Identification of the correct safety factors for scaffold foundations.
Foundations are a necessary part of any scaffold, whether it is a supported scaffold, a suspended scaffold, or an aerial lift. Webster’s dictionary describes a foundation as “the natural or prepared ground or base on which some structure rests.” Webster goes on to describe a base as “a bottom support; that on which a thing stands or rests.” Without a foundation, or base, the scaffold is useless. Think about it: if a supported scaffold, that is a temporary elevated platform that is supported by rigid legs or posts, doesn’t have a solid foundation, it will collapse. The same is true for aerial lifts such as scissors lifts or boom lifts, where it is very important that the foundation is strong enough to support the machine.
What about suspended scaffolds, those elevated temporary platforms that are supported by non-rigid means such as ropes? Do they need foundations? You may want to answer no since the rigging that supports the rope is typically on the roof of the structure. But you would be wrong. While the word foundation is typically used to describe the lowest level of a building and is usually in the ground, for scaffolding it means much more than that. Think in terms of Webster’s definition for a base: “a bottom support; that on which a thing stands or rests.” In the case of suspended scaffolds, the “thing” is the rigging, such as a cantilever beam, while the “bottom support” is the roof of the building or other structure supporting the rigging. In other words, all scaffolds need foundations; it’s just that the foundation for suspended scaffold may be on the roof of the building.
This brings us to an interesting question about the strength of foundations: what safety factor is required for scaffold foundations? Should it be adequate as specified in the federal Occupational Safety & Health Administration (OSHA) Construction Industry supported scaffold standards or should it have a safety factor of four as specified in the capacity standards? But wait, there’s more! The OSHA Construction Industry suspended scaffold criteria specifies that “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.)” [29 CFR 1926.451(d)(1)] For suspended scaffolds this means the supporting surface, such as the roof of a building, should have a safety factor of 4. For example, if you had a 1,000 pound load supported by a beam that cantilevered 18 inches past the edge of the roof, and the beam had a backspan of 10 feet, the fulcrum load would be 1,150 pounds while the required counterweight at the back of the beam for such a situation would have to be 600 pounds. In our example the roof would have to support 1,750 pounds of actual weight. This is like parking a couple of Harley Davidson Electra Glide Classics on the roof. Picture that in your mind! Frankly, my experience suggests that not too many suspended scaffold erectors give this loading thing much thought. But then, they probably don’t think about parking Harleys on the roof either. Applying a safety factor of 4, the roof would have to support 4,600 pounds at the fulcrum. That’s a lot of load. At the back end of the beam the roof would have to support 2,400 pounds meaning that the roof would have to support 4,000 pounds + 2,400 pounds for a total of 6,400 pounds. In other words, the roof would have to hold the equivalent of a Chevy Crew Cab pickup truck. Is this really necessary? How many roofs do you think can hold a load of this magnitude? Do the standards really require this?
While the snappy quick answer may be yes, the best way to answer this is to determine what the hazard is and what the intent of the standard is. The hazard, of course, is that the roof collapses under the load of the hoist. Therefore, the intent of the standard is to make sure you don’t collapse the roof while using a suspended scaffold; not a bad reason for having the regulation. The tricky part is how to determine if the roof will have a 4 to 1 safety factor against collapse. Related to that question is determining how much of the roof you can use to support the rigging. Since the fulcrum is often a point load, there is a real possibility of having the fulcrum poke a hole in the roof. That would not be good. Therefore, this load has to be spread out. The same may hold true for the back end, depending on how the counterweights are rigged.
Most outrigger applications are designed by “experience,” that is gut feel as to the strength of the roof. If the roof happens to be new concrete, your gut just might be right. On the other hand, if the roof is a hundred years old and decayed, your gut may not be right at all and you’ll get indigestion, not to mention what the roof might be doing.
The bottom line is that, just like the rigging, the supporting surface (the roof) must also have a safety factor of 4. In our previously mentioned example, the actual load that has to be supported is 1,750 pounds, two Harleys. Depending on the roof construction, for example the direction of the support beams and the design live load, you may be okay. For illustration, if the roof design live load is 20 pounds per square foot (psf), and the outrigger beams are spaced at least 20 feet apart, the roof just might work with the required safety factor. Of course, if the live load includes the design snow load, and it snows, your safety factor will melt away before the snow does!
In other words, if you have been guessing about the roof strength, you may have a correct safety factor —or not.