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Cantilever Beam

Do You Know Suspended Scaffolds?

By | Cantilever Beam, Hoists, Scaffolding | No Comments

Are you familiar with suspended scaffolds?  Do you know the difference between a suspended scaffold and a hanging scaffold?  Well, here’s a chance to show your friends and neighbors how well you know suspended scaffolds.  Take this quiz and see if you are the best of the best.

The answers are at the bottom of the page—no cheating!

 True or False

  1. ____A suspended scaffold is the same as a hanging scaffold.
  2. ____Outrigger scaffolds are one type of suspended scaffolds.
  3. ____You don’t need to utilize personal fall protection on a Multi-point Suspended Scaffold.
  4. ____Suspended scaffold users do not need any training if they are not operating the hoists on a suspended scaffold.
  5. ____Access is not required for a suspended scaffold.
  6. ____Counterweights for a cantilever beam can be ice or Jell-O.
  7. ____The safety factor for wire suspension ropes is at least 8.
  8. ____Counterweights cannot be used to stabilize outrigger beams on Mason Multi-point suspended scaffolds.
  9. ____Guardrails are not required on two point suspended scaffolds if all the occupants are wearing personal fall arrest   equipment.
  10. ___Guardrails or equivalent are required on Boatswains’ chair scaffolds.
  11. ___Outrigger beams secured directly to the roof do not require tiebacks.
  12. ___Suspended scaffolds shall be designed by a competent person and installed under the supervision of a qualified person, competent in scaffold erection.
  13. ___Vertical pickup means a rope used to support the horizontal rope in catenary scaffolds.
  14. ___Tiebacks only need to be one half the strength of the suspension ropes since they are there for back-up, not suspension.
  15. ___Sand can be used as a counterweight provided it is in a sealed strong metal container.

 

Now for the tough part, fill in the blank!

  1. When wire rope clips are used on suspension scaffolds, there shall be a minimum of ________ installed per connection.
  2. A stage rated for two workers or 500 pounds can support ________workers.
  3. Ropes shall be inspected for defects by a competent person prior to each ___________.
  4. Manually operated hoists shall require a _________crank force to descend.
  5. Wire rope clips shall be installed according to the __________recommendations.
  6. A two-point suspended scaffold is supported by _________ suspension ropes.
  7. Two-point suspended scaffold platforms shall not be more than ______inches wide unless it is designed by a ________person to prevent _________conditions.
  8. Suspension scaffold means one or more platforms suspended by _____ or other _______means from an overhead structure.
  9. The toprail of a suspended scaffold guardrail system must be able to withstand a force of at least ________pounds.

 

True or False Answers:

  1. False.  A hanging scaffold is constructed with rigid tubes while a suspended scaffold hangs from ropes.
  2. False.  Outrigger Scaffolds are a type of supported scaffold.
  3. True.  You need to install a guardrail system.
  4. False.  All scaffold users need training.
  5. False.  Proper access is required for all scaffolds.
  6. False.  The ice may melt and you might eat the Jell-O.
  7. False.  The minimum safety factor is 6.
  8. True.  The beams must be anchored to the supporting structure.
  9. False.  A guardrail system and PFE is required.
  10. False.  How do you attach a guardrail to a chair?
  11. True.
  12. False.  Suspended scaffolds shall be designed by a qualified person and installed under the supervision of a competent person, qualified in scaffold erection.
  13. True.
  14. False.  Tiebacks must be equal in strength to the suspension rope.
  15. True.  While not recommended, as long as the sand cannot leak out, it’s okay.
Fill in the Blank Answers:
  1. 3
  2. Depends on the weight of the workers.  You can put 5 on if they only weigh 125 pounds each.  Alternatively, if Bubba weighs 400 pounds, only he can be on it.
  3. Workshift.
  4. Positive.
  5. Manufacturer’s
  6. 2
  7. 36, qualified, unstable
  8. Ropes, non-rigid
  9. 100

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.

Kept in Suspension

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

A review of the standards proves that using a suspended scaffold, such as a two point suspension scaffold typically used for high rise building maintenance and repairs, isn’t any more dangerous than walking down the street that is below it.  In fact, I surmise that a perusal of statistics will show that walking down the street is probably more hazardous than working on a suspended scaffold.  Check out the standards; suspension ropes have to be six times stronger than the load that will be on them.  How can these ropes possibly break?  Obviously somebody is misbehaving.  Is it you?  How well do you know the standards?  Here is a quiz to test your understanding of the federal OSHA standards regarding suspended scaffolds, those scaffold platforms supported by ropes or other non-rigid means.  (Do you suppose the workers who installed the suspended scaffold in the photo would ace this quiz?)

True or False:

  1. The minimum safety factor for suspended scaffold components is 6.
  2. It is permissible to use u-bolts for rope connections supporting workers as long as there are at least three u-bolts.
  3. Outrigger beams can only stick out from the face of the building 24 inches.
  4. Multi-point suspended scaffold users are not required to utilize personal arrest equipment in addition to a guardrail system.
  5. All workers on a multi-level two point suspended scaffold must wear personal fall arrest equipment and be tied off to individual vertical lifelines.
  6. An adjustable scaffold is a suspension scaffold if the mechanical back-up anti-fall dog (level) is inoperable.
  7. All suspended scaffold platform users suspended over water must have life preservers.
  8. All suspended scaffold platform users suspended over water must have at least one lifesaving skiff.
  9. Counterweights for cantilever beams can be water, if frozen, or Jello®.
  10. Counterweights shall not be removed from the beam until the scaffold is disassembled.

Fill in the blank:

  1. All outrigger beams not stabilized by direct connections shall be secured by ________.
  2. When wire rope clips are used on suspension scaffolds, there shall be a minimum number of _________ clips per connection.
  3. Suspension scaffolds shall be inspected before each __________ by a ________ person.
  4. Two point suspension scaffold platforms shall be no more than ________ inches wide unless designed by a qualified person.
  5. The toprail height on a two point suspended scaffold shall be between ______ inches and ________ inches.
  6. The toprail strength on a two point suspended scaffold shall be at least _____ pounds.
  7. All suspended scaffold users shall be trained by a __________ person.
  8. The minimum anchor strength for the personal fall arrest anchor is _______ pounds unless designed by a ___________ person and maintains a safety factor of at least _______.
  9. If an outrigger beam cantilevers 24 inches beyond the fulcrum and the centerline of the counterweight is 12 feet behind the fulcrum, and the load on the suspension rope is 1,000 pounds, the counterweight must be ___________ pounds.
  10. All suspended scaffold installations must be done under the supervision of a _______ person _______ in scaffold erection, using _________and ___________ persons.
  11. Counterweights shall be secured by a _____________ means to the outrigger beam to prevent accidental displacement.

The answers are on page xxxx.  There is no pressure here but remember: you pass if you get all the answers correct and you fail if you get from 1 to 21 wrong; OSHA doesn’t permit partial compliance!

Answers

  1. F
  2. F
  3. F
  4. T
  5. F
  6. T
  7. T
  8. T
  9. F
  10. T
  11. TIEBACKS
  12. 3
  13. WORKSHIFT, COMPETENT
  14. 36
  15. 36 AND 45
  16. 100
  17. QUALIFIED
  18. 5000, QUALIIED
  19. 667 POUNDS
  20. COMPETENT, QUALIFIED, TRAINED, EXPERIENCED
  21. MECHANICAL

 

pic1-kept-in-suspension

Balancing Act

By | Cantilever Beam, Resources, Safety Hazards | No Comments

I recall being on a jobsite some years ago where I saw an incredible cantilever beam rigging system for suspended scaffolding.  Okay, it was a lot of years ago.  But anyway, it was a 4×4 wood timber about 12 feet long stuck out over the edge of the roof about a foot or so.  On the cantilevered end was a ¾” manila rope hanging down 10 floors.  On the other end of the timber were a couple of bags of sand.  Actually, this wasn’t the incredible part.  The incredible part were the two guys who climbed over the parapet on the edge of the roof down to the platform that was connected to the manila rope.  They were old guys!  Frankly, I didn’t think that anybody that used rigging like this lived long enough to be old.

Fast forward to about seven years ago inChicago.  I was riding the “El” downtown from the airport enjoying the sights when we stopped at a station not far from where my great grandfather built carriages and wagons.  There on an apartment building was a two point suspended scaffold.  Yep, you guessed it; cantilevered timber beams with manila suspension ropes.  Nobody was around at the time so I have no clue what they were using for fall protection but one can only wonder.

Fast forward a couple of years to a few years ago.  Mom, being the conscientious mother she always was, liked to take pictures of various scaffolds and send them to me.  She got pretty excited when she discovered that there was a scaffold hanging on the side of her building where the masons were tuck pointing the walls.  I got pretty excited when I saw the pictures and noticed that the guys were hanging by frayed manila ropes—well actually the scaffold was hanging by frayed manila ropes, and these guys had no fall protection!  Incidentally, the masons got pretty excited when Mom told them I taught OSHA safety.  You gotta love moms.

What’s the point here?  Well, first Mom was quite a person.  Second, it’s amazing that after 35 years of OSHA regulations, we still have workers using suspended scaffolds that are not only unsafe but don’t come close to complying with safety standards.  I’m not suggesting that you can’t use manila rope or wood; I am saying that you must know what is safe and not safe before jumping over the side of the building.

Outrigger beams are always an interesting subject because of the ingenuity of the erector.  My observations over the years have taught me that some people just have no clue about loads, forces, and what is safe.  Take counterweights for example.  Sand bags and other similar flowable weights are just not a good idea if the contents can flow out.  A cantilever beam without a counterweight won’t be a cantilever very long.  How about brick and block.  This type of counterweight seems to be appealing due to its availability and ease of handling.  Might be appealing but what happens when the mason decides he needs your counterweight to complete the wall, or worse yet, your clever counterweight falls off your clever beam.  I suspect you won’t feel so clever.  There’s a reason we want purpose built counterweights of known weight with a mechanism to positively attach them to the beam and it isn’t to make the system more expensive.

The cantilever beams, also known as outrigger beams, brings out the creativity of supposed rigging experts.  I suspect that some of these renown experts assume that if it is made of steel and looks like an “I” beam, then it should work.  What they apparently don’t understand are things like the bending stress, stability, section modulus, unbraced length and other slick engineering concepts that determine the suitability of a beam for a given situation.  Can you use wood?  Of course you can, if the compression perpendicular to grain, the bending stress, unbraced length, and the section modulus are within safe limits.  Don’t know how to determine those things?  Then don’t guess.  Same holds true for steel or aluminum.  What about the portion that sticks out beyond the face of the building.  Some erectors assume that if it doesn’t bend too much and appears to be safe, it must be safe.  Don’t be so hasty in making that snappy decision.  A steel beam can take a lot of punishment before suddenly failing.  I’ll bet you wouldn’t want to be relying on that beam when it decides not to support you anymore.  The longer the cantilever, the more complicated the situation becomes.  Stresses and factors that are insignificant with small cantilever become critical factors in longer cantilevers.  Again, if you aren’t aware of these changing factors, don’t tempt fate.

The Scaffold Industry Association (SIA) publishes suspended scaffold safety codes and survey sheets that can be used to calculate the magnitude of the counterweight for a given cantilever and load.  This is good information to have for an outrigger system; but it is only one part of the required analysis.  Don’t forget that beam.  After all it’s what keeps you swinging.