suspended platform rigging stall load rated load

Suspended Scaffold Rigging

On August 30, 1996 OSHA published its final rule for the standard 1926. With it, they revised the construction industry safety standards which regulate the design, construction, and use of scaffolds. In paragraph 1926.451(d) the criteria for Suspension Scaffolds are stated. Subparagraph (1) states the following:

“All suspension scaffold support devices, such as outrigger beams, cornice hooks, parapet clamps, and similar devices, shall rest on surfaces capable of supporting at least 4 times the load imposed on them by the scaffold operating at the rated load of the hoist (or at least 1.5 times the load imposed on them by the scaffold at the stall capacity of the hoist, whichever is greater).”

This means there are two ways to set up your suspended scaffold rigging safely. However, only one of them is the right way and which one depends entirely on the situation. In our professional opinion, this results in a number of challenges that should be unnecessary.

Before we continue, let’s have a look at OSHA’s definitions of stall load and “rated load”.

“Stall load means the load at which the prime-mover of a power-operated hoist stalls or the power to the prime-mover is automatically disconnected.”

“Rated load means the manufacturer’s specified maximum load to be lifted by a hoist or to be applied to a scaffold or scaffold component”

Rigging for stall load

A decade after OSHA published the final rule, the Scaffold, Shoring & Forming Institute Inc. (SSFI) released a technical bulletin: “Rigging for Stall Load of a Hoist”. In it, they provide some critical notes to this safety requirement. They address four problems regarding rigging for stall load:

Problem 1: The user doesn’t know what the stall load of a hoist is.
Problem 2: The stall load varies significantly with influences like power supply, running time and internal temperature.
Problem 3: Nobody defines how to test for stall load.
Problem 4: The manufacturers themselves don’t know the stall load.

The fourth problem is a result of that of the second. Manufacturers know the range of the stall load, but the exact value on a specific moment are condition dependent. One could argue however, that the safest thing to do is to use the highest value as the base of your calculation.

Overload protection

Because most modern hoists are capable of lifting more than their rated load, we highly recommend using an overload detection device. Although generally not required by OSHA (but mandatory in Europe’s EN1808), an overload device prohibits the hoist from lifting more than its rated load, so this will avoid serious hazards. In practice, the overload device will ensure the stall load is equal to the rated load of the hoist. Therefore, in these situations the surface (often a roof) should be capable of supporting at least 4 times the load imposed on them by the scaffold operating at the rated load of the hoist.

Rigging for rated load of a hoist

When rigging for the rated load of a hoist, workers may still be faced with some challenges. Although the load a surface is exposed to is largely determined by the working load limit of the hoist, there are other important variables that influence it.

In case of outrigger beams, the surface also has to counterweights and the weight of the outrigger itself. The load of counterweights will also vary, depending on the configuration of the inner beam length and the outreach.

Surface

When the variables are known and are taken into account, the next question arises: what exactly is “surface”? OSHA does not provide a clear definition of this term. The earlier mentioned outrigger beam may easily cover a surface of 6’ by 16’. However, this rigging equipment will most likely be resting on either foot plates or castor wheels. This results in a point load on the plates or wheels, which have a much smaller surface to divide the load.

In this situation the suspended scaffold (load force) is hanging at the front, the counterweights (effort force) are at the back and the fulcrum is in between. This is called a first class lever. In these peak load will rest entirely on the fulcrum, which is the front two plates/wheels.

outrigger beam fulcrum

The fulcrum load is calculated using the following formula: F1 = F2 * (L1+L2)/L2
In this example, the rates load of the hoist is 1,100 lbs. (F1). This results in a fulcrum load (F2) of 1,390 lbs. The surface under the wheels or plates should be able to withstand 2780 lbs. (1,390 times 4, divided over 2 supports). That is a very high force to withstand.

To resolve this issue, we highly recommend to always use base plates to rest the supports on. These plates will help distribute the point load more evenly over a larger surface, requiring less capacity per square inch of the surface.

Final evaluation

Before the suspended scaffold is used, OSHA requires that “direct connections shall be evaluated by a competent person who shall confirm, based on the evaluation, that the supporting surfaces are capable of supporting the loads to be imposed.” (1926.451(d)(3)(i))

Resulting in the need of a competent person always having to calculate all variables. And this every day and every time the scaffold is moved or its configuration altered.

The challenge of evaluating the safety of the system doesn’t stop there. The same competent person will have to determine and/or verify if the surface has enough load capacity stated by OSHA. But where can this information be found? The solution given, is to ask building management which may be familiar with the roof’s rated load and may have experience with such matters. Otherwise, an engineer may have to be consulted. But is the information they have access to (still) correct? Although this information is essential for working safely, they are not responsible for the work that has to be done. The competent person is. This is a lot of responsibility resting on the shoulders of said person, given not many information is at hand.

These “questions”, variables and doubtful requirements really make you wonder: how much does this standard really contribute to workers’ safety? Now, 20 years later, perhaps it is time for a standard which is clearer and easier to adopt to a specific situation, making construction work at height safer.

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