Tunnel-boring machines (TBM) are usually supported by I-shaped beams which would reduce the drainage ability of open channel. To investigate the influence of I-shaped beams on the roughness coefficient of the open channel, theoretical analysis on the equivalent additional roughness coefficient produced by I-shaped beams was conducted by introducing the concept of equivalent hydraulic parameters. In addition, the water depth of semicircle open channel with I-shaped beams under different flow rates was measured in a model test of a practical project, and the overall roughness coefficient of I-shaped beams section was calculated through assumed trial method and piecewise average method. Results show that the theoretical values calculated by strip method combined with equivalent additional roughness coefficient method agree well with the test values. In conclusion, the roughness coefficient of open channels with vertical bars can be preliminarily estimated by theoretical calculation methods presented in this paper.