بررسی حد نهایی پایداری تاق آهنگ بر روی پایه در برابر شتاب افقی ثابت زمین موازی صفحۀ قوس

با توجه به قرارگیری بخش بزرگی از میراث معماری ایران در مناطق لرزه خیز، توجه به بررسی سازمان یافته‌ رفتار این سازه ها در زلزله حائز اهمیت است. هدف پژوهش حاضر بررسی سریع و پارامتریک گونه ای از این سازه ها جهت یک دریافت کلی از رفتار آنها در برابر زلزله بر اساس مشخصات هندسی آنها بوده است. این نوشتار، با تمرکز بر روی رفتار موازی صفحۀ قوس در سازۀ متشکل از تاق آهنگ بر روی پایه، به این پرسش پرداخته که حد نهایی پایداری این‌گونه سازه ای، در تناسبات مختلف هندسی در اثر شتاب افقی ثابت زمین به چه میزان است؟ در این مسیر، اثر تغییر نسبت ضخامت به دهانۀ قوس و میزان چیدن پای قوس نیم دایره و نسبت ضخامت پایه به دهانه، برای یک ارتفاع مشخص، بر پایداری سازه بررسی شده است. تحلیل، در حالت حد نهایی و با استفاده از خط رانش انجام شده و نتایج با استفاده از روش دوران صفحۀ زمین بر روی مدل فیزیکی اعتبارسنجی شده است. نتایج نشان می دهند می توان با چیدن شانۀ قوس تا ارتفاعی مشخص و انتخاب ضخامت مناسب، پایداری تاق را در برابر شتاب افقی زمین به سطح مطلوبی رساند و پس از آن، میزان پایداری سازه وابسته به نسبت عرض به شعاع می باشد. افزایش چیدن شانۀ قوس پس از یک میزان مشخص، باعث کاهش میزان حد نهایی پایداری سازه می شود. افزایش غیر ضروری ضخامت قوس، نیز، احتمال فروریختگی پایه را در نتیجۀ وقوع مکانیسم قوس قوی- پایۀ ضعیف افزایش می دهد.

limit state analysis of barrel vault supported by walls against constant horizontal ground acceleration

a significant portion of iran’s traditional architecture can be observed in areas that are vulnerable to earthquakes. conservation involves knowledge of this heritage, as well as appropriate analysis and design. this underscores the need for careful study of these numerous structures and their risk of collapse during earthquakes. on the other hand, there are many such structures with unknown material properties, making analysis expensive, time-consuming, and unreliable. therefore, it is necessary to understand the overall structural behavior of such structures. the current research aims to quickly study a structure type based on general assumptions in order to understand its behavior in relation to its geometric features in the context of iranian vernacular architecture and allow for primary judgment in design and analysis. this article focuses on the behavior of a barrel vault supported by walls in its arch plane and addresses the question: what is the stability threshold of this type of structure to constant horizontal ground acceleration for different geometrical proportions? the problem is investigated by a limit state analysis using thrust line, based on heyman’s assumptions. the results are verified with the tilt table method and 48 physical models. the influence of various factors on the stability threshold of a structure is discussed. these factors include the ratio of the thickness of the semicircular arch to its radius, the height of the brick infill interlocked with the haunch of the arch, which changes the embracing angle, and the ratio of the width of the wall to the radius of the arch. the results show that as the embracing angle of the arches decreases and the thickness of the arch increases, the arch stability threshold increases. by selecting a suitable geometrical proportion for arches, the vault, despite its 2d behavior, can withstand lateral acceleration very well. therefore, in most cases, the weakness of the wall determines the lateral stability threshold of such a structure. in some cases, by increasing the thickness of the semicircular arch, a more economical design can be achieved for the arch and wall complex, but this strategy is only useful for areas with low earthquake risk. as it requires a huge wall in the high-risk areas, it makes the structure more expensive to build. thus, using an arch with a small embracing angle is advised. moreover, using a thick arch leads to a more destructive mode of collapse: overturning of the wall. the negative effect of cracks in walls also decreases using arches with small embracing angles. furthermore, the experience of assembling physical models shows that this method makes the structure easier to build. this validates the traditional strategy employed by master builders of designing thin vaults with small embracing angles. the brick filling of the arch haunch up to a certain threshold significantly increases the lateral stability of the structure consisting of the arch and wall; however, increasing the filling level more than this threshold reduces the overall stability. because this reduction in the studied proportions is not significant, removing the extra filling cannot be considered an effective technique for increasing the overall lateral stability of existing buildings. in addition, cracks in the wall reduce the stability threshold. this problem, especially with a slender wall, leads to a significant reduction in the lateral stability threshold. in some proportions of arch, e.g., when the embracing angle is 90 degrees, the negative effect of cracking walls is controlled to some extent.  physical models show that in the case of a thin semicircular arch, an analytical model based on an ideal mathematical model cannot be reliable. in other cases, with a small embracing angle, physical models can withstand lower lateral loads, around 50% of the nominal threshold of lateral stability, than analytical models. the data from this study can inform decisions on the stabilization and design of masonry structures.