مطالعه رفتار لرزه ای قاب بتن آرمه در معرض حمله کلرایدی با رویکرد احتمالاتی در بستر زمان

حمله کلرایدی یکی از مخرب‌ترین پدیده‌هایی است که در سازه‌های بتنی اثراث نامطلوبی بر بتن و فولاد به جای می‌گذارد. خوردگی میلگردها از یک سو و آسیب بتن از سوی دیگر می‌تواند بطور چشمگیری منجر به کاهش ظرفیت لرزه‌ای سازه‌های بتنی در بستر زمان گردد. با توجه به این موضوع، مدلسازی زوال مقاطع بتن آرمه پیش از انجام تحلیل‌های غیرخطی به منظور ارزیابی رفتار لرزه‌ای آنها ضرورت دارد. در این راستا نشریه 360 توصیه می‌نماید که برای در نظر گرفتن اثرات خوردگی مقاطع بتن آرمه، روابط ممان-انحناء مورد استفاده در تعریف مفاصل پلاستیک به کمک یک عدد ثابت تحت عنوان ضریب آگاهی اصلاح گردد. با توجه به وابستگی فرایند زوال سازه‌های بتنی تحت حملات شیمیایی به زمان و همچنین وجود عدم قطعیت‌های مختلف در مدل‌سازی این پدیده، به نظر می‌رسد که لحاظ نمودن اثرات خوردگی تنها با یک ضریب ثابت کافی نبوده و می‌بایست پژوهش‌های بیشتری در این خصوص انجام گردد. در این راستا در مقاله حاضر رفتار لرزه‌ای یک قاب خمشی بتن آرمه در عمر مفید 50 ساله تحت حمله کلرایدی بر وجوه خارجی ستونها مورد مطالعه قرار گرفت. بدین منظور در ابتدا انتشار کلراید با توجه به قانون فایک مدل‌سازی و سپس میزان آسیب در میلگرد‌ها و بتن به کمک نرم افزار متلب محاسبه گردید. به منظور افزایش دقت مدل‌سازی، از یک چهارچوب احتمالاتی مبتنی بر شبیه‌سازی مونت کارلو جهت درنظر گرفتن عدم قطعیت‌ها استفاده شد. در گام بعد، منحنی های ممان-انحناء مقاطع با استفاده از نتایج مدل‌سازی زوال استخراج گردید و با روابط توصیه شده در نشریه 360 مقایسه شد. در پایان رفتار لرزه‌ای قاب خمشی مذکور به کمک تحلیل غیرخطی استاتیکی (پوش آور) در دو حالت استفاده از نتایج ممان-انحناء بدست آمده از پژوهش حاضر و نشریه 360 مورد مطالعه قرار گرفت.

probabilistic-based investigation of reinforced concrete frame seismic behavior under chloride attack

chloride attack is one of the most destructive phenomena that has an adverse effect on concrete and steel materials in reinforced concrete structures. corrosion of rebars and damage of concrete can significantly reduce the seismic capacity of these structures over time. accordingly, it is necessary to model the deterioration of reinforced concrete sections before performing nonlinear analysis to evaluate their seismic behavior. in this regard, instruction for seismic rehabilitation of existing buildings (code no. 360) recommends that in order to consider the corrosion effects of reinforced concrete sections, the moment-curvature relationships used in the definition of plastic joints are corrected by a fixed number called the knowledge factor. due to the fact that the deterioration process of concrete structures under chemical attacks is time-dependent and also there are various uncertainties in modeling this phenomenon, it seems that considering the effects of corrosion with only one constant factor, is not enough and in this regard, more research needs to be done. in this regard, in the present paper, the seismic behavior of a reinforced concrete flexural frame with a lifetime of 50 years under chloride attack on the external aspects of the columns was studied. for this purpose, in the first step, chloride diffusion is modeled according to fick’s law and then the measure of damage in rebars and concrete was calculated using matlab software. in order to increase the modeling accuracy, a probabilistic framework based on monte carlo simulation was used to consider the uncertainties. in the next step, moment-curvature curves of the sections were extracted using the results of deterioration modeling and were compared with those recommended by code no. 360. after that, the seismic behavior of the flexural frame was studied using static nonlinear analysis (pushover) based on the moment-curvature results obtained from the present study and the recommendations of code no. 360. a summary of the results obtained in this study can be expressed as follows: corrosion due to chemical attacks can change the behavior of reinforced concrete members over time from deformation-control to force-control. for this reason, the type of failure mechanism of these structures changes from ductile to brittle. in correcting the moment-curvature diagrams of reinforced concrete flexural frame columns using the knowledge factor of code no. 360, it is necessary to pay attention to the actual behavior of the member subject to corrosion. using the method used in this research, it is possible to predict the actual behavior of concrete sections under the chloride attacks during the lifetime of the structure based on the modeling results of cross-sectional deterioration. for the studied moment frame, it was concluded that in the first half of the structure life, the use of a knowledge factor 0.75 to modify the curvature, is appropriate to correct the behavior of column sections subject to corrosion. but in the second half of the life of the structure, it is better to correct the moment-curvature relationship by applying the knowledge factor to the moment. in this study, the diameter of the rebars, ductility of steel, and the compressive strength of concrete were considered as indicators of damage due to chloride attacks. based on statistical calculations, it was concluded that the determination of the reduction in diameter of rebars over time has a higher uncertainty than the other two parameters. therefore, further research is needed to provide a suitable solution to more accurately estimate this parameter.