Experimental studies have been carried out to investigate the features of a composite flat frame deformation that arise in the process of its gradual installation and loading. During the first stage only precast elements (columns and beam precast parts) are assembled, which are further loaded with some weight that simulates its own weight in a real structure, the weight of other precast elements and that of monolithic concrete. Thus, at the first stage of existence the load is perceived only by the precast elements of the flat frame. Subsequently, at the second stage, without removing the previously applied load, the monolithic concrete is laid, which, having gained the required strength is included in the deformation process, taking an additional applied load that simulates the weight of floor structures, partitions, curtain walls and operational load. The motivations for conducting experimental research were as follows: the carcass of a composite building (is simulated by a flat double two-story frame in the experiment) in real conditions is built in stages, which is expressed in serial installation of individual components. These design features lead not only to the inclusion of separate parts of composite elements in the deformation process at different times, but also to a significant change in the design scheme as a whole (the formation of continuous beams and floor slabs, an appearance of a rigid junction of beams with columns, an increase in the degree of static indefinability of the system, and so on). Experimental studies of the stress-strain state of composite flat frames were performed, taking into account the phased installation process and changes in the design scheme. Precast parts of experimental flat frames (columns, beam precast parts) are made of heavy concrete, and monolithic beam parts – of light concrete (expanded- clay concrete). The conducted research allows us to state that the phased installation and involvement in the deformation process of both individual elements of the composite carcass and the constituent parts of the elements significantly change the picture of the carcass deformation.