scaffold foundation Archives | DH Glabe & Associates

What is the Foundation for the Foundation?

By | Aerial Lifts, Cantilever Beam, Resources, Scaffolding | No Comments

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.

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.

How Firm is It?

By | OSHA Standards & Regulations, Resources, Scaffolding | No Comments

There are regulations addressing the question. There are standards addressing the question. There are codes that address the question and there are industry customs and practice that address the question. But the real question isn’t how firm it is but rather what do we mean by firm? As in, scaffold legs shall bear on adequate firm foundation. It would seem that the statement (or more precisely, requirement) is somewhat of an oxymoron. Can a foundation not be firm? Can it be adequate but not firm? The Federal Occupational Safety and Health Administration (OSHA) regulation requires: “Supported scaffold poles, legs, posts, frames, and uprights shall bear on base plates and mud sills or other adequate firm foundation.”


At best, this regulation is confusing; at worst, misleading. To further confuse the issue, the sub-regulation mandates that: “Footings shall be level, sound, rigid, and capable of supporting the loaded scaffold without settling or displacement.” So, is a footing a foundation? What if the scaffold is supported by the sloping floor of a water tank? And how sound should it be? If nothing else, these regulations should tell the reader that the OSHA Standards surely are not instructions on how to construct scaffolds. A little research into the dictionary tells us that firm means solid, not easily moved or disturbed. That sure makes sense for a scaffold. So far so good, but what happens when we use a mud sill. If the sill is truly on mud, there goes our firm foundation. And the way the regulation is written, it sounds like that mud sill is firm all the time. If it’s sitting in the mud, it won’t be firm and besides, no matter how good the mud or the sill is, it still needs a foundation to support itself and the scaffold!


If you’re still reading, by now you probably figured out that the regulation addressing foundations wasn’t written too well. That’s why we need to look beyond the letter of the law and learn its’ intent. What is the intent? We don’t want your scaffold to fall over because your firm foundation became infirm. A scaffold leg on concrete doesn’t need a sill, unless of course you very recently poured the concrete. But that scaffold leg still needs a base plate. Obviously the leg won’t go through the concrete if it doesn’t have a base plate but the concrete can damage the bottom of the tube. Consequently, a base plate is required. Is a sill (notice I left out the mud) required when the scaffold, and base plate, is supported by concrete? No, but it’s not a bad practice to have a sill – it helps provide some friction and keep everything from sliding. Do we always nail the base plate to the sill? Not required, but why not hammer a nail in it; you won’t lose the sill that way.


For many of us, experience has demonstrated that if the scaffold is on soil, or dirt, (same thing except to soils engineers) a sill is required. The question is, how big should the sill be. The answer, in true engineering terms is; it depends. Hire that soils engineer and he/she can test the soil for you but that’s not too practical for most of us. How about using that other approach called the heel test. If you’re up to your knees in mud, chances are you won’t find a sill big enough to support the scaffold. But if your heel makes only a slight impression and the soil is compacted, you can be reasonably assured (the “reasonably” is lawyer jargon) that a sill 10 inches by 12 inches will function reasonably well. This rule of thumb (or heel) assumes the soil is compacted, protected from washout or other stability threatening environmental event, and the scaffold leg has no more than 2000 pounds bearing done on the sill. Remember, if you are not comfortable with your ability to make the decision about sills, get the help of a qualified person!


Now that we have briefly discussed a couple of things about base plates, sills, and firm foundations, read the regulation again. Think about its intent. Think about that scaffold leg load that has to be supported by the foundation. Does it need a sill and firm foundation to spread the load so we don’t lose the scaffold and hurt someone? Don’t forget, avoiding injury and death is the real intent of this regulation.