In the modern, rapidly developing world, polymeric materials are widely used. The results of various new studies allow them to be widely used as materials for new products, as well as to apply them when strengthening the supporting structures of buildings and structures. This article discusses the issue of increasing the carrying capacity of lap-joints using FRP sheets. The classification of various types of fiber-based composite materials is given. The mechanism of reinforcement of steel plates with fiber with the analysis of the work of a polymeric material is presented; loading diagrams of experimental samples are presented. The analysis of the obtained results allows us to conclude about the joint work of materials, as well as the possibility of using polymer materials based on carbon fiber to increase or restore the bearing capacity of metal structures. A review of studies of metal structures reinforced with carbon fiber in different countries is given. The results of tests conducted in different countries allow us concluding that polymeric materials can be used for structures subjected to cyclic loading, as well as dynamic effects.

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The results of the studies are presented which is an extension of the earlier authors’ work of combined bolted-welded joints for building steel structures. Data on the parameters of the stress-strain state of finite-element models of physical specimens of bolted, welded and bolted-welded connections are given. These specimens are manufactured and are being prepared for testing. There are some special features of design of physical specimens and numerical models: the conditions which have been created for realization of limit state, conditioned by crushing of bolt edge of a hole in a thinner piece and its consequent breaking (poking out). Subsequently, it is supposed to compare data on the parameters of the numerical models and physical samples. Considering computer modeling of bolted-welded specimens, it is assumed that welds are made (imposed) after a finite-element model of the bolt connection has been loaded to a certain part of its bearing capacity according to the crushing criterion. Such conditions correspond to the operation of a bolted joint reinforced at partial unloading by welds. Thus, it is assumed that the bolted joint will be deformed largely before it is reinforced with welds. According to the authors' hypothesis, it will create favourable conditions for joint operation of the bolted joint and welds during its loading after reinforcement. The generally obtained numerical simulation data confirm the above hypothesis on the possible joint operation of welds and bolted joint, in which the limiting state in terms of buckling is realized and which is reinforced by partial unloading with welds.

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One-story buildings with steel frames are used in many cases for the output production for various purposes. The main vertical bearing element of such frames are the steel columns. With the presence of the technological lifting mechanisms in the building in the form of bridge supporting cranes in such columns, the crane part is designed lattice, and a special step (ledge, beam) is provided in the column design for supporting the crane girders. The columns are determined for a significant proportion of the metal consumed on the steel frame. Reducing the metal columns is an urgent task. Improving the efficiency of the design solutions is possible based on the study of the structures’ actual operation factors, in particular – the columns stability, and the accumulation of data to clarify the standard design methods for sustainability.