August 20, 2013
Let's say you want to paint a window frame on the side of a house, which is 14 feet directly above a level cement sidewalk. You have a 24-foot extension ladder and, to be safe, you want the ladder to remain in place on the sidewalk while you're painting. If you know the weight of the ladder and the coefficient of static friction between the ladder and the sidewalk, you can calculate the maximum angle the ladder can make with the sidewalk before it slips. Of course, it helps to have a background in physics and Newtonian mechanics to do the calculation.
Or, you could just apply the quarter length rule: Place the ladder's base a distance away from the wall or upper support equal to one-quarter the effective working length of the ladder. That will approximate the "optimum resistance to sliding, strength of the ladder, and balance of the climber," which happens to be 75.5 degrees.
The current American National Standards Institute ladder standard (ANSI A14, 2007) sets 75.5 degrees as the optimum set-up angle and suggests using the quarter length rule to achieve it. But the standard doesn't say anything about how 75.5 degrees became the "optimal" value. One of the earliest references dates to 1949 and "Regulation 211: Equipment and tools" (Geneva, Switzerland) in the Model code of safety regulations for industrial establishments for the governance of governments and industry. However, it wasn't until 1958 that H.A. Hepburn presented a series of papers in the British Journal of Industrial Safety, which explored the mechanics of ladder use. In those papers, he examined the quarter-length rule, the dynamics of ladder loading, and the theory of ladder footing. Hepburn emphasized that ladder use can be accurately modeled because ladders obey the laws of physics.
Hepburn concluded that the quarter length rule was valid and "every prospective user of a portable ladder should be instructed on [the rule] and reminded of it at regular intervals." When the rule could not be followed, Hepburn wrote that "tying the ladder, footing it, or staking it would be required." By "footing," Hepburn meant that when a person was climbing a ladder, a weight equal to or greater than the person climbing should be applied to the lowest rung to keep it stable.
In 1990, ANSI added the so-called anthropometric method to the A14 standard as another way to approximate the 75.5-degree set-up angle. Many ladder users are probably familiar with the method, if not the name. Just stand at the base of the ladder with toes against the rails and arms extended horizontally; the proper angle is achieved when the palms of the hands touch the top of the rung at shoulder level. Since then, researchers have found that the method is effective, but often results in ladders set up at angles less than 75 degrees, especially short extension ladders.
This year, NIOSH introduced yet another way to achieve the optimal 75.5-degree up angle – a smartphone app that uses a patented easy-to-read multimodal inclination indicator for ladder positioning. To position a ladder, just hold your phone flat against the side rail, next to the front edge. Move the ladder until you hear a beeping sound and you've achieved the proper angle!
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But remember: the information in this newsletter is intended to highlight safe work practices, but it does not replace Oregon OSHA workplace safety and health rules.
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