Horizontal Fall Arrest Systems: Rigid Systems vs. Flexible Line Systems
If the system is not user friendly, it will not be used and will be a detriment to the company's overall fall protection program.
- By W. David Lough
- Sep 01, 2004
THERE are hundreds, if not thousands, of available fall protection systems and temporary anchors that end users can buy, install, and use when and where they are required. For example, a worker can use an ANSI-approved anchorage connector (strap) in conjunction with an ANSI-approved energy absorbing lanyard and ANSI-approved full body harness to create a compliant fall arrest or fall restraint system in any number of situations. In addition, manufacturers have created innovative pieces of equipment that can be adapted to different structural shapes so that with proper training, workers can create individual anchors in most areas of their working environment.
All of these systems can provide the worker with adequate fall protection in a relatively small work area, but what can be done when workers require a larger range of movement?
Traditionally, this situation has been overcome using horizontal fall arrest systems. Horizontal systems allow the worker unimpeded, three-dimensional movement over a much greater work area that is limited only by the horizontal length of the system.
Designed by a Qualified Person
Once a company decides a horizontal system is needed, a number of requirements must be met. First and foremost is that the horizontal system be designed by a Qualified Person. (According to OSHA 1926 Subpart (m), "Qualified Person" means one who, by possession of a recognized degree, certificate, or professional standing, or who, by extensive knowledge, training, and experience, has successfully demonstrated his ability to solve or resolve problems relating to the subject matter, the work, or the project.)
One of the first decisions the qualified person should make is whether a rigid or flexible system is best for the situation at hand. This can be a difficult decision that is sometimes made by end users based on their requirements and systems they have seen or used in other locations. But making this decision based solely on the end user is not recommended. This is not to say the information gathered from the end user based on the task at hand and its experience is not of utmost importance; it is, and it should be the cornerstone of any decision made. If the system is not user friendly, it will not be used and will be a waste of funds and a detriment to the company's overall fall protection program. There are, however, several other factors that should be taken into consideration before a decision is made. This article will describe a number of these factors and explain their importance.
End and Intermediate Anchor Forces
One of the most important factors that affect the decision to install a rigid or flexible system is the forces that are exerted on the anchor structure. In many cases, the horizontal system must be connected to existing building structure. Depending on the age of the building, the structural members may not be originally designed with fall protection in mind.
When examining a two-worker, 100-foot flexible horizontal system, it is not unheard of that the actual end anchor forces can easily be in the range of 3,000 to 6,000 pounds. When designing for a safety factor of two, we can expect the end anchors must be able to withstand a force of 6,000 to 12,000 pounds, minimum. In many situations, this requires that the existing structure be reinforced to withstand the high end anchor forces or a horizontal lifeline energy absorber be incorporated into the system to reduce the end anchor forces. This can increase the required clearance.
For a rigid system, end anchor forces are not as high because we are concerned with only the direct forces combined with the system's weight, rather than vector forces on the anchors. These forces can be as low as 1,800 pounds (with safety factor of two totaling 3,600 pounds) when considering a two-person system. In most cases, there is no requirement for reinforcement of the existing structure when installing a rigid system. This would have to be determined by a qualified person.
Connecting to Existing Structures
Depending on the type of building structure and the end anchor forces developed by the chosen system, the existing structure may require reinforcement. This can be required regardless of whether a rigid or flexible system is chosen, but it is more common when using a flexible system due to the higher forces. If it is required for either system, it may be more cost effective to install a horizontal lifeline because these require fewer attachments to existing structures than a rigid horizontal fall arrest system.
Typically, a long span horizontal lifeline can easily span 100 feet. "Off the shelf" engineered systems, depending on the manufacturer and type of system, can have a multitude of spans spaced from 30 to 50 feet. Rigid rail systems normally can span 10 to 20 feet, depending on the type of steel section and the size. Usually, the longer the span distance, the heavier the steel section, so a balance must be achieved. Thus, the rigid system is not too heavy and does not increase the requirement for reinforcement of the existing structure.
Clearance is another important factor when determining which type of horizontal system to use for a specific situation. Many systems have been designed to withstand the forces exerted on the components and end anchors; however, if the worker strikes the ground anyway (even if only the worker's feet contact the ground), the system has failed.
With a rigid horizontal fall arrest system, there are no additional clearance requirements for dynamic sag and no need for a horizontal lifeline energy absorber. With a flexible horizontal lifeline, there is usually a great deal of dynamic sag in the system, which will increase the required clearance below the worker. This dynamic sag will change depending on the rigidity of the end anchors, temperature changes, the type and size of cable, the distance between anchors or intermediate anchors, whether or not the system is a single span or multiple span system, use of a horizontal lifeline energy absorber, and the number of workers who may fall into the system at one time.
Under typical circumstances, sags in these cable systems can easily vary from 2 to 6 feet. When you take into account additional clearance requirements that can include free fall (up to 6 feet), workers' height (averaging 6 feet), energy absorber deployment (3.5 feet), harness stretch (1 to 2 feet, depending on the harness) and any margin of safety you may require (2 feet), it is not hard to see that in many cases where limited clearance is available, a flexible system may not be the best solution. Installing a horizontal lifeline energy absorber also will drastically increase the dynamic sag and therefore the required clearance.
In some companies, cost unfortunately will be the most important factor for deciding which type of horizontal system to choose. Everyone understands cost is very important, but companies should investigate and weigh accordingly all the factors involved in their specific situation to ensure the system will function properly, be user friendly, be structurally sound, and stop the worker before he contacts the ground.
There are five costs associated with any horizontal fall arrest system: engineering costs, supply costs, installation costs, training, and maintenance costs. In many cases, engineering costs, training costs, and maintenance costs will be similar for a flexible fall arrest system or rigid fall arrest system. Where the difference in cost is most noticeable is in the supply and installation costs.
For several popular "off the shelf" cable systems, the connecting means and pass through systems make up a great deal of the cost, and a certified installer is required for installation. Rigid systems are usually a bit more costly for supply--and with the rise in steel prices they have been getting increasingly expensive. But if the structure must be reinforced to accommodate the forces of the horizontal lifeline, the cost spread between these systems shrinks greatly.
Installation costs for each system also can vary if the structure to which it is being attached is unquestionably strong. If this is the case, thee cable systems will be installed much more quickly and cheaply. (There are fewer connection points due to the greater spans, and cable is much easier to lift into place than heavy structural steel.) Once again, if reinforcement of the existing structure is required for the cable system, then the installation cost gap narrows and both systems can be very comparable, as far as cost is concerned.
There are many types of flexible and rigid systems on the market, both permanent and temporary. This article does not mean to encompass all possible systems or hazards and only intends to give an outline of what at a minimum should be examined to make an educated purchasing decision.
In many instances, the buyer will use the same type of horizontal system for all situations. This is a good idea in some cases because it will reduce the need for training on a number of different systems, reduce system compatibility issues, and may reduce costs for installation, supply, and maintenance. This may not be the best idea if the hazard areas differ a great deal; as we have illustrated, one system may not function for all areas and tasks.
The rigid system is typically the best solution simply based on the fact the worker won't fall as far as when he is connected to a flexible system, because of the elimination of any dynamic sag and horizontal energy absorber deployment. In any case, where you stop the worker from falling farther, you decrease the chance there may be an incident where the worker is injured.
From a cost standpoint, flexible fall arrest systems typically are cheapest. In the end, safety professionals must balance the cost and effectiveness of the system to prevent an injury.
This article originally appeared in the September 2004 issue of Occupational Health & Safety.