بررسی تأثیر نسبت ارتفاع تیر به عرض مقطع ستون در حالت شکست و عرض مؤثر دال در قاب‌های خمشی بتن مسلح

نوع مقاله : پژوهشی

نویسندگان

مرکز تحقیقات سازه، پژوهشگاه بین‌المللی زلزله‌شناسی و مهندسی زلزله

10.24200/j30.2024.63024.3269

چکیده

اغلب کدهای طراحی بین‌المللی، بندهائی را برای دست‌یابی به رفتار شکل‌پذیر و جلوگیری از رفتار تُرد و خطرناک در قاب‌های بتن مسلح در نظر گرفته‌اند. هدف از بندهای مذکور، دست‌یابی به روش ستون قوی- تیر ضعیف در طراحی قاب‌های بتن مسلح است. بررسی شکست‌های پرتکرار در زمین‌لرزه‌های قوی دهه‌های اخیر نشان می‌دهد که حالت شکست تیر قوی- ستون ضعیف منجر به بروز خسارت‌های شدیدی در قاب‌های خمشی بتن مسلح شده است. این حالت شکست رایج می‌تواند به دو علت اصلی ایجاد شود؛ اول اینکه، در قسمت طراحی لرزه‌یی قاب‌های بتن مسلح برای شکل‌پذیری‌های مختلف، رابطه‌یی مابین ارتفاع تیر و عرض مقطع ستون تعریف نشده است، که این موضوع می‌تواند در نتیجه‌ی فرآیند طراحی موجب طرح تیرهای با مقاومت خمشی و سختی بیشتر نسبت به ستون شود، که نهایتاً باعث خسارت شدید به ستون‌ها و درنهایت، انهدام آن قاب‌ها طی زلزله‌های بزرگ شود. دوم اینکه، اثر بتن سقف و آرماتورهای مدفون در آن در مقاومت خمشی منفی تیرها یا به درستی تخمین زده نمی‌شود و یا در محاسبه‌ی لنگر خمشی منفی تیرها لحاظ نمی‌شود. که این امر نیز نهایتاً می‌تواند منجر به ضعف خمشی ستون‌ها نسبت به تیرها در یک اتصال شود. برای ارزیابی تأثیر نسبت ارتفاع تیر به عرض مقطع ستون در عملکرد لرزه‌یی قاب‌های خمشی بتن مسلح، چند مدل محاسباتی در چارچوب یک مطالعه‌ی پارامتریک مدل‌سازی و تحلیل شد‌ه‌اند. بدین منظور و در ابتدا، پس از راستی‌آزمایی یک مدل آزمایشگاهی با استفاده از روش اجزاء محدود و در نرم‌افزار آباکوس نسبت به توسعه‌ی مدل‌های عددی اقدام شده است. پارامتر‌های بررسی‌شده، شامل: ارتفاع و طول تیر طولی (تیر و بارگذاری جانبی در یک جهت) مدل، راستی‌آزمایی‌شده هستند. یافته‌های مطالعه‌ی عددی حاضر نشان می‌دهد که نیاز مبرمی برای درنظرگرفتن تبصره‌ی جدیدی در قسمت طرح لرزه‌‌یی قاب‌های بتن مسلح درخصوص محدودساختن نسبت ارتفاع تیر به عرض مقطع ستون به عدد 25/1 وجود دارد.

کلیدواژه‌ها

موضوعات


عنوان مقاله [English]

INFLUENCE OF BEAM HEIGHT-COLUMN WIDTH RATIO ON SEISMIC BEHAVIOR OF RC MOMENT RESISTING FRAMES

نویسندگان [English]

  • A. Khalili
  • F. Nateghi Alahi
PhD Candidate, Structural Research Center, International Institute of Earthquake Engineering and Seismology
چکیده [English]

Most reputable international design codes have included provisions for achieving ductile behavior and avoiding brittle and hazardous behavior in reinforced concrete frames. These provisions aim to achieve the concept of the weak beam-strong column, where plastic hinges during an earthquake occur first in the beams. Analysis of frequent and repetitive failures in strong earthquakes in recent decades of reinforced concrete structures shows that the strong beam-weak column failure mode typically leads to severe damage in these structures. The frequent occurrence of this failure mode can be attributed to two main factors. Firstly, stiffer beams are often used against more flexible columns due to the absence of seismic provisions that limit the relation between beam height and column width, which results in column severe damage to the columns and, finally, collapse of them. Secondly, the effect of the cast-in-situ slab in increasing the negative flexural strength of the beam is often underestimated or ignored, leading to the flexural strength of the columns being less than that of the beams. A series of computational models were created and analyzed in a parametric study to assess the impact of the ratio of beam height to column width on the seismic performance of MRFs. Before that, the FEA performance was validated by comparing its results with experimental data. The findings emphasize the urgent need for a new seismic provision that limits the beam height to column width ratio to a maximum of 1.25. Also, it is indicated that the values of effective slab width obtained from the provisions of ACI and EC8 are insufficient to ensure the implementation of the weak beam-strong column design methodology. Findings demonstrate that the value of effective slab width is minimally impacted by the beam height when the ratio of longitudinal beam height to column width ( h_lb⁄C ) is less than 1.5.

کلیدواژه‌ها [English]

  • Finite element analysis
  • strong beam-weak column failure mode
  • beam height to column width
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