Technical theory on rod bending in a plane. Geometry on rod deformation and force geometry. Basic unknowns and basic equations, static and mathematical classification of girders. Theorems on girder energy. Principles of virtual motion, principle of virtual forces and their application. Influential functions, influential lines and their application. Statically determinate girders: determining the support reaction and forces in cross sections of solid and trussed girders; knot method, decomposition method, element replacement method, application of virtual displacement principle. Construction of influencel lines: static method, kinematical method, element replacement method. Deformation of statically determinate girders. Determining of displacements and rotations of girders; geometric solution, application of virtual force method. Statically kinematical analogy, determining of displacement diagrams for solid and trussed girders. Theorems on mutual actions. Construction of influence lines for deformation sizes.