Fall Protection Anchors

Anchors must comply with SANS 50795:1996

Persons that select, install and test anchors classified as “basic anchors” must be competent to perform that work. {Basic anchors are limited to class “A1” anchors as defined in SANS 50795:1996 set into not less than C20/25 quality concrete}

Rope access anchor

Basic principals of forces in anchors

  • Axial tension force: Those actions that produce axial tension stresses in anchors and act in a direction that coincides with the axis of the anchor.
  • Shear force: Those actions that produce shearing stresses in anchors and that act at right angles to the axis of the anchor, close to the face of the concrete.
  • Bending force: Those actions that produce bending stresses in anchors due forces that act at right angles to the axis of the anchor but not close to the surface thus causing lever-arm moments.

Figure 1.  Basic forces in anchors. Note the position of the applied force relative to its location and direction on the anchor bolt

Figure 2.  Typical load actions to which fall arrest / rope access anchors are exposed

Figure 2.  Bending of anchor due to bending moment

  • Bending moments caused by lever arms

In most cases, Rope access / Fall arrest anchors are “post-fixed” anchors i.e. installed after the building has been completed and waterproofed. This means that the anchors penetrate the waterproofing membranes and plaster render on the concrete walls.

The thickness of the waterproofing and plaster must be established as this affects the cantilever length of the anchor. See Figure 3

Figure 3.  Practical issues – anchors through non-load bearing materials

Many plate type hook-on points or cast metal types that are designed to lay flat against the concrete surface, and is at right angles to the anchor bolt can only take loads at right angles to the axis of the installed anchor, whereas hook-on points such as cast eye nuts cannot take any loads at right angles to the axis of anchors.

Additional precautions need to be taken with the use of eye nuts to ensure that lateral loads are never applied. See figure 4 below.  

Figure 4. Eye nut loading limitations. Loads at right angles are prohibited

Load testing of anchors

All Rope access / Fall arrest anchor bolts should be subjected to a load test to verify correct installation. Load testing must be done at the prescribed minimum load and in the direction of the applied force.

In many cases anchors are merely tested for resistance to axial loading i.e. what is known as “pull-out” tests but ignoring the reality of bending moments and shear forces.

Load testing of anchors

All chemical anchors must follow the instructions of the manufacturer and special attention be paid to the minimum depth and drill diameter. Minimum edge distances and the condition of the concrete is of cardinal importance.

IMPORTANT NOTE:

The use of anchors set into brick walls, precast concrete block walls or any other materials other than in-situ cast concrete should be referred to a professional engineer that has experience in this field.

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