شناسایی ویژگی‌های دینامیکی سازه بادگیر بر اساس تحلیل حوزه فرکانس و حوزه زمان داده‌های آزمایشگاهی

بادگیرها عناصر معماری منحصربه‌فردی هستند که با هدف تنظیم شرایط محیطی در معماری گذشتۀ کشورمان و به‌ویژه خانه‌های تاریخی شهر یزد استفاده می‌شده‌اند. علیرغم ارزش تاریخی و کارکردی این عناصر، رفتار دینامیکی سازه آنها مورد مطالعه چندانی قرار نگرفته است. انجام آزمون‌های شناخت ویژگی‌های دینامیکی بر روی سازه‌های تاریخی باید با آسیب حداقل به این سازه‌ها همراه باشد. به همین دلیل در سال‌های اخیر استفاده از آزمون غیر مخرب ارتعاش محیطی مبتنی بر تنها پاسخ و نه عامل تحریک، برای شناخت رفتار واقعی دینامیکی این نوع از سازه‌ها گسترش یافته است. بر این اساس، هدف مقاله حاضر شناسایی ویژگی‌های دینامیکی سازه بادگیر از طریق تعیین فرکانس‌های طبیعی، شکل مودی و نسبت میرایی مودهای مختلف دینامیکی با انجام آزمون ارتعاش محیطی است که به این منظور آزمون مذکور بر روی دو نمونه منتخب از بادگیرهای خانه‌های قاجاری شهر یزد انجام شد. قبل از انجام آزمون، به‌منظور تخمین شکل مودهای سازه‌ها جهت تعیین نقاط نصب شتاب‌سنج‌ها، تحلیل مقادیر ویژه بر روی مدل اجزای محدود هر دو سازه انجام شد. در نهایت تحلیل مودال عملیاتی بر روی نتایج آزمایش در نرم‌افزار آرتمیس و با استفاده از دو روش مبتنی بر جستار قله در حوزه فرکانس و یک روش شناسایی زیر فضای تصادفی در حوزه زمان، انجام شد و ویژگی‌های مودال استخراج و نتایج با هم مقایسه شدند.

dynamic identification of the wind-catcher structure based on frequency- and time-domain analyses of experimental results

the present paper addresses the structural behavior of a tower-like historic structure called wind-catcher. the wind-catchers are important architectural elements in persian architecture. these unique elements were used in old iran, in particular in the world heritage city of yazd, to create cross ventilation and passive cooling for protecting houses’ residences against hot environmental conditions. however, their structural performance under dynamic loads (earthquake and heavy wind) is a matter of concern due to their relative high height, high slenderness ratio, and poor material and construction details. nevertheless, evaluating their dynamic behavior is out of the scope of the relevant research in literature. dynamic identification tests are known as one of the most robust tools for evaluating the dynamic behavior of structures. in the case of heritage constructions, the tests should respect the minimum intervention principle, which addresses the use of diagnosis/strengthening methods with minimum damage to the building under restoration. for this reason, the use of ambient vibration tests to identify the real dynamic behavior of such structures has received significant attention in recent years. the test is an output-only vibration test, where only the vibration response is measured during the service conditions of a structure (under wind, sound, or traffic vibrations).accordingly, this paper aims to identify the wind-catcher dynamic behavior, namely the determination of natural frequencies, mode shapes, and damping ratios of its different modes, with minimum damage to the structure. for this purpose, the ambient vibration test was conducted on two wind-catchers belonging to two houses located in yazd’s historic fabric. the sampled structures, which date back to about 200 years ago, are classified as high-height wind-catchers of the city houses.to obtain an overall and initial estimation from the modal behavior of the studied structures, a finite element model implemented in the diana software was employed to conduct eigenvalue analysis before conducting the tests. the numerical results were also used to determine the installation points of accelerometers to be sure of identifying the real modes of the tested structures. a macro-modeling approach was used to simulate masonry as a homogeneous isotropic material in a continuum element. more than 100,000 solid elements were used to model each of the studied structures, adopting initial values for material properties (young modulus and specific weight) from literature. it was also assumed that the translational degree of freedoms of both structures are restrained where they are connected to the underlying building. as a result, four accelerometer sensors were adopted for the purpose of in-situ ambient vibration tests. the sensors were placed on the four corners of the structures’ top level.next, operational modal analysis (oma) was performed on the test results using frequency- and time-domain signal processing methods implemented in the artemis software. the frequency domain decomposition (fdd) method and its enhanced version (efdd), which are based on frequency peak picking, were employed as frequency-domain methods. the extended unweighted principal component (upcx) technique, a subset of stochastic subspace identification (ssi) methods, was also adopted. finally, the results obtained were compared. the comparison showed that the modal parameters obtained from different oma techniques have relatively similar values, implying that the obtained results represent the real modal behavior of the structures under study. the paper results can be used to obtain an updated numerical model for which material properties, geometrical features, and boundary conditions are calibrated with respect to experimental results. the calibrated model is able to simulate the test-obtained modal parameters. updated numerical models are necessary for structural health monitoring, detection of existing damage, safety assessment, and seismic vulnerability evaluation of structures.