Introduction. This article analyzes the requirements of current regulatory and technical documents for the protection of buildings and structures from progressive collapse, as well as individual protection methods. The feasibility of improving the load-bearing reinforced concrete elements calculation theory, not only during the element’s service life up to the first special limit state, but also during the special limit state is discussed. Reserves for the strength and deformability of flexural elements are presented.

Aim. To experimentally verify the hypothesis of the neutral axis of a flexural element displacement during loading and to refine its magnitude, including during the special limit state.

Materials and methods. The experimental study was conducted by testing prototypes of flexural elements. A single-span, simply supported reinforced concrete beam was loaded with two concentrated loads through a distribution crossbeam at one thirds of the span. This article describes the experimental setup, characteristics, and prototypes, as well as the measuring instruments used to obtain experimental data on changes in the stress-strain state of a flexural element at all stages of its operation, including the stage of reduced bearing capacity (softening).

Results. This article presents the first part of the results of an analysis of experimental studies (loading stage from 0,8 Mult to Mult), aimed, among other things, at determining the criteria for the special limit state of flexural reinforced concrete elements. An analysis of the calculated and experimental values of the relative deformations of concrete and reinforcement, as well as the bending moments determined at the deformation stages, is presented. A methodology for determining the stress-strain state of a structure at various loading stages has been developed and presented for discussion. This methodology is based on the equilibrium equation for external and internal forces acting in the cross-section of a reinforced concrete element, taking into account changes in the position of the neutral axis.

Conclusions. The error in the theoretical results obtained using the authors’ methodology for determining the stress-strain state of a structure, compared to experimental data obtained at the loading stage from 0,8 Mult to Mult, was up to 3–5 %, depending on the deformation stage. The hypothesis for changes in the neutral axis position, outlined in the developed methodology and confirmed by experimental research, will be published in the authors’ subsequent papers.