The clear width at and below the handrails at spiral stairways shall be not less than 26 inches (660 mm) PHOTO 52 and the walkline radius shall be not greater than 24½ inches (622 mm) PHOTO 53. Each tread shall have a depth of not less than 6¾ inches (171 mm) at the walkline DRAWING 54. Treads shall be identical, PHOTO 53 and the rise shall be not more than 9½ inches (241 mm) PHOTO 52. Headroom shall be not less than 6 feet 6 inches (1982 mm) PHOTO 55.