The article considers the issues of increasing the structural efficiency of rational steel combined trusses by improving their stress-strain state through the development of new structural forms. The main task is to create new rational structural forms of roof systems, with smaller dimensions and material consumption compared to existing analogues, and also so that they meet the technological requirements for the arrangement of communications. The criterion of full stress design by the rationalization method is applied, which ensured the simultaneous finding of topology and dimensions. The principles of structural efficiency are presented, on the basis of which a new structural form of the truss is proposed - a combined small-element truss with a span of 30 m. A new approach is proposed for creating a new structural form of a combined truss, the essence of which is to analyze (by numerical experiment) the stress-strain state of the truss stiffening girder. An algorithm for searching for an effective structural solution for rational combined trusses is developed. The proposed new structural form of a combined truss with a span of 30 m provides a reduction in the mass of steel compared to the typical one by 12.2%. A rational and effective combined steel truss with lattice elements of the “Vierendeel” type was obtained, which allowed to reduce material consumption, reduce the costs of their manufacture and expand the area of use.