## 1 Introduction  
Strengthening of civil engineering infrastructure has gained significant attention due to deterioration problems of structures and need for meeting up-to-date design requirements [1]. One of the basic factors that causes the unsatisfactory condition of the existing infrastructure is corrosion of reinforced steel in concrete, which causes damage of concrete, loss of reinforcing steel and in some cases failure of construction [2]. Taking into consideration the existing concrete infrastructures both in Europe and worldwide, there is a large interest for the research in the field of strengthening of concrete structures. In addition, the most common reasons for strengthening the existing structures are damage to structures due to earthquakes, changes in the purpose of structures and the implementation of additional loads. Strengthening of RC structures can be achieved in several ways: by reducing static influences (by changing the static system or by subsequent external prestressing), increasing the load-bearing capacity of the cross section, changing the state of stress, etc. Increasing the load-bearing capacity of the cross-section is the most common type of strengthening of RC structures and can be achieved by increasing the dimensions of the concrete cross-section,adding steel reinforcement or adding reinforcement made of composite materials, such as FRP materials. As their name suggests, FRP materials are made up of high-strength fibers embedded in a polymer matrix. Depending on whether aramid, carbon or glass fibers are used for reinforcing the composite material, there are AFRP (Aramid Fiber Reinforced Polymer), CFRP (Carbon Fiber Reinforced Polymer) and GFRP (Glass Fiber Reinforced Polymer) materials [3]. Even though there are several researches about strengthening of reinforced concrete beam, the majority of papers discusses behavior of simple beams strengthened by the FRP laminates. In the experimental research presented in literature, behavior of RC continuous beams made of conventional concrete strengthened with the FRP reinforcement was compared to an unstrengthened control beam. Conclusions of such researches, some of which are listed below, indicate both increases in bearing capacity and reduction of deformations of strengthened beams. Existing studies on the strengthening of continuous RC beams indicate that the strengthening of the negative moment zone alone has a significant effect on increasing the bearing capacity of such beams [4]. According to [PERSON] et al. [5], although many in situ RC beams are of continuous constructions, there has been very limited research on the behavior of such beams with externally applied FRP laminates. For example, research related to the behavior of continuous RC beams strengthened by CFRP (strips, sheets or laminates) is presented in [6; 7; 8; 9], while [10] deals with strengthening using the glass FRP strips by the EB method. In [8] and [9], continuous RC T-beams are investigated. In addition to the papers that generally deal with experimental research of strengthened beam girders, there is a significant number of papers in which methods for modeling reinforced RC beams using FEM analysis are proposed. Numerical analysis using Abaqus software of RC beams strengthened with hybrid FRP sheets are presented in [11; 12]. Hybrid FRP sheets were prestressed and externally bonded to the concrete surface. While [11] deals with flexural behavior of the beams under the action of live and dead load, Ref. [12] deals with fatigue properties of examined beams. Good agreement of numerically obtained results with experimental data was observed in both papers. [PERSON] et al. [13] developed a viscoelastic solution for the interface stress distribution in a strengthened RC beam. The validation of this solution is verified by FEM analysis.