Can Scaffolds Support This?

By David H. Glabe, P.E. / May 21, 2017

Spring is in the air, the birds are chirping and scaffolds are being built. Can life get any better? It used to be that contractors feared winter in the northern regions of North America. Cold temperatures, snow, wind and generally miserable conditions prompted owners and contractors to curtail outdoor activities. That was then; now construction charges ahead, fearless and courageous against even the nastiest of weather. Once again science and progress has prevailed! Improved, clothing, materials, equipment and methods allow construction to continue in any environment.
To facilitate these activities, it was common to enclose supported scaffolds against the weather. But times have changed; cold weather isn’t the only reason to enclose scaffolds. Containment of debris, tools and workers are now common reasons to enclose scaffolds. Enclosures are also used to advertise, block the work activities from pedestrians and even hide the workers who might be gawking at the pedestrians. Enclosing supported scaffolds is now a year around activity in all areas of North America, on all types of projects in all types of conditions.

Unfortunately, workers have false perceptions concerning supported scaffolds that are enclosed, including the perception that the forces on enclosed scaffolds are not as severe in summer as they are in winter; the perception that using open netting results in lower forces than using solid material; that no additional measures must be taken when a scaffold is enclosed and; site conditions have little effect on an enclosed scaffold.

The truth of the matter is that all scaffolds must be designed by a qualified person, that is, someone who can demonstrate the ability to properly design a scaffold, whether it is enclosed or not. Since designing for wind forces is a necessarily complicated matter, it is common that the qualified person for this design work is a Professional Engineer qualified in such activities. Of course, anyone can take a shot at the design (and unfortunately it is often the case), but the results can be fatal due to a gross underestimation of the forces developed by the wind. So, what is so complicated about wind design? Here are a few factors that must be considered:

Wind Forces

It is absolutely true that the force applied to a scaffold and its enclosure from the wind can be calculated. Short of a meteor falling out of the sky, there is no such thing as a “freak act of nature.” Those who argue so because their scaffold fell over need to be retrained. More accurately, an enclosed scaffold can be designed for a certain maximum wind speed; if the wind is expected to be higher than the design speed, either the scaffold must be dismantled, the enclosure removed, or additional measures must be taken to ensure the stability of the scaffold.

Wind Speed

Obviously, the wind velocity (speed) is the main factor in determining wind forces on a scaffold. However, choosing the correct wind speed for a specific location isn’t that easy. Although wind charts have been developed for North America that indicate maximum design wind velocities, choosing the correct velocity is just the starting point. In fact, there are numerous areas of the continent that have “special wind regions” that require additional investigation to determine the expected wind velocity. One example is along the east side of the Rocky Mountain range, extending from Montana down through Colorado and into New Mexico. At certain times of the year, Chinook winds, that is winds that drop down the east slopes of the mountains, reach as high as 100 mph. Similar winds, called the Santa Ana winds, occur in southern California. These winds don’t occur throughout the year; if your enclosed scaffold is erected during the right time of the year you don’t have to design for these winds; but watch out if the job is delayed and the scaffold is still standing when a Chinook wind hits!

Stability Ties

The key to scaffold success is to adequately design the scaffold and its connection to the adjacent structure. While U.S. federal OSHA and other agencies specify the minimum tie requirements for supported scaffolds, the tie spacing most likely will be grossly inadequate for any substantial enclosed scaffold. While #9 or #12 wire may suffice for a connection of an unenclosed scaffold, it typically is never adequate for an enclosed one. In other words, the ties for an enclosed scaffold must be designed for the anticipated tension and compression loads that are expected to occur. For those who choose to wing it and do something such as doubling up the ties should expect to see their scaffold take wing and fly like a kite. Keep in mind that it is not uncommon to have ties (and the adjacent structure) designed to hold several thousand pounds or more.

Adjustment Factors

When a qualified person designs an enclosed scaffold, he or she must consider these factors:

  • The height of the scaffold
  • The geographical location of the scaffold
  • The location of the scaffold relative to the surrounding structures
  • Surrounding Structures
  • Shape of the Scaffold/Structure (e.g. round or square)
  • Local Wind History
  • Partial or Full Enclosure
  • New construction or demolition
  • Existing structures—are the windows open or closed?

Time of year

This is not a complete list but it gives an idea of the potential complexity of the analysis and design.

Enclosure Porosity

Porosity is the fancy word for how many and how big are the holes in your enclosure material. If you are using netting, the holes can be quite small or they can be big. If the holes are over 2 inches in diameter, such as plastic fencing, porosity can be considered. Otherwise, the prudent scaffold designer will consider the netting as a solid material for the simple reason that the holes can become plugged. Snow and ice can easily plug the most porous netting in winter while sawdust, sand, asbestos (why you would use netting to try to contain asbestos is the more important question - you really need retraining!), stucco, plaster and other fine materials will also have an adverse effect on the airiness of your material regardless of the time of year.

While this article doesn’t cover all the factors that must be considered by the qualified person when designing an enclosed scaffold, it offers a glimpse into the complexity of the situation. Merely “doubling up the ties” and “this is the way I have always done it” is not a prudent approach; it just shows you are lucky. And while being lucky may work in craps or roulette, it has no place in the design of an enclosed supported scaffold. Is your life worth a throw of the dice?

Tags: Scaffold Bracing Scaffolding Blog Scaffold Components allowable scaffold load Resources

Previous Post Concrete Formwork - Under Pressure!
Next Post DHG Hiring A Structural Engineer

David H. Glabe, P.E.

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