بررسی اثر مؤلفه قائم زلزله‌ حوزه نزدیک بر رفتار لرزه‌ای پل‌های جداسازی شده

محمود میناوند, محسن غفوری آشتیانی, منصور ضیائی‌فر

چکیده


در آیین‌نامه‌های طراحی پل نظیر آشتو رابطه مستقیم و صریحی برای لحاظ نمودن اثرات مؤلفه قائم زلزله در طراحی تکیه‌گاه‌های الاستومری ارائه نشده است. در عوض آشتو در راهنمای طراحی جداگرهای لرزه‌ای خود برای لحاظ نمودن مؤلفه قائم زلزله بدون در نظر گرفتن شتاب مؤلفه قائم، نوع خاک، فاصله تا گسل و...، افزایش و کاهش مقدار بار مرده به میزان 20 درصد را پیشنهاد می‌کند. درصورتی‌که در نواحی نزدیک گسل، مؤلفه قائم زلزله شدید بوده و باعث افزایش قابل‌توجه پاسخ‌ها در پل می‌شود. در این مطالعه به‌صورت موردی پل طبقاتی صدر تحت اثر مؤلفه قائم تعدادی شتاب‌نگاشت زلزله دارای خصوصیات نزدیک گسل قرار گرفته است و تحلیل تاریخچه زمانی غیرخطی برای هر یک از آنها انجام شده است. نتایج نشان می‌دهد مؤلفه قائم زلزله در نواحی نزدیک گسل باعث افزایش قابل‌توجه پاسخ اعضای پل نظیر افزایش حداکثر نیروی محوری پایه‌های پل، شتاب عرشه، برش و لنگر خمشی در مقطع عرضی عرشه می‌شود و بیشتر از مقدار افزایش یا کاهش 20 درصدی بار مرده مد نظر آیین‌نامه است. برای بررسی رفتار جداسازهای لرزه‌ای ناشی از ازدیاد نیروی فشاری تحت اثر مؤلفه قائم زلزله، مدلی ساده از جداسازها که قابلیت لحاظ نمودن تغییرات سختی در راستای قائم و افقی و انجام تحلیل پایداری کمانشی را دارا می‌باشد تهیه شد. نتایج نشان می‌دهد افزایش نیروی محوری فشاری باعث کاهش سختی جانبی جداسازها می‌شود و افزایش جابه‌جایی جانبی باعث افزایش تغییر شکل محوری جداساز شده و همچنین منحنی چرخه‌ای نیرو-‌جابه‌جایی جداسازها دچار کاهش سختی و مقاومت جانبی می‌شود. علاوه بر آن، روابط معمول در محاسبه ظرفیت کمانشی جداگرها تا حدودی دست بالاست و استفاده از مدل‌های تحلیل کمانشی ارجحیت دارد. برای کاهش اثرات مؤلفه قائم زلزله در سازه‌های جداسازی شده، جداسازی سه‌بعدی از طریق ایجاد هم‌زمان انعطاف‌پذیری قائم و افقی در جداسازها پیشنهاد شده است. نتایج نشان می‌دهد کاهش پاسخ پل نظیر شتاب قائم وسط دهانه، برش و لنگر خمشی عرشه با کاهش سختی قائم جداسازها روند نزولی دارد.  

 

Investigation of Near-Fault Earthquake Vertical Component Effect on the Seismic Behavior of Isolated Bridges

Mahmood Minavand1, Mohsen Ghafory-Ashtiany2* and Mansour Ziyaeifar3

1. Department of Structural Engineering, Science and Research Branch, Islamic Azad University, Tehran, Iran

2. Professor, Structural Engineering Research Center, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran, *Corresponding Author, e-mail: ashtiany@iiees.ac.ir

3. Associate Professor, Structural Engineering Research Center, International Institute of Earthquake Engineering and Seismology (IIEES), Tehran, Iran

 

In bridge design codes such as AASHTO, there is not a direct and explicit relation for considering the effects of the earthquake vertical component in the design of elastomeric supports. In lieu, AASHTO recommends a 20% reduction or increase of the dead load amount in its seismic isolators design guide for considering the earthquake vertical component without taking into account the acceleration vertical component, soil type, distance to fault, etc. While the earthquake vertical component in near-fault areas are strong and significantly increase responses in the bridge. In this study, Sadr bridge located in northeast of Tehran in which low damping rubber seismic isolators are used, is investigated under the effect of the vertical component of some earthquake accelerograms with near-fault properties. Besides, the non-linear time history analysis is performed for each of them. Results show that the earthquake vertical component in near-fault areas will significantly increase the response of bridge members such as the increase of the maximum axial force of the bridge piers, deck acceleration, shear and bending moment in the deck cross section and is much higher than the 20% increase or decrease of the dead load given in the code. In order to investigate the seismic isolators behavior due to the increase of the axial compressive force under the effect of the earthquake vertical component, a simple model of isolators with the capability of considering the stiffness variations in the vertical and horizontal directions and performing the buckling stability analysis was prepared, and the isolators responses for three different lateral loadings including the application of monotonic transverse displacement, cyclic loading and earthquake loading were obtained. For all three lateral loadings, the results show that the increase of the compressive axial force decrease the isolators’ lateral stiffness and increasing the lateral displacement will increase the isolators’ axial deformation. Besides, the hysteresis curve of the isolators will experience a reduction in the stiffness and lateral strength. In the following, the buckling capacity amount obtained from the isolation model with the capability of buckling analysis is compared with the buckling capacity amount obtained from the conventional recommended relations. The results show that the conventional relations slightly overestimate the buckling capacity and the use of buckling analysis models is preferable. In order to decrease the effects of the earthquake vertical component in the isolated structures, three-dimensional isolation is recommended through simultaneous application of vertical and horizontal flexibility in the isolators. The results show that the decrease in bridge response such as the midspan vertical acceleration, deck shear and bending moment has a descending trend with the isolators’ vertical stiffness reduction.       

Keywords: Vertical Component of Earthquake, Base Isolated Bridge, Near-Fault Earthquake, Buckling of Base Isolator, 3D Isolation.


موضوع


مؤلفه قائم زلزله، پل جداسازی شده، زلزله حوزه نزدیک، کمانش جداساز، جداسازی سه‌بعدی.

تمام متن:

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مراجع


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