ارزیابی آسیب‌پذیری قاب‌های ضعیف بتن مسلح مقاوم شده با روش‌ها و ترازهای مختلف به کمک منحنی شکنندگی و تحلیل هزینه فایده

با کمک راهکارهای مختلف سازه‌ای مانند افزودن عناصر جدید لرزه‌بر و یا افزایش مقاومت و شکل‌پذیری عناصر موجود، ظرفیت سازه‌های ضعیف در برابر تحریک زلزله تا حد پذیرش آیین‌نامه بهسازی افزایش می‌یابد. اما با توجه به احتمالاتی بودن اثر تحریک زلزله، لازم است اثر این مقاوم‌سازی بر روی عملکرد احتمالاتی سازه سنجیده شود. در این مقاله، با توجه به تنوع راهکارهای مقاوم‌سازی و امکان انجام آن در ترازهای مختلف، چارچوبی با استفاده از منحنی‌های شکنندگی استفاده شده است، تا ضمن لحاظ اثرات احتمالاتی رخداد زلزله، به کمک تحلیل هزینه‌فایده بهترین و مناسب‌ترین راهکار مقاوم‌سازی انتخاب گردد. بدین‌منظور، دو روش اضافه کردن دیوار برشی بتن‌آرمه و استفاده از ورق‌های الیاف کربنی مسلح پلیمری (cfrp) برای مقاوم‌سازی سازه‌های ضعیف بتنی استفاده شده است. آسیب‌پذیری سازه‌های مقاوم شده در ترازهای مختلف مقاوم‌سازی،  نسبت به سازه ضعیف اولیه ارزیابی شده است. نتایج تحلیل‌ها برای سازه‌های 5، 8 و 15 طبقه مورد بررسی، نشان داده است که استفاده از ورق‌های cfrp همواره اقتصادی‌تر است، ولی چون دیوارهای برشی در شدت‌های بالای زلزله، احتمال فراگذشت از حدود عملکردی آستانه فروریزش را بیشتر کاهش می‌دهند، در سازه‌های بلند که آسیب‌پذیرتر هستند، نسبت ‌فایده به هزینه، دو روش به یکدیگر نزدیک شده‌اند.

Vulnerability Assessment of Retrofitted Weak Reinforced Concrete Frames with Different Methods and Levels Using Fragility Curves and Cost-Benefit Analysis

PurposeDifferent structural solutions are employed to enhance the capacity of weak structures under the earthquake excitations to satisfy the target of the rehabilitation regulations. These retrofitting methods include adding new seismicresistant components or increasing the strength and ductility of existing components. Given the probability of the earthquake occurrence, it is necessary to evaluate the effect of different retrofitting scenarios on the probabilistic performance of the structure. In this paper, a framework using fragility curves is presented to select the most efficient retrofit scenario in terms of the costbenefit analysis. For this purpose, two methods of adding reinforced concrete (RC) shear walls and use of polymeric reinforced carbon fiber (CFRP) sheets are employed to retrofit weak RC frames of 5, 8 and 12stories. Different retrofitting scenarios for RC frames were compared using the proposed framework.MethodologyFollowing the presented framework, the damage indices of structures of each scenario are extracted from the incremental nonlinear dynamic analysis. Then, the fragility curve, the damage probability matrix, and the expected annual damage costs are obtained. In the last step, different scenarios were compared using costbenefit analysis. The benefits and costs included in the costbenefit analysis were the reductions of the annual damage cost and cost of each structural retrofitting scenario, respectively. The higher the benefitcost ratio, the more economical the scenario is. IDARC [1] software was employed for dynamic nonlinear structural analysis. For modeling the hysteresis deteriorations in dynamic analyses, stiffness, resistance, and pinching deterioration parameters were employed. These parameters are obtained based on the relationships proposed in [2] to accommodate the hysteresis of the numerical modeling with the experimental one. The weak RC frames were retrofitted with the basis rehabilitation target by the Iranian rehabilitation Guidelines [3], signifying that the performance level of the structure is life safety (LS) under the design seismic risk (0.25 g). Also, for better evaluation of the retrofitting methods, retrofitting was conducted at two higher levels.ConclusionsThe results of the nonlinear dynamical analysis showed that shear walls can reduce the interstory drift ratios significantly more than CFRP sheets do. The results of the costbenefit analysis revealed that retrofitting with the CFRP sheets is a more preferable method than retrofitting with the shear walls in terms of the economic approach, especially for the shorter height structure (5story frames). It is because the cost of executing shear walls and shear wall foundations is very expensive. By increasing the structural height (from 5 to 8 and 15 stories), CFRP sheets outperformed shear walls, while for the 15story frame, adding the CFRP sheets was a better solution than adding the shear walls. For high and middleheight structures (8 and 15story frames), the differences in the costbenefit ratio of the two methods were ignorable. The CFRP sheets further reduced the exceedance probability of damages at low earthquake hazard levels, while shear walls further reduced the probability of damage occurrence and damage exceedance (especially highperformance levels such as collapse prevention). This is especially the case for higherrise structures where the collapse probabilities are higher than for the shorter structures, leading to a close costbenefit ratio of the two retrofitting methods.References1. Valles, R.E., et al. (2009) IDARC2D Version 7.0: a Computer Program for the Inelastic Damage Analysis of Buildings. NCEER, State Univ. of New York at Buffalo, technical report MCEER090006.2. Bakhshi, A. and Asadi, P. (2013) Probabilistic evaluation of seismic design parameters of RC frames based on fragility curves. Scientia Iranica, 20(2), 231241.3. Strategic Oversight Deputy, Technical System Affairs (2013) Guideline for Seismic Rehabilitation of Existing Buildings, Publication No. 360, (1st revision) (in Persian).