In steel structures, the combination of both a bending moment and bimoment actions on I-beam members is possible in many cases. In the elastic stage of work, the material bearing capacity check is based on maximum normal stresses, which for symmetric I-beams are the sum of moment and bimoment stresses. The maximum stresses are compared with the calculated resistance. With the development of plastic strains, the bearing capacity of the section is higher than in the elastic stage. The article examines the combined effect of a bending moment and bimoment on I-beams. The paper presents recommendations to check the I-beam bearing capacity, taking into account the development of plastic deformations.