scaffold stability Archives | DH Glabe & Associates

Pop Quiz: 30 Questions About Scaffolding

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It is somewhat surprising how creative workers can get when it involves scaffolding.  Just when it seems all the questions have been answered, along comes a question that raises an issue that was never addressed.  Challenge yourself to these questions and see if your answer agrees with the one given at the end of this article.

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Are You Stable?

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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.

Will Your Knee-Out Work?

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A description of the proper use of a knee-out.

There are two issues that need to be addressed when considering the use of a knee-out in your scaffold.  The first issue involves the stability of the scaffold while the second issue involves leg loading.  Stability can be a real problem if the base width of the scaffold that the knee-out is attached to is narrow compared to the size of the knee-out.  While “off the shelf” knee-outs normally do not exceed 45 inches in the horizontal dimension, a knee-out can be any size you want—if you know how to design and construct it.  Let’s say you have a scaffold that has a base width dimension of 5 feet.  You decide to install a knee-out on the outside leg of the scaffold that happens to be 7 feet, measured horizontally.  If you don’t have enough weight in the base scaffold, the whole thing will fall over.  Of course, the clever, or not so clever, scaffold erector assumes the weight of the scaffold will be the “counterweight” for the knee-out.  Imagine what happens if the knee-out gets loaded up with plank and materials that weigh more than the scaffold equipment or, better yet, somebody decides to dismantle the base scaffold first before dismantling the knee-out.  The dismantling may not take as long as you thought!

Knee-outs have a direct impact on the leg to which it is attached.  Assuming that an upper scaffold leg is supported by the knee-out and built up from there, there are two types of forces that the supporting scaffold leg must support, vertical forces and horizontal forces.  The vertical loads from the knee-out are transferred into the supporting leg and presumably down to the scaffold foundation.  The connection to the leg at the top of the knee-out has to resist a horizontal force that wants to pull the leg outward while the bottom connection of the knee-out wants to push the leg inward.

Since supported scaffold legs, normally a steel tube for frame, systems, and tube & clamp supported scaffolds, can handle vertical/axial loads efficiently, the vertical force is no big deal as long as the total load of the knee-out vertical load and the supporting scaffold leg do not exceed the allowable load for the supporting leg.  Remember, the supporting leg is basically holding up two legs, and the loads on those two legs.

The horizontal forces are a little trickier.  Round tubes can handle vertical loads well but do a really lousy job of handling horizontal loads, exactly the horizontal forces/loads that a knee-out applies to a round tube.  What is a designer to do?  Well, the qualified designer knows that bracing is required to transfer the imposed loads properly so none of the scaffold components are overloaded.  This load transfer can be achieved in a variety of ways.  The first requirement is to install a horizontal member at the knee-out connection so that at least two legs are connected horizontally.  Then a vertical diagonal is required to transfer the load down to the next runner location, typically one frame down if it is a frame scaffold, and 6’-6” or so if it is a systems scaffold or tube & clamp scaffold. This process is repeated until either the vertical legs can handle and disperse the horizontal loads to other legs, or you have transferred the loads through the bracing down to the foundation.  How do you know when that occurs?  Well, there are two ways; analyze the scaffold or try it out and see if it bends!  I strongly recommend the analysis method rather than the guess method—workers’ lives are at stake here.

Another issue that comes up, and is usually not considered by erectors guessing and “winging it” involves the diagonal member that transfers the leg load supported by the knee-out to the supporting leg.  If that member is installed at a 45 degree angle, the force/load in the member is almost 1.5 times (1.404 to be exact) more than the leg load it is supporting.  And since this diagonal member is in compression, it also must be braced when the length exceeds its ability to support the expected load.  This is the stuff of qualified designers, typically qualified engineers who can develop an appropriate design for the specific situation.

If you can’t ascertain the loads the knee-out is subjected to, if you cannot calculate the horizontal forces applied to the supporting scaffold, if you cannot figure out how to transfer the applied loads so the scaffold can handle them, don’t guess at it; leave it to the experts to design it for you.

A Perfect Scaffold

By | Fall Protection, OSHA Standards & Regulations, Resources, Scaffolding, Scaffolding Platforms | No Comments

Is there such a thing?  A friend asked me about it the other day; he wondered if I had ever written about a “perfect scaffold.”  Well, I don’t ever recall specifically writing about a perfect scaffold.  I know I have written about safety aspects of scaffolds.  I have written about the components that are part of a perfect scaffold.  I have written about the strength, the stability, the safety of a scaffold but I don’t recall ever having written or described a perfect scaffold.

So what constitutes a perfect scaffold?  It is a scaffold that is constructed so that it can be used by workers without injurious incident.  It is a scaffold that won’t kill the guy that is on it.  It’s a scaffold that has access to the platform and the platform provides access to all work areas.  It is a scaffold that is stable and designed to carry the intended load.  It is a scaffold that allows the worker to go home to his/her family after the workshift.  It is a scaffold that complies with all the applicable standards.  Specifically, the perfect scaffold will include the following:

Fall Protection:  On a supported scaffold, a complete guardrail system, including top and mid rails, will be installed around the perimeter of all platforms except along the edge that is within 14 inches of the work surface (18 inches for stucco/plastering applications).  Alternatively, a personal fall arrest system with conveniently available anchors will be used.  Fall protection height will be determined by the competent person.  In no case will that height exceed 10 feet.  For suspended scaffolds, both a guardrail system and personal fall protection is required on single and two point scaffolds.  For multipoint suspended scaffolds a guardrail system is required although personal fall protection may also be required, as determined by the competent person.

Access:  All scaffold platforms require access whenever there is more than a 24 inch break in elevation.  While there are numerous access choices, typically ladders, stairs, access frames, or direct access are used.  The first step cannot be more than 24 inches.  If you choose to jump out the window onto the scaffold, make sure you are within 14 inches horizontally and 24 inches vertically of the platform.

Falling Object Protection:  All platforms need falling object protection if there is exposure to employees working below.  The easiest solution is to not have employees working above each other.  If that is not possible, screens, debris nets, catch platforms, toeboards or canopies are required.

Electrocution:  It is unhealthy and therefore unwise to come into contact with energized electrical wires unless of course you want to become energized.  As a rule, stay at least three feet away from 300 volt power lines and ten feet away from 50,000 volt lines.  Above 50,000 volts, really stay away.  Consult the standards for the specific distances for specific voltages.

Foundation:  All scaffolds need a foundation, including suspended scaffolds.  For suspended scaffolds, make sure the rigging is properly sized for the intended load.  All components must be mechanically fastened so they act as one unit and you don’t lose critical pieces, such as the counterweights.  Make sure all safety devices are attached, particularly the tiebacks, to proper anchors.  For supported scaffolds, make sure that all scaffolds legs have base plates, no matter who is using the scaffold and no matter whether you are on concrete or not.  Sills may be required.  The qualified person will determine the size of the sill, based on the leg load and the capacity of the supporting soil/surface.

Stability:  All supported scaffolds should be constructed so they don’t rock and roll.  This is done through the proper use of bracing, including cross, diagonal, straight, or combination therefore, and ties, to the existing structure.  If your scaffolds rocks, something is wrong.  For suspended scaffolds, you may be rocking and a rollin’ but it better be controlled!

Strength:  In spite of the opinion of some scaffolders, all scaffolds must be designed by a qualified person, an individual who knows the capacity of the scaffold being designed and the intended use of the scaffold.  While bracing may keep a scaffold from falling over, an overloaded scaffold will ultimately fail and fall down.  This is not good for the occupants of the scaffold.  Besides, it really messes up the scaffold.

There you have it, the perfect scaffold.  It’s so easy to describe, but it seems so difficult to construct.  Maybe it’s because nobody is complying with US Federal OSHA Construction Industry Standards 29 CFR 1926.451(a)(6), 29 CFR 1926.451(f)(7)  and 29 CFR 1926.454, subsections a, b, and c.  Look them up; see if you agree with me.


By | OSHA Standards & Regulations, Resources, Safety Hazards, Scaffolding, Scaffolding Planks | No Comments

A friend with the Occupational Safety and Health Administration (OSHA) sent this sketch to me. He asked for my opinion concerning the safety of the installation and whether this scaffold complied with the OSHA Standards. What would you answer?


We can analyze this scaffold two ways:


1. Is this scaffold safe?

2. Is this scaffold in compliance with applicable standards?


Let’s look at the safety issue first. The platform is at two levels. Assuming that the frame is 5 feet tall and 5 feet wide, both platforms are less than five feet above the level below. Is this safe? If the platform is cantilevered over a boiling vat of sulfuric acid, you may want to consider fall protection. On the other hand, if the scaffold is in the local pillow factory, there probably isn’t any problem. What about the width of the platform? What about the relative relationship between the two platforms? What about having the lower platform supported by an intermediate horizontal support, typically a member that is about 1 inch in diameter? Is there safe access? What work activity is this scaffold supporting? These are questions that the competent person should ask when evaluating this scaffold.


Typically the scaffold industry accepts the fact that a work platform should be at least 18 inches wide, meaning that two 2×10 planks is the minimum platform width. (This fact is confirmed by OSHA, ANSI, and Cal/Osha.) It would appear that this scaffold meets those criteria. However, it seems that a hazard could exist due to the gap between the platforms, especially since these platforms are at approximately the same elevation. A worker, concentrating on his/her work, may quickly look over, see a platform, and think it is continuous; stepping backward would be disastrous!


An intermediate ledger supports the lower platform. Is this permissible? Sure it is, limited only by the strength of the supporting member. Under normal circumstances, this smaller diameter ledger will still be stronger than the typical platform. In fact, the beauty of these frames is that they can support platforms at different elevations, and they can support the side brackets at various elevations too.


Access via the frame is probably not possible due to the overhang of the plank. A clamp-on ladder or other safe means of access would be required. Finally, there is a danger of the scaffold over-turning due to an excessive load on the side bracket platform. The scaffold would require counterweights or a tie to another substantial structure to be sure that the scaffold would remain stable.


Analyzing the scaffold based on the applicable Federal OSHA Standards will produce similar results as the first analysis. This shouldn’t be surprising since the standards have been established to address hazards such as the ones already described. The most common question that is asked regarding a scaffold such as this one concerns the width of the platform. Regulation 29CFR 1926.451(b)(1) frankly states that “each platform on all working levels of scaffolds shall be fully planked or decked between the front uprights and the guardrail supports as follows…” If you think that this means that the platform should be across the full width of the frame, take another look at the side bracket platform. Where is the upright? Where is the guardrail support? The upright is behind the platform, and there is no guardrail support! What’s a worker to do? For that matter, what’s a compliance officer to do? Ask the questions: What is the intent of this regulation? What hazard does this regulation address? It’s simple, the regulation addresses fall protection. Since this platform is only 5 feet above the level below, there is no hazard, based on the OSHA fall protection standards. The standard, in particular 29CFR 1926.451(b)(2), requires that all frame scaffold platforms shall be a minimum 18 inches wide except in certain specific circumstances. The platforms on this scaffold comply with this regulation.


Do any standards address the intermediate ledger that supports the lower platform? The answer is found in 29CFR1926.451(a)(1) which specifies that “each scaffold and scaffold component must be capable of supporting, without failure, its own weight and at least 4 times the maximum intended load applied or transmitted to it.” As long as the live load applied to the ledger is no more than 25 per cent of the maximum load it can support, we are in compliance with the regulations.


As described earlier, access via the frame rungs/ledgers, is not permitted due to the assumed overhang of the plank. (If there is no overhang, and if the rungs/ledgers are spaced properly, the frame could be used as access.) As described in the standards, safe access includes stairs, clamp-on ladders, portable ladders, and direct access. Access between platforms would not be necessary if the horizontal distance between the platforms is less than 14 inches, and the vertical distance between platforms is no more than 24 inches. (See 29CFR1926.451(e)(8))


Finally, stability for this scaffold is addressed in the supported scaffold criteria sub-paragraph. Several regulations require that supported scaffolds, including frame scaffolds, shall always remain plumb and level.


In summary, this analysis shows that good construction practice can, in many instances, exceed the standards. This isn’t surprising since the standards are minimum requirements for the construction and use of scaffolds. The analysis also shows that:


1. The minimum width of a typical frame scaffold platform is 18 inches.

2. Workers on platforms more than 10 feet above the level below must have fall protection.

3. Platform fall protection may be required at lower heights, depending on exposure to hazards.

4. The platform does NOT have to extend all the way across the full width of the frame although this may be the easiest way to provide fall protection.

5. Access shall be provided for all platforms more than 24 inches above the level below.

6. Supported scaffolds must remain stable and plumb at all times.