A new approach to assessing the degree of damage to buildings during earthquakes using statistical modeling is proposed. Using the Monte Carlo method, synthetic databases with specified statistical characteristics of damageability of reinforced concrete frame buildings were obtained. After transforming the elements of these databases, new statistical characteristics of damageability were obtained with variation coefficient values less than 0.30. The research results can be used to determine the seismic load calculation through the permissible damage coefficient.

The effect of reinforcement ratio and compressive strength of concrete on the stability of reinforced concrete elements at different ratios of longitudinal force to torque is examined. For the purposes of the study, an analytical solution is used for reinforced concrete bars, which takes into account the change in stiff-ness under the combined action of axial compressive force and torque, considering the nonlinear relationship between stresses and strains according to Model Code and the change in strength and deformability of concrete under complex stress-strain state according to the model of G.A. Geniev. For the reinforced concrete elements considered in the study, the stability region boundaries for the combined action of the axial compressive force and torque are plotted. It is shown that at combined loading by axial force and torque for small values of axial force N one should expect failure due to loss of strength of sections under the action of torque M_t. For elements made of concrete of different compressive strength classes, but with close values of effective reinforcement ratio α_s, the dimensionless axial force α_n and dimensionless torque α_m decrease as the concrete strength class increases.

 In accidental design situations, the loading mode of reinforced concrete structures includes two typical stages such as long-term deformation under constant or slowly varying load and dynamic impact. The purpose of the presented research was to reveal the peculiarities of stress-strain state of concrete under two-stage static-dynamic loading caused by an emergency situation. To achieve this purpose, the experimental investigations of concrete specimens in the form of prisms were carried out under different loading modes, including quasi-static and dynamic loading. The influence of the presence or absence of a long-term loading stage was also considered. It was shown that long-term loading at a stress level of 0.6 of the ultimate strength had a positive effect on the strength of concrete both in quasi-static tests and in dynamic loading. The hardening coefficient in quasi-static tests was 1.07 for the first series specimens and 1.10 for the second series specimens. The dynamic hardening was 1.20 for the first series specimens and 1.32 for the second series specimens.

The issues of protecting buildings from special, usually non-design, impacts, including progressive collapse as a result of emergency impacts, are the subject of many scientific works and regulatory documents have been developed regulating calculation and design provisions for application at the design stage. At the same time, the issues of strengthening existing buildings and structures at the stages of major repairs, reconstruction (if measures, for example, for protection against progressive collapse were not envisaged at the design stage, then bringing the structures to the requirements of the standards is a complex, often impossible task), as well as restoration of load-bearing structures after emergency situations remain open. In accordance with the current regulatory documents, the calculation for a special limit state is carried out with reduced coefficients for loads and taking into account the dynamic hardening for the design resistance of materials. After an emergency impact, it is necessary to perform a calculation for limit states taking into account all reliability factors, which usually leads to a deficit in the bearing capacity of a significant number of load-bearing structures taking into account the damage to individual structures and the redistribution of forces to others that are not damaged. It is for this operational stage after the special impacts considered in this article that there are no special requirements and assumptions in the operation of the building’s load-bearing structures.

The aim of the work was to study the influence of the span, slab thickness, grade and thickness of the corrugated sheet on the deflections of the profiled sheeting at the concreting stage of the composite slab. The object of the study was orthotropic composite slabs made using on permanent formwork in the form of profiled sheeting of grades H75, H144, H153 according to GOST 24045–2016 and TRP200 according to GOST R 52246, with a thickness of 0.7–1.5 mm. A calculation and analytical method of research was used, based on regulatory documents in force in the Russian Federation. Based on the results of the study, the influence of deflections and strength of the corrugated sheeting on the applicability of various spans of corrugated sheeting in the range from 3 to 6 m at the concreting stage of the composite slab was analyzed. Recommendations are proposed to limit the use of small thicknesses of corrugated sheets of 0.7–1.0 mm for spans over 4 m for slabs less than 250 mm thick with grades of corrugated sheets H114 and H153 according to GOST 24045–2016. Recommendations are given for the installation of temporary inventory supports for all spans of composite concrete floors when using corrugated sheets of 0.7–0.9 mm thickness for grade H75 according to GOST 24045–2016. The obtained data can be used in the design of composite concrete floor slabs and in the inspection of the technical condition of erected structures.