In recent years there are more and more structures made of composite materials, especially in the form of thin-walled shells, being applied in various fields of technology. When using composite materials such as concrete or fiberglass, reinforcing elements are often placed along the axes of the curvilinear coordinate system of the shell, and in this case, the structure can be considered as orthotropic. There are a lot of papers on the calculation of orthotropic shells, but they do not adequately investigate a number of important factors that influence the stress-strain state of the shell and its stability. In particular, the calculation of reinforced shells does not take into account such factors as in-plane shear, shear and torsional stiffness of ribs, etc. The paper presents the mathematical model of deformation of thin orthotropic shells of revolution, based on the model of Timoshenko – Reissner. The model takes into account the design of reinforcement with the shear and torsional stiffness of the ribs, geometric nonlinearity and also the irregular shape of the shell. Possibility of application of methods and algorithms which are used in the study of isotropic shells is shown. The presented model investigates the stress-strain state and stability of thin orthotropic reinforced shells of revolution more adequate

Introduction: Usually, to analyze statically indeterminate rod systems, the classical displacement method and preprepared tables for two types of rods of the main system are used. A mathematically correct representation of local loads with the use of generalized functions makes it possible to find an accurate solution of the differential equation for the equilibrium of a beam exposed to an arbitrary transverse load. Purpose of the study: We aimed to obtain analytical expressions for functions of deflection, rotation angles, transverse forces, and bending moments depending on four types of local loads for beams with different boundary conditions, so as to apply accurate solutions in the displacement method. Methods: We propose an analytical form of the displacement method to analyze rod structural models. For beams exposed to different types of transverse load (uniformly distributed force, concentrated force, or a couple of forces), accurate analytical solutions were obtained for functions of deflection, bending moments, and transverse forces at different types of beam ends’ restraint. This is possible due to the fact that concentrated load and load in the form of the moment of force can be specified by using unit column functions. By transforming Mohr’s integrals, using integration by parts, we show that the system of canonical equations of the displacement method was obtained based on the Lagrange principle. Results: Based on the analysis of a statically indeterminate frame, the effectiveness of the proposed analytical method is shown as compared with the classical displacement method.

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In this work, derivation of equations of a mixed type for shallow shell constructions of an arbitrary type is carried out by means of the variational method. Such equations are more simplified equations of the shell theory, as compared to equations in displacements, but in case of some types of fixing of shell edges (for example, in case of pin-edge and movable fixing) they are more convenient. The mathematical model of shell deformation is based on the Kirchhoff-Love hypotheses, geometrical nonlinearity is taken into consideration. The full functional of shell energy is used for derivation of equilibrium equations and the third equation of strain compatibility in the middle surface of a shell, its minimum condition (the first variation of the functional has to be equal to zero) giving place to these equations. The stress function is entered in the middle surface of the shell in such a way as to make the first two equilibrium equations vanish identically. Thus, the third equilibrium equation and the equation of strain compatibility give the equation of a mixed type in relation to the deflection function and the stress function in the middle surface.

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