Archive for March, 2012
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.
March 25th, 2012 by David H. Glabe, PE
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.
March 1st, 2012 by David H. Glabe, PE
A description of the proper use of a knee-out.
There are two issues that need to be addressed when considering the use of a knee-out in your scaffold. The first issue involves the stability of the scaffold while the second issue involves leg loading. Stability can be a real problem if the base width of the scaffold that the knee-out is attached to is narrow compared to the size of the knee-out. While “off the shelf” knee-outs normally do not exceed 45 inches in the horizontal dimension, a knee-out can be any size you want—if you know how to design and construct it. Let’s say you have a scaffold that has a base width dimension of 5 feet. You decide to install a knee-out on the outside leg of the scaffold that happens to be 7 feet, measured horizontally. If you don’t have enough weight in the base scaffold, the whole thing will fall over. Of course, the clever, or not so clever, scaffold erector assumes the weight of the scaffold will be the “counterweight” for the knee-out. Imagine what happens if the knee-out gets loaded up with plank and materials that weigh more than the scaffold equipment or, better yet, somebody decides to dismantle the base scaffold first before dismantling the knee-out. The dismantling may not take as long as you thought!
Knee-outs have a direct impact on the leg to which it is attached. Assuming that an upper scaffold leg is supported by the knee-out and built up from there, there are two types of forces that the supporting scaffold leg must support, vertical forces and horizontal forces. The vertical loads from the knee-out are transferred into the supporting leg and presumably down to the scaffold foundation. The connection to the leg at the top of the knee-out has to resist a horizontal force that wants to pull the leg outward while the bottom connection of the knee-out wants to push the leg inward.
Since supported scaffold legs, normally a steel tube for frame, systems, and tube & clamp supported scaffolds, can handle vertical/axial loads efficiently, the vertical force is no big deal as long as the total load of the knee-out vertical load and the supporting scaffold leg do not exceed the allowable load for the supporting leg. Remember, the supporting leg is basically holding up two legs, and the loads on those two legs.
The horizontal forces are a little trickier. Round tubes can handle vertical loads well but do a really lousy job of handling horizontal loads, exactly the horizontal forces/loads that a knee-out applies to a round tube. What is a designer to do? Well, the qualified designer knows that bracing is required to transfer the imposed loads properly so none of the scaffold components are overloaded. This load transfer can be achieved in a variety of ways. The first requirement is to install a horizontal member at the knee-out connection so that at least two legs are connected horizontally. Then a vertical diagonal is required to transfer the load down to the next runner location, typically one frame down if it is a frame scaffold, and 6’-6” or so if it is a systems scaffold or tube & clamp scaffold. This process is repeated until either the vertical legs can handle and disperse the horizontal loads to other legs, or you have transferred the loads through the bracing down to the foundation. How do you know when that occurs? Well, there are two ways; analyze the scaffold or try it out and see if it bends! I strongly recommend the analysis method rather than the guess method—workers’ lives are at stake here.
Another issue that comes up, and is usually not considered by erectors guessing and “winging it” involves the diagonal member that transfers the leg load supported by the knee-out to the supporting leg. If that member is installed at a 45 degree angle, the force/load in the member is almost 1.5 times (1.404 to be exact) more than the leg load it is supporting. And since this diagonal member is in compression, it also must be braced when the length exceeds its ability to support the expected load. This is the stuff of qualified designers, typically qualified engineers who can develop an appropriate design for the specific situation.
If you can’t ascertain the loads the knee-out is subjected to, if you cannot calculate the horizontal forces applied to the supporting scaffold, if you cannot figure out how to transfer the applied loads so the scaffold can handle them, don’t guess at it; leave it to the experts to design it for you.