SafetyAdvice: Understanding PFAS Anchorage
[Editor's note: This article is the first installment of a three-part series on anchorage points and personal fall arrest systems. In this article, the concept of anchorage is defined and the roles of the qualified person and competent person are explored.]
OSHA Subpart M [1926.500(b)] defines anchorage as "a secure means and point of attachment for lifelines, lanyards or deceleration devices." For physics buffs, it may also be identified as the terminus of the final force vector created by the arrested impact of a falling mass. Beyond the anchorage, the dissipation of the negative acceleration must occur without quantifiable structural failure in the mechanism or material supporting the anchor. While a suitable anchor point, under these criteria, may be easily identified on an open steel structure, it may prove problematic on a steep-pitch, wood-framed, shingled roof or a flat, hot tar and ballast roof. Anchor selection is not for those who refuse to consider the consequences of their selections. The OSHA standard goes on to describe the criteria for anchor points used for attachment of personal fall arrest equipment (PFAS) shall be "capable of supporting at least 5,000 pounds (22.2 kN) per employee attached."
This is the primary force provided by OSHA meant to quantify the minimum strength of any PFAS anchor point. The ANSI reference standard (Z359.1) for "Safety Requirements for Personal Fall Arrest Systems, Subsystems and Components, Revised 1999," Section E2.3, further defines an anchorage as: "A secure means of attachment to which the PFAS is connected. An anchorage is generally a fixed structural member required for the stability and other purposes of the structure itself. Examples include a beam, girder, column or floor. An anchorage connector (AC) is a component or subsystem which attaches to the anchorage and has means to permit secure and functional connection of the rest of the PFAS when the anchorage alone would not (i.e., brackets, rings, chokers, collars, davits, trolleys, etc.)."
It is generally accepted that the OSHA/ANSI 5,000 pound limit is sufficient tensile strength of materials to resist the impact force with which a worker under 310 pounds should not exceed at the terminus of a six-foot arrested free fall. This statement is based on sections 1926.502(d)(15)(i) and (ii), which state: "Anchorages used for attachment of personal fall arrest equipment shall be ... capable of supporting at least 5,000 pounds (22.2 kN) per employee attached, or shall be designed, installed, and used ... as part of a complete personal fall arrest system which maintains a safety factor of at least two; and under the supervision of a qualified person."
Section 1926.502(16)(v) goes into further description of the entire PFAS designed by the qualified person: "Personal fall arrest systems, when stopping a fall, shall ... have sufficient strength to withstand twice the potential impact energy of an employee free falling a distance of 6 feet (1.8 m), or the free fall distance permitted by the system, whichever is less."
This means all the components of a personal fall arrest system, including each and every anchorage point, shall be capable of resisting this terminal impact force (TIF) without failure, with a minimum two-times safety factor. An anchor point must be designed to actually withstand 5,000 pounds of impact force even if the maximum intended Terminal Impact Force will not exceed 2,500 pounds. Consider the fact that many personal fall arrest systems are not simply linear, but multi-level and multi-directional. Those points at which the system changes vectors also require anchor force analysis prior to design and construction.
According to 1926.502(d)(16) the maximum limit of the terminal arresting force (TAF) applied to an employee wearing a Class III full body harness, lanyard and deceleration device shall not exceed 1,800 pounds (8 kN). The ANSI standard stipulates that the maximum combined dead weight (mass) of the arrested fall victim (including his clothing and tool belt) shall not exceed 310 pounds (140 kg). If the worker's total mass is calculated to be greater than 310 pounds, "then the employer must appropriately modify the criteria and protocols" of the personal fall arrest system and all of its components, including the anchor points, to meet or exceed the two-times safety factor. This is typically achieved by consulting with the fall protection manufacturer for a custom-designed and built harness, lanyard, lifelines, hardware and anchors. It would be difficult to further assess all of the contributory force vectors without a total, documented FP system design consultation by a qualified, professional engineer.
The Qualified PersonThe responsibility to select the structure that will serve to absorb the terminal forces of an arrested fall shall fall on the shoulders of the qualified person (QP). What criteria must the employer consider before designating a QP for fall protection? OSHA defines a qualified person (1926.32) as "One who, by possession of a recognized degree, certificate or professional standing, or who by extensive knowledge, training, and experience, has successfully demonstrated the ability to solve or resolve problems relating to the subject matter, the work or the project." It may not be a position requiring a state engineer's license, but it must be filled by one who has "extensive" and "demonstrable" qualifications.
The QP never selects an inadequate anchor point, such as a vent stack, small supply or waste piping, electrical conduit, ductwork, ladder, or guardrail. Any structures of unknown strength or unsubstantiated stability would be excluded. A suitable anchorage point may be a site-built, independent, stand-alone structural feature or a component of some existing structure. It must be able to withstand not only it's own dead load without failure but also all live loads, such as personnel, any environmental loads (wind, snow, ice) and applied loads such as accumulated equipment and machinery loads. This can be a problem. How can a QP analyze and calculate the residual capacity of a structure to resist an additional shock load (TIF) if the materials in question have been either accidentally damaged, physically modified or repaired or degraded by environmental exposures over many years? Not all anchorage points are considered engineered, with a complete load analysis, approved material strength values and a stamped set of assembly drawings showing load points and distribution vectors. The role of the QP to select and evaluate anchor points can be seen as one of the most difficult tasks on the PFAS erection team.
The Competent PersonOnce the PFAS system is designed, the implementation of a site-specific fall protection plan is the responsibility of the on-site competent person (CP) as environmental conditions, structural configurations and ongoing construction events change constantly. For instance, Section 1926.502(d)(19) states: "Personal fall arrest systems and components subjected to impact loading shall be immediately removed from service and shall not be used again for employee protection until inspected and determined by a competent person to be undamaged and suitable for reuse."
Unlike the QP, this designated competent person (CP) has much more individual authority to promptly manipulate manpower and materials as he deems necessary to provide the required minimum site safety factors and mitigate any other hazards. As an employee designated as competent, he has the employer's total authority on the site to control events, acts and conditions as he thinks necessary. He may also terminate any and all operations on a particular work site when he feels that the conditions are immediately dangerous to life and health (IDLH). This might be the case whenever the structural integrity of an anchor point designed and specified by the QP is in doubt due to any number of criteria. Anchor points are the absolutely last links in the vertical or horizontal fall protection system designed and installed by the QP. They should be the first point for the CP to inspect pre-shift and regularly during the shift.
Under the worst-case scenario, the maximum terminal impact forces will arrive practically "instantaneously" with the fall event. Subforces will rapidly redistribute themselves to many widespread points throughout the support structure, often followed by rebound impacts, deflections and pendulum stresses. How all of these dynamic forces affect any static loads already applied can create a very complex set of problems to solve. Prior to the arrest, shock loads reaching the anchor points, all the components of the entire personal fall arrest system are affected in turn. Immediately everything is in motion. Wire rope and synthetic webbing and stitching stretch out to their elastic limits. Subsystem anchors and load transfer points deflect and elongate. Knots, clamps, swages, hooks, rings and carabiners lengthen and distort. Both large and minute portions of the total kinetic energy are transferred into many other potential forms of energy such as motion, heat and sound. The conservation of momentum states that matter and energy can be neither created nor destroyed - they only change state. The QP shall have all of the documentation analyzing the potential transfer of this energy until TIF is applied on all the primary/secondary PFAS anchor points. In addition to this analysis, the QP shall provide any "out-of-service" inspection criteria deemed necessary for the competent person to apply once the PFAS has been put in use. Anyone who cannot perform these operations is not considered a qualified person.
[Next month, part two of the three-part series on anchorage safety focuses on anchorage force testing. Part three will concentrate on the safe work practices involved with directly connecting your PFAS system to your primary and secondary anchor points.]