The article presents a method of solving complex systems by means of decomposition into simple systems, for which the determination of the solution is not complicated. It is also shown in a simple way how to determine the diagrams of internal (cross-sectional) forces, without the need to arrange mathematical functions in successive load intervals. The article also presents the engineering method of determining displacements using the principle virtual work, using the so-called multiplication of the graphs of internal forces by the method of Vereshchagin. The solutions of the presented examples are also included, determined with the use of the Autodesk Robot Structural Analysis computer program. The article is a help for students of the Faculty. Civil engineering, engineering studies.