تجزيه ‌و تحليل غيرخطي سازه‌هاي بتن‌آرمه تحت بارگذاري مونوتونيك و دور‌ه‌‌يي رفت و برگشتي با استفاده از روش اجزا محدود

Nonlinear Analysis of Reinforced Concrete Structures under Monotonic and Reversed Cyclic Loading using the Finite Element Method

در اين مطالعه از تجزيه و تحليل اجزا محدود غيرخطي براي پيش‌بيني رفتار سازه‌هاي بتن‌آرمه تحت بارگذاري مونوتونيك و دوره‌يي استفاده شده است. براي شبيه‌سازي رفتار غيرخطي بتن در فشار و كشش از سطح تسليم HISS، كه مي‌تواند تغييرشكل‌هاي كشسان و خميري را در نظر بگيرد، استفاده شده است. براي حالات باربرداري و بارگذاري مجدد از معادلات ارايه‌شده‌ي دساي استفاده شده است، كه در عين ساده‌بودن مي‌تواند شبيه‌‌سازي رضايت‌‌بخشي از بارگذاري دوره-‌يي ارايه دهد. مدل‌ پيشنهادي به صورت برنامه‌يي به زبان فرترن در قالب اجزا محدود نوشته شده است، كه در آن براي مدل‌سازي بتن از المان‌هاي 8 گره‌يي سرنديپيتي و براي آرماتورها از المان‌هاي 2 گره‌يي استفاده شده است. به‌منظور بررسي توانايي مدل پيشنهادي در پيش‌بيني رفتار غيرخطي سازه‌هاي بتن‌آرمه تحت بارگذاري مونوتونيك و دوره‌‌يي، يك قاب و يك تير مدل‌سازي شده و نتايج بارـ تغييرمكان براي آن‌ها به‌دست آمده است. مقايسه‌ي بارـ تغييرمكان به‌دست‌آمده از اين سازه‌ها با نتايج آزمايشگاهي تطابق خوبي داشته است، كه نشان‌دهنده‌ي توانايي مدل پيشنهادي در پيش‌‌بيني رفتار غيرخطي سازه‌هاي بتن‌آرمه است.

In this study, for predicting the behavior of reinforced concrete structures under monotonic and cyclic loading, a nonlinear finite element analysis has been used. For simulation of the nonlinear behavior of concrete under pressure and tension, the hierarchical single-surface (HISS) plasticity model has been used that can take into account elastic and plastic deformations. It involves a single continuous yield surface, unlike some previous models that include multiple and discontinuous yield surfaces, which can introduce computational difficulties. This model, which allows for isotropic and anisotropic hardening and associated and non-associated plasticity characterizations, can be used to represent material responses based on the continuum plasticity theory. Cyclic and repetitive loading, involving loading, unloading and reloading, occur in many problems, such as dynamics, earthquakes and thermo-mechanical responses. The unloading response is often nonlinear, However as a simplification, it is often treated as linear elastic. Here, both linear and nonlinear elastic simulations are included. During unloading and reloading, Plastic deformations can occur and can influence the overall response of the structure. In this study, the Desai equations, which that are simple but can provide satisfactory simulation for the unloading and reloading states of reinforced concretes under cyclic loading are used. The elastic-perfectly plastic behavior of steel reinforcement is assumed. Then, in order to study the capability of the proposed models in the analysis of nonlinear finite elements of reinforced concrete structures under monotonic and cyclic loading, a program was written in FORTRAN language in a framework of the finite element. This program used eight- noded serendipity elements for concrete and two-noded elements for reinforced concrete. In order to study the capability of the proposed model in predicting the behavior of reinforced concrete structures under monotonic and cyclic loading, one beam and one frame have been modeled and the results of the load - displacement were found for them. Comparison between the load-displacement obtained from these structures with laboratory results show good agreement, and shows the capability and potential of the present model in investigating the nonlinear behavior of reinforced concrete structures.