Anchor Points: It looks nice but is it safe? The basic issues

Discussion in 'Fall Protection' started by Neil Enslin, Mar 17, 2019 at 9:09 AM.

  1. Neil Enslin

    Neil Enslin Moderator

    Anchor Points: It looks nice but is it safe?
    The basic issues



    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 SANS50795:1996 set into not less than C20/25 quality concrete}

    Rope Access level 3 technicians are not competent if their qualification is only that of a level 3 technician.

    The person must display a level of engineering proficiency and practical expertise that is required by anchor point installers. Theses are covered by the outcomes contained in a new QCTO draft qualification under development and nearing completion.

    As an interim measure, the IWH Professional Body will work with industry to finalise a skills programme in the next month or two, as a first step towards this competency.

    Topics to be covered include but are not limited to:

    • Reaction loads and direction of applied loads
    • Test loads as per SANS 50795:1996
    • In-service loads
    • Resultant forces
    • Combined shear and Tensile forces and limitations
    • Bending moments and bending stress
    • Allowable stress in anchor bolts/ size/ mechanical grade
    • Supporting structures /substrates
    • Load testing vs pull-out testing
    • Minimum anchor size and embedded depth
    • Minimum edge distances and anchor spacings
    Note!Any installation of anchors that exceed the criteria of “basic anchors” shall only be done after the design of such systems have been undertaken / approved by competent engineering professionals.


    Brief discussion regarding some of the above topics


    Basic principles of forces on Rope access / Fall arrest 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 produces 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 produces 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.
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    Figure 1.Basic forces in anchors. Note the position of the applied force relative to its location and direction on the anchor bolt

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

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    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

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    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.

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



    Load testing


    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.

    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 is beyond to scope of competency envisaged in the proposed course and must be avoided. If in doubt, refer the matter to a professional engineer that has experience in this field.


    Information supplied by Saiosh Corporate Member

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    source: https://www.saiosh.co.za/news/442112/Anchor-Points-It-looks-nice-but-is-it-safe.htm