تبعات دو پارامتری بودن طیف‌های طراحی استاندارد 2800 (A×B) و ارائه ضرایب N_a و N_v برای ارزیابی اثرات جهت‌داری در حوزه نزدیک

نوع مقاله : مقاله پژوهشی

نویسنده

دانشکده مهندسی عمران، دانشگاه علم و صنعت، تهران، ایران

چکیده

این نوشتار بر آن است که طیف های استاندارد 2800 را که عمدتاً ناشی از دو پارامتری (A×B) بودن آن است، مورد ارزیابی قرار داده و نقاط ضعف آنرا با توجه به اصول امروزه مهندسی زلزله شفاف‌سازی نماید. نمونه‌هایی از آن شامل شکل طیفی یکپارچه از پیش تعریف شده در بازه های شتاب طیفی ثابت و حداکثر سرعت ثابت، عدم وابستگی پریود گوشه (Ts) به بزرگا و فاصله ساختگاه تا گسل و نوع خاک، عدم وابستگی شکل طیف به فرآیند تحلیل خطر ساختگاه، عدم وابستگی شتابهای طیفی به فاصله ساختگاه تا گسل و بی ارتباط بودن پارامتر s به مشخصه ای از طیف طراحی است. روش ساده ای برای ارزیابی پارامتر جهت‌داری وابسته به زمان تناوب موجود N(T) ارائه گردیده است. 120 دیتا در حوزه نزدیک دو شهر آمریکا روی چهار نوع خاک تهیه گردیده که منتج به ارائه دو پارامتر Na(T) (برای بازه حداکثر شتاب ثابت) و Nv(T) (برای بازه حداکثر سرعت ثابت) شده است. شتاب‌های طیفی حاصل از این دو پارامتر برای ارزیابی طیف های استاندارد 2800 با اعمال اثرات پارامتر جهت داری موجود N(T) مورد استفاده قرار گرفته است. مقایسه نتایج حاصله با طیف های موجود استاندارد 2800 گویای آن است که پارامتر یاد شده نمی تواند به درستی منعکس کننده اثرات جهت داری در حوزه نزدیک باشد که با توجه به ساخت و سازهای چشمگیر در سال های اخیر و تهدید زلزله های قوی از اهمیت ویژه ای برخوردار است. چنین نتیجه گیری شده است که اصلاح طیف های موجود استاندارد 2800 امری اجتناب ناپذیر است.

کلیدواژه‌ها

موضوعات

عنوان مقاله [English]

consequences of the Recently Modification of Iran Seismic Design Code (No. 2800) (Due to A*B) and Proposing Forward Directivity Effect Coefficients (Na and Nv)

نویسنده [English]

  • A. Nicknam
Iran University of Science and Technology, Tehran, Iran
چکیده [English]

This article intends to criticize and discuss the challenging of the design spectra in Iran seismic design (No. 2800, 4th version) mainly due to its two parametric form (A×B) and highlight its weaknesses regarding the current earthquake engineering principals. Examples of the discussed points include the predefined spectral shape as a fixed envelope of spectral ordinates at the constant maximum acceleration and velocity ranges, the dependency of corner period (Ts) to those of constant spectral acceleration, meaningless of the defined parameter (S) in spectral shape formula. The design spectral acceleration in the third and fourth 2800’s code are compared and assessed. A simple methodology for assessing the existing period dependent forward directivity effects parameter, N(T) is presented. The real spectral ordinates data at 120 near source sites from the two cities in the United State on four site soil types are used to develop the presented two parameters as representatives of FD-pulse effects; Na(T1) (for the constant maximum spectral ordinate range of period) and Nv(T) (for constant maximum velocity spectral ordinate range of period). The average values of the obtained design spectra are compared with those of the 2800’s third and fourth versions and the differences are the results are assessed. The resulted considerable differences confirm the necessary modification of the currently used design response spectra in the both period ranges of; the constant maximum acceleration spectra and acceleration spectra at the constant maximum velocity.

کلیدواژه‌ها [English]

  • Maximum constant acceleration design spectra
  • maximum constant velocity design spectra
  • corner period (Ts)
  • spectral shape
  • forward directivity effects parameter Na(T) and Nv(T)

مراجع

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نشریه مهندسی عمران امیرکبیر
دوره 49، شماره 1
خرداد 1396
صفحه 197-208

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