Fall Arrest Clearance Calculations Made Easy

Session No. 617

Fall Arrest Clearance Calculations Made Easy

Greg Small, P.Eng., M.Eng., P.E. High Engineering Corp Calgary, AB

Introduction

How often, before any of us start work using a Fall Arrest System, do we pause to actually figure out how far we would fall before the System stops us? When was the last time any of us actually calculated our clearance?

The following are possible reasons why we may not be evaluating our clearance:

? Required Clearance can be complex to assess. It requires knowledge about how the equipment and systems perform, spatial reasoning and some mathematical calculation. While a Fall Protection course can teach about the equipment, our students may not have the inherent skills. When exams include clearance calculation, these are the questions most frequently answered incorrectly. Nonetheless, students pass because they get enough of the other questions right, and leave the course with certificates testifying they understand Fall Protection well enough to be safe.

? When we use Fall Arrest Systems at significant heights, it is "frighteningly" obvious that we have more than enough clearance, no matter what type of Fall Arrest System we are using. We focus on having the right equipment, system and anchorage. There is truly no safety reason why we must accurately know the Required Clearance, so we may get in the habit of NOT considering it.

? As we get closer to the ground, where knowing our clearances becomes important, we also feel much more comfortable. What we are doing may be exactly the same as what we trusted at greater heights. We may be complacent when we notice that no one else around us seems concerned about clearance. Perhaps we are unwilling to admit that we cannot remember how to figure it out and simply trust that "if they feel safe then I should feel safe". This is known as a "herd" mentality, when everyone assumes that someone else, who knows more than us, would speak up if there were a problem.

In summary.... most people neglect clearance because they probably don't understand it well enough. It is in our nature to trust something we don't understand when we do not have the capacity to prove it.

Available vs. Required Clearance. When we need to determine if we are safe, we must understand two types of clearance, what is "Available" as compared to what is "Required".

Available Clearance is the room below an anchorage or platform, through which workers can freely fall, before encountering a surface or object that may injure them in stopping or deflecting the fall.

Required Clearance, also known as Calculated Clearance, is the greatest distance below an anchorage, or the platform a worker might fall from, before the Fall Arrest System will stop (arrest) the fall.

A safe and effective Fall Arrest System must ensure that Required Clearance is less than the Available Clearance.

Available Clearance is most easily determined through direct measurement, such as by dropping a tape measure from the platform a worker is standing on. There are few problems with knowing it accurately.

As discussed above, Required Clearance is the more difficult type of clearance to assess, since we must figure out how far we might fall according to how our Fall Protection System behaves.

The purpose of this paper is to simplify these calculations, so that more (ideally all) workers and their supervisors can efficiently and safely assess Required Clearance. The final formulae and Pictograms provided at the conclusion of this paper will hopefully be useful to Workers and the Competent and Qualified Persons who may be working with them.

Two Different Types of Fall Arrest Systems = Two Clearance Calculation Methods.

There are two distinct types of Fall Arrest Systems; those that minimize slack between the user and the anchorage (Automatic Length Systems), and those that do not (Fixed Length Systems). For the purposes of this paper, these are defined as follows:

"Fixed Length" (FL) Systems

In this paper, "FL Systems" refers to "Fixed Length Lanyard or Lifeline Systems".

These systems connect the worker to an anchorage using a fixed length lanyard or lifeline. Sometimes the length of this equipment can be manually changed, but there is no automatic adjustment to maintain the shortest possible length between the user and the anchorage. A Vertical Lifeline that uses a manual fall arrester would be classified as an FL System.

These systems generate Free Fall (explained in the next section) according to how much closer the worker is to the anchorage than the length of the lanyard or lifeline.

"Automatic Length" (AL) Systems

In this paper, "AL Systems" refers to "Lifeline Systems that automatically adjust to minimize the length of the connection between the Worker and the Anchorage". This feature minimizes the

Free Fall, to reduce the Required Clearance. Examples of AL Systems include Self Retracting Lifelines (SRLs) and Vertical Lifelines (VLLs) that use Automatic Fall Arresters (Rope Grabs).

Clearance for each type of system (FL vs. AL) is most easily calculated by a different method, with different rules about what must be included in the calculation.

Although either method may be used to calculate Required Clearance for both FL and AL Systems, this can get quite complicated when the method that is best for FL systems is used on AL Systems, and vice-versa. Qualified Persons should possess the spatial reasoning and calculation skills to be able to do this, but other people may not.

In part, because it is possible to calculate clearance using only one of the two methods, training programs sometimes choose to teach only one clearance calculation method. This leaves workers, at best, with the tools to easily determine clearance for the one type of system, but very confused if they try to apply what they learned to the other type.

It takes a bit longer to teach both clearance calculation methods, including the need to develop a clear understanding of which method is applied to an FL vs. an AL System, but the results, in my experience, have been a greatly improved ability to evaluate Required Clearance.

In this paper, we go a step further, and apply the FL and AL calculation methods to four key types of systems. We then provide Pictograms to help users identify which formulae to use, and show some examples of just how easy it can be to determine Required Clearance.

What factors must be considered in Required Clearance Calculations?

The following factors must be considered in determining Required Clearance in AL and FL Systems.

Free Fall (FF)

This is the distance the worker falls freely, with nominally no force applied to slow him or her down. Free Fall takes all of the slack out of the Fall Arrest System and includes the distance required for arresting devices, such as Fall Arresters and Self-Retracting Lifelines (SRLs) to activate or lock-up.

Unfortunately, spatial reasoning and math are required to figure this out in FL Systems.

One of the objectives of this paper is to develop clearance calculation methods that eliminate the need to determine Free Fall.

Deceleration Distance (DD)

This is the distance a worker travels while the system applies force to arrest the fall. Many parts of the Fall Arrest System can be involved in dissipating the energy generated during a fall, including deployment of Personal Energy Absorbers (PEAs), braking mechanisms on SelfRetracting devices (SRLs), and the Anchorage System itself can deflect. Each component absorbs energy as it deploys, stretches, deflects or sags. This is complex, and usually requires a Qualified Person to determine how much energy gets apportioned to each part of the system, in order to accurately determine the total Deceleration Distance.

One of the objectives of this paper is to develop clearance calculation methods that reduce or eliminate the need to determine Deceleration Distance. This is done by making conservative assumptions about how far things deflect, stretch or deploy. This is not a new idea by any means, since most people are already taught to assume full deployment of their Personal Energy Absorbers.

Harness Stretch

The D-ring usually flips up and slides up the webbing when a fall is arrested. The webbing in the harness also stretches, and some Harness models use highly elastic webbing, which is considered more comfortable to wear.

Depending on the model, harness stretch varies between 1 and 2.5

feet (0.3 to 0.75 m). To keep things simple, we usually specify clearance

based on a conventional harness (that would stretch 1 foot (0.3 m)), and

instruct workers to add in an additional 1.5 feet (0.45m) if using a

"stretch" harness. It is useful to have a term defining the additional harness stretch, so this paper will use the term, "XH".

Figure 1 Comparing

Harness Stretch

Worker Stretch-Out

A standing worker starts and finishes the fall in a vertical body orientation, and (nominally) does

not get any longer. The torso begins the fall from a higher elevation than for someone who is

kneeling on the platform. This affects the Free Fall in FL Systems, and the clearance in the AL

Systems.

Workers who kneel or lie on the work surface will straighten out and hang vertically. They will elongate by 2.5 to 4 feet (0.75 to 1.2m) in the vertical direction, depending on their initial kneeling or lying position. Kneeling while working is common. Lying down is rare, and largely eliminates the risk of falling when you are close to an unprotected edge. For these reasons, our focus when teaching Worker Stretch-Out should be on the case of kneeling workers.

Worker Stretch-Out must be considered when determining clearance below the platform because it is the governing case for AL Systems anchored above the worker.

Figure 2 Kneeling workers need 2? ft (0.75m) more clearance

For FL Systems (or AL Systems anchored behind or lower than the worker's Dorsal Dring), standing workers will have more Free Fall than kneeling workers, generating more fall energy, requiring greater deployment of PEAs or SRLs, and thus more clearance. Thus we only need to add extra clearance for Stretch-Out of kneeling workers when using AL systems, such as SRLs and VLLs that are anchored above the worker.

Combined Worker and Harness Stretch (XW)

It is a common practice to combine Worker Stretch-Out and Harness Stretch into a single factor, XW.

The ANSI Z359.6 and CSA Z259.16, standards for the "Design of Active Fall Protection Systems", specify XW as 1 foot (0.3m) for a worker falling from a standing position when wearing a regular harness.

If the worker is not standing at the start of the fall, we must add 2.5 or 4 feet (0.75 or 1.2m) respectively if falling from a kneeling or lying position, but as discussed above, this is only applicable for AL Systems anchored above the worker, such as SRLs and VLLs.

If the worker is wearing a stretch harness, we must add XH, discussed above in Harness Stretch.

Height of Worker at Fall Arrest (HW)

At Fall Arrest, the worker is fully stretched out. In a regular Harness, we assume the worker height is 6 feet (1.8m) from Dorsal D-ring to toes.

The initial body position does not affect the Height of Worker at Fall Arrest.

If the worker is wearing a stretch harness, we must add XH, discussed

above in Harness Stretch.

Figure 3

Height of worker

Swing Fall Distance (SFD)

Gravity will always pull workers to the lowest possible elevation the system will allow, directly below (or opposite) the anchorage (or a point the line deflects over). Workers who are connected to an anchorage system that is

(D-ring to toes) at Fall Arrest is nominally 6 ft (1.8m)

not immediately overhead will drop in elevation as they swing from the location where the

system starts to apply arrest forces until they come to rest wherever gravity pulls them.

Unfortunately, figuring out the vertical drop in a swing fall also requires mathematical (geometry) and spatial reasoning skills, making it as difficult as figuring out Free Fall.

Well meaning people often offer simple rules of thumb, such as "keep your line within 30 degrees of vertical", however this does not help much with clearance calculations for a couple of reasons:

? The worker is not told how much clearance to add if they follow this rule. ? The reason the worker is not told how much clearance to add is because it actually depends

on the length of the line between the worker and the anchor. At 30 degrees, a 10-foot (3m) line will create a Swing Fall of 1.34 feet (0.41m) whereas a 100-foot (30.5m) line gives a swing fall of 13.4 feet (4.1m), which is extremely dangerous for sideways impacts.

A good alternative to attempting to teach mathematical swing calculations is to show workers in the field how to measure or estimate the length of line between the anchorage and where they will be working, and then to measure from the anchorage to the platform or edge they may fall from. We can visualize where gravity will pull us, or perhaps experiment with a weight on a string. The swing fall distance is the difference between these two measurements, and can actually be determined without subtraction if you don't retract the tape from the first measurement and read the

Figure 4 Swing Fall

requires understanding

of geometry

 ................
................

In order to avoid copyright disputes, this page is only a partial summary.

Google Online Preview   Download