STRUCTURAL MEMBER USED FOR RAINWATER STORAGE LAMINATED STRUCTURE

A structural member for building a rainwater storage stack structure by being stacked up vertically in alternating orthogonal directions inside a water storage tank provided underground is provided, which enables precise and stable stacking, and formation of a rainwater storage stack structure with a small number of stacking and with a larger capacity for rainwater. The structural member 1 has hollow rectangular frusta 2 arranged front and back and right and left and coupled at lower ends thereof with a bottom plate part 3a of a constant width, each frustum having a rectangular top wall 2a and slanted peripheral wall plates 2b extending from four side edges of the top wall 2a. Protrusions 4 and 4 are protruded at both ends of the top walls 2a, so that the lower ends of the slanted walls on both sides of the rectangular frusta 2 of upper structural members 1 are supported on the top walls 2a of the lower structural members 1. The lower ends engage with the opposite inner faces at the lower ends of the protrusions 4 and 4 so as to support load from above. The bottom plate parts 3a are provided with engaging recesses 5 that are placed over the protrusions 4 and 4 of the structural members 1 arranged below to prevent unwanted forward, backward, leftward, or rightward displacement of the structural members 1.