Many people do not have first hand experience with fall arrest and tie-back roof anchors (for window washing systems and fall protection equipment) and are not versed in high-rate energy performance methods.
An ideal fall arrest and tie-back roof anchor is designed to meet the strength and force standards contained in Federal OSHA 29CFR1910.66 Appendix C.
To arrest a fall in a controlled manner, it is essential that there is sufficient energy absorption capacity in the system. Without this energy absorption, the fall can only be arrested by applying large forces to the worker and to the anchorage, which can result in either (or both) being severely affected.
A full understanding of the “force requirement” is a complex process. A force-type review takes into account the energy consideration in the roof anchor design. These loads can be considered “high-rate-energy forces”. Actual loads on the user, anchorage and structure can vary widely.
The designer must recognize that the anchor, securement and structure can see various loads because of the varying:
- user weight,
- height of fall,
- geometry, and
- type of rigging equipment used
Many people do not have first hand experience with fall arrest and tie-back roof anchors (for window washing systems and fall protection equipment) and are not versed in high-rate energy performance methods. Anchors are often over designed or under designed and may not adequately support a worker and the rigging equipment in the event of a failure.
Drop Test Procedures
Strength Test: a test weight is dropped once using 300 pounds plus or minus 5 pounds (135 kg plus or minus 2.5 kg) and should be used to test a safety anchor.
The drop test must be performed with a non-elastic wire rope lanyard. The lanyard length should be 5 feet plus or minus 2 inches (1.83 m plus or minus 5 cm) as measured from the fixed rigid anchorage.
The test weight should fall without interference, obstruction, or hitting the floor or ground during the test. Any breakage or slippage which permits the weight to fall free to the ground should constitute failure of the anchor and therefore the anchor does not pass the strength test.
Force Test: consists of dropping the respective test weight using a five foot shock absorbing lanyard. The maximum elongation distance should be recorded during the force test. The intent in the force test is to control and measure the applied arresting forces and loads. A system design fails the force test if the recorded maximum arresting force exceeds 2,520 pounds (11.21 Kn) when using a body harness.
The force test is most often used when designing horizontal life line parts and components. Understanding these force principles will help designers understand the anchor performance and design.
“Normally this means that the anchor eye may bend or yield, yet the anchorage or structure will be protected.”
Good design will include a factor of safety or proper engineering multiple over the allowable working load. Proper design will ensure that the anchor deforms in order to absorb energy, yet at the same time will ensure that the final securement method or studs cannot fail.
The critical connections must include an increased importance factor of 1.9 vs. 1, if redundancy is not built into the design. It should also be mentioned that adhesive anchors have higher design requirements because of the possible abnormal deterioration of concrete and aging loss; redundancy in anchor bolts must also be considered.
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