ارزيابي ضريب بزرگنمايي تغييرمكان پيشنهادشده در آيين نامه هاي 6-ASCE7 و ويرايش چهارم استاندارد 2800 براي قابهاي خمشي ويژه بتني با و بدون ديواربرشي

Evaluation of Proposed Deflection Amplification Factor in ASCE7-16 and Standard No. 2800 (4th Edition) for Special Moment-Resisting Frame with/without Shear Wall

معمولاً در آيين‌نامه‌هاي تدوين‌شده بر اساس شيوه‌ي «طراحي بر مبناي نيرو»، مقادير بارهاي جانبي ناشي از زلزله، توسط ضريب رفتار (R)، كاهش يافته و بنابراين تغييرمكان‌هاي جانبي حاصل از تحليل استاتيكيِ خطيِ نيز، به صورت كاهش‌يافته نتيجه مي‌گردند. از اين رو لازم است كه اين تغييرمكان‌ها پس از تحليل سازه، تحت بزرگ نمايي قرار گرفته تا تخميني از مقادير واقعي اين تغييرمكان‌ها حاصل شود. اين عمليات در اغلب آيين‌نامه‌هاي لرزه‌اي، معمولاً به وسيله‌ي اعمال ضريبي تحت عنوان ضريب بزرگنمايي تغييرمكان (Cd)، صورت مي‌پذيرد. بدين منظور در اين مقاله، ضريب بزرگ نمايي تغييرمكان (Cd) براي سيستم‌هاي باربر جانبي از نوع قاب خمشي بتن‌آرمه ويژه با و يا بدون ديوار برشي بتن‌آرمه، تحت ارزيابي واقع گرديده است. در اين راستا، براي هر يك از سيستم‌هاي باربر جانبي يادشده، 3 عدد قاب ساختماني 3، 7 و 11 طبقه در نظر گرفته شده است. اين قاب­ها علاوه بر تحليل‌هاي استاتيكي خطي، در معرض ركوردهاي حوزه دور معرفي شده در دستورالعمل FEMA P695 تحت تحليل ديناميكي خطي و ديناميكي غيرخطي تحت قرار گرفته ­اند و در انتها نيز ضريب Cd براي هر كدام از قاب‌ها محاسبه و استخراج گرديده است. نتايج حاكي از آن است كه مقادير پيشنهادي آيين‌نامه‌هاي ASCE7-16 و استاندارد 2800 (ويرايش 4) براي ضريب يادشده در سيستم‌هاي باربر جانبي مذكور كافي نبوده و به منظور تخمين واقع‌‌گرايانه‌‌تري از عملكرد سازه در زلزله، مقادير Cd بزرگتري تقاضا گرديده است. اين مقادير براي سيستم‌‌هاي باربر قاب خمشي بتن‌آرمه ويژه با و يا بدون ديوار برشي بتن‌آرمه پيشنهاد شده است

Response modification factors are used to reduce the lateral loads in "force-based design" method. Naturally the calculated lateral displacement of the structures in the linear static analyses is smaller than actual values. Hence, deflection amplification factor (Cd) is needed to consider a realistic estimation of nonlinear displacements. Most seismic design codes such as ASCE7-16 and standard No. 2800 (4th edition) propose this factor for different lateral force-resisting systems. This paper evaluates the proposed deflection amplification factor for special reinforced concrete moment-resisting frames with/without shear wall. For this purpose, a set of 2D reinforced concrete frames with 3, 7 and 11 story are designed based on standard No. 2800 (4th edition) and implemented in Opensees software in each case without considering the soilstructure interaction. In this regard, beams and columns are modeled using concentrated plasticity method with “Elastic Beam Column Element” in the middle and “Zerolength Element” at the end of elements. Moreover, “SFI-MVLEM” element is used for modeling of shear walls. Nonlinear behavior in two ends of the beams and columns is assigned by “Modified Ibarra-Medina-Krawinkler Deterioration Model with PeakOriented Hysteretic Response” model which has been developed by Ibarra et al. (2005). This model is defined using the proposed equations by Haselton et al. (2007). For the shear walls, uniaxial behavior of steel reinforcements and concrete sections are simulated by Steel02 and ConcreteCM, respectively. Studied frames are verified using Hatzigeorgiou and Liolios (2010) and Liu et al. (2020) study for special momentresisting frame with/without shear wall, respectively. In addition, linear static analysis, linear and nonlinear dynamic analyses are applied to 3, 7 and 11 story frames with two lateral force-resisting systems. In this regard, 22 far-field ground motion records which have been introduced in FEMA P695 are used as seismic scenarios. These records are scaled based on Standard No. 2800 (4th version) to have identical spectral acceleration with the design spectrum for the fundamental period (T) of each studied frames. For this purpose, each record is normalized to its peak ground acceleration and records are scaled so that the average acceleration spectrum of all records was above the design spectrum in 0.2T to 1.5T range. In order to evaluate the deflection amplification factor and Cd/R, in addition to maximum roof drift ratio, inter-story drift ratio is also used for each frames due to concentration of structural damage in certain floors of a multistory structures and, consequently, creating larger lateral displacements in those floors. The calculated Cd coefficients are compared to the proposed values in ASCE7-16 and standard No. 2800 (4th version) for all special reinforced concrete moment-resisting frames with/without shear wall. This comparison shows that the Cd coefficients which have been proposed in above-mentioned seismic design codes are not appropriate and more realistic estimate of the structural performance in earthquake has demanded larger Cd values. Moreover, Cd and Cd/R values are changed with the height of special reinforce concrete frames with/without shear wall.