Alternating tread-type stairs have a series of steps between 50 and 70 degrees from horizontal, attached to a center support rail in an alternating manner so that a user of the stairs never has both feet at the same level at the same time. (See Figure D-12.)
(1) You must ensure that alternating tread-type stairs are designed, installed, used, and maintained in accordance with approved manufacturer's specifications, and have the following:
(a) Stair rails on all open sides;
(b) Handrails on both sides of enclosed stairs;
(c) Stair rails and handrails of such configuration as to provide an adequate handhold for a user grasping it to avoid a fall;
(d) A minimum of 17 inches between handrails;
(e) A minimum width of 22 inches overall;
(f) A minimum tread depth of 8 inches;
(g) A minimum tread width of 7 inches; and
(h) A maximum rise of 9 1/2 inches to the tread surface of the next alternating tread.
(2) You must ensure that alternating tread-type stairs have not more than a 20-foot continuous rise. You must provide one or more intermediate platforms in accordance with WAC 296-24-76515 where more than a 20-foot rise is necessary to reach the top of a required stair.
(3) You must ensure that stairs and platforms are installed so the top landing of the alternating tread stair is flush with the top of the landing platform.
(4) You must ensure that stair design and construction sustains a load of not less than 5 times the normal live load, but never less strength than to carry safely a moving concentrated load of 1,000 pounds.
(5) You must ensure that treads are equipped with slip-resistant surfaces.
(6) You must ensure that where a platform or landing is used, the width is not less than the width of the stair nor less than 30-inch depth in the direction of travel. You must ensure that stairs are flush with the top of the landing platform.
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