material and the microstructural geometry. However, the selected material has to be manufactured, and manufacturing is by no means a perfect process. As a result, there is always a threat that the properties of the final material differ from the ones expected. The current study addresses this issue by exploring the effect of geometrical imperfections on the uniaxial properties of regular open-cell foams. Such an investigation requires an appropriate approach, capable of relating the microstructural characteristics to the effective behavior of the material. An existing homogenization model was used for this purpose.The model assumes the open-cell foam to be constructed from a repetition of the same unit-cell. The unit-cell in its original state has a regular structure, i.e. it comprises four identical members forming equal angles with respect to each other; the point at which these members coincide is called the unit-cell vertex. Imperfection was introduced in the form of small perturbations in the position of the unit-cell vertex. Accordingly, homogenization was employed to obtain the uniaxial stress-strain response of various perturbed microstructures. The results were then quantified in terms of four uniaxial parameters. The next step was to compare these parameters with their counterparts that had been obtained for the regular microstructure. The consequence of this was to identify how the uniaxial response of originally regular, open-cell foam could vary when its microstructure was subjected to geometrical imperfections. In particular, this investigation revealed that different uniaxial parameters altered linearly (or pseudo-linearly) as the perturbation of the microstructure increased.Keywords: Constitutive modeling, homogenization, open, cell foams, geometrical imperfection
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Pages 63-72Ultra High Performance Concrete (UHPC) is an excellent material for the creation of filigree components. Its high ductility and endurance make it the ideal replacement for other materials, such as steel, in many areas. Ultra high performance concrete has very high endurance due to its low capillary porosity. Its high resistance to deicing salt makes it an interesting choice for use in surfaces frequently exposed to deicing salt. Ultra high performance concrete (UHPC) has the potential to greatly simplify construction joints in accelerated bridge construction by reducing the development length of reinforcing steel.The use of UHPC in highway bridges has recently begun in the connection between modular precast components. The potential for increased safety and quality that comes from the use of prefabricated components is enticing; however, there is also the recognition that the use of these components frequently necessitates the use of field cast connection details. Conventional construction practices for such connection details can result in reduced long-term connection performance compared to joined components. In this study, the bond strength between UHPC and reinforcing bars is investigated using pullout and spliced beam specimens. Thirty six pullout specimens and ten beam specimens were manufactured and tested. The effects of different properties such as concrete cover and top bar effect on bond strength are studied by pullout tests. The strength and ductility of 9 full scale flexural beams fabricated from two precast components and joined together with UHPC are investigated. The bond strength of spliced bars in joints is the main parameter of this investigation. One beam without spliced bars was cast as a reference specimen. Mid-span displacement and the applied load were obtained using a hydraulic jack, a load-cell, a LVDT and a data logger set. The load versus displacement relationship of beams was determined using the experimental data. A ductility ratio was used to evaluate the ductility of the specimens. Comparison between the load-displacement relationship and the ductility ratio of different specimens indicates that using UHPC in the connecting region of precast members leads to simplification of the connection details without loss of ductility and flexural strength of the beams.Keywords: Bond strength, connection, ductility, precast members, splice length, UHPC
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Pages 73-80The importance of non-structural components in the seismic performance based design of buildings is well known nowadays. Impaired Non-Structural Components (NSC) in earthquakes could cause heavy loss of life and property and loss of serviceability. The response of NSC depends on various parameters, among them are the behavior of the primary structure, location of non-structural elements in the structure, interaction between NSC and the structure, type and number of NSC attached points, the geometry, shape and condition of the mass distribution of NSC and etc. Researchers have used different methods for calculating NSC responses. Among them are the direct method and the Floor Response Spectrum (FRS) method. In the first method, analysis is performed by modeling the primary structure and NSC simultaneously. In case the weight of the NSC is less than 10% of the total weight of the structure and 20% of the weight of the floor, then, the effect of NSC on the structure can be neglected and simpler methods can be implemented. One of the simpler methods is the FRS method. In this method, the behavior of the primary structure at the attached point or points of NSC is etermined, eglecting their interaction. Then, the obtained FRS is used as an input for analyzing non-structural elements. This method is used in many design codes in order to determine the design force on the NSC. The present research studies the effect of various parameters, including the inelastic behavior of the system, natural periods of primary and secondary systems and type of structural system, on NSC response. A method for calculating the NSC response is suggested, which accounts for the location of the NSC and the inelastic behavior of the structure more precisely. For this research, a variety of moment resisting frames, eccentric and concentric braced frames were examined. These frames are designed in accordance with ASCE 2005 requirements. 15 records of far-field earthquakes and 15 records of near-field earthquakes were chosen for performing parametric studies. To perform expanded parametric studies on various frames with different stiffness and strength, modified shear building models for these frames were constructed. The shear building frame is set to have an equivalent lateral force-deformation behavior in each storey of a given MRF, CBF or EBF frame. Consequently, the base shear-roof displacement curve of both frames became approximately the same. It has been demonstrated that the structure system type does not significantly affect the suggested relations. Moreover, in this study, it has been shown that earthquake intensity causes minimum acceleration (in the first floor) to decrease, and peak cceleration (in higher floors) to increase.Keywords: Non, structural components, earthquake intensity, shear building, near, field effects, floor response spectrum
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Pages 81-93Rotational components of ground motion including rocking and torsional components have been eccentrically ignored for a long time, first, because the rotational effects were thought to be small for structures and, second, due to their small amplitude, they cannot be measured using standard seismic devices. Recently it has been shown that the rotational component of ground motion can have noticeable effects on the dynamic response of structures, and many structural failures and the damage caused by earthquakes can be linked to differential and rotational ground motion. The main purpose of this paper is presentation of a proper formulation for dynamic analysis of concrete gravity dams under the correlated translational and rotational components of ground motion due to earthquakes. The rocking component of earthquake acceleration has been obtained using the corresponding available translational components, based on transversely isotropic elastic wave propagation in 2-D space and the classical elasticity theorem between rotational and translational motions. Using the mentioned approach, it becomes possible to consider frequency dependent wave velocities and the incident wave angle of the earthquake to generate the rotational components of ground motion. For this purpose, two translational components of different earthquake accelerations have been adopted to generate their relative rotational components, based on SV and SH wave incidence. The results are compared with other work and show very good agreement. Using translational and obtained rotational components of ground motion, a dynamic analysis of the Pine Flat dam has been performed for six earthquake accelerations. Analyses have been done using the finite element method, considering dam-reservoir interaction. The dam and reservoir are modeled using a Lagrangian approach, and different water levels of the reservoir are considered. The material behavior of the dam and reservoir is considered to be elastic, linear, isotropic and homogeneous, and, also, the foundation is assumed to be rigid. Results are shown that depend on maximum rocking components and their frequency content. Content, these components can be effective on the linear dynamic response of concrete gravity dams and cannot be negligible in some cases.Keywords: Rotational components of earthquake, frequency dependent wave velocity, dam, reservoir interaction, Lagrangian, Lagrangian approach
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Pages 95-106In order to perform a stress-deformation analysis of a geotechnical system, it is essential to know the in-situ stress state of the ground. The in-situ vertical stress can be simply determined if the depth and the soil density are known. However, it is more difficult to determine the in-situ horizontal stress. This is because it depends on several other soil characteristics, such as stress history and over consolidation history.Throughout the years, several additional sensors have been developed in order to supplement the data collected with the CPT test. Among the wide variety of sensors developed, the lateral stress module represents a promising tool for estimation of in situ lateral stress conditions from the interpretation of lateral stress penetration data. However, the popularity of the so called lateral stress cone has declined over the years due to constraints in both the instrumentation and the interpretation of measured data.In the present paper, a database of calibration chamber tests results is provided. Using the database, the effects of different soil parameters (such as relative density, in-situ vertical stress, in-situ horizontal stress,constraint modulus, and over consolidation ratio) on cone tip resistance have been studied.Among these parameters, relative density and in-situ horizontal stress are found to be the main effective parameters and a correlation between these parameters and cone tip resistance has been suggested. With a given relative density, the relationship can be used to estimate the in-situ horizontal stress of the soil upon measured cone tip resistance.The relationship has been validated with calibration chamber test database. Besides, its effectiveness has been compared with two other empirical correlations suggested by other researchers. The results indicate that the correlation proposed in this study can predict cone tip resistance better than the other two available correlations.
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Pages 117-124Wavelet transform is a mathematical tool which indicates the time variations of frequency in a signal. On the other hand, the endurance time method is a new dynamic pushover procedure where the performance is assessed based on the length of the time interval needed to satisfy required performance objectives. In this paper, endurance time acceleration functions (ETAF) are generated for the compatible design spectrum of standard No. 2800 for soil type (II) with wavelet transform and optimization algorithms. To generate endurance time functions, in the first step, wavelet analysis is performed to decompose a series of random data to several levels, each level covering a special range of frequencies. Each level is then divided into a number of equal time intervals and each interval of time is multiplied by a variable. Unconstrained optimization algorithms are used to calculate the variables and minimize the error between the response and the target spectra. The method applied is a modification of the data in the time and frequency domain. For this purpose, the efficiency of the heuristic algorithm (GA), the classic algorithm (quasi-Newton algorithm) and their combination, for the simplified objective function (duration of this objective function is 5.12 seconds, which consist of 512 acceleration points in 0.01s time steps) is evaluated. The algorithm which determines the minimum error in the specified iteration is selected as the best method to optimize. Employing this optimization method, the main acceleration function (duration of this objective function is 20.48 seconds, which consist of 2048 acceleration points in 0.01s time steps) is produced. Finally, error values of the acceleration functions produced are compared with ETA20d acceleration functions. The target time of the functions is 10th of a second when the response of a SDOF system with a damping ratio of 5% equals the codified template design spectrum.
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Pages 125-135One of the most important defects occurring in reinforced concrete structures is the corrosion of reinforcements under undesirable environmental conditions. Various innovative methods have been studied by different esearchers to prevent corrosion. If a typical concrete structure, reinforced by steel rods, encounters corrosive environments like salts, acids and chlorides for a length of time, it will lose strength to some extent. In addition, the corrosion of the steel, which is located inside the concrete, might expand and damage it. Longitudinal rods have larger diameters compared to stirrups and, therefore, they demonstrate lower corrosion effects and are a better alternative. Stirrups with lower diameters and more proximity to the environment are subject to corrosion and, therefore, it is better to use FRP stirrups, which are resistant to corrosion. Precast stirrups have a special manufacturing process and are also expensive and rare. So, using them in small scale rojects is not economic. Hence, it seems necessary to find a new method for manufacturing FRP stirrups that is economically viable as well as showing a good performance under external forces. The present research experimentally investigates replacing steel stirrups with stirrups made by FRP filaments under cyclic loads. So, three concrete connections were made and tested. A longitudinal reinforcement with steel stirrups was designed as the reference connection. Steel longitudinal reinforcements were used in all three connections. One specimen of FRP stirrup connections is exactly similar to the reference. As with the other two FRP stirrup connections, the space between the stirrups is twice that of the reference specimen. The stirrups in the last two specimens were made of FRP filaments, in the laboratory. The diagrams of moment-drift, moment-curvature and moment-strain have been obtained for steel and FRP stirrups. Energy absorption and hardness changes have been calculated for all three specimens. All three stirrups went under cyclic loads, and FRP stirrups showed a better performance compared to steel stirrups.Keywords: RC Joint, FRP stirrup, panel zone, ductility
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Pages 137-146Generally, steel frames are classified as rigid, semi-rigid and flexible frames, according to their connection stiffness. But, in the analysis of steel frame buildings, it is customary to represent joint behavior by an idealized model, either as a rigid or as a pinned joint. However, typical connections in actual structures do not behave in either a perfectly rigid or a perfectly pinned manner. Previous investigations have shown that most connections in braced simple steel frames using angles do not behave in a perfectly pinned manner and exhibit some rotational stiffness. Consequently, an idealized pinned assumption for these types of connections can lead to unrealistic predictions of the response and strength of steel structures. The objective of the current study is to evaluate the effects of this partial rigidity of the beam-to-column onnection on the actual behavior of steel frame buildings. Particular attention is devoted to double web angle connections and top and seat angle connections, which are customary types used in most practical cases. In this paper, some basic concepts related to the beam-to-column connection, such as fixity percentage and end fixity factor, are illustrated in detail and then calculated for customary steel frames. For this purpose, particular classes of braced steel buildings, those composed of three, five and eight stories with different span length, are modeled as 2-D frames, and nonlinear static analysis is performed. According to the results, connection fixity quantities are less than 10 percent. Also, some changes in element forces occurred due to the actual semi-rigid behavior of flexible connections. Generally, considering the nonlinear behavior of connections in braced simple steel frames, the designing forces of beams decrease (up to 18 percent in our models), and the designing forces of columns increase. These increases in the designing forces of outer columns, especially in upper stories, are noticeable.Keywords: Simple steel frame, beam, to, column connection, partial rigidity, web angle, seat angle
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