The seismic response of framed structures is often accompanied by sudden failure caused by the exhaustion of shear stresses in the beam-column connection. Determining the shear forces responsible for these phenomena involves calculating the forces transmitted to the connection from the beam, through its reinforcing bars, and through the concrete between the beam and column. In the current codes, these forces are determined capacitively based on the amount of inserted in the beam-column connection reinforcement. However, this does not answer the question of how large the shear forces actually are. In the present paper, a cantilever beam loaded with a linearly distributed load with intensity at the free end of the beam is considered. The full dimensions and mass properties of the beam materials are taken into account. Expressions are derived for the magnitudes of the resulting support reactions, corresponding to the reinforcing bars and the concrete support. A limit stage was considered, which makes it possible to track the changes in the values of the support reactions both before the appearance of a crack between the beam and the column, and during its growth. Expressions are derived for asymmetric beam cross-sections. The shear force value at the beam-column joint was calculated based on the exact values of the beam forces. The results are compared with those recommended in the literature. A comparison has been made with Eurocode. The proposed exact method gives results that differ from the adopted in the literature by 5.47% to 21.53%, depending on the stage of crack development. The difference between the new exact method and that of the Eurocode ranges from -3.38% to 16.97%, based only on the largest shear force value determined by the exact method.