ABSTRACT Recently, precast segmental concrete structure with external tendons combined with span-by-span construction method is one of the innovative structures and methods in bridge construction, because of substantial cost and time saving in construction. The comprehension of the shear behavior of segmental concrete beams with external tendons, therefore, is required. In addition, the prediction of the shear carrying capacity is an important part in design such structure. In the segmental concrete structure, a segmental joint represents the location of discontinuity of non-prestressed steel reinforcement. The section of segmental joint can be weaker to compare with other sections in segmental concrete beams. The shear behavior and the shear carrying capacity of segmental concrete beams, therefore, are strongly affected by the shear transfer mechanism of segmental joint. In order to investigate the shear behavior and shear carrying capacity of segmental concrete beams, many studies were concentrated on the male-female shear key to evaluate the shear transfer across a segmental joint. The studies on the male-female shear key seem to be suitable for the segmental joints close to the supports in the simply supported beams where the joint opening is insignificant. Due to the effect of moment, segmental joint may open. The compression force and the shear force are complexly transferred across an opened segmental joint. The effect of the shear transfer mechanism across an opened segmental joint on shear behavior and shear carrying capacity of segmental concrete beams with external tendons is necessary. In this thesis, experimental and analytical investigations are conducted to examine the effect of joint opening on shear transfer mechanism of segmental joint, the shear behavior and shear carrying capacity of segmental concrete beams with external tendons. The simplified method, the truss model, with a high accuracy is then proposed to predict the shear carrying capacity of segmental concrete beams with external tendons. In the first set of experimental investigation, the effects of joint position, prestress level and joint type, i.e. epoxy joint and dry joint were carried out. The experimental results showed that the shear behavior and shear carrying capacity of segmental concrete beams with straight external tendons were strongly affected by a local behavior of keyed segmental joint. The local behavior of keyed segmental joint, however, was influenced by joint position and prestress level. Joint opening and height of joint opening increased when joint position moved close to the loading point and prestress level decreased. The height of joint opening affected the contact area of segmental joint which resulted in the change of the compression strut transferring across an opened segmental joint. As joint position moved close to the loading point the shear failure mode changed from the shear compression failure mode to the flexural iii shear failure mode. As prestress level decreased the shear failure mode changed from the shear compression failure mode to the diagonal tension failure mode. With the same tendency of joint position and prestress level, the shear carrying capacity of segmental concrete beams decreased. At the ultimate stage, the shear transfer mechanism and shear carrying capacity of segmental concrete beams with dry joint was similar to that with epoxy joint; even through the concrete stress at the lower fiber was about 6 N/mm2. Analytical investigation by finite element method (FEM) for segmental concrete beams with straight external tendons showed that the compression stress flow transferred across an opened segmental joint were found to have two tendencies and affected by joint position. These two tendencies of the compression stress flow from the loading point and its thickness were then evaluated by formulae. The modified model for segmental concrete beams with straight external tendons had been proposed based on the simplified truss model. The shear carrying capacity assessed from the modified model had provided a better accuracy in comparison with the existing prediction models for segmental concrete beams with straight external tendons. The collected experimental results were also compared to confirm the applicability of the modified model in the wider range of segmental concrete beams such as bonded or unbonded internal tendons. In the second set of experiment, the contribution of draped tendons on the shear behavior and the shear carrying capacity of segmental concrete beams with external tendons were examined. It was found that deviator force and local behavior of segmental joint were affected the shear behavior and shear carrying capacity of such segmental concrete beams. FEM results showed not only the deviator force and the local behavior of the segmental joint but also the transfer mechanism of prestressing force from an anchorage which governed the compression stress flows in the shear span. The modified model was extended to predict the shear carrying capacity of segmental concrete beams with draped external tendons when an inclined angle of draped external tendons was larger than 2 deg. When an inclined angle of draped external tendons was smaller than 2 deg., the modified model for segmental concrete beams with straight external tendons could be applied with considering the effect of small inclined angle. The shear carrying capacity predicted from the extended modified model for draped tendons showed a good accuracy in comparison with the existing prediction models for segmental concrete beams with draped external tendons. The shear behavior of segmental concrete beams with external tendons in this study was examined. The effects of joint position, prestress level and joint type, deviator force and transfer mechanism of prestressing force from an anchorage were evaluated. The good prediction of the shear carrying capacity of segmental concrete beams with external tendons from the proposed simplified models was obtained by comparing to the existing models.