In the buildings of the pre-revolutionary construction period, overlaps are laid, representing a roll of concrete or brickwork in the form of cylindrical vaults on steel beams. In such historic buildings, the replacement of structural elements is undesirable, so it is necessary to know the exact nature of the structure in order to avoid unnecessary replacement of the elements. Usually in such designs, when carrying out verification calculations, the joint work of the steel beams and the vault is not taken into account - only the steel beams are calculated, and the vault is taken into account only as an additional load of its own weight. According to the results of these calculations, steel beams do not meet regulatory requirements, whereas in reality they can withstand high loads. The article explores the discrepancy between the results of traditional calculations and the actual nature of the work of the collaboration of steel floor beams and vaults. The article considers the calculation of the approximate model of the structure realized with the use of the software complex "SCAD". The actual stress-strain state of the structure is determined. The influence of various factors on the distribution of moments is investigated.