rigging 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.

How Much More?

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

A description of the strength and safety of suspended scaffolds

How much more can we do? Will more standards increase the safety of suspended scaffolds? Will training increase the safety of suspended scaffolds? Will more safety features increase the safety of suspended scaffolds? Wait—is there a problem that needs to be addressed or are suspended scaffolds safe? It is always interesting when I first meet someone and tell them I am involved with scaffolding. Sooner rather than later they tell me how dangerous it all must be and how could I still be alive? And when I tell them that a properly constructed scaffold, including one hanging down the side of a building by ropes is completely safe, they just insist that I must not know what I am talking about.

If you look at the statistics, they are correct; unfortunately they missed the word “properly” when I say that a properly constructed suspended scaffold is safe. The injury and fatality statistics do indicate that there are issues with scaffolds, particularly suspended scaffolds, those scaffolds that have “one or more platforms suspended by ropes or other non-rigid means from an overhead structure.” A commonly seen example is the scaffold platform that is used by workers to perform maintenance to the face of a high rise building. And of course, it is also commonly seen on the evening television news when there is a scaffold failure—it makes great footage for the local and national news crews.

I thought it would be an educational exercise to see how many US federal OSHA scaffold standards specifically address and/or require safety items for a typical single or two point suspended scaffold. There are four primary components of a suspended scaffold, the rigging, the platform, the hoist, and the personnel safety protection. Each of these components has requirements that go beyond the basic function of that component. For example, the rope that the platform hangs from is much stronger than what is required to support the load. This is called the safety factor; its purpose is to make sure that the scaffold doesn’t collapse because an errant worker damaged the suspension rope while using the scaffold. So, here is the list I came up with—can you add to it?

Rigging: 6 to 1 safety factor on the rope (wow that is high!); 4 to 1 safety factor on the beams and counterweights (that’s high too!); Design by a qualified person; Installation under the supervision of a competent person; A tieback rope to hold the rigging in position and not let it slide off the roof; non-flowable counterweight; For multi-point suspended scaffolds, the design to be completed by a qualified Professional Engineer; When it is secured directly to the structure it shall be evaluated by a competent person; Counterweights shall be secured to the outrigger beam; Ropes shall be inspected before each workhshift (Actually the whole scaffold shall be inspected before each workshift); 3 wire rope clips shall be used (instead of the normal 2 required for rigging materials);

Platform: The normal width shall not exceed 36 inches; The fasteners securing the platform to the hoist shall have a 4 to 1 safety factor; The platform shall have a 4 to 1 safety factor; The platform shall not swing (In spite of the fact that we sometimes call them “swing scaffolds”); A guardrail system shall be installed on all open sides of the platform.

Hoist: The rope shall be long enough or have a stop so the end of the rope doesn’t pass through the hoist; The hoist shall be tested by a qualified testing laboratory; The hoist shall have an operating over-speed brake; The hoist shall have a 4 to 1 safety factor.

Personnel Protection: All occupants shall wear and utilize proper personal fall protection. The anchor for the vertical lifeline shall be independent of the scaffold rigging; The lifeline anchor shall hold 5,000 pounds or be designed by a qualified person to have a 2 to 1 safety factor; The employer shall provide for prompt rescue of employees in the event of a fall or shall assure that employees are able to rescue themselves; The user has to have training in the proper use of the scaffold, including all the safety items.

That’s quite a list. Let’s sum it up: The rigging, hoist and platform have to be really strong. The suspension rope has to be really, really strong. Not only does the platform have to have a guardrail system so the worker doesn’t walk off the platform, but the worker has to utilize personal fall protection so she doesn’t fall in case the platform disappears. In the event of that occurrence, the worker who is hanging around must be promptly rescued. (Yeah, like that always happens—that’s what makes the evening news so interesting. Nobody planned to get the guy down that’s hanging on the side of the building.)

So how can it be that suspended scaffolds fail? Everything has more strength than is required, the workers are trained, the equipment is constantly inspected for defects, and a rescue plan is in place. Perhaps the evening news should be more thorough in its investigation. You figure it out—do we need more rules or do we just have to use the ones we have?