Anchorages
HOT TIP!
Anchorage strength is a key consideration in the overall effectiveness of your fall arrest system. Anchorage elevation is often overlooked, and of equal importance! Every effort should be made to select an anchorage that is higher than the elevation of your dorsal D-ring. Doing so will provide you with the following advantages:
• Reduced free-fall distance • Reduced pendulum effect or swing-fall • Reduced fall arrest forces • Reduced forces transmitted to the anchorage • Reduced arrest or deceleration distance • Reduced clearance requirements
Fall clearances are discussed in greater detail later on in this guide. However imagine the typical six foot worker. The height (while standing) of their dorsal D-ring is going to be about 5' above the walking- working surface. If the worker were to tie-off to an anchorage at a 5' elevation using a 6' energy- absorbing lanyard, their free-fall distance will be 6' and the typical deceleration distance is going to be 3.5'. If an anchorage were selected at an elevation of 7.5', their free-fall distance is reduced to 3.5', and their deceleration distance will be reduced by about a foot. Their overall clearance requirements would be reduced by three and a half feet!
Every measurable aspect of the fall arrest is improved by elevating the anchorage, and the risk of injury due to contact with a lower level or an object in the fall path is greatly reduced.
Anchorages: Breaking It All Down
There are two basic sets of guidance for anchorage strength: 1) The OSHA regulations stipulate that a fall arrest anchorage must support a static weight of 5,000 pounds per user attached, or 2) that a system must be designed to provide a safety factor of two. The 5,000 pound rule is pretty simple. That number was originally derived in an effort to help users visualize the strength of the anchor. If you can’t imagine a full-sized automobile suspended from the anchorage you intend to tie-off to, then you may need to rethink your anchorage. The 2:1 rule is the basis for a great deal of fall protection design logic. The objective here is to ensure that any design is developed with consideration of the anticipated loads, and that the minimum strength target was sufficient to provide a margin for error. ANSI/ASSE Z359.2 and Z359.6 are exceptional resources for fall protection planning and design, and are useful end-user references. The best practice is to involve a qualified person or professional engineer in the selection and designation of anchorages. This will ensure that the strength requirements are being met.
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