Tapered plate girders often form part of large-scale structures such as long continuous bridges or industrial buildings where due to considerable loads the higher resistance is required. There are several important reasons choosing non-prismatic girders. First of all, their tapered shape with gradually changing inertia allows for more effective stress distribution inside the web-panel and contributes to steel reduction and thereby to decrease the overall cost of the structure. Trapezoidal shape of such panels also may be desirable in structures with non-standard shape for example where pre-formed service openings are needed. Although rectangular plate girders were studied in many occasions in last few decades, the latest investigations have shown that the structural behaviour of tapered panels is more complex and different distribution of the internal forces takes place. Due to a lack of design rules for assessment of ultimate shear resistance of tapered plate structures with considerable angle of a slope (> 10 degrees), this research is focused on searching for a solution of the problem. The main body of the thesis is composed of four independent publications where each of them summarizes different phase of the research. The most relevant issues related to tapered panels discussed in the papers were: the critical shear load in tapered simple-supported plates, the influence of geometrical and structural imperfections, the optimal position of the longitudinal stiffener, the Resal effect, and finally the ultimate shear resistance of stiffened and unstiffened tapered plate girders. Nevertheless, the main objective of this work was the development of a reliable design tool to assess of the ultimate shear resistance of non-prismatic plate girders. The methodology applied in the research consists of the following stages: study of the bibliography and initial theoretical research, development of a numerical model, execution of two experimental programs, development of a wide parametric study, analysis of the experimental and numerical results, comparing them with those obtained according to EN 1993-1-5, and finally - development of a new design proposal for the assessment of the ultimate shear resistance for tapered steel plate girders. The PhD research was supported by two experimental programs focused on the structural behaviour of tapered plate girders. In the first program, transversally stiffened members subjected to shear and shear-bending interaction were tested. The second experimental program was focused on longitudinally stiffened tapered plate girders under shear. Results obtained from the experimental tests were used for the verification of the numerical model. Plate girders reveal tendency to possess a significant post-buckling resistance. This phenomenon can be observed as a diagonal tension field developing within the web-panel. In both experimental tests and numerical analyses, this characteristic behaviour was observed. Using verified the numerical model, a wide parametric study for a large number of tapered plate girders was carried out. All numerical results presented in this research were compared with those obtained according to EN 1993-1-5 and discussed. Finally, a new design method for the assessment of the ultimate shear resistance of tapered steel plate girders was presented. The new design proposal is based on the currently valid - Rotated Stress Field Method. The procedure maintains its simplicity and improves considerably results obtained for non-prismatic panels. This new reliable design tool, valid for any geometry and any typology of tapered steel plate girders, provides a solution of the main objective defined in this research.