ارزيابي شكنندگي لرزه اي و حاشيه ايمني فروريزش قاب هاي خمشي بتن آرمه تحت اثر خوردگي آرماتورها

Evaluation of Seismic Fragility and Collapse Margin Ratio of Reinforced Concrete Moment Resisting Frame under Steel Reinforcement Corrosion

يكي از مهمترين اهداف طراحي و تعمير و نگهداري سازه ها تامين ايمني آنها در برابر حوادث و بحران هاي طبيعي نظير زلزله است، كه نيازمند تامين مقاومت كافي و عملكرد مطلوب و مورد انتظار سازه ها ميباشد. عوامل مختلفي نظير خوردگي آرماتورها بر وقوع خرابي و ميزان آسيب در سازه هاي بتن آرمه تاثير ميگذارند. عملكرد لرزه اي و قابليت اعتماد سازه هاي موجود از شرايط محيطي و نقص هايي كه در طول عمر سازه بوجود مي آيند، تاثير مي پذيرد و در نتيجه اين عملكرد متفاوت از عملكردي خواهد بود كه در هنگام طراحي سازه فرض مي شود. خوردگي آرماتور سازه هاي بتن آرمه يكي از عوامل اصلي افزايش آسيب پذيري اين سازه ها است. در اين مطالعه جهت بررسي شكنندگي لرزه اي و آسيب پذيري سازه ها تحت اثر خوردگي دو سازه قاب خمشي بتن آرمه 3 و 7 طبقه برمبناي پلاستسيته ي متمركز مدلسازي شده است و دو سناريوي خوردگي به صورت 10% و 20% كاهش سطح مقطع آرماتور و اثرات منفي آنها به اعضاي سازه اي اين مدل ها اعمال شده است. سپس با استفاده از تحليل استاتيكي غيرخطي و تحليل ديناميكي افزايشي (IDA) و استخراج منحني هاي شكنندگي، عملكرد و شكنندگي لرزه اي اين سازه ها مورد بررسي قرار گرفته است. نتايج نشان مي دهد كه در اثر خوردگي احتمال خرابي و شكنندگي لرزه اي سازه ها افزايش و حاشيه ايمني فروريزش سازه ها (CMR) كاهش يافته است بطوريكه تحت سناريوي خوردگي 20% احتمال خرابي سازه 3 طبقه عليرغم افزايش همچنان زير 10% بوده ولي در سازه 7 طبقه احتمال خرابي از مقدار مجاز آيين نامه اي (10%) فراتر مي رود و نياز به بهسازي دارد.

One of the most concerns about design and maintenance of structures in civil engineering is the safety of structures in the events of natural disasters, including earthquakes, which requires adequate resistance and providing expected performance of structures. Different factors can have an impact on the occurrence of damage and the damage content in structures and, consequently, the loss of economic assets as well as human health and life safety during earthquakes. Normally, high alkaline property of concrete, PH about 13, forms a protective oxide layer on the reinforcement steel surface. The Carbon dioxide in the atmosphere or the chloride ion in the concrete environment especially in the coastal zone, along with the moisture and the oxygen can penetrate through the concrete pores and micro-cracks and can reach the rebar surface. Then, they cause rebar corrosion inside the concrete by destroying the protective oxide layer on the steel surface. Chloride ions reach the passive layer according to the explained pattern and they begin to react in the passive layer when the amount of chloride ions exceed the critical value and cause the perforation corrosion. Therefore, the performance of deteriorating structures can be different from the desirable performance of pristine structures. Corrosion of steel reinforcement in reinforced concrete (RC) structures is one of the main factors in increasing the vulnerability of RC structures. Due to corrosion, mechanical properties of steel involving yield and ultimate stresses, their corresponding strains, and the elasticity modulus of steel will change. Also the cross-sectional area of steel reinforcement decreases. Furthermore, after cracking, the mechanical properties of concrete will change. In this study, in order to investigate the seismic fragility and vulnerability of RC structures due to steel reinforcement corrosion, two buildings involving a 3-storey and a 7-storey RC moment frames are modeled based on the lumped plasticity model for considering nonlinearity. Two corrosion scenarios of 10 and 20 percent reduction of steel reinforcement cross section and their effects applied to the structural members of these RC frames. Then, seismic performance and the fragility of these two RC frames are investigated using nonlinear static analysis (pushover analysis) and incremental dynamic analysis. Fragility analysis results show that the probability of failure and seismic fragility of RC structures increased due to reinforcement corrosion. Therefore, fragility curves shifted to the left due to corrosion, illustrating the increase in the probability of damage at different spectral accelerations. The safety margin of the collapse of the 3 and 7-storey structures also decreased due to corrosion. For example, as a result of 20 percent corrosion scenario, safety margin of three-storey structure decreased by 16.5 percent and the safety margin of seven-story structure decreased by 28 percent. Results also illustrate that the collapse margin ratios of both structures (CMR) are reduced for 10 percent corrosion scenario. Although the probability of failure increased for 3-storey RC frame, it remains below 10 percent. However, for 7-storey RC frame, the probability of failure exceeds 10% (allowable failure probability adopted by the code) and the frame needs to be rehabilitated.