During operation, steel beams, under the influence of natural and climatic factors, aggressive environments or under conditions of an increased level of temperature and humidity conditions in the building, are affected by surface corrosion. The distribution of corrosion-affected areas along the length and height of the beam section, as well as its level, depends on various factors. During corrosion, as a result of rust formation, the cross section of steel beams is damaged and their bearing capacity is reduced. To determine the residual strength of beams in bending or shearing, it is necessary to perform an instrumental examination and verification calculations of sections damaged by corrosion. Based on the obtained results, is made a decision regarding the general technical condition of the structure and the scope of measures for its repair or strengthening, as well as the operating conditions of the building site on which it is located. The main issue that needs to be resolved when examining building structures with damaged sections is the assessment of the presence of an emergency condition in them, in which their instantaneous destruction can occur, and the possible (forecast) period of their operation before the emergency condition. A methodical approach to determining the boundary criteria for the depth (thickness) of corrosion in damaged sections of steel I-beams is proposed. The use of limiting criteria for corrosion damage to sections in steel structures will make it possible to clearly distinguish between their technical conditions (unsuitable for normal operation or emergency) during work on their examination and assessment of their bearing capacity.