mobile scaffolds Archives | DH Glabe & Associates

Aerial Lift or Mobile Scaffold?

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

In the year 2000, at the turn of the century, the U.S. Federal Occupational Safety & Health Administration, OSHA, issued a Letter of Interpretation wherein it opined that aerial lifts known as scissors lifts (see illustration) are not aerial lifts but instead are mobile scaffolds.  The opinion was based on the fact that the revised OSHA scaffold standards for construction reference an American National Standards Institute, ANSI, standard that does not include scissors lifts.  This ANSI standard, known as ANSI A92.2-1969, was written in 1969, before the proliferation of aerial lifts that we have today.

Why was this outdated standard used, you ask?  Well, at the time the review of the original scaffold standard was initiated, A92.2-1969 was the ANSI standard that was applicable.  And since this was the only applicable standard, OSHA was required to work within the constraints of this standard to determine if scissors lifts were aerial lifts or not.  Since scissors lifts were not specifically mentioned in A92.2-1069, OSHA concluded that scissors lifts could not be included in the Aerial Lifts section of the revised standards.  However, OSHA also concluded that scissors lifts are scaffolds and therefore the other scaffold standards apply.  This includes 29 CFR 1926.451-General Requirements, and 29 CFR 1926.452-Additional Requirements for Specific Scaffolds.  Finally, OSHA concluded that since scissors lifts have wheels, they are Mobile Scaffolds and therefore must comply with 29 CFR 1926.452(w)-Mobile Scaffolds.  This interpretation of the standards relies on the accuracy of the assumption that scissors lifts are not aerial lifts and the assumption that scissors lifts are scaffolds.  Accepting these assumptions validates OSHA’s interpretation; not accepting these assumptions results in an entirely different conclusion.

I suggest an alternative interpretation to this dilemma since the industry generally recognizes that scissors lifts are aerial lifts and the Mobile Scaffold standards just don’t apply.  Here’s the argument:  The preponderance of information indicates that it was never intended for scissors lifts to be classified as Mobile Scaffolds.  A review of the preamble to the revised scaffold standards clearly indicates that the writers of the revised standard knew that additional ANSI standards existed: “OSHA recognizes that the A92 Committee has updated A92.2-1969 and has adopted other A92 standards which address technological advances and evolving safe industry practices regarding elevating and rotating work platforms.” (Federal Register, August 30, 1996, p 46095)  Furthermore, the writers also recognized the unique attributes of aerial lifts and the fact that they are just not the same as a typical supported or suspended scaffold.  How do I know that?  Besides stating in the preamble “…that the requirements of §1926.451 and §1926.452 do not apply to this type of equipment,”  the Scope and Application of Subpart L (29 CFR 1926.450(a)) clearly states that “The criteria for aerial lifts are set out exclusively in §1926.453 (Aerial Lifts) of this subpart.”  This exclusion is restated at the beginning of the General Requirements where it is confirmed that “This section does not apply to aerial lifts, the criteria for which are set out exclusively in § 1926.453.”  All this clarifies the applicability of standards but it does not necessarily clarify whether scissors lifts are scaffolds as described in § 1926.451 and §1926.452 or whether they are aerial lifts and consequently must comply with § 1926.453.  I believe the answer to this question exists within § 1926.453-Aerial Lifts and in the preamble for the revised standards.

1926.453-Aerial Lifts includes a note at the end of the section that points the reader to Non-mandatory Appendix C.  This appendix “lists examples of national consensus standards that are considered to provide employee protection equivalent to that provided through the application of ANSI A92.2-1969, where appropriate.”  Appendix C lists seven ANSI standards for aerial platforms, including ANSI A92.6-1990, Self Propelled Elevating Work Platforms. In case you are wondering, that’s the technical description for scissors lifts.  (See the illustration)  Furthermore, the OSHA writers explained in the preamble that “This Appendix is provided to serve as a guide to employers required to provide appropriate employee protection under § 1926.453, Aerial Lifts.  This Appendix reflects the proliferation of equipment-specific ANSI A92 standards since the adoption of ANSI A92.2-1969.”  Looks to me like a scissors lift is an aerial lift, not a Mobile Scaffold.

I can appreciate the constraints under which OSHA must operate.  The rulemaking process requires that the agency must comply with the legal restrictions that are in place to ensure that standards and regulations are not randomly (or intentionally) manipulated.  However, in this case, where it is clear in the industry that a scissors lift is an aerial lift, perhaps a little manipulation might be a good thing.  One last suggestion if I haven’t convinced you:  Read the Mobile Scaffold standards, § 1926.452(w)-Mobile Scaffolds and see how well they apply to a scissors lift.  The first standard requires that the scaffold “shall be braced by cross, horizontal, or diagonal braces, or combination thereof, to prevent  racking or collapse of the scaffold…Scaffolds shall be plumb, level, and squared.”  Does this make sense for a scissors lift?  How about “Where leveling of the scaffold is necessary, screw jacks or equivalent means shall be used.”  Or this one: “Caster stems and wheel stems shall be pinned or otherwise secured in scaffold legs or adjustment screws.”  And finally, “Before a scaffold is moved, each employee on the scaffold shall be made aware of the move.”  If you are operating the controls, do you talk to yourself?

The ANSI standard for scissors lifts, A92.6 is comprehensive, straightforward and very specific to scissors lifts.  Use this document.  You can purchase it directly from the Scaffold Industry Association at a very reasonable cost!

Hot Wheels

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

As with all scaffolds, there are design, construction, and safety issues with mobile scaffolds.  The idea here is to discuss some engineering issues, leaving the obvious safety issues to the “competent person, qualified in scaffold construction.”  Now that I think about it, perhaps the safety issues aren’t so obvious so let’s cover those first.  Make sure you have fall protection, falling object protection, access, adequate strength, a decent platform that remains in place, and don’t do something stupid.  Now that we have the safety features in place, proper design, in combination with proper use, makes the mobile scaffold such an excellent productivity tool.

What is it that makes the mobile scaffold safe, or conversely, unsafe?  The center of gravity, an engineering term that describes the stability of a mobile scaffold, is one significant factor.  Another factor is the strength of the casters and other components.  Another factor is the forces required to move the scaffold.  These forces are horizontal, vertical or both.  A qualified designer of mobile scaffolds must consider these factors, and of course the user of the scaffold must understand how to safely drive the scaffold (or at least push it around).

The Construction Industry scaffold standards from the Federal Occupational Safety and Health Administration, OSHA, address these issues as does both the American National Standards Institute, ANSI, scaffold standards and the Scaffold Industry, SIA, Codes of Safe Practice.  Specifically, the federal standards, of which the construction standards are the best source, identify the hazards described above, that is stability, strength, and dynamic forces.

What is the significance of the strength of the various components?  Well, I doubt you want the scaffold collapsing while you are on it.  Therefore you need to know your limitations.  The typical scaffold caster is usually the limiting factor.  Hallway scaffolds, those narrow scaffolds commonly used by drywall installers, have a capacity of about 250 pounds.  Frame scaffold casters, on the other hand, will have a capacity of approximately 500 pounds unless you buy one of those cheap casters of unknown capacity.  Larger frame scaffold casters, and those used with systems scaffolds will have a capacity in excess of 1,000 pounds.  These caster capacities are usually adequate for most mobile scaffold uses and are almost always less than the leg capacity unless, of course, you buy one of those cheap scaffolds of unknown strength.  The bottom line is to find out what your caster can hold before the ball bearings begin to fall out!

The stability of the scaffold is very important to the occupant of the scaffold for apparent reasons.  It’s just not a good idea to have the scaffold fall over, whether it is occupied or not.  How do we ensure that it won’t tip?  By making it big enough and not pushing it over.  If the mobile scaffold has a big enough base, both in width and length, the scaffold will remain standing, absent any other forces.  Except for California, the maximum height to base ratio is 4.  (In California it’s 3 to 1 and no, it’s not because they have earthquakes.)  This means the height can be no more than 4 times the minimum base.  For example, if you have a mobile scaffold that is 5 feet wide by 8 feet long, the maximum height is 5 feet times 4 equals 20 feet.  If you want to go higher, then make the base bigger.  But be careful – you may be overloading the casters because of all that extra scaffold weight.  The sky is the limit, no pun intended, but the higher you go the heavier it gets and pushing it around gets to be a real challenge.

How much does it take to push over a mobile scaffold?  The snappy answer is: not much.  The force needed to move the scaffold horizontally and the force needed to push it over are not the same although the untrained scaffold user may inadvertently be applying a force to knock it over all the while thinking that she is applying the force to move it horizontally on the floor.  Worse yet, if the casters aren’t rolling, due to maybe a small obstruction, a horizontal force at the top of the scaffold will quickly become a force that will knock the scaffold over.  In engineering terms, we call that instability.  For the user who is riding the scaffold down to disaster, it may be referred to in other terms.  Here is what is going on.  When you push against the side of the scaffold, you are trying to get the mass of the scaffold moving.  If you push close to the bottom of the scaffold, all your efforts will go to moving the scaffold.  As you push more, the scaffold slowly begins to move, converting a static (non-moving) condition into a dynamic (moving) condition.  The weight of the scaffold obviously influences the amount of force needed to get the scaffold moving.

Now, another factor comes into play here; the center of gravity.  The center of gravity is an imaginary point in the scaffold that is defined as the center point of all the vertical loads of the scaffold including the scaffold components, platforms, and the folks on the scaffold.  Typically, this point is in the middle of the scaffold but if there are cantilevered platforms the center of gravity will shift towards the direction of the cantilever.  If the cantilever is big enough, or the weight on the cantilever is big enough, or the folks on the scaffold are leaning out over the guardrail, the center of gravity shifts to the outside of the scaffold base, and the trouble begins.  The users get real excited because it is at this point that the scaffold begins to tip.  The same thing can happen when the scaffold is pulled along from the top by grabbing onto the roof trusses, for example.  While it may take a force of say 100 pounds to get the scaffold going, if the bottom isn’t going anywhere and the top is, the center of gravity begins to shift and the force needed to pull the scaffold over reduces to as little as 20 pounds; this is when the scaffold begins to tip.

Right about this time, the errant user has just experienced basic physics and now realizes the error of his ways. He begins to head to the other end of the scaffold in an attempt to makes things right.  Unfortunately he forgot to pin the casters into the scaffold leg and they fell out during the tipping maneuver;  the rest of the story gets real ugly.  And that is why the OSHA standards require that: “Manual force used to move the scaffold shall be applied as close to the base as practicable but not more than 5 feet (1.5 m) above the supporting surface.”  That is also why the standards also require you to pin the casters to the legs.

And what about surfing the scaffold—the technique of “jerking” the scaffold so it moves horizontally?  What do you suppose that does to the forces and stability of the scaffold?


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

People do the most amazing things. Take the “scaffold” application illustrated in the two photographs. The photos, contributed by Mr. Steve Schuler of Carl Schuler Masonry Construction Company in Waterloo, Iowa, depicts a scaffold constructed of lumber and mounted to a Ford Pick-up Truck. A cursory review of the two photos suggests that the scaffold might be in violation of applicable standards and industry practices. In fact, it’s common to quickly dismiss this scaffold as a ludicrous example of somebody’s idea of a safe scaffold. But wait; what exactly is wrong with this scaffold? Apparently somebody thought this scaffold is okay or they would never have constructed it this way. Let’s look at this wood scaffold (and perhaps a rolling tower) using a methodical approach to evaluating the scaffold so that all aspects are considered when determining the strength, safety, and workability of the scaffold.


A good place to start is by referring to the Five Most Serious Violations pertaining to scaffolds that has been developed by the Federal Occupational Safety and Health (OSHA) Training Institute:


• Unsafe Access

• Falls

• Electrocution

• Struck by Falling Objects

• Scaffold Collapse


What is the access? The photos do not illustrate any typical form of access, such as a ladder or stairway, for any of the platforms. While access could be provided directly from the building, there is no evidence this is the case. (Could it be that the access is mounted to another truck and the operator hasn’t arrived at work with it yet?)


How about fall protection for the workers. Fall protection is required by law for workers who are on scaffold platforms more than ten feet above the lower level. While the lower platform does not require fall protection, the upper platforms appear to be more than ten feet above the ground, thus requiring fall protection. Since there are no guardrails, it must be assumed that the workers will be wearing personal fall protection equipment. What do you suppose might be chosen as the 5000 pound anchor for the lifeline?


Electrocution does not appear to be a hazard, based on these photos. However, since the scaffold is mobile, a “rolling tower,” it is very possible that the scaffold can be driven to another location where interference with electrical lines could be a real possibility. Remember that the minimum distance the scaffold must be from 300 volt lines is 3 feet. Would the driver of this truck even be able to see the electrical lines, much less maintain the minimum distance?


Falling Object Protection is required whenever there is the possibility that objects may fall off a platform and hit a worker below. Typically, protection is provided by toeboards or barricades. Neither is visible in these photos. (In this case, it might be the scaffold itself that will fall and strike the worker below, based on these photos!)


The fifth serious hazard, scaffold collapse, addresses a multitude of sins, including, but not necessarily limited to, platform construction, foundation, component strength, stability, materials used, and bracing. The platforms appear to be more than 18 inches wide, meeting industry and OSHA standards, but are they strong enough to support the anticipated loads? Cleverly, the platforms appear to hinge up for some reason, perhaps to clear the local bridge abutments on the drive home. (Can it be assumed that the scaffold and truck are not driven home each night since it appears over height?) Note that when the scaffold is in position, it is supported by wood and concrete blocks under the bumpers and under the front and rear “outrigger” legs, effectively locking out the truck springs and providing the required stability. Good idea; too bad the erectors used wood blocks and concrete blocks as sills. So much for stability.


From an engineering standpoint, an analysis of the strength of this scaffold could become rather difficult and cumbersome. It is unclear as to how the erector could prove that a sufficient safety factor is being provided. Starting with the truck, the question immediately arises as to whether the truck is a half ton or three quarter ton pick-up. Maybe Ford Motor Company can give us some advice on this. The legs might be adequate since they appear to be 4×4’s with excellent bracing. Unfortunately the real strength is unknown because the connections are unknown. Additionally, since the loads that will be applied to the scaffold are unknown, there is no way to determine if the scaffold has sufficient strength. What grade is the lumber? That is a major factor in determining the strength of a wood scaffold.


As a rolling tower, or more precisely, a mobile scaffold, it can be assumed that the wheels are pinned to the legs but, are there “positive wheel and/or wheel and swivel locks?” (It’s assumed that the rear wheels don’t swivel but some Chevy owners might argue the point.) If one assumes that a power system is being used to propel this mobile scaffold, then one must also assume that the power system was “designed for such use.” (29CFR1926.452(w)(4). Can we assume there will be no riders of the scaffold? And finally, the question that begs the answer; what happens when the “outriggers” are raised and the scaffold is prepared for moving to the next location? That must be quite a sight!


By now, you may be thinking, why wasn’t this guy visited by OSHA? This installation was reported to OSHA in October, 1998, according to Mr. Schuler. As of December 22, 1998, the scaffold, with truck attached, or the truck, with scaffold attached, was still on the project, the same project where a crane tipped over and injured several workers. Where is the training, where is the competent person?


Rolling Safely

By | Resources, Scaffold Bracing, Scaffold Components, Scaffolding, Scaffolding Platforms | No Comments

Previous articles discussed scaffold regulations and scaffold training requirements, issues that obviously are important but don’t necessarily tell us how to erect or use a mobile scaffold properly.

Mobile scaffolds, also known as rolling towers (formerly known in the old OSHA regulations as manually propelled mobile scaffolds), are commonly used when access to heights is required for a short period of time. These mobile scaffolds work very well but unfortunately there are a surprisingly high number of accidents associated with mobile scaffolds. These accidents are generally due to misuse of the scaffold although improper assembly of the parts and pieces contribute to the accidents. Consequently, this article addresses both the proper assembly of the scaffold and the correct use of mobile scaffolds.


Important Assembly Points


Using manufacturers’ guidelines, applicable regulations, and common sense, here are the important things to remember when assembling a mobile scaffold:


• The casters should be double locking so that the wheel cannot turn nor can the caster rotate.

• The casters must be pinned or bolted to either the screwjack or to the frame leg.

• The casters must be strong enough to support the anticipated load. (Most common 8 inch scaffold casters have a safe capacity of about 500 pounds.)

• The screwjacks should extend up into the frame leg at least 12 inches, preferably more.

• The mobile scaffold must always be plumb and vertical. Adjust the screws as required.

• Never have more than 12 inches of adjustment between the bottom of the frame leg and the top of the caster.

• All of the frames must be pinned together.

• All of the frames must have cross braces attached firmly.

• A horizontal diagonal brace is to be installed as close to the bottom of the scaffold as possible. This brace keeps the scaffold square.

• The horizontal diagonal brace should be installed approximately every 20 feet vertically on scaffolds 5 feet wide and approximately every 12 feet on scaffolds less than 5 feet wide.

• Provide access to all platforms of the scaffold. This can be a built in ladder, a clamp on ladder, or even a stairway if the scaffold is a very large mobile scaffold.

• If clamp on ladders are used, install them on the width side of the scaffold, not the length side of the scaffold.

• The ladder should extend at least 36 inches above the top platform unless there is a hand hold above the platform such as an access gate panel or guardrail system.

• A full guardrail system, consisting of both top rail and mid rail must be installed on all sides of the mobile scaffold.

• A toeboard must be installed on all open sides of the mobile scaffold unless other means of falling object protection is provided.

• The scaffold height must never exceed three or four times the minimum base dimension, depending upon where you are working. For example, California requires the base to be at least one third the height of the scaffold. This means that on a scaffold that is five feet wide, the height is limited to 15 feet, measured from the ground or floor to the top of the platform.

• If outriggers are used to increase the width of the mobile scaffold, be sure that they are securely fastened to the frames and properly braced.

• All planks used for the platform must be secured from movement. Hook plank are the best choice since they don’t hang over the ends and can be easily secured from movement.



Proper Use of a Mobile Scaffold


Despite proper assembly of a mobile scaffold, accidents can easily happen due to wrong use of the scaffold. Here are some things you should do to make sure that the mobile scaffold is used safely:


• Make sure everybody that will use the mobile scaffold is trained in the proper use of mobile scaffolds.

• Do not modify the scaffold unless you know the regulations and know what effect the modification will have on the stability and safety of the scaffold.

• Watch out for power lines when moving mobile scaffolds from location to location. (Rubber casters are insulators; you are a great conductor!)

• Always push the scaffold as close to the bottom of the scaffold as possible, but no more than 5 feet above the base.

• Never, never, pull yourself along from the top of the scaffold while riding it.

• Don’t ever use powered means to move the mobile tower unless it has been specifically designed to be moved by that method.

• Always lock the casters before getting on the scaffold to work.

• Take care when climbing the mobile scaffold so that you don’t pull the scaffold over. If necessary, climb on the inside of the tower.

• Do not remove the guardrail system unless alternate forms of fall protection are provided. If you do remove the guardrail system, reinstall it before anybody else uses the mobile scaffold.



Riding Mobile Scaffolds


A lot of accidents occur because people ride mobile scaffolds while they are being moved. You are strongly discouraged from riding mobile scaffolds because of the high inherent danger involved with riding mobile scaffolds. Manufacturers, suppliers, and the Scaffold Industry Association strongly discourage this practice. However, OSHA allows people to ride mobile scaffolds under certain conditions. They include:


• The surface on which the scaffold is being moved is within 3 degrees of level, and free of pits, holes, and obstructions.

• The height to base width ratio of the scaffold during movement is 2 to 1.

• Outriggers, when used, are on both sides of the scaffold.

• When power systems are used, the propelling force is applied directly to the wheels and does not produce a speed in excess of 1 foot per second.

• No employee is on any part of the scaffold which extends outward beyond the wheels, casters, or other supports.


Using a mobile scaffold safely is the responsibility of all workers. Using the guidelines above, safety information provided by your supplier, and following the applicable regulations, will result in a safe work environment for you.