Is An Enclosed Scaffold Different?

By David H. Glabe, P.E. / November 1, 2004

Why did my enclosed scaffold fall over?  Why is there asbestos (or lead–take your pick) contaminating the worksite?  I followed the regulations; the scaffold shouldn’t have done this to me!  Ah, the lament of the uniformed scaffold user.  It seems that enclosed scaffolds have a propensity to fall over.  What can be causing this phenomenon?

Scaffolds are generally enclosed for two reasons, either for containment or for weather protection.  Containment keeps whatever is inside the scaffold from getting out and weather protection keeps whatever is outside the scaffold from getting in.  For example, a scaffold will be contained so that lead paint being removed won’t scatter and do harm.  On the other hand, a scaffold will be enclosed so that the workers on the platforms inside the scaffold are protected from nasty weather conditions.  Either way, wrapping a scaffold can dramatically and adversely change the forces on a scaffold; a qualified designer will take these changes into consideration when designing the scaffold.  Unfortunately, an unqualified designer won’t know to take these changes into consideration and most likely will experience a disappointing result.

Besides the scaffold structure itself, there are several parameters that must be incorporated into the design and erection of a scaffold that will be wrapped in an enclosure system.  These parameters include the enclosure material that will be used, the tying system that will be used, the environmental circumstances, the scaffold configuration, and geography.

The enclosure material will directly impact the loads applied to a scaffold and the scaffold tie system.  The enclosure material may be solid, such as plastic “shrink wrap”, or may be porous, such as screening.  Screening can be described by its porosity, that is, the ratio of solid area to openings.  For example, a 50% material would have half of the material solid and half of the material open.  Depending on the size of the openings, air and some debris could go through screening.  On the other hand, a 100% solid enclosure material, such as solid plastic sheeting or plywood, would contain everything inside the scaffold and allow nothing through.  Since the horizontal forces on the scaffold and scaffold tie system are a result of the solid area exposed to the wind, the type of enclosure material being used must be known when designing the scaffold.

Any tie system used to secure the scaffold upright and plumb must withstand the forces applied to it.  While the U.S. Federal Occupational Safety and Health Administration, (OSHA), specifies maximum permissible vertical and horizontal tie spacing (minimum tie requirements) in its’ standards, it must be remembered that these are minimum standards and only provide stability to the scaffold; this minimum tie spacing does not provide strength against wind forces, particularly if the scaffold is enclosed.  Typically, a qualified person who is familiar with the forces that occur, should design the ties used on an enclosed scaffold.  Untrained erectors, not qualified to make the appropriate decision, casually assume that “doubling up” the ties is sufficient.  Trust me, this assumption can lead to disastrous results.  Geographic conditions and the scaffold itself affect the forces that occur when a scaffold is wrapped.  These conditions and factors include the height of the scaffold, the length of the scaffold, the width of the scaffold, the shape of the adjacent structure, the location of the structure and the scaffold, the proximity of other adjacent structures, the terrain, the local weather conditions and the material being used for the enclosure, to name a few.

So, what are the forces on the individual ties?  Can a standard wire tie handle the load?  As stated earlier, it is impossible to accurately state the loads on a wrapped scaffold without a specific analysis of the scaffold in question.  However, a generic example can be used to illustrate the forces involved.  If the wind speed at the project is 20 mph, the basic force in the middle of the scaffold is approximately 1 pound per square foot.  If the wind picks up to a 40 mph breeze, the load increases to 4 pounds per square foot and with a 60 mph breeze, the force increases to 9 pounds per square foot.  Using the OSHA tie spacing standards as a reference, the force on the tie, at 20 miles per hour, would be at least 800 pounds.  Applying the 4 to 1 safety factor, the tie should be designed for at least 3,200 pounds.  At 60 miles per hour, the force on the tie would be a minimum 7,020 pounds.  Applying the standard safety factor, the tie must be designed for at least 28,000 pounds.  You would need quite a bit of wire to handle that kind of force!  Keep in mind that these forces have not taken into consideration the factors that will increase the load substantially, in some cases doubling or even tripling the load on the tie.  These increases in forces occur at corners, roofs, and other areas where structures or the surrounding terrain affects the wind speed.

Wrapping scaffolds should not be taken lightly.  Qualified scaffold designers should always be consulted before erecting the scaffold.  When the scaffold is enclosed, you must have a designer who understands how enclosures affect the scaffold; this is for your safety and the safety of your fellow workers.

Tags: Scaffolding enclosed scaffold Resources scaffold tie system wrapping scaffolds

Previous Post Why Is It Better?
Next Post Should You Be Committed?

David H. Glabe, P.E.

See what David H. Glabe has to say about construction engineering and the scaffolding industry.