The subject elaborated within the scope of this dissertation is the bending of the straight beams. In the first part of the dissertation, the types of beam bending are presented together with theoretical assumptions. For each type of bending, the theoretical background of strain and stress analysis is presented. Moreover, equitation for determining normal, shear, and principal stress are derived as well as for the principal stress trajectories. The critical points of the cross-section at which the largest normal and shear stress appear are defined, and the calculation of the cross-section strength is given. The theoretical guidelines for selecting the section type is given, as well it is given ideal cross-section and the efficiency of the cross-sectional area. Finally, the application of derived formulas is shown by a numerical example of calculating the bending load of the beams. The maximum normal and shear stress are determined, and diagrams of their distribution along the section are shown.