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cantilever beam 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.

The Power of the Beam

By | Cantilever Beam, Resources | No Comments

The Amazing Inside Story of how a Cantilever Beam Works!

Cantilever beams, also known as outrigger beams, are frequently used to support the end of a rope from which a suspended scaffold hangs.  Have you ever wondered how that beam works, especially if you are on the other end of the rope?  Well, here’s the story.

A cantilevered beam is one component of an assembly that consists of a number of parts and pieces that provide the necessary support for the loads that are hanging on the rope.  The beam is designed to use the advantage of leverage; this reduces the amount of force on the rear end of the beam (that would be the other end from where the rope is connected).  Of course, the beam cannot do the work alone.  It must have support towards the front end and the rear end.  The support at the front is called the fulcrum or front support (that’s clever engineering jargon).  The cantilever of the beam is measured from this front support to the point of rope attachment.  This is a critical dimension since the beam has to be strong enough to transfer the load from the rope back across the fulcrum and then to the rear end.  At the rear end is the other support.  Yep, you guessed it, it’s the rear support, also known as the “inboard end”.  This is where the counterweight is located or where the beam is connected directly to the supporting structure.  Now, in order for the whole system to work, the counterweight has to be big enough, if used, or if the beam is attached directly to the structure holding everything, then the connection has to be strong enough and the structure has to be strong enough.  So, how strong does it have to be, you may ask?  Well, strong enough.

Actually, this is where it gets interesting.  The fulcrum load can get rather large, depending on how much the beam sticks out.  And the counterweight can get pretty big too, particularly since you need four times what is actually required to keep the beam from going over the edge of the building.  Incidentally, don’t tell the erection crew that they are carrying 4 times the required counterweight up the stairs; you’ll have a mutiny on your hands.  Other than make the erection crew work harder, there is a very good reason for the extra counterweight.  In engineering terms it is called the safety factor.  In laymen’s terms, the extra counterweight is for typical jobsite screw-ups, such as overloading the suspended scaffold.

Where can the system go wrong?  Unfortunately, there are several places where the unqualified designer can make a fatal error.  First is in the supporting structure.  If the cantilever beam is installed on the roof, the roof has to hold the load.  I’m always surprised how casual some people can be about the strength of a roof, particularly on an older building or one where the maintenance is lax and structural damage has occurred.  While a structural analysis of the roof is typically not within the scope of the typical scaffold installation, it is also typical that the individual charged with the investigation of the roof’s strength will need an accurate submittal of load information as a result of the scaffold loads on the cantilever beam.

The second opportunity for a fatal error is with the beam itself.  The beam has limits.  Just because the beam is 16 feet long doesn’t mean you can cantilever it 8 feet, or for that matter 15 feet.  Funny things start to happen as a beam is cantilevered; the beam likes to wander sideways out there at the front end where the rope is connected.  While most people expect the beam to deflect, that is, start to droop (another one of those engineering terms) few people expect it to wander.  Unfortunately, like an unsupervised teenager, if it wanders too much it gets into big trouble.  Depending on the shape of the beam, too much sideways wandering can make the beam roll, deflect vertically and fail.  If the scaffold users are lucky, the beam will just fold over and the scaffold occupants will wind up on the evening news.  If unlucky, the beam will break and the scaffold will collapse and fall to the street below.  The good news in this scenario is that assuming the scaffold users are utilizing personal fall arrest equipment, like they are supposed to, they’ll be saved from the fall but will still wind up on the evening news.  Hopefully nobody on the street below will get hit by falling debris.

The third possible fatal error is losing the rear end support.  If counterweights are used, they must be mechanically connected to the beam; that is, the counterweights cannot be precariously stacked on top of the beam or haphazardly wired to the beam.  In fact, the counterweights must be specifically designed for the beam and the connections.  If the beam is directly connected to the supporting structure, not only does the connection hardware have to be strong enough but the roof structure components must be able to support the load.  In many cases this will require the services of a qualified Professional Engineer.

The fourth fatal error involves the lack of knowledge of the designer, erector and/or user.  If any of these participants does not have the training and expertise to correctly complete his or her obligations to the project, disaster can occur.  Qualified design is essential; correct installation, according to the design, is imperative; pre-workshift inspections of the rigging are crucial, and; correct scaffold usage, by trained workers, is critical to the safety of the project.

The fifth fatal error, which follows from the fourth fatal error, is lack of training.  All the equipment in the world won’t save you from an early demise if you do not know how to use it.  Training is the key!  And, where can you get that training?  Go to www.scaffold.org for starters.