The paper considers the influence of concrete class reduction on the strength of reinforced concrete structures in the monolithic frame. In the course of the study, the strength assessment is performed by comparing the required reinforcement in vertical and horizontal load-bearing structures made of concrete of class B25, as well as of concrete of low classes B18 - B24. Three load combinations are considered, one main and two special load combinations. The special load combinations take into account seismic load and possible local failure of structures. The maximum percentage of reinforcement area increase obtained in the course of the calculation with concrete class reduction in the basic combination is 71%, in the special combination 34.6%. It was found that at low percentages of reinforcement of structures in the range of 0.1 - 0.6%, the reduction of concrete strength significantly affects the load-bearing capacity of the structure, and the required reinforcement can increase up to 71%. At higher reinforcement percentages, in the range of 1.3%, the change in required reinforcement is insignificant.