PURPOSE: To increase shared shearing force, to ensure the strength and tenacity of a boundary and to economize wall thickness and the arrangement of reinforcements by splicing a center core and an outer circumferential frame by beams made of steel frames and arranging X-shaped reinforcements to the RC-made short beams of an outer circumferential frame and short beams among mutual center core walls.

CONSTITUTION: Center cores B having wall structure and outer circumferential frames A having tube structure are spliced by long-sized steel framed beams 3. Each span ratio of RC-made short beams 2 for the outer circumferential frames A to short beams 4 among mutual center core walls is reduced. Consequently, since shear fracture and shear adhesive fracture are easy to be generated, X-shaped reinforcements 5 are arranged to the beams of the section, thus ensuring strength and rigidity. Fixing end sections are bent, and a plurality of X-shaped reinforcements 5 are bundled by constraint reinforcements 7, and buried to a center wall 10 in the X-shaped reinforcements 5 of the short beams among mutual center cores, thus housing the shape and length of the fixing sections in small size. Accordingly, the shared shearing force of the outer circumferential frames A is increased, and the shared shearing force of the center cores B is lightened, thus reducing wall thickness and the quantity of the reinforcements arranged.