Tarps and other enclosure materials, such as plastic sheeting, are typical materials used to create a desirable work atmosphere. Many scaffolds are enclosed in screening and debris netting—I recall one resort project in Aruba where the scaffold was wrapped in a mesh to ensure, so I was told, that construction debris would not blow into the adjacent swimming pool. In reality it was there so the guests below couldn’t see the less than productive construction workers staring at them! And, of course, now that outdoor temperatures in North America are slowly falling, thoughts of a cozy work environment on a supported scaffold become more frequent, resulting in more scaffolds being wrapped in some type of enclosure so that work can continue. It is interesting that wrapped scaffolding has been frequently discussed and written about and yet each year scaffolds fall over because somebody wrapped the scaffold without giving much thought to the effects that the enclosure would have on the stability of the scaffold. Of course, one of the keys to a successfully constructed scaffold is making sure that the scaffold doesn’t fall over; this is especially important for the individuals who happen to be using the scaffold!
The concept of stability is straightforward: The forces that want to knock the scaffold over have to be resisted. How can this be done? While there may be a number of methods that can be used, there are three that are most commonly used by scaffolding designers and erectors:: tying the scaffold to another strong structure that can resist the forces; guying the scaffold tower to a suitable anchor that can resist the forces, and; making the scaffold large enough so the size and weight of the scaffold are adequate to keep the scaffold from falling over. Since the stability of asupported scaffold is desirable, standards and regulations have been written to address the issue. The U.S. Federal Occupational Safety & Health Administration, OSHA, requires that “Supported scaffolds with a height to base width ratio of more than four to one (4:1) shall be restrained from tipping by guying, tying, bracing, or equivalent means….” [29 CFR 1926.451(c)(1)] The standard goes on to require that when the scaffold is tied to an existing structure, it has to be tied at a frequency of no more than 30 feet horizontally and 26 feet vertically for scaffolds wider than 3 feet, and 20 feet vertically for scaffolds 3 feet and narrower. (In California the requirements are more restrictive.)
Unfortunately, this regulation can be very misleading for the simple reason that it doesn’t address varying field conditions. Keeping in mind that the OSHA scaffolding standards are minimum requirements and not directions or instructions, the qualified person who designs the scaffold shall determine the proper means and methods for ensuring the stability of a scaffold. Also keep in mind that a qualified person will not guess at what is required to ensure scaffold stability. Unfortunately, the reality is that too many scaffold erectors and users think that experience is a great method for determining what it will take to keep the scaffold from falling over. While the OSHA mandated requirements may work for a scaffold not wrapped in plastic, the same tying requirements will be woefully inadequate for a scaffold wrapped in a tarp and subjected to a violent winter storm. (Lucky for many wrappers, the enclosure material rips into pieces and blows off before the scaffold is yanked from its’ moorings!) When a scaffold is wrapped in a quality enclosure, that is a netting or enclosure that is resistant to tearing, the scaffold instead will rip, bend and ultimately fail.
Interestingly, #9 wire is often used to secure a scaffold to a structure. While this can work with an open scaffold design, it very rarely is adequate for a wrapped scaffold, even if the ties are “doubled up.” Remember, guessing never has worked well as a substitution for a properly designed and erected scaffold.
So, what is the worker to do? The answer is easy, logical, and in compliance with the applicable standards and good scaffolding engineering practice. Have a Qualified Person design the scaffold. In the case of a wrapped/enclosed scaffold, it will probably take the skills and expertise of a Qualified Professional Engineer who can design the scaffold for the anticipated forces at the specific scaffold location and for the specific time of year that the scaffold will be exposed to external forces from the wind and other environmental conditions.
If you think that you are qualified to design an enclosed scaffold answer yes or no to these statements. (If you answer no to any of them, you are not qualified to design an enclosed scaffold):
I know where to find the information that tells me what the design wind loads are for my scaffold location;
I am familiar with the American Society of Civil Engineers (ASCE) Standard, Minimum Design Loads for Buildings and Other Structures wind loading criteria;
I know the strength of #9 wire and why it shouldn’t be used for wrapped scaffolds;
I can calculate the forces that are a result of a 100 mph breeze;
I know how to calculate overturning moments and forces due to pressures;
I know what the effects of a partially wrapped scaffold are;
I know what happens if the windows are open;
I know what effects a building corner or roof has on a wrapped scaffold;
I know my limitations.