ارائه‌ی روابط جدید ظرفیت میرایی هیسترزیس قاب‌های خمشی فولادی در سطح عملکرد ایمنی جانی مورد نیاز روش­­ طراحی مستقیم مبتنی بر تغییر مکان

نوع مقاله : Articles

نویسندگان

1 دانشگاه آزاد اسلامی، واحد علوم و تحقیقات

2 پژوهشگاه بین‌المللی زلزله‌شناسی و مهندسی زلزله

چکیده

هدف اصلی این مقاله، ارائه‌ی رابطه‌ی مناسبی برای محاسبه‌ی نسبت میرایی هیسترزیس برای قاب­های خمشی فولادی و مقایسه‌ی آن با رابطه‌ی ارائه شده در روش طراحی مستقیم مبتنی بر تغییر مکان است. نکته‌ی کلیدی در روش طراحی مستقیم بر اساس تغییر مکان، مدل کردن میرایی هیسترزیس به‌وسیله‌ی نسبت میرایی ویسکوز معادل (EVD) 1 با استفاده از روابط و فرمول­های مبتنی بر شکل­پذیری می­باشد. خطا در برآورد میرایی ویسکوز معادل می­تواند به خطا در محاسبه‌ی نیروها و برش پایه در این روش منجر شود. در این مطالعه برای به دست آوردن این نسبت، مدل‌های مختلف قاب­های خمشی فولادی با بهره‌گیری از تحلیل­های متنوعی مورد مطالعه قرار گرفته است. 30 مدل‌ قاب­ خمشی، با تعداد طبقات 3، 6، 9، 12 و 15، به دو شکل 3 دهانه و 6 دهانه، و تحلیل‌ها شامل: تحلیل استاتیکی غیرخطی، تحلیل دینامیکی غیرخطی با استفاده از هفت شتاب­نگاشت ساختگی، تحلیل تاریخچه زمانی دینامیکی بر اساس دو پروتکل رکورد سینوسی یکنواخت، یکی مبتنی بر زمان تناوب اولیه و دیگری مبتنی بر زمان تناوب در سطح عملکرد ایمنی جانی و در نهایت تحلیل بار افزون رفت و برگشتی با استفاده از پروتکل جابه‌جایی سینوسی فزاینده انجام شده است. استنتاج حاصل از محاسبات نسبت میرایی در سطح عملکرد ایمنی جانی بر اساس روابط یاکوبسن و جنینگز، مقادیر حاصل از روابط میرایی معادل در دستورالعمل FEMA-440 و نیز مدل‌سازی سازه‌ی یک درجه آزاد، اختلافاتی را با فرمول تجربی پیشنهاد شده در پیش‌نویس دستورالعمل DBD2012 نشان می‌دهد. لذا، در محدوده‌ی شکل‌پذیری سازه‌های مورد مطالعه، به جای فرم رابطه‌ی موجود برای محاسبه‌ی نسبت میرایی هیسترزیس، از فرم رابطه‌ی نمایی استفاده شد که با نتایج به دست آمده هماهنگ‌تر است. در انتها نیز با برقراری ارتباط بین زمان تناوب اولیه و معادل سازه، رابطه‌ی جدیدی برای محاسبه‌ی نسبت میرایی هیسترزیس مبتنی بر نسبت زمان تناوب معادل به زمان تناوب اولیه در سازه‌های قاب خمشی فولادی پیشنهاد گردیده است. Hysteretic Damping Capacity of Steel Moment-Resisting Frames in Life Safety Performance Level Required in Direct Displacement-Based Design MethodIn recent years, interests in utilizing performance-based design to achieve earthquake-resistant structures have grown. One of a robust procedure in this category, which is presented in 1993 by Prof. Priestley, is Direct Displacement-Based Design (DDBD) method. Extensive and developed researches have shown that DDBD has a great potential to overcome existing shortcomings of the force-based design method. During last decade, DDBD, which was initially proposed for designing RC buildings and bridge piers, are developed for steel structures. DDBD has two main factors: hysteretic damping capacity and yield displacement of the building. Accurate estimation of these parameters is very important to determine proper value of design base shear of the building under consideration. These factors were firstly estimated experimentally for concrete structures but now, they are intensely studied analytically by many researchers for various forms of steel structures. In this regard, this paper attempts to consider more realistic estimation of equivalent viscous damper capacity of moment resisting steel frame structures and its influence on determined base shear values.In this paper, 30 different moment-resisting frames with various numbers of stories (3, 6, 9, 12 and 15) and spans (3 and 6) are studied. For each frame, different analysis methods are carried out: (1) nonlinear static or pushover analysis, (2) nonlinear time history analysis employing synthetic accelerograms, (3) nonlinear time history analysis employing two sinusoidal protocols with different excitation frequencies: initial and effective frequencies, (5) nonlinear static cyclic analysis using an incremental sinusoidal displacement protocol, (6) simple linear analysis of an equivalent single-degree-of-freedom (SDOF) model of the structure subjected to a sinusoidal load, and finally (7) the proposed relation in the Model Code for the Displacement-Based Seismic Design of Structures, DBD12. Comparing the results shows that the equivalent damping ratio obtained using DBD12 relation for life-safety (LS) level is significantly lower than the values obtained by the analyses conducted in this study. This means that the determined base shear for designing such steel building is much more than values for a safe building. In other words, steel buildings using relations of DBD12 tend to be stiffer and stronger than needed. Hence, a new relation is derived to determine the hysteretic damping of MR steel frame structures in the LS performance level as a function of a ductility coefficient. Furthermore, using the relationship between the initial and effective period mentioned in ATC40, another practical relationship is proposed as a ratio of the effective period over the initial period of the considered building. Keywords: Direct Displacement-Based Design (DDBD) Method; Equivalent Viscous Damping; Nonlinear Static Analysis; Effective Period; Sinusoidal Loading Protocol   

کلیدواژه‌ها


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