The floor of catwalks should have non-skid surfaces. Catwalks should have handrails on both sides, a mid-rail, and a toe board (figure 1). Avoid rough edges, bolt ends, and other protrusions.

The minimum width of a catwalk is 12 in. This minimum width requires walking "cat-fashion" (steps aligned fore-and-aft, figure 2). When used for emergency evacuation, or where personnel must carry a load, a minimum 16 in. width is preferable.(2)

Figure 1 - Catwalk layout and dimensions. (3)

Figure 2 - Minimum catwalk widths. (2)

Note: Additional information on Stairway and Ladder design is available online from the U.S. Department of Labor Occupational Safety and Health Administration OSHA 3124 1997 (Revised).

Use ladders where the desired rise from the horizontal is at an angle of 50° or more, or where a stairway is not practical. Use a stair ladder for inclines between 50° and 75°. Use a vertical ladder for angles greater than 75°. Use stairs for rises between 20° and 50°. Use ramps for inclines less than 20°.

Ladder Design Principles

• The first step from the ground must be reachable by the shortest expected user, and at least two handholds must be accessible to this person while he or she is still on the ground.
• The steps or rungs must accept the user's shoe, with the shoe being placed for a firm step; i.e., the midpoint of the shoe, not just the toe, must rest solidly on the step. Each step or rung should be sufficiently wide so that the person can stop and rest both feet on it.
• Plan the position of each succeeding step and its associated handhold so that the user's final entry into the vehicle will be compatible with sitting in the seat. Remember that when a person climbs a ladder, the hand and leg that are making the next move are on opposite sides of the body. If handholds and steps are not planned to conform to this natural "climb pattern," the person will more than likely end up with the wrong foot ready to enter the workstation; i.e., usually a person cannot hold and step from the same side without swinging. Do not create a system that forces the user to step on a hubcap, tire, or other irregular surface.
• Provide sufficient clearances for a person exiting the workstation to turn around and face a vertical ladder to climb down.
• The specific contour of handrails must follow the gripping, pulling, and supporting patterns associated with both entry and exit; entry and exit are generally quite different and sometimes require more than one handrail geometry.

Non Vertical (Stair) Ladders

Non vertical ladders should have flat horizontal treads (as opposed to round rungs) and two handrails. The most familiar example of this type is the ship's ladder, which usually rises at an angle of 68° from the horizontal (50° - 60° is a preferable range), with a clearance for only one person. Use separate up and down ladders for simultaneous two-way traffic. Two-way ladders should use a maximum tilt angle of 60°, preferably with a double handrail in the center. (2)

Figure 3 shows recommended dimensions for stair ladders. The optimum height between treads is 8½ to 9 inches. Treads should be open (without risers) and provided with non-skid surfacing. The depth of the tread depends upon the angle of the ladder. As a rule, the rear of each tread should overlap the front of the tread immediately above, varying from 1 in. for a 70° ladder to 3 in. for a 50° ladder. Although portions of the shoe may extend beyond this point, this design will be in contact with the weight-bearing portion of the shoe sole. Fasten metal screening to the underside of the ladder to prevent the foot from slipping through. When two or more flights of such ladders are one above the other, solid metal sheets instead of screening will protect those on the lower ladder from falling dirt particles, etc. Provide handrails with a non slip surface on both sides of the ladder. The handrail diameter should be between 1 1/4 in. and 1 3/8 in., with a spacing of 21 in. to 24 in. (figure 4). The recommended handrail clearance from an adjacent wall is 3 in. (2)

Figure 3 - Non vertical ladder dimensions. (2)

Figure 4 - Handrail arrangement. (2)

Vertical Ladders

Use round rungs to provide both hand grips and foot supports for vertical ladders (inclines between 75° and 90°). Figures 5 and 6 shows the recommended dimensions of such a ladder.

The optimum height between treads is from 11 to 12 in. The optimum width of the rungs is 18 to 21 in. Allow 30 in. (90° inclines) to 36 in. (75° inclines) of clearance on the climbing side of the ladder. Allow 6 to 8 in. on the wall (or opposite) side of the ladder. If ladders are used to provide permanent access to several levels, they should be offset at each level and protected by guardrails around the opening at the top of each ladder. (2)

Figure 5 - Vertical ladder design. (2)

Figure 6 - Vertical ladder design. (2)

Stairs

Stairs should rise from the horizontal at an angle of between 20° and 50°. The preferred angle is between 30° and 35°. This rise angle automatically determines the ratio of riser height to tread depth, but the minimum riser height should be 5 in. (2) and the maximum 11 in. (3) Some example riser heights and tread depths are shown in table 1. The optimum tread depth is 9 1/2 in. to 10 1/2 in. plus a 1 in. to 1 1/2 in. overhang (figure 7). (2) These dimensions provide depth such that, in descending the stairs, the ball of the foot, does not extend beyond the front edge of the tread, and the heel comfortably clears the overhang of the step above.

Figure 7 - Stair tread dimensions. (2)

Avoid long continuous flights of stairs. Where space permits, provide landings every 10 to 12 treads.(2) In addition, enclosed stairs should have a handrail on at least one side. (2) Figure 8 shows the recommended height of handrails. The width of stairs (between handrails or between a wall and handrail) should be as shown in figure 9.

For open stairways and landings, provide a guardrail halfway between the handrails and treads. In addition, provide screen guards between the guardrail and floor for landings where the stairway is at right angles to the landing (figure 10). (2)

Figure 8 - Recommended handrail heights. (2)

Figure 9 - Recommended stair widths between handrails. (2)

Figure 10 - Use of guards and guard rail when stair flights are at right angles. (2)

1. Human Factors Design Handbook. Wesley E. Woodson, Barry Tillman, Peggy Tillman, McGraw-Hill, Inc., 1992.
2. Human Engineering Guide to Equipment Design. Joint Army-Navy-Air Force Steering Committee, Wiley-Interscience Publication, New York, 1972.
3. SAE J185 Access Systems for Off-Road Machines. Society of Automotive Engineers, Warrendale, PA, 1988.
4. Ergonomic Design for People at Work. Human Factors Section of the Eastman Kodak Company, Van Nostrand Reinhold Company, 1983.