آیین نامه های لرزه ای، گذشته، حال و چشم انداز آینده

آیین‌نامه‌های لرزه‌ای از ایده ساده ضریب زلزله آغاز و طی یک‌صد سال به شکل امروزی درآمدند. درک ضوابط پیچیده و به‌کارگیری درست آن در گرو شناخت مبانی نظری این ضوابط است اما متاسفانه در این باره کمتر بحث شده است. در این مقاله پیدایش و سیر تحولات آیین‌نامه‌های لرزه‌ای و چگونگی تعیین ضریب رفتار، طیف طرح، توزیع نیرو، و تکیه فزاینده بر تحلیل دینامیکی تشریح و دقت مفروضات آیین‌نامه بحث شده است. پرسش اساسی این است که ضوابط آیین‌نامه چگونه می‌تواند سازه را علیرغم فقدان مقاومت کافی ایمن سازد؟ کلید این معما در ضریب رفتار است که باید تغییرشکل ایجاد شده را در محدوده ظرفیت شکل‌پذیری نگاه دارد. رابطه ضریب رفتار با تغییرشکل لرزه‌ای، صعوبت تخمین ظرفیت شکل‌پذیری، چگونگی برآورد ضرایب رفتار فعلی، و مسئولیت آیین‌نامه‌های طراحی در ارائه روش محاسبه و تامین شکل‌پذیری از محورهای اصلی این گفتار است و نشان داده شده کاهش شدید ظرفیت شکل‌پذیری در سازه‌های فولادی به خاطر بروز پدیده جهش کرنش و در قاب‌های بتنی به خاطر زوال مقاومت فشاری ستون تابعی از انحراف (دریفت) است. از این‌رو، انحراف و میزان خسارت سیستم‌های لرزه بر سخت مانند دیوار برشی، مهاربند و میانقاب بسیار کمتر از قاب بوده و از حاشیه اطمینان بسیار بیشتری برخوردارند. جایگزینی طیف انحراف به‌جای طیف طرح پیشنهادی برای آینده است که قابلیت سیستم‌های لرزه بر در کاستن از انحراف را به‌صورت خودکار لحاظ نموده و برخلاف آیین‌نامه‌های فعلی درجه اطمینان مشابهی را برای انواع سیستم‌های تحت پوشش فراهم می‌سازد.

seismic codes: past, present and future outlook

seismic codes began with the simple concept of the earthquake coefficient and, over the course of a century, evolved into their current form. understanding the complex regulations and applying them correctly depends on a solid grasp of the theoretical foundations of these codes. unfortunately, there has been little discussion on this matter. this article explores the origin and evolution of seismic codes, the determination of the response modification factor r, the design spectrum, force distribution, and the increasing reliance on dynamic analysis. it also discusses the accuracy of the assumptions in the codes. a key question is how the code regulations can ensure the safety of a structure despite insufficient resistance. the key to this dilemma lies in the response modification factor, which should keep the deformation within the limits of the structure’s deformation capacity. the relationship between the r  and seismic deformation, the difficulty in estimating the deformation capacity, how current values of r are estimated, and the responsibility of design codes in providing methods for calculating and maintaining sufficient ductility are the main topics of this article. it is shown that the significant reduction in deformation capacity in steel structures is due to the occurrence of the strain rise phenomenon, and in concrete frames, it results from the loss of column compressive strength, due to the drift. therefore, in stiff seismic-resistant systems such as shear walls, braces, and infill walls are more reliable than flexible frames as they experience much lower drifts. drift spectra are suggested to replace the existing seismic design spectra as they can easily appreciate the adequacy and potential of seismic resistant systems in reducing seismic drifts, and, unlike the current practice, provide a uniform level of confidence for all types of structural systems.the concept of the seismic coefficient was recommended by italian engineers in 1909. in the absence of accelerographs, it was arbitrarily assumed to be equal to 0.125. this oversimplified concept was soon spread throughout the world and was used for seismic design of all sorts of structures. the advent of accelerographs in the 1930s and thousands of recorded accelerograms demonstrated a vast gap between the assumption and reality. however, the concept survived miraculously and remains almost intact to this day, although it has undergone a disguising process that has made it unrecognizable using scientific-looking frameworks. the introduction of the response modification factors in 1978 was the most important step. they were used as adapters to keep the seismic forces unchanged despite employing seismic spectra hugely greater than the original values of seismic coefficients. it is shown that these values of r do not guarantee parity, and some forms of structures may suffer far more damage than others. the present seismic codes face two unknown facts: i) the actual deformation demand in severe earthquakes, ii) the actual deformation capacity of different types of structures. without being able to equate these two, the codes would lack the necessary rational basis for judging seismic stability of structures.the present pattern for the distribution of seismic forces is shown to relate to a rigid bar rotating about its base. it is also shown that the ever-growing emphasis on linear dynamic analysis does not guarantee more accuracy due to excessively nonlinear behavior in strong earthquakes. the adequacy of a distribution model can only be asserted via nonlinear dynamic analyses. some of these models are presented. furthermore, the adequacy of acceleration spectra for displacement-based design is also discussed. it is shown that they are not suitable for estimating the nonlinear displacement response and do not lead to a competent design for structures with insufficient lateral stiffness such as bending frames. on the contrary, the suggested drift spectra look promising as they can adequately predict the seismic drift, taking into account the actual strength and stiffness of the structure.