Hoists and elevators are invaluable productivity enhancement devices used in project construction and renovation. It would be impossible to complete today’s projects without hoists and elevators to move workers and materials vertically. The Federal Occupational Safety and Health Administration, OSHA, and other agencies have standards that apply to the fabrication, installation, and use of hoists, elevators, and other similar devices. However, understanding the standards that apply to the devices used to transport workers and materials is only a part of the picture. Knowing how these devices interact with scaffold products is essential if safety is to be realized.
In a broad sense, vertical transport platforms can be generally classified in a manner similar to scaffolds. The platforms can be supported by masts/columns or they can be supported by suspension ropes. Within this broad categorization, many variants can be utilized to get the platform from the ground to where you want to go. For example, an elevator in a 50 story building will be operated and supported by wire ropes while an elevator in a 2 story building will be operated by a hydraulic ram. In other words, the one is supported by ropes while the other is supported by an adjustable column mounted on the lowest floor. What does this have to do with scaffolding? Well, nothing unless you decide to use the scaffold to support the hoist or elevator. Then it can be pretty important to know what is going on.
Let’s look at mast supported hoists and elevators first. Keep in mind that there are very significant differences between material hoists and elevators used to transport humans. However, the principle behind the operation of each is similar. A mast or column supports the vertical load that is applied. Depending on the design, the mast or column may also support the horizontal loads that will be applied to it. What are these two loads? The vertical load is the downward load that is a result of the weight of the car, the occupants, and the mast itself. If this load is directly over the column or mast, there will be minimal horizontal loads. On the other hand, if the car is cantilevered to the side of the mast, there can be a significant horizontal force that has to be restrained. If the mast in question is attached to a building, for example, the hoist components, and the building, will restrain the forces. If the mast however is attached to a scaffold, the scaffold had better be designed to handle those loads. Since supported scaffolds are typically designed to support vertical loads only, a very qualified designer is required for these additional horizontal forces. In fact, due to the potential complexity of the horizontal and vertical forces, some scaffold companies prohibit attaching large hoists and elevators to the scaffold. This is not to say that it cannot be done; rather, these scaffold companies are very concerned about safe scaffold/hoist installation and use.
What about small hoists that transport material, such as scaffold components. Can they be attached to scaffolds? Sure, provided the scaffold is designed to support both the vertical and horizontal loads that will be imposed, not only by the hoist, but also by the materials that are being transported. Impact loads also require consideration as the sudden starting and stopping of the machine can have an adverse effect on the ability of the scaffold to withstand the stresses. If the hoist is mounted on the side of a scaffold tower the eccentric load caused by the hoist being outside the legs of the scaffold can produce significant forces that may exceed the capacity of the connection between the scaffold and the structure, resulting in cataclysmic results. Consequently, the qualified designer must carefully consider all the forces that will occur and design accordingly.
Hoist platforms supported by suspension ropes typically apply only a vertical load to the supporting tower or apparatus. These loads must be supported by the scaffold legs and just as important, by the members that directly hold the suspension ropes. Again, a qualified person must design the entire structure, considering the impact, live, and equipment weight in the design. The suspension cables must be strong enough for the loads and must be rigged according to the manufacturer’s recommendations. Access gates and guardrail systems must be carefully designed into the scaffold so that there are no open sides or ends of the scaffold deck when the hoist platform is absent. Lighting and signaling may also be a necessary part of the installation.
Even simple hand operated hoists can be disastrous in untrained hands. The most basic hoist involves a rope that goes through a pulley supported by an arm attached to a scaffold leg. Since these hoists have a very low load rating, you would think nothing could go wrong. Unfortunately, workers can experience unwelcome results if they haven’t been trained properly. The first tell tale sign that all is not right is a bent hoist arm. Somebody hasn’t been told the load limit! Next, the operator (the guy on the ground pulling on the rope) is standing a substantial distance away from the hoist, creating a potentially destabilizing horizontal force on the scaffold. As the worker pulls and pulls, the load gets higher and higher and the horizontal force gets bigger and bigger. About that time, the scaffold falls over and everybody is surprised. The real surprise is that everybody is surprised.
If you are going to hoist materials or people, and you want the scaffold to support it, make sure you consider all the forces or you may find your vertical travel quickly becoming horizontal travel!