Roof Anchor Design Principles

The selection of proper roof anchors is critical and requires an in depth knowledge of fall window washing, suspended maintenance, and fall protection codes and rigging methods.  Also, recognizing that each building is different, most architects, developers, and general contractors find the selection of a professional roof anchor company a daunting task.

Lifelines

A standard lifeline is 5/8 inch (1.59 centimeters) in diameter and has a breaking strength of 5,000 pounds (2,267.96 kilograms).  However, tying off to an anchor using a knot in a rope can reduce its strength by 50% (or more) to the cutting action of the lifeline or lanyard.

Therefore, good design must ensure that the anchor eye is greater in diameter than the rope itself.  The proper anchor eye for a possible knot connection is 3/4 inch (or 1.9 centimeters).  This design principle will ensure that the anchor eye will not cut, damage, or weaken the rope.

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Flate Plate Anchors

Special caution should be noted when considering a flat thin plate anchor.  These anchor designs are intended to be used with a snap hook or D-ring connection.  However, if a knot is tied to these anchors the rope is more likely to sever and fail.

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D-Rings and Snap Hooks

D-rings and snap hooks are designed with minimum tensile strength of 5,000 pounds (2,267.96 kilograms).  D-rings and snap hooks shall be proof-tested to a minimum tensile load of 3,600 pounds (1,632.93 kilograms) without cracking, breaking, or suffering permanent deformation.

Other problematic compatibility issues include the use of non-locking snap hooks or D-rings that are sized incorrectly and are not compatible with the anchor diameter and/or inside dimension of the anchor.  These incompatible dimensions, relative to the snap hook, will cause the connected object to depress the snap hook keeper and release unintentionally.

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Forged Eye Bolts

Eye bolts are rated along the axis of the bolt and its strength is greatly reduced if the force is applied at an angle to this axis in the direction of shear.

Generally roll out can occur using small eyebolts of 1/2 inch (1.27 centimeters) and 5/8 inch (1.59 centimeters) diameter.  These eyebolts are also too rigid so when put to a fall arrest test they will damage the securement stud and cannot be designed with redundancy.  They are too small and should not be used as part of a fall arrest system.

Also, it should be stated that forged eyebolts, non-shoulder, and shoulder eye bolts are not designed and approved by the manufacturer as a life safety product.  These anchors are designed for vertical loading not angle loading.   Also, recognizing that forged eye bolts are rigid, the securement bolt is almost always damaged under a test load or cyclical normal use conditions.

A hook must be compatible when the diameter of anchor to which the snap hook is attached is greater than the inside length of the snap hook when measured from the bottom (hinged end) of the keeper to the inside curved top of the snap hook.  Thus, no matter how the D-ring is positioned or moved (rolls) with the snap hook attached, the D-ring cannot touch the outside of the keeper, thus depressing it open.

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Why 5,000 pounds?

The design rule is that 5,000 pounds (2,267.96 kilograms) fracture or pull out is sufficient to accept all tie-back and life line loads that are likely to be subjected to the anchor using conventional equipment on a roof top.

The anchor eye, base, or base plate should accept this energy and load not the securement studs to support a worker in the event of a fall.  The codes state that the 5,000 pounds (2,267.96 kilograms) based on the weight of a worker being 250 pounds (113.4 kilograms), experiencing a force of gravity multipled by 10 times a safety factor of 2.

250 x 10= 2,500 x 2= 5,000 pounds

While the energy absorbing lanyards hold in excess of 5,000 pounds (2,267.96 kilograms) when fully absorbed, most limit the load during the fall to under 1,250 pounds (566.99 kilograms).

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