مدل المان اليافي براي مدل‌سازي رفتار لرزه‌يي ساختمان‌هاي بتن مسلح قديمي با آرماتور ساده

A FIBER ELEMENT MODEL FOR SEISMIC EVALUATION OF OLD R.C. BUILDINGS

غالب سازه‌هاي بتن مسلح ساخته‌شده‌ي قبل از سال 1970، براساس آرماتورهاي ساده طراحي شده‌اند. از آنجا كه تعدادي از اين ساختمان‌ها هنوز مورد بهره‌برداري قرار مي‌گيرند ارزيابيِ لرزه‌يي آنها حايز اهميت است. در اين تحقيق يك مدل اليافي اصلاح‌شده براي ارزيابي لرزه‌يي رفتار اين ساختمان‌ها پيشنهاد شده كه در آن، تغييراتي در مشخصه‌هاي رفتار تنش ـ كرنش مصالح فولادي اعمال شده تا تاثير ساده‌بودن آرماتور لحاظ شود. براي كاليبراسيون مدل عددي از يك دسته نتايج آزمايشگاهي استفاده شد و به‌منظور صحت‌سنجي مدل عددي، نتايج آن با نتايج نمونه‌هاي آزمايشگاهي ديگر مقايسه شده است. نتايج حاصله بيان‌گر انطباق خوب نتايج مدل المان اليافي اصلاح‌شده با نتايج آزمايشگاهي است. همچنين با استفاده از مدل المان اليافي اصلاح‌شده رفتار يك قاب دو‌بعدي از يك ساختمان بتن مسلح قديمي با آرماتور ساده با تحليل‌هاي استاتيكي غير‌خطي و ديناميكي افزايشي ارزيابي شد.

The structural safety of existing constructions is one of the key issues of modern earthquake and structural engineering. Many existing old concrete buildings are designed according to obsolete seismic codes or even without any specific seismic provisions. Before the 1970ʹs, when deformed bars found a wider application, plain rebars were extensively used in RC structures. Currently, there is no generally accepted practice for the seismic evaluation of RC buildings with plain reinforcements. The stiffness, strength, ductility and energy dissipation behavior of RC members with plain reinforcements are significantly influenced by the bond-slip. The current study deals with the seismic behavior of RC buildings reinforced with plain rebars. In this study, a tailored fiber element model has been proposed, which can be used for evaluating the seismic performance of RC structures reinforced with plain rebars. The slipping characteristics of the plain rebar have been calculated using a semi-analytical approximation and incorporated in this model. To this aim, an altered stress-strain property has been assigned to the steel fibers in tension. To evaluate the seismic performance of RC buildings reinforced with plain rebars, the cross section of the structural members needs to then be modeled by fiber elements and the new but softer stress-strain constitutive model should be introduced to the steel fibers. The bond-slip behavior is included into two end segments in all beam-column RC members. The length of the end segments, or the strain penetration length, has been considered to be proportional to the plastic hinge length at two ends of each RC member. The ratio between the strain penetration and plastic hinge lengths has been obtained from calibrating the proposed model against experimental data. For cyclic loadings, the energy degradation factor (for steel fibers with bond-slip effects) has also been defined through the calibration of the proposed fiber element model against experimental data. The calibrated model has then been validated against other sets of experimental results from other researchers, who carried out monotonic and cyclic tests on RC members with plain rebars. In general, a reasonable level of agreement has been perceived between the numerical and experimental results, under both monotonic and cyclic loading conditions. As a practical case study, the model proposed has been implemented for the seismic evaluation of an important historical existing RC building with plain reinforcements. Two dimensional frames from this old reinforced building with plain rebars have then been studied. Both non-linear static and dynamic types of earthquake excitation have been considered. The model proposed has been found to well present a degraded performance for the structure with plain reinforcements, in comparison with a corresponding structure with deformed reinforcements. It has also been noticed that the simple approach proposed by FEMA273, for the seismic modeling of RC buildings with plain reinforcements, yields considerably conservative predictions for the seismic performance of the structure studied.