The aim of the work - experimental investigation on crack propagation and deformation in high-strength fiber reinforced concrete beams with round cross-sections under combined bending and torsion for the development of practical methods of crack resistance, deformation and strength analysis of such structures, and also for the accumulation of new experimental data on resistance under combined loading. Method is experimental-theoretical. Results. Deflection plots and force-deformation relationships for high-strength fiber reinforced concrete beams with round cross-sections under combined bending and torsion are determined experimentally. Principal deformations in terms of elongation and compression of concrete for the experimental beam structures with high torsion to bending moment ratio are determined. It is established that for high-strength fiber reinforced concrete structures of circular cross-section, generally, development of one-two discrete cracks is observed, therefore the circular shape of the cross-section slightly reduces the concentration defined by the material structure of high-strength concrete. On the basis of the conducted investigation on high-strength fiber reinforced concrete structures with circular sections, new experimental data on the combined stress-strain state in the studied areas of resistance is obtained, such as: values of generalized cracking, and failure, load, its level relative to the ultimate load; distance between the cracks at different stages of crack propagation; crack widths at principal reinforcement axis level, at a double diameter distance from the principal rebar axes and also along the entire crack profile at various stages of loading; coordinates of nonplanar crack formations; patterns of crack formation, development and opening in reinforced concrete structures under combined bending and torsion.

On an energy basis, without involving the apparatus of dynamics of structures, an analytical solution of the problem was obtained to determine the dynamic additional loads in complexly stressed reinforced concrete elements of multi-storey frames in a transcendental state caused by special emergency effects. The stress state, crack patterns and fracture patterns in the crossbars of the considered frame structures are determined by the complex stress state of the frame elements caused by the combined action of torsional and bending moments. The assessment of the bearing capacity of such structures was carried out by the diagram method based on the use of energy ratios in the structural elements before and after a special impact, which makes it possible to determine the maximum dynamic deformations, curvatures and forces in the first half-wave of vibrations using the results of nonlinear static calculations according to the primary and secondary design schemes.

A computational model is presented for determining deformations in complexly stressed beams of a structurally non-linear reinforced concrete frame in a transcendental state caused by a special emergency action in the form of a sudden removal of one of the structural elements. The determination of the stress-strain state after the formation of spatial cracks in the reinforced concrete elements of the frame was carried out using the calculated spatial section. At the same time, dynamic additional loads in the design section of the reinforced concrete frame element from a special impact are determined on an energy basis using the “bending moment-curvature” and “torque-angle of rotation” diagrams. The considered calculation algorithm includes the determination of forces in compressed concrete, longitudinal and transverse reinforcement for primary and secondary design schemes and verification of special limit state criteria. The results of a numerical analysis of the effect of cracking, the ratio of torque and bending moment on the stress state in transverse and longitudinal reinforcement and on the dissipative properties of a frame structural system are presented

The paper presents a new structural solution of the precast-monolithic frame for prefabricated residential and civil buildings of various storeys manufactured from prefabricated industrial reinforced concrete elements. The precast frame structures include L-shaped and inverted U-shaped precast elements, installed in the longitudinal and transverse directions and forming a frame, hollow-core slabs and bracing perforated beams of the outer contour, on which fencing non-bearing wall structures are supported within each storey. The computational model of the precast-monolithic building frame was developed using different degrees of discretization at different stages of the analysis. This allowed to obtain both a general picture of structural system deformation in the limiting and over limit states caused by special and emergency actions, and a detailed picture of the stressed state in concrete and reinforcement of structural elements before and after cracking. The paper provides the results of the comparative analysis of the effectiveness of application of the proposed structural system in the mass construction as compared with the applied constructions of large-panel buildings. It has been shown that the application of the proposed structures of panel-frame elements allows considerably reduce the material capacity, cost and transportation expenses of the reinforced concrete frame by up to 30 %, ensuring the mechanical safety of the building.

Experimental determination of the parameters of the force resistance of reinforced concrete structures aimed at protecting them from emergency beyond design impacts is an important direction in improving the safety of buildings and structures. In this connection, the purpose of the study was an experimental assessment of the deformation parameters in the complexly stressed elements of reinforced concrete frames under special impact in the form of a sudden column removal. Experimental studies were carried out for two frames, one of which was tested when removing the middle column, the second - when removing the extreme. Experimental two-span structures of reinforced concrete frames are designed with three floors in height, reinforcement was made with spatial reinforcing cages that provide resistance to torsion with bending. The results of experimental and theoretical studies of reinforced concrete frame structures under special influences and an assessment of displacements, cracking and destruction of the considered complex-stressed structural elements under such influences are presented. It is established that the type of stress state, the formation and width of crack opening significantly affect the dissipative properties of the structural system.