Method for assembling rack and rack

A rack comprising:

• a plurality of floors each of which stores a plurality of bins for containing items, and allows a transportation robot to run along a surface thereof;
• and
• a plurality of support columns supporting the floors, each of the plurality of support columns having a main body, an insertion plug at one end of the main body and an accommodation plug at another end of the main body opposite to the insertion plug,
• wherein each of the plurality of floors includes a plurality of floor modules making up part of each of the floors, and each of the floor modules is detachably supported by the accommodation plug with a floor reception part;
• wherein at least one of the floor modules is detachable from the support columns of each floor to secure a route for an operator to go down to a lower floor from an upper floor of the plurality of floors.