Scaffold Bracing

Pop Quiz: 30 Questions About Scaffolding

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It is somewhat surprising how creative workers can get when it involves scaffolding.  Just when it seems all the questions have been answered, along comes a question that raises an issue that was never addressed.  Challenge yourself to these questions and see if your answer agrees with the one given at the end of this article.

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Can Scaffolds Support This?

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Spring is in the air, the birds are chirping and scaffolds are being built. Can life get any better? It used to be that contractors feared winter in the northern regions of North America. Cold temperatures, snow, wind and generally miserable conditions prompted owners and contractors to curtail outdoor activities. That was then; now construction charges ahead, fearless and courageous against even the nastiest of weather. Once again science and progress has prevailed! Improved, clothing, materials, equipment and methods allow construction to continue in any environment.
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Industrial Strength Bracing

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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!

How Does the Scaffold Hold That Load?

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The OSHA standards require a scaffold user to have training. One of the requirements of that training is that the user must know “The maximum intended load and the load carrying capacities of the scaffold used” [29 CFR 1926.454(a)(4)].  (In case you are wondering, erectors are suppose to know this too.)  So, where do you find that information?  The manufacturer should have that information.  And the manufacturer gets that information either by testing the scaffold, that is load the scaffold until it fails, or by engineering analysis.  And how is it that a scaffold can support a load?  I have explained this before but it has been a long time so here it is again.

Have you ever wondered why a scaffold can support loads?  Have you ever wondered how an engineer can determine what a scaffold leg can support?  Do you wonder who thought up the way to answer those questions?  Wonder no more.  Thanks to Swiss physicist Leonhard Euler, (typically pronounced “Oiler”), a method was developed to determine the strength of a column (which is what a scaffold leg is). Back in 1774, Mr. Euler discovered that a column would not buckle until the load reached a certain amount.  This load, known both as the critical force and also as Euler’s Load, is affected by certain characteristics of the column.  These characteristics include the support conditions at the end of the column, the distance, or length, between support points, the shape of the column, and the material of the column.  Based on these parameters, Mr. Euler developed a formula that determined the critical load.

While scaffolds can be manufactured using a variety of materials, steel is the most common material used.  (Euler’s equation can be used with any material, including aluminum, fiberglass, plastic, and wood.)  Scaffolding is typically constructed with round tube, which is equally strong in all directions.  Second to a round tube is a square tube which exhibits similar qualities.  Rectangular tube may be used but the strength will be higher in the direction of the long face of the tube compared to the short face.  Other shapes may be used if the manufacturer determines that there may be an advantage.  While the shape of the material, and the material itself will help determine the capacity of a particular scaffold leg load, the characteristic that affects the strength of scaffolds more than the material or shape is the distance between points of support.  Depending on the scaffold type, these points of support may be cross braces, diagonal braces, horizontal braces, or ties to an existing substantial structure.  For example, in a tube and coupler scaffold, the length between points of support is usually 6’-6”.  The horizontal members and the diagonal members are connected to the legs at these intervals.  Therefore the length of the scaffold leg (column) is 6’-6”.  For a systems scaffold, the support points will occur where the horizontal and diagonal members are attached to the leg.  This usually occurs at 6’-6” to 7’-0”, similar to a tube and coupler scaffold.  A frame scaffold, on the other hand, will have points of support at the cross brace studs in one direction, and at the location of the horizontal members that are welded to the legs in the other direction.

The distance between the points of support is critical to the strength of the scaffold leg.  Reducing the distance between the horizontal members on a tube and coupler scaffold or systems scaffold by 50 per cent can more than double the strength of the scaffold leg.  Conversely, increasing the distance between horizontal members by 50 per cent, (not recommended,) can reduce the capacity by substantially more than 50 per cent.  Therefore, if one is to deviate from the standard erection procedures, it is important to verify the design prior to construction. Similarly, removing a critical cross brace from a frame scaffold can drastically reduce the capacity of the scaffold.  This is not to say that braces cannot be removed. A frame scaffold will have sufficient bracing as long as the scaffold leg is braced to at least one other leg.  However, removing the incorrect brace may result in a scaffold that is unable to provide the anticipated support.  This is not good!

Frame scaffolds also differ from tube and coupler, systems, and wood pole scaffolds in another aspect in that additional bracing is provided by the horizontal members that are welded into the frame.  Since this bracing varies between frame styles and manufacturers, all frames are not equal in capacity.  In fact, a review of scaffold load charts will show that frame capacities vary dramatically, depending on the height of the frame, the location and spacing of the brace studs, and the pattern of the frame horizontal members.

Mr. Euler’s formula has been used successfully for quite a long time.  Other formulae have been developed since Mr. Euler’s work and refinements have been made to these formulae so engineers can accurately predict the capacities of columns under a broad range of circumstances.  But keep in mind that the basic premise is still accurate: The capacity of a scaffold leg can be easily affected by modification of the distance between support points.  If you cannot accurately determine the effects these modifications will make, don’t modify the scaffold.  In other words, don’t mess with Mr. Euler’s formula!

Bracing Opportunities, PART 2

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Last month, we discussed the importance of bracing and the effect that bracing has on the strength and stability of a scaffold. We also discussed the effects the brace connection to the scaffold leg have on the strength of the overall scaffold. Finally, we discussed that there are various types of bracing, including cross-bracing, diagonal bracing, tie bracing to an existing substantial structure, and horizontal bracing, that can be used to brace scaffolds. This month we’ll discuss bracing variations for frame scaffolds.

As many scaffold users and erectors are aware, a frame scaffold must have some sort of cross bracing in order to be stable. Figure 1 shows the most basic type of frame scaffold tower that can be erected. While only one side of the tower is shown, this cross bracing is on both the front and the back of the scaffold. (For discussion purposes, the front of the scaffold is the side closest to the face of the structure.) The cross braces are attached to the studs of the scaffold frame leg, and the cross brace members are attached to each other where the members cross each other. This connection is critical since the attachment of the brace to the leg usually gives no bending strength because the brace normally can rotate on the pin. The First Rule of Scaffold Bracing can be developed from Figure 1.

“Each scaffold leg must be braced to at least one other leg and/or to an adjacent properly braced structure.”

Note that the rule states that each leg must be braced, not necessarily cross-braced. In the case of Figure 1, cross braces have been used and this tower meets the requirements of the First Rule. Figure 2, consisting of several independent towers also meets the requirements of the rule. Figure 3, which uses horizontal rails to connect the independent towers of Figure 2 together, also obeys the rule. What is the advantage of Figure 3 over Figure 2? Since horizontal ties are required on tall scaffolds at each end of a scaffold and every thirty feet in between, each tower of Figure 2 would be considered an independent scaffold, requiring ties. The scaffold of Figure 3 would be considered one scaffold since all frames are connected to each other, thus reducing the number of horizontally located ties. Figure 4 is another alternative that meets the requirements of the first rule and reduces the number of horizontal ties. Continuous cross bracing is desirable, as illustrated in Figure 5, since it results in a more rigid structure. This also allows for the removal of selected braces without jeopardizing the structural integrity of the scaffold. Remember that the cross braces in the end bays can never be removed since the end leg will lose its bracing to another scaffold leg, violating the First Rule of Scaffold Bracing.

Other bracing alternatives can be used when access to the face of the work is required. Figure 6 illustrates a scaffold that is properly braced, complies with the First Rule of Scaffold Bracing, provides reasonable stiffness to the scaffold, and allows for open (non-cross braced bays) in alternate bays. This bracing scheme can be used on both the front and the back of the scaffold. Another method, used primarily in the western United States, uses a mixture of cross, horizontal braces called goosers, and additional ties to the structure. Figure 7 illustrates this method. Cross braces are used on the end bays and every third or fourth bay horizontally. The horizontal goosers are installed on the top ledger (horizontal member) of each frame, at the front leg. Ties to the structure are installed every twenty feet horizontally (every other leg), and every thirteen feet vertically (every second frame). In this type of installation, the ties to the structure must not only take tension and compression loads but select ties must also be installed so they can resist sidesway loads of the scaffold legs. It is important to note that this design will result in a lower allowable leg load than what is published in manufacturers’ leg load charts due to the greater spacing between braces. While it is recommended that continuous cross bracing be used on the back of the scaffold, the gooser/cross brace combination is often used on the back leg too, further decreasing the capacity of the scaffold.

Other patterns of bracing may be used, provided the scaffold has been designed by a qualified person. If the First Rule of Scaffold Bracing is applied, any problems caused by insufficient bracing will be minimized. Some things to remember about bracing frame scaffolds include:

1. Just because you’ve “always done it this way,” doesn’t necessarily mean its right.

2. Just because the scaffold didn’t fall over when a brace was removed doesn’t mean the brace isn’t needed.

3. Don’t underestimate the importance of the ties to the adjacent structure.

4. Don’t overestimate the strength of the adjacent structure.

5. Removal of critical cross braces and ties solely because “they’re in the way” is really stupid.

6. Don’t tamper with the bracing if you don’t understand it.

7. A qualified person is required for all scaffolds.

8. If in doubt, cross brace the front and back legs on all bays.

9. It is unlawful to erect, dismantle, or alter a scaffold except under the supervision of a competent person, qualified in scaffold design.

Proper bracing of any scaffold is directly related to the strength and stability of the scaffold; don’t let your next scaffold be a bracing statistic. Remember, “Each scaffold leg must be braced to at least one other leg and/or to an adjacent properly braced structure.”

Bracing Opportunities

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Questions are frequently asked about the correct bracing of scaffolds. Scaffold users turn to the standards looking for the answer, only to be disappointed by the lack of information that could be used as “instructions.” The standards acknowledge the necessity and importance of bracing by requiring that “uprights shall be plumb and braced to prevent swaying and displacement.” Since there are so many different ways to properly brace each type of supported scaffold, it is impossible to dictate a simple solution that would apply in all cases. This is why there are no simple, specific bracing guidelines. The question remains: What is correct bracing of scaffolds?


To appreciate the importance of bracing, you must understand its purpose and use. Bracing, as it is used in scaffolding, provides strength and stability to supported scaffolds. Simply stated, bracing provides strength by controlling the “unbraced length” of the scaffold leg and bracing provides stability by ensuring the scaffold leg doesn’t fall over. Beyond that, though, the concept of bracing can get a little more complicated. So, let’s look at strength and stability and see how bracing affects each one.


Bracing defines, and controls, the unbraced length of the leg. The unbraced length is the vertical distance between points of support. These points of support do not allow the legs to move sideways. The points of support in supported scaffolds are the cross brace studs on frame scaffolds, node points with horizontals on systems scaffolds, and the horizontal members in tube and coupler scaffolds. The unbraced length is one of the factors that determine how much a scaffold leg can support. Other factors include the shape of the leg, the size of the leg, and the material that is used. In other words, a wood leg is not as strong as the same size steel leg, a 3 inch diameter leg is stronger than a 1 inch diameter leg, and a round leg is equally strong in all directions compared to a rectangular leg that is stronger in one direction than in the other direction.


Since scaffolding equipment is assembled or erected from items that are manufactured, we typically cannot change the material, the shape or the size. Therefore, the only thing that we can do is change the unbraced length. This can be good and bad. By increasing the number of horizontal members on a tube and coupler or systems scaffold, the amount of weight the scaffold leg can support increases significantly. On the other hand, removing horizontal members will definitely decrease the capacity of the scaffold leg. Remove too many, and your scaffold collapses. The same results occur with frame scaffolds. While it is somewhat difficult to increase the frequency of cross braces, it is very easy to remove cross braces. Remove the wrong cross brace and you have a scaffold that may not be able to hold any load because you just increased the unbraced length. It is interesting to note that at least one scaffold manufacturer has additional cross braces studs (connection points) that allow the erector to install additional cross braces. This increases the capacity of the frame by reducing the unbraced length.


Bracing is also required to provide stability for the scaffold. The stability is a result of connecting adjacent legs of a scaffold together in such a way that the legs form a larger base. The connection between the brace and the leg is a very important part of this bracing. On tube and coupler scaffolds, the connection is what we call a fixed connection, provided you use a rigid right angle clamp and not a swivel clamp! A systems scaffold’s connection varies, depending on the manufacturer. Some connections are more rigid than others are, although all of them have some degree of rigidity. The diagonal braces that are installed on the scaffold supply additional rigidity. In fact, it is the diagonal braces that transfer all the loads to the legs. Too often, tube and coupler, and systems scaffolds, are constructed without any additional diagonal braces. In this case, the erector is relying on the connection between the horizontal and the leg to supply all the bracing. This is not good since scaffolds are not designed to support or transfer these loads. The scaffold will stand as long as the load is minimal, and there are no horizontal forces on the scaffold. At some point, however, the connection will become overloaded, break, and the scaffold will fall over. This is definitely not good.


On frame scaffolds, the connection is what is called a pin connection. The brace can rotate on the pin but cannot move sideways. In this case, it is the connection between the diagonal members of the cross brace that makes the scaffold stable. Remove that center rivet or bolt from the cross brace and you have nothing. Use horizontal rails, such as guardrails, in place of a required cross brace, and you have nothing. The scaffold falls down.


Can other types of bracing be used to brace scaffolds? Yes there are. Bracing the scaffold to an adjacent structure that is properly braced is one common method. We typically call this type of brace a tie. If the scaffold is surrounded by a stiff structure, diagonal or cross bracing may not be required. In other instances, the plank may provide bracing. Horizontal braces, in conjunction with additional ties to the structure, can be used. The configuration of the scaffold may provide bracing.


As you can see, there are many opportunities to provide the correct bracing for scaffolds. Unfortunately, there are an equal number of opportunities to leave out important bracing. Keep in mind that frame scaffolds don’t necessarily require continuous bracing but they certainly require the right bracing. Tube and coupler scaffolds, and system scaffolds, may stand without diagonal bracing, but you just don’t know how long they’ll stand. Do you want to take that chance?


Next month, we’ll look at various examples of adequate bracing. In the meantime, if you don’t understand bracing, leave it up to the experts. After all, the standards are very specific in requiring that a qualified person shall design each and every scaffold.

Rolling Safely

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Previous articles discussed scaffold regulations and scaffold training requirements, issues that obviously are important but don’t necessarily tell us how to erect or use a mobile scaffold properly.

Mobile scaffolds, also known as rolling towers (formerly known in the old OSHA regulations as manually propelled mobile scaffolds), are commonly used when access to heights is required for a short period of time. These mobile scaffolds work very well but unfortunately there are a surprisingly high number of accidents associated with mobile scaffolds. These accidents are generally due to misuse of the scaffold although improper assembly of the parts and pieces contribute to the accidents. Consequently, this article addresses both the proper assembly of the scaffold and the correct use of mobile scaffolds.


Important Assembly Points


Using manufacturers’ guidelines, applicable regulations, and common sense, here are the important things to remember when assembling a mobile scaffold:


• The casters should be double locking so that the wheel cannot turn nor can the caster rotate.

• The casters must be pinned or bolted to either the screwjack or to the frame leg.

• The casters must be strong enough to support the anticipated load. (Most common 8 inch scaffold casters have a safe capacity of about 500 pounds.)

• The screwjacks should extend up into the frame leg at least 12 inches, preferably more.

• The mobile scaffold must always be plumb and vertical. Adjust the screws as required.

• Never have more than 12 inches of adjustment between the bottom of the frame leg and the top of the caster.

• All of the frames must be pinned together.

• All of the frames must have cross braces attached firmly.

• A horizontal diagonal brace is to be installed as close to the bottom of the scaffold as possible. This brace keeps the scaffold square.

• The horizontal diagonal brace should be installed approximately every 20 feet vertically on scaffolds 5 feet wide and approximately every 12 feet on scaffolds less than 5 feet wide.

• Provide access to all platforms of the scaffold. This can be a built in ladder, a clamp on ladder, or even a stairway if the scaffold is a very large mobile scaffold.

• If clamp on ladders are used, install them on the width side of the scaffold, not the length side of the scaffold.

• The ladder should extend at least 36 inches above the top platform unless there is a hand hold above the platform such as an access gate panel or guardrail system.

• A full guardrail system, consisting of both top rail and mid rail must be installed on all sides of the mobile scaffold.

• A toeboard must be installed on all open sides of the mobile scaffold unless other means of falling object protection is provided.

• The scaffold height must never exceed three or four times the minimum base dimension, depending upon where you are working. For example, California requires the base to be at least one third the height of the scaffold. This means that on a scaffold that is five feet wide, the height is limited to 15 feet, measured from the ground or floor to the top of the platform.

• If outriggers are used to increase the width of the mobile scaffold, be sure that they are securely fastened to the frames and properly braced.

• All planks used for the platform must be secured from movement. Hook plank are the best choice since they don’t hang over the ends and can be easily secured from movement.



Proper Use of a Mobile Scaffold


Despite proper assembly of a mobile scaffold, accidents can easily happen due to wrong use of the scaffold. Here are some things you should do to make sure that the mobile scaffold is used safely:


• Make sure everybody that will use the mobile scaffold is trained in the proper use of mobile scaffolds.

• Do not modify the scaffold unless you know the regulations and know what effect the modification will have on the stability and safety of the scaffold.

• Watch out for power lines when moving mobile scaffolds from location to location. (Rubber casters are insulators; you are a great conductor!)

• Always push the scaffold as close to the bottom of the scaffold as possible, but no more than 5 feet above the base.

• Never, never, pull yourself along from the top of the scaffold while riding it.

• Don’t ever use powered means to move the mobile tower unless it has been specifically designed to be moved by that method.

• Always lock the casters before getting on the scaffold to work.

• Take care when climbing the mobile scaffold so that you don’t pull the scaffold over. If necessary, climb on the inside of the tower.

• Do not remove the guardrail system unless alternate forms of fall protection are provided. If you do remove the guardrail system, reinstall it before anybody else uses the mobile scaffold.



Riding Mobile Scaffolds


A lot of accidents occur because people ride mobile scaffolds while they are being moved. You are strongly discouraged from riding mobile scaffolds because of the high inherent danger involved with riding mobile scaffolds. Manufacturers, suppliers, and the Scaffold Industry Association strongly discourage this practice. However, OSHA allows people to ride mobile scaffolds under certain conditions. They include:


• The surface on which the scaffold is being moved is within 3 degrees of level, and free of pits, holes, and obstructions.

• The height to base width ratio of the scaffold during movement is 2 to 1.

• Outriggers, when used, are on both sides of the scaffold.

• When power systems are used, the propelling force is applied directly to the wheels and does not produce a speed in excess of 1 foot per second.

• No employee is on any part of the scaffold which extends outward beyond the wheels, casters, or other supports.


Using a mobile scaffold safely is the responsibility of all workers. Using the guidelines above, safety information provided by your supplier, and following the applicable regulations, will result in a safe work environment for you.