Fluid forces reacting to structures, which are usually modeled as added mass, should be taken into consideration in response analyses of structures immersed in liquid. In this study, added masses for a predominant mode in base excitation of multiple-hexagonal-bar systems were investigated. Shaking table experiments were conducted with models consisting of up to 127 hexagonal bars, which have relatively small aspect ratios. The results showed that the actual added masses for the predominant mode are smaller than those determined by conventional analyses with the assumption of a two-dimensional fluid region because of axial pressure distribution. A formula was proposed to evaluate the added mass with the three-dimensional effect, using a simplified analysis model. Another formula was also proposed to calculate the participation factor. Both formulas were verified by making a comparison between analytical and experimental results.