Bridges are important structures in all developing countries such as Iraq. On the other hand, construction of Precast adjacent box girder bridges is expanding due to proper seismic performance, ease of implementation and economic savings in construction costs. One of the important structural elements in Precast adjacent box girder bridges is the shear key, which, if well designed and implemented, will have an important effect on its seismic behavior. Therefore, it is very necessary to evaluate the behavior of the shear key in order to prevent financial and human losses after the earthquake. The main goal of this research is to investigate the factors affecting the shear behavior of connecting shear keys to prefabricated concrete beams in the bridge. In this research, it is assumed that the shear key is made of UHPC concrete and the other two concrete pieces are made of HSC concrete. The parameters investigated in this research are HSC concrete compressive strength, UHPC concrete compressive strength, HSC concrete reinforcement details, FRP rebar, number of steel rebars passing through the shear key, diameter of steel bars passing through the shear key, reinforcement details of the shear key. The number of shear keys and the shape of the shear keys. For this purpose, in this research, firstly, in order to check the accuracy, the laboratory sample of the direct shearing test of the shear key is modeled using ABAQUS finite element software, and then the modeling results are compared with the laboratory results. After that, using ABAQUS software, the direct shear test is simulated for 9 groups of numerical models with different specifications. Finally, load-displacement diagram, shear capacity and equivalent plastic strain contour for numerical models are discussed. The modeling validation results show that the numerical results are in acceptable agreement with the laboratory results. Also, the results of this research show that increasing the strength of HSC concrete has a significant effect (up to about 40% compared to the reference model) on shear capacity, but increasing the strength of UHPC concrete has a small effect (up to about 4% compared to the reference model) on the capacity. A shear will have a shear key. Also, the details of HSC concrete reinforcement will have a small effect, but the details of UHPC concrete reinforcement in the shear key will have a large effect on the shear capacity. The use of FRP rebar compared to steel rebar in UHPC concrete reinforcement has increased the shear strength by 54% and 49%, respectively, compared to the reference model. Also, the type of reinforcement of the shear key, the number of reinforcements passing through the shear key and the diameter of the reinforcements are effective on the shear capacity and toughness (under the load-displacement diagram). So, in the numerical model in which rebar with diameter 6, 12, 18 and 24 was used, the shear capacity was 38, 49, 64 and 85% higher than the reference model (numerical model without rebar). By increasing the number of shear keys, the amount of toughness and shear capacity has increased. So, the shear capacity of the model with two shear keys and three shear keys is 0.83 and 0.72% higher than the reference model (SH1-160), respectively. Also, the use of a shear key whose dimensions have doubled has a lower shearing capacity compared to the use of two shear keys. Considering that the toughness of SH7-160 numerical model is higher than all numerical models and also its shear capacity is close to the highest shear capacity among all laboratory models, so it can be said that the shape is more suitable and optimal for The shear key is hexagonal and according to the numerical model SH7-160.
