The study of the stress-strain state of a monolithic reinforced concrete building frame depending on the types of materials used is necessary to assess their rational use. In construction practice, when erecting buildings made of monolithic reinforced concrete, heavy concrete is used as the main material. Already at the beginning of the last century, lightweight concrete established itself as a material from which large blocks were made, used as a piece building material for low-rise construction and five- and eight-story buildings. The article considers the possibility of using lightweight concrete as the main material of a monolithic building frame. The use of lightweight concrete involves a different work of the spatial frame of the building and, accordingly, other reinforcement, which significantly affects the technical and economic indicators due to its low weight compared to heavy concrete. The paper examines the main load-bearing structures of the monolithic frame: columns, beams and floor slabs in comparison with a similar frame made of heavy concrete. Numerical modeling of the volumetric stress state using the finite element method is performed. Analysis of the results of two variants of a monolithic frame showed savings in reinforcement of the mentioned load-bearing structures by 39.7% for columns, 10.9% for beams and about 18% for floor slabs.