Archive for the ‘Scaffolding’ Category
April 9th, 2013 by David H. Glabe, PE
Properly Identifying Scaffold Types and Foundations
Is your scaffold supported? I sure hope so since an unsupported scaffold isn’t much of a scaffold. Can a scaffold be supported but yet not supported? Yes it can but that’s a dumb question. Obviously all scaffolds require support, some from the ground and some from above, such as the roof of a building. Do we care if a scaffold is supported but yet not supported? Well, you should since you need to know the type of scaffold you have if you expect to have a safe scaffold and one that is in compliance with recognized standards. The big question is: how are scaffolds classified?
There are three types of scaffolds: Supported Scaffolds, Suspended Scaffolds and Aerial Platforms. Supported Scaffolds are temporary elevated platforms that are supported by rigid means such as legs, pole or uprights. Suspended scaffolds are temporary elevated platforms that are supported by non-rigid means such as chains or ropes. Aerial platforms, or at least aerial devices are described by the American National Standards Institute, ANSI, as “Any device, extensible, articulating, or both, which is primarily designed and used to position personnel. The device may also be used to handle material, if designed and equipped for that purpose.”
When determining what type of scaffold you may have, and of course you want to do this so you know which OSHA standards apply, you need to ask yourself a few questions. The first question to ask is probably the most important: Is the elevated platform actually a scaffold? This is important since the answer determines which regulations and standards apply. If your platform isn’t elevated, it’s not temporary, and it isn’t supporting employees or materials or both, it isn’t a scaffold. On the other hand, if you’re standing on a table attempting to change a light bulb, and your boss told you to do it, then you have a scaffold. In fact, you have a Supported Scaffold since the platform (the tabletop) is being supported by rigid legs. Of course, I’m sure you used a ladder to get to the table top since you certainly don’t want to violate any safety standards.
Once you have determined that your platform is a scaffold, then you need to determine if it is a Supported Scaffold, a Suspended Scaffold of an Aerial Platform. As stated earlier, if the platform has a rigid support, it is a Supported Scaffold. Besides tables, other examples of Supported Scaffolds include Tubular Welded Frame Scaffolds, Systems Scaffolds, Tube & Coupler Scaffolds, Pump Jack Scaffolds, Horse Scaffolds and Tank Scaffolds.
If your platform is elevated and supported by a rope or two, then you have a Suspended Scaffold. Examples include the porch swing your neighbor stood on to knock down the wasp nest, Single and Two Point Suspended Scaffolds such as a window washer’s scaffold, Catenary Scaffolds and a Multi-point Suspended Scaffolds. Remember that a Suspended Scaffold is a temporary elevated platform supported by non-rigid means. Besides wire rope this can also include chains, hemp rope, clothes line (if it’s strong enough), and slings.
As with any categorization, there has to be an exception or two, something to get everyone confused. And since I don’t want to disappoint you, scaffolds have their exceptions. The first, which isn’t really an exception but more of a clarification, is the hanging scaffold. A hanging scaffold is a scaffold normally constructed of rigid metal tubes and is hanging down from a structure above. While the usual assumption is that this type of scaffold is a Suspended Scaffold such is not the case! Since it is constructed of rigid tubes, it is a Supported Scaffold even if it is “suspended” from above. These scaffolds are referred to as Hanging Scaffolds or Hung Scaffolds.
Masons’ Adjustable Supported Scaffolds are the common exception to the supported/suspended classification. A Masons’ Adjustable Supported Scaffold has a platform that is supported by suspension ropes and a redundant (back-up) tower support. The suspension ropes are supported by rigid towers constructed of metal tubes. This results in a tower that is both a Suspended Scaffold and a Supported Scaffold. Consequently both the OSHA Construction Supported Scaffold Regulations [29 CFR 1926.451(c)] and the OSHA Construction Suspended Scaffold Regulations [29 CFR 1926.451(d)] may apply, depending if the platform is supported by ropes only or also by the tower itself.
You are in luck if the temporary elevated platform and its supporting structure is a Boom Supported Elevating Work Platform (boom-lift), Self-propelled Elevating Work Platform (scissors lift), a Vehicle-Mounted Elevating and Rotating Aerial Device (truck mounted boom lift), or a Mast-Climbing Work Platform (mast climber), because neither the Supported Scaffold nor the Suspended Scaffold regulations apply. In fact, you need to use the specific American National Standards institute (ANSI) standards for these devices.
Supported Scaffold or Suspended Scaffold or Aerial Platform, you get to decide which one it is by what is holding up the platform—it’s that easy. However, if you are using a table as a scaffold, you may want to rethink that idea. If your platform is suspended from your mother’s clothesline you really want to rethink that one. Want help in identifying the type of scaffold you’re dealing with? The best way is to get proper scaffold training from a qualified trainer.
February 28th, 2013 by Donald McCuskey, P.E.
An Introduction to Panel Points and Why They are Important
After reviewing many user manuals for scaffold, it is clear that scaffold manufacturers understand the strong and weak locations of each of their respective systems. However, I have not seen any scaffold user or technical manual attempt to explain to their users why they always show scaffold tie placements at the intersection of horizontal and vertical diagonal members (typical for system scaffold). In this brief article, I’ll attempt to demonstrate some reasoning behind tie location placement. We will be introducing the concept of “panel points” and their importance in scaffold construction.
The typical definition of a panel point is “the point of intersection where a web (or webs) meets a chord”.
Okay, that probably doesn’t help you much. See Figures 1 & 2 for a better graphical representation of what we’re talking about here (note the deflected shape of the scaffold for effect). As the above definition is generally used for structural trusses, a better way to define panel points for scaffolding would be as follows: “the intersection where horizontal bearers, runners, or vertical diagonals meet a vertical leg”.
Now that you know what a panel point is, let’s dig into why it is important. The magic (physics) of a panel point is straightforward. If a load is applied directly to the panel point, the supporting members are loaded primarily in tension or compression with little or no member bending forces. Members have larger capacities when they are only required to transfer tension/compression forces compared to bending and tension/compression forces.
As a simple illustrative example, pretend you are a scaffold leg. Now pretend you need to hold Dave Glabe’s prized “D. Victor Saleeby Bronze Eagle Award” over your head, say 150 lbs (Dave feeds his award three solid meals a day and it has reached an impressive girth). In this instance, you would be a compressive member transferring weight of the bronze Eagle through your body and into the ground. If you wanted to mimic a combination compression and bending member, have someone tie a rope around your waist and pull on you. Not so easy to hold that 150 lbs. eagle over your head now, is it? With this image in your head, you are now one with the scaffold leg and a step closer to scaffold enlightenment!
So remember, panel points keep scaffold members in tension/compression, not bending plus tension/compression. Therefore, slapping a significant load bearing tie in the middle of a 10 foot scaffold tube would probably not be the best idea! Attaching or directing a load to a panel point is better than attaching or directing a load at any other spot. Of course, we are talking about scaffold in general terms and there are always exceptions. Don’t think this article gives you the knowledge to do anything in violation of the OSHA standards or the manufacturer’s recommendations. When in doubt, always consult with a Qualified Person.
February 4th, 2013 by David H. Glabe, PE
Should the scaffold you just erected in the local oil refinery comply with the US federal Occupational Safety & Health Administration (OSHA) General Industry Standards or the Construction Industry Standards? While you might think this is a question with an easy and simple answer, it isn’t. Does it matter which standards apply? That question has two answers: from a compliance standpoint, yes. From a safety standpoint, not so much.
Erecting a scaffold in an existing oil refinery would suggest that the General Industry standards would apply since the refinery is completed and not being constructed. This would be an incorrect conclusion: Determining which set of standards apply to a specific situation is much more complex. While OSHA has a definition for “construction,” there is no definitive regulatory description of “maintenance” or “general industry” where it would be clear to the employer as to which standards to utilize for ensuring compliance but more importantly, that the workplace is safe for his/her employees.
OSHA defines “construction work” as: “work for construction, alteration, and/or repair, including painting and decorating.” [29 CFR 1910.12(b)] Additionally, OSHA points out in one of its Letters of Interpretation (LOI): “Also relevant to the distinction between construction and maintenance are the Davis-Bacon Act regulations. In essence, 29 CFR 5.2(i) defines construction work as ‘generally includ[ing] construction activity as distinguished from manufacturing, furnishing of materials, or servicing and maintenance work.’” [LOI-Knobbs, Nov. 18, 2003] A dictionary definition of “construction” is: “The act or process of putting together parts.” [Webster, 2000]
In the Knobbs LOI, OSHA refers to a definition for “maintenance” that was in one of its directives by stating: “In OSHA’s directive on the general industry confined space standard, the Agency stated that maintenance involves ‘keeping equipment working in its existing state, i.e., preventing its failure or decline.’” (emphasis added) [LOI-Knobbs, Nov. 18, 2003] In a LOI dated February 1, 1999, OSHA describes maintenance: “Maintenance means keeping equipment or a structure in proper condition through routine, scheduled or anticipated measures without having to significantly alter the structure or equipment in the process.” While this may clarify the difference between “construction” and “maintenance” in OSHA’s mind, it does little to clarify it for the typical scaffold erector or his/her employer. Research indicates that OSHA has wrestled with the issue for some time since there is more than one Letter of Interpretation addressing the topic.
Before discussing what might describe a work activity as “construction” or maintenance,” it would be instructional to describe which factors do not determine whether the work activity is “construction” or not. First the name of the company has nothing to do with the matter. For example, just because your company is called Dave’s Construction does not mean that all your work is construction. Second, who the employees are has nothing to do with determining the applicability of the standards. Third, whether it is “in-house” employees or an outside contractor has nothing to do with the type of work being performed.
“Construction is not limited to new construction, but can include the repair of existing facilities or the replacement of structures and their components,” declares OSHA [ibid] The project’s scale and complexity must be considered in making a determination. The physical size of the object that is being worked on can be a factor. It can be considered construction “because it is a complex task in view of the steps involved…” [ibid] Probably the most clarifying statement is OSHA’s declaration in the Knobb LOI that “it is not the personnel which will determine whether work will be considered maintenance or construction, but the work itself.” In the same LOI, OSHA points out that while the work may be done during a scheduled “maintenance outage,” that alone will not qualify it as maintenance.”
Finally, in a LOI dated August 11, 1994, OSHA tells its Regional Administrators “where an activity cannot be easily classified as construction or maintenance even when measured against all of the above factors, the activity should be classified so as to allow application of the more protective 1910 or 1926 standard, depending on the hazard. In such cases the situation should be issued in the alternative with the emphasis on the more protective standard.” Wow! That appears to say that if you are not sure, find a standard that fits the situation, no matter where it comes from. How bizarre can it get? Here’s how.
In a Letter of Interpretation dated April 17, 2006, OSHA was asked if permanent guardrails that were 36 inches high would suffice during construction in a “General Industry facility.” The existing facility apparently had guardrails on its platforms that were an acceptable 36 inches high, in compliance with the General Industry requirements for guardrails. However, construction was going to occur at this same facility. Subpart M of the Construction Standards requires that guardrails shall be installed between 39 and 45 inches. Consequently, in this instance, OSHA stated that a temporary guardrail would have to be installed adjacent to the lower existing permanent 36 inch high guardrail to protect employees during the construction activities. Justification for this requirement was that “construction activities often include carrying tools and materials that are heavy, awkward to handle, and, in the case of large materials, can sometimes block the employees’ view.” In other words, if the employee is blindly stumbling along dragging heavy tools, he better have a higher guardrail. Otherwise, it’s okay to have a lower guardrail for the permanent employees of the facility. Amazing.
Keep in mind that the OSHA standards are minimum requirements and they are promulgated to address safety hazards in the workplace. Since it is OSHA’s standards, it can manipulate and interpret them in any way it chooses. However, for your protection, do what is right to protect yourself and your fellow employees, not only from hazards that may cause serious injury or death, but also from ridiculous interpretations, and ultimately, ineffectual citations.
December 17th, 2012 by David H. Glabe, PE
Is a mast-climbing work platform a scaffold? This is a common question due to conflicting information provided by OSHA. The quick answer to the question is yes, but determining the applicability of OSHA standards for mast climbers requires a bit of research, a real understanding of what a mast climber is, and common sense.
While developing the revised scaffold standards that went into effect in 1996, the evidence indicates that federal OSHA determined that aerial work platforms, as described in the ANSI A92.2-1969 standard, are in fact scaffolds.
The original 1971 OSHA standards placed the aerial work platform regulations in Subpart N, Cranes and Derricks. However, the SAIA pointed out that since OSHA determined that aerial work platforms are scaffolds, the aerial work platform standards should be included in the revised scaffold standards. OSHA agreed and placed the aerial lift standards in a new section recognizing that aerial lifts were a unique type of scaffold. The Scaffold Standards Scope and Application, 29 CFR 1926.450(a) states: “The criteria for aerial lifts are set out exclusively in 1926.453 of this subpart.” This is also stated at the beginning of the Scaffold General Requirements, 29 CFR 1926.451: “This section does not apply to aerial lifts, the criteria for which are set out exclusively in §1926.453.”
In addition, it is acknowledged in the preamble to the 1996 revised scaffold standard that: “OSHA recognizes that the A92 committee has updated A92.2-1969 and has adopted other A92 standards which address technological advances and evolving safe industry practices regarding elevating and rotating work platforms. The Agency has determined that compliance with the pertinent A92 standards adopted by ANSI since 1969 will provide employee safety at least equivalent to that attained through compliance with ANSI A92.2-1969.” This opinion was restated in a note to § 1926.453 (found at the end of section 1926.453): “Non-mandatory Appendix C lists examples of national consensus standards that are considered to provide employee protection equivalent to that provided through the application of ANSI A92.2-1969, where appropriate.” Simply stated, if you comply with the ANSI standards, OSHA will recognize that you are working safely, and you will be in compliance with 29 CFR 1926.453.
If you are using a mast climber and complying with the requirements of the specific ANSI A92.9 standard for mast-climbing work platforms the assumption is that your company is in compliance with the OSHA standards because of OSHA’s note concerning mast-climbing work platforms in the mandatory standard, 1926.453. Furthermore, A92.9 is listed in Non-Mandatory Appendix C of the OSHA scaffold standard and included in OSHA’s note.
Sounds easy, right? Not so fast.
On Aug. 1, 2000, OSHA issued a Letter of Interpretation arguing that scissor lifts are not aerial work platforms but are Mobile Scaffolds, a concept that is based on the fact that scissor lifts were not included in the A92.2-1969 standard; this is a theory that conflicts with the industry’s understanding and categorization of scissor lifts as an aerial lift demonstrated in the ANSI A92.6 Self-Propelled Elevating Work Platforms standard.
It then follows that since mast climbers, like scissor lifts, are not listed in A92.2-1969, and using OSHA’s flawed logic, mast climbers are not aerial lifts and must comply with the scaffold regulations in 29 CFR 1926.451 and 452. While this is a ridiculous proposition, and contrary to the statements in the Scope and Application of the scaffold standard, let’s see what happens when applying an alleged applicable OSHA standard.
First, which OSHA regulations apply? Using Figure 1 as an example, you can see that there are wheels and a mast; perhaps it can be considered to be a Mobile Scaffold. One OSHA Mobile Scaffold standard requires that “platforms shall not extend outward beyond the base supports of the scaffold.” Note the platform extensions in Figure 1—looks like we have a problem! How about the requirement that “caster stems and wheel stems shall be pinned or otherwise secured in scaffold legs”? This doesn’t seem to work so well either
Now consider this mast climber to be a regular Supported Scaffold, rather than a Mobile Scaffold. Perhaps these standards are a lot easier to use. These regulations require that the scaffold has to be tied to the adjacent structure at each end and every 30 feet in between. Since there is only one mast, that would mean at each corner. Vertically, a tie isn’t needed until the mast is four times higher than the minimum width. This typically does not conform to the manufacturers’ requirements and will probably result in mast failure. Without belaboring the situation, you can see there are problems attempting to apply non relevant standards.
The solution is obvious, simple and has already been addressed by OSHA in § 1926.453. Since OSHA’s note in § 1926.453 accepts the ANSI standards as applicable and acceptable standards, not only will you be safe, you will also be in compliance with the OSHA standards assuming you apply the appropriate ANSI standard. It’s as easy as that! If anybody tells you differently, they are wrong: Refer them to A92.9 and tell them to carefully read the note at the end of § 1926.453. If you don’t have the applicable ANSI A92 standard, download a copy from www.saiaonline.com for a nominal fee.
Figure 1- Mast Climber: Single Mast
November 8th, 2012 by David H. Glabe, PE
While some may believe there are no useful regulations, and certainly the actions of some scaffold users would confirm that belief, the OSHA standards (regulations) governing how we use scaffolds are actually quite, well useful. Included are exciting minimum requirements such as who is to inspect scaffolds and who is to supervise the construction of scaffolds. If you are a user of scaffolds, don’t stop reading! Here’s a highlight of what you will find when you read section 29 CFR 1926.451(f) – Use of Scaffolds:
(1) Don’t overload scaffolds. If you don’t know how strong a scaffold is, or how much stuff you are placing on the scaffold, it is impossible for you to comply with this easy to understand regulation.
(2) Don’t use lean-to or shore scaffolds. If you think shore scaffolds are only used along the seashore, you’re all wet. Imagine taking a sawhorse, cutting it in half and leaning both halves against a wall, about 8 feet apart. Throw some planks across the top and you have a lean-to scaffold. Don’t do this.
(3) Have a Competent Person inspect the scaffold prior to each workshift. This one is serious; inspections help spot any improper modifications and developing problems that may exist. Remember, a Competent Person can identify hazards and has the authority to do something about it.
(4) Repair any busted scaffold components or have them removed from use, before getting on the scaffold.
(5) Don’t move your scaffold horizontally with folks on it unless it has been specifically designed for that use by a qualified registered professional engineer. This doesn’t apply to Mobile Scaffolds which, under certain conditions, can be moved with or without folks on them.
(6) Don’t get too close to power lines. If the hum is too loud or your hair is standing on end, you are too close! As a rule of thumb, 3 feet up to 300 volts, 10 feet up to 50,000 volts and an additional 0.4 inches for every 1,000 volts above that. I have no idea how you would measure that although I strongly discourage using a metal tape measure!
(7) Erect, dismantle, move, or alter a scaffold only under the supervision of a Competent Person, Qualified in scaffold erection, using trained and experienced workers. This is important. If you don’t know how to erect scaffolds, don’t assume you’re an expert—you’re not. And don’t screw with the scaffold; ask for help.
(8) Slippery scaffolds can be dicey. Unless you are using the scaffold for a ski jump, stay off it until the slipperiness is removed. Of course, if you are the slipperiness removal technician, then get up there and remove the oil, ice or whatever is making it slippery.
(9) If you are swinging hoisted loads, don’t let them hit the scaffold. You would think this is logical but apparently not if we have a regulation for it.
(10) Make sure the suspension rope matches the hoist and brake size. Brakes sized for a ¾ inch rope won’t stop if you have a ¼ inch rope. Oh-oh!
(11) Don’t burn, melt or eat the suspension rope holding you.
(12) Don’t work on a scaffold in storms or a high wind, as determined by your Competent Person. If the wind is so high that you think you will be blown off the scaffold, then you must utilize personal fall protection or put up a windscreen. (really, it actually says this!)
(13) Don’t let stuff (debris) pile up on the platform.
(14) Don’t use upside down 5 gallon buckets (and other similar items) to increase the height of the scaffold. Here’s a novel idea: if the scaffold isn’t high enough, have a trained and experienced erector build it higher.
(15) Don’t use ladders on scaffold platforms unless the scaffold platform is big enough to provide stability against overturning forces from the ladder, the platform is secured to prevent movement, the ladder is stabilized due to platform deflection, and the ladder legs are secured so they don’t come off the platform (duh). (Of course, you could have the trained and experienced erector from # 14 build the scaffold higher.)
(16) Wood platform plank cannot deflect more than 2 inches for a 10’-0” span, 1-3/8 inches for a 7’-0” span and 1 inch for a 5’-0” span. If the deflection is more than that, you either have too much load on the plank, or you have crummy plank, or both. If you don’t know how much weight you can put on the plank, reread # 1 and get help.
(17) Don’t weld from a suspended scaffold unless you know what you are doing. This means you know what precautions must be taken to protect the scaffold so that you don’t burn off the ropes holding you in the air, you don’t fry the hoist, and you don’t melt the aluminum components. If you haven’t been trained in the necessary rigging techniques, stay off the scaffold with your stinger; you may kill yourself.
As I stated earlier, these are highlights of the minimum requirements for the safe use of a scaffold. Don’t assume this covers everything that you may be doing on the scaffold. Rules and regulations do not make up for the stupid stuff you may do. Just because it isn’t listed in the standards doesn’t mean it is safe. If you are unsure about whether you are working safely, then get off the scaffold and get training, or retraining. Scaffolds are safe when constructed correctly. It’s the untrained user that will make that scaffold unsafe.
October 4th, 2012 by David H. Glabe, PE
Tarps and other enclosure materials, such as plastic sheeting, are typical materials used to create a desirable work atmosphere. Many scaffolds are enclosed in screening and debris netting—I recall one resort project in Aruba where the scaffold was wrapped in a mesh to ensure, so I was told, that construction debris would not blow into the adjacent swimming pool. In reality it was there so the guests below couldn’t see the less than productive construction workers staring at them! And, of course, now that outdoor temperatures in North America are slowly falling, thoughts of a cozy work environment on a supported scaffold become more frequent, resulting in more scaffolds being wrapped in some type of enclosure so that work can continue. It is interesting that wrapped scaffolding has been frequently discussed and written about and yet each year scaffolds fall over because somebody wrapped the scaffold without giving much thought to the effects that the enclosure would have on the stability of the scaffold. Of course, one of the keys to a successfully constructed scaffold is making sure that the scaffold doesn’t fall over; this is especially important for the individuals who happen to be using the scaffold!
The concept of stability is straightforward: The forces that want to knock the scaffold over have to be resisted. How can this be done? While there may be a number of methods that can be used, there are three that are most commonly used by scaffolding designers and erectors:: tying the scaffold to another strong structure that can resist the forces; guying the scaffold tower to a suitable anchor that can resist the forces, and; making the scaffold large enough so the size and weight of the scaffold are adequate to keep the scaffold from falling over. Since the stability of a supported scaffold is desirable, standards and regulations have been written to address the issue. The U.S. Federal Occupational Safety & Health Administration, OSHA, requires that “Supported scaffolds with a height to base width ratio of more than four to one (4:1) shall be restrained from tipping by guying, tying, bracing, or equivalent means….” [29 CFR 1926.451(c)(1)] The standard goes on to require that when the scaffold is tied to an existing structure, it has to be tied at a frequency of no more than 30 feet horizontally and 26 feet vertically for scaffolds wider than 3 feet, and 20 feet vertically for scaffolds 3 feet and narrower. (In California the requirements are more restrictive.)
Unfortunately, this regulation can be very misleading for the simple reason that it doesn’t address varying field conditions. Keeping in mind that the OSHA scaffolding standards are minimum requirements and not directions or instructions, the qualified person who designs the scaffold shall determine the proper means and methods for ensuring the stability of a scaffold. Also keep in mind that a qualified person will not guess at what is required to ensure scaffold stability. Unfortunately, the reality is that too many scaffold erectors and users think that experience is a great method for determining what it will take to keep the scaffold from falling over. While the OSHA mandated requirements may work for a scaffold not wrapped in plastic, the same tying requirements will be woefully inadequate for a scaffold wrapped in a tarp and subjected to a violent winter storm. (Lucky for many wrappers, the enclosure material rips into pieces and blows off before the scaffold is yanked from its’ moorings!) When a scaffold is wrapped in a quality enclosure, that is a netting or enclosure that is resistant to tearing, the scaffold instead will rip, bend and ultimately fail.
Interestingly, #9 wire is often used to secure a scaffold to a structure. While this can work with an open scaffold design, it very rarely is adequate for a wrapped scaffold, even if the ties are “doubled up.” Remember, guessing never has worked well as a substitution for a properly designed and erected scaffold.
So, what is the worker to do? The answer is easy, logical, and in compliance with the applicable standards and good scaffolding engineering practice. Have a Qualified Person design the scaffold. In the case of a wrapped/enclosed scaffold, it will probably take the skills and expertise of a Qualified Professional Engineer who can design the scaffold for the anticipated forces at the specific scaffold location and for the specific time of year that the scaffold will be exposed to external forces from the wind and other environmental conditions.
If you think that you are qualified to design an enclosed scaffold answer yes or no to these statements. (If you answer no to any of them, you are not qualified to design an enclosed scaffold):
I know where to find the information that tells me what the design wind loads are for my scaffold location;
I am familiar with the American Society of Civil Engineers (ASCE) Standard, Minimum Design Loads for Buildings and Other Structures wind loading criteria;
I know the strength of #9 wire and why it shouldn’t be used for wrapped scaffolds;
I can calculate the forces that are a result of a 100 mph breeze;
I know how to calculate overturning moments and forces due to pressures;
I know what the effects of a partially wrapped scaffold are;
I know what happens if the windows are open;
I know what effects a building corner or roof has on a wrapped scaffold;
I know my limitations.
September 7th, 2012 by David H. Glabe, PE
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
- ____A suspended scaffold is the same as a hanging scaffold.
- ____Outrigger scaffolds are one type of suspended scaffolds.
- ____You don’t need to utilize personal fall protection on a Multi-point Suspended Scaffold.
- ____Suspended scaffold users do not need any training if they are not operating the hoists on a suspended scaffold.
- ____Access is not required for a suspended scaffold.
- ____Counterweights for a cantilever beam can be ice or Jell-O.
- ____The safety factor for wire suspension ropes is at least 8.
- ____Counterweights cannot be used to stabilize outrigger beams on Mason Multi-point suspended scaffolds.
- ____Guardrails are not required on two point suspended scaffolds if all the occupants are wearing personal fall arrest equipment.
- ___Guardrails or equivalent are required on Boatswains’ chair scaffolds.
- ___Outrigger beams secured directly to the roof do not require tiebacks.
- ___Suspended scaffolds shall be designed by a competent person and installed under the supervision of a qualified person, competent in scaffold erection.
- ___Vertical pickup means a rope used to support the horizontal rope in catenary scaffolds.
- ___Tiebacks only need to be one half the strength of the suspension ropes since they are there for back-up, not suspension.
- ___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!
- When wire rope clips are used on suspension scaffolds, there shall be a minimum of ________ installed per connection.
- A stage rated for two workers or 500 pounds can support ________workers.
- Ropes shall be inspected for defects by a competent person prior to each ___________.
- Manually operated hoists shall require a _________crank force to descend.
- Wire rope clips shall be installed according to the __________recommendations.
- A two-point suspended scaffold is supported by _________ suspension ropes.
- Two-point suspended scaffold platforms shall not be more than ______inches wide unless it is designed by a ________person to prevent _________conditions.
- Suspension scaffold means one or more platforms suspended by _____ or other _______means from an overhead structure.
- The toprail of a suspended scaffold guardrail system must be able to withstand a force of at least ________pounds.
True or False Answers:
- False. A hanging scaffold is constructed with rigid tubes while a suspended scaffold hangs from ropes.
- False. Outrigger Scaffolds are a type of supported scaffold.
- True. You need to install a guardrail system.
- False. All scaffold users need training.
- False. Proper access is required for all scaffolds.
- False. The ice may melt and you might eat the Jell-O.
- False. The minimum safety factor is 6.
- True. The beams must be anchored to the supporting structure.
- False. A guardrail system and PFE is required.
- False. How do you attach a guardrail to a chair?
- False. Suspended scaffolds shall be designed by a qualified person and installed under the supervision of a competent person, qualified in scaffold erection.
- False. Tiebacks must be equal in strength to the suspension rope.
- True. While not recommended, as long as the sand cannot leak out, it’s okay.
Fill in the Blank Answers:
- 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.
- 36, qualified, unstable
- Ropes, non-rigid
July 2nd, 2012 by David H. Glabe, PE
The following is a clarification of OSHA Standard 29 CFR 1926.451(c)(2)(i) which requires that: “Footings shall be level, sound, rigid, and capable of supporting the loaded scaffold without settling or displacement.”
This standard addresses the hazard of a foundation that is insufficient to support the scaffold. The intent of the standard is to require that scaffold foundations are adequate; that is, they have sufficient strength, are stable, and the footing compensates for non-level surfaces that can introduce horizontal forces which have not been restrained.
The claim has been made that swivel screwjacks cannot be used with scaffolds since they bear on non-level surfaces. This claim is incorrect since the screwjacks are being used to create the level surface that the standard requires. While it might be argued that the screwjack is part of the scaffold and consequently must bear on a level surface, this argument is without merit for the simple fact that the swivel screwjack is specifically used for the purpose of bearing on sloped surfaces.
Standards have never precluded the use of swivel jacks with scaffolds. The truth is quite the opposite: swivel screwjacks are used to create the level surface that is required so that scaffold legs are stable. Interestingly enough wedges and shims are also used to create a level surface. By disallowing swivel jacks, it can be argued that wedges and shims cannot be used since they are not part of the foundation but rather are a part of the scaffold. In other words, if it were not that the scaffold is at a specific location, the wedges and shims would also not be there.
In summary, swivel jacks are a permissible component to be used in the construction of a scaffold. As with all scaffolds, the scaffold shall be designed by a Qualified Person who will address the issue of horizontal forces when designing the foundation.
May 23rd, 2012 by David H. Glabe, PE
A description of proper scaffold bracing techniques for power plant boilers and similar industrial applications.
When it comes to scaffold bracing, when is there enough bracing? Certain untrained erectors, and users, assume that if the scaffold isn’t falling down, then there is enough bracing; not a very smart, practical or safe approach. And then there is the question of what type of bracing are you talking about? Is it stability bracing, bracing that keeps the scaffold from falling over, or is it the bracing that gives a supported scaffold its’ strength?
Stability bracing typically includes the connection to an adjacent structure to make sure the scaffold stays erect. Strength bracing is the bracing that is necessary to make sure the scaffold legs can support the anticipated load that will be applied to them. Strength bracing can take several forms, depending on the type of scaffold and the design chosen by the qualified designer. For example, a tubular welded frame scaffold uses cross braces for bracing. A cross brace consists of two tubular or angular lengths of metal connected at the middle to form an “x”. The four ends of the brace have holes so the brace can be connected in four locations by sliding the holes over pins welded to the frame legs. This is called a pin connection. For systems scaffolds, bracing is accomplished by using a single diagonal member that is connected to adjacent legs in a vertically diagonal direction. The connection is a rigid connection rather than a pinned connection. However, the bracing effect on each accomplishes the same goal which is to make sure the scaffold leg can support the anticipated load.
A major factor in the strength of a supported scaffold leg is what engineers refer to as the “unbraced length.” A scaffold of a given diameter and material will support decreasingly smaller loads as the unbraced length of the leg gets longer. In other words, a tube 12 inches long will support a lot more load than a tube 12 feet long. When it comes to systems scaffolds, the standard unbraced length is typically 6’-6” (for systems scaffolds based on the metric system). In other words, scaffold erectors are used to installing horizontal runners every fourth connection point on the leg, resulting in an unbraced length of 6’-6”. And this is what should be done. But the bracing doesn’t end with providing a horizontal support every so often. Without some kind of additional bracing, the scaffold will simply deflect sideways, resulting in a catastrophic collapse.
This additional bracing can be either vertical diagonal bracing or other bracing that provides equivalent support, such as an adjacent scaffold or an adjacent structure. Take, for example, a power plant boiler. For those of you not familiar with a power plant boiler, picture a half gallon milk carton upside down with the “vee” shaped top now at the bottom. Picture the milk carton 175 feet high, 100 feet long and 60 feet wide. (That’s like the height of the Statue of Liberty!) Now, scaffold the interior of the milk carton using systems scaffold utilizing a 30” diameter access opening in the bottom of the milk carton. Hey, nobody said boiler scaffolds were easy to construct.
When the qualified designer chooses bracing for a scaffold in this situation, she can use diagonal bracing, the boiler walls, or a combination of the two to provide the required lateral support for the legs. As you can imagine, just the weight of the scaffold will exert a considerable load on each scaffold leg. In other words, the bracing is critical to the success of the design. If vertical diagonal bracing is chosen by the designer, the design is straightforward. Typically, the diagonal bracing is installed every fourth bay (depending on the manufacturer) and in both directions. Remember, a scaffold in an upside down milk carton is three dimensional, in other words, multiple bays wide and multiple bays long.
An alternative bracing scheme is to use the walls of the boiler for the bracing. This is effective when the bracing is designed properly, installed according to the design, and not tampered with by the scaffold user. This is critical since using the boiler walls effectively requires the legs to be “bumped” against the opposite walls of the boiler. In other words, a continuous line of horizontal runners must extend from wall to wall with bump tubes against the wall at each end. There is no room for error in this type of bracing since removal of a single bump tube will immediately affect the unbraced length of the scaffold leg and instantaneously decrease the capacity of the scaffold leg, possibly resulting in a catastrophic failure. (If the individual tampering with the bracing is lucky, the resulting failed scaffold will wedge against the walls of the boiler, avoiding a catastrophic collapse which would kill the misbehaving scaffold user and his fellow employees.)
The bottom line here is that bracing is critical for the ability of a supported scaffold to support a load. Whether diagonal bracing is used for a systems scaffold or whether another structure is used to provide the bracing doesn’t matter as long as it is done correctly.
It is up to the designer and erector of the scaffold to get it right. That is why we have standards and regulations that require that scaffolds be designed by a qualified person, an individual who knows what he/she is doing. And of course, we expect the scaffold to be constructed accordingly to the design (we have a regulation for that too). And finally, we don’t want users messing with the scaffold (yep-there’s a regulation for that too), especially if it’s inside an upside down milk carton!
March 27th, 2012 by David H. Glabe, PE
Connections play a big part in the proper erection of a scaffold. Knowing how connections work, which products to use, and their strengths are important for both erectors and users.
Being well-connected may suggest that you have a strong bond with another person or at least you may have influence over another person’s behavior and action. Unfortunately, this article is not about that type of connection-you’ll have to go somewhere else for advice on being personally well-connected. But what about your scaffold; is your scaffold well connected? And what kind of connections are we talking about?
There are all kinds of connections found in scaffolding. In engineering terms, there are shear connections, tension connections, compression connections, moment connections, and bearing connections. These connections can be provided by bolts, nails, screws, wire, welds, glue, adhesives, tape, bubble gum, string, wire rope, friction devices, u-bolts, swaged fittings, fist-grips, expansion anchors, coupling pins, retainer pins, studs, rivets and bungee cords. Well bubble gum might be a reach but the rest are legitimate; the choice of connection depends on the required strength of the connection and the application. For example, using string to attach a frame scaffold to a building will only provide a tension tie (it works only for pulling, not pushing) and the string probably does not have the required strength. On the other hand, you could weld the same scaffold to the structure but then the weld would have to be cut when the scaffold is dismantled-probably not a good choice for this application.
Myths are pervasive in the scaffold business and often include connections. Can wire, specifically # 9, 10 or 12 gauge wire be used for connections? Do we need high strength bolts for everything scaffold related? Can I use duct tape? Are friction connections bad? And can I hang a supported scaffold by its coupling pin? The easy answers, in order, are: Maybe, maybe, doubtful, no, and perhaps.
Let’s start by looking at the issue surrounding the use of wire. Wire is often used to provide a connection between a supported scaffold and a structure to provide stability so the scaffold doesn’t fall over. While the federal Occupational Safety & Health Administration, OSHA, specifies that supported scaffolds be tied to a structure at certain intervals, it does not specify the strength. Therefore, anything can be used, including wire, string and duct tape, provided it is sufficiently strong. On the other hand, California OSHA, (CalOSHA), allows the use of #10 or double wrapped #12 wire to connect the scaffold to the structure. This is an interesting concept since it assumes that these size wires are adequately strong regardless of the circumstances; this is a bad approach since wrapping the scaffold with enclosure material will probably overload the wire connection. In another common application, scaffolders frequently want to secure a scaffold leg to a coupling pin using #9 wire in place of a retainer pin. This could be a bad idea since the wire may not be able to handle the shear (karate chop) load.
Clamps/couplers are commonly used with supported scaffolds, providing a rigid or swivel connection between two tubes. The clamps primarily rely on friction to provide the connection and there are those (whoever those are) that say this is bad—you should never rely on friction for the connection. They (whoever “they” are) apparently don’t realize that they (same folks) rely on friction to walk, drive, stop, sit, or eat. In spite of the “friction myth,” scaffold clamps work because trained scaffold erectors understand that the clamp has to be properly tightened to ensure a proper connection.
And what about the myth of high strength bolts? I have no idea where this myth started but for some reason everyone (whoever “everyone” is) thinks that only high strength bolts can be used for scaffold connections. Sure, if a high strength bolt is used as a connection on an aerial lift, for example, then you better replace it with the correct high strength bolt. But, come on, regular everyday bolts work for many applications. If you want to use high strength bolts everywhere that’s fine with me; just don’t tell me it’s required.
And what about that coupling pin/connector that aligns one scaffold leg on top of another? Since its primary purpose is to provide alignment what strength is required? Well, it doesn’t have to be very strong unless you decide you want to hang your scaffold (as opposed to suspending a scaffold from a rope). Now the coupling pin must have sufficient strength to hold up the entire scaffold that’s hanging. Usually the coupling pin isn’t strong enough. And guess what– the bolts have to be strong enough too. May I suggest having a qualified person design that connection before you kill someone? By the way, don’t necessarily rely on the manufacturer; he/she may have no idea what to tell you.
As for those funny little connectors that secure a suspended scaffold wire rope to an anchor, its best to make sure that they are installed correctly. These connectors, whether u-bolts, fist grips, or swaged fittings, they all rely on friction. In this case, definitely follow the manufacturer’s recommendations for torque specifications since this is the key to safe use. And, don’t forget that the correct size and quantity of u-bolts or fist grips are required.
That brings us to the use of duct tape. Applicable standards and good engineering practice dictate that all connections must have adequate strength to support 4 times the anticipated load. If you can tell me the strength of duct tape in tension, I’ll be happy to design a suspended scaffold for your use. Will it be a single point or two point suspended scaffold platform that you want? Oh wait, I forgot that you want to go up and down with suspended scaffold. I think the hoist is going to be a tricky one to design! Perhaps we should stick (no pun intended) to something a little more conventional.