Hot Wheels: Scaffolding Safety

By David H. Glabe, P.E. / October 1, 2009

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?

Tags: Fall Protection Scaffolding Scaffolding Platforms American National Standards Institute mobile scaffolds OSHA OSHA Standards & Regulations Resources Scaffold Industry Association

Previous Post The Standard Standard
Next Post Random Numbers

David H. Glabe, P.E.

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