Patent attributes
An upper floor member 30 unitarily includes a floor panel portion 30a for forming a floor surface 12, a first wall portion 30b extending downwards from circumferential edges of the floor panel portion 30a and continuous in the circumferential direction, and a second wall portion 30c spanned between opposed portions of the first wall portion 30b and progressively increased in thickness toward the center of the floor panel portion 30a. The floor panel portion 30a is polygonal in shape, and the first wall portion 30b is progressively increased in thickness toward the center of each side. The first wall portion 30b extends downwards to the vicinity of the lower floor, and its lower end is supported by a support means 18. Thus, a double floor structure which is inexpensive, has a high stiffness, good workability, and offers a high vibration-damping performance can be provided.