The Power of the Beam

By David H. Glabe, P.E. / February 1, 2011

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.

Tags: beam cantilever beam Resources scaffold suspended scaffold training

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David H. Glabe, P.E.

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