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frame scaffolds Archives | DH Glabe & Associates

Wide Open Spaces

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Remove the crossbrace; it’s in the way!  If the scaffold doesn’t fall over, apparently the brace isn’t needed!  Those people who put in all the braces don’t know how to build scaffolds!  Before you get too excited, the foregoing statements are not true—please keep reading!

Scaffolds are temporary elevated platforms that provide access to work surfaces.  Ideally, that access should be clear of obstructions so the workers’ progress is not blocked.  In other words, the workers desire, and many times, require wide open spaces to work.

What does this mean for the various types of scaffolds that are used for a variety of work activities and circumstances?  What effect do the components and configuration of those components have on worker access?  For example, what effect do the components of a suspended scaffold have on access?  Typically, suspended scaffolds rarely have any obstructions between the suspension ropes. Suspended scaffold suspension ropes normally do not create a problem since the suspension ropes are next to or behind the worker. Supported scaffolds, on the other hand, present a different situation.  Vertical legs are placed at frequent intervals that can easily interfere with the work activity. In the case of a frame scaffold, the legs are commonly spaced no more than 10 feet apart and connected by crossbraces.  These legs and braces can present an obstacle to the worker and can easily impede the work.  Fortunately, side brackets can solve this dilemma by placing the worker in front of the scaffold leg, (towards the work surface,) allowing an open space that runs the full length of the scaffold.  Tube and coupler scaffolds and systems scaffolds can also use side brackets or they can be configured in such a way that the diagonal bracing is minimized.  Recently introduced modular scaffolds, such as Waco Exprescaff™, Patent Sprint™, and Safway SL Frame System™, solve the problem of intrusive crossbracing by designing out the traditional cross bracing.

One solution to the crossbrace problem with frame scaffolds has been used primarily in the western United States for many years.  The front crossbraces are replaced with a horizontal brace installed on the top frame ledger, adjacent to the inside legs.  Scaffolds configured in this way, with only horizontal braces, are a common sight where stucco, plastering and other similar work is being performed, since the workers require an open space to install the backing and the various coats of finish in an uninterrupted sequence. Is this safe?  Is it similar to the new modular scaffolds?  Is this practice in compliance with applicable standards?  The simple answer is: It depends.

All supported scaffolds, that is, scaffolds built from the ground up, respond to the same engineering principles.  Consequently, proper and sufficient bracing is mandatory no matter what kind of supported scaffold is being used.  Removing crossbraces from a frame scaffold without replacing them with another form of bracing is asking for trouble.
Frame scaffolds require crossbraces for stability and strength.  Removing critical crossbraces can result in substantially decreased capacity and dangerous instability.  However, the capacity and stability of the scaffold can be reinstated by substituting alternate forms of bracing.  These alternate forms of bracing can be; ties to the structure; horizontal bracing; and diagonal bracing.  In addition, increased leg diameter can reduce the frequency of the bracing.  For example, the new modular scaffolds utilize diagonal bracing on the outside of the scaffold at specified intervals, larger diameter legs, and hook plank that becomes a part of the bracing system.  Additionally, certain modular systems utilize the guardrail system as a portion of the bracing system.  Can frame scaffolds be configured the same way?  Probably not.  However, selective crossbrace removal and more frequent, specifically designed ties, can result in a scaffold that will be adequate for the intended purpose.

What is a properly braced scaffold and who determines the design?  A qualified person does.  This individual, who has the experience, education, training, and expertise, knows what to do when crossbraces are removed and what is required to provide sufficient support and stability.  Typically, these scaffolds will have a configuration that has at least the following:

•    Tension and compression ties to the structure every 20 feet horizontally and every other tier vertically.
•    Tension and compression ties at the ends of the scaffold.  These ties will be at 45 degrees to the structure to provide added stiffness to the legs.
•    Crossbraces every tier vertically on the end bays.
•    Crossbraces in every third, fourth or fifth bay horizontally, depending on the circumstances.
•    Additional leg stiffeners and/or crossbraces in the scaffold, depending on the height of the scaffold and the type of scaffold equipment being used.
•    Crossbraces on the outside of the frame every tier vertically on the end bays and every other bay in between.  (Alternatively, the scaffold will be continually crossbraced.)

Scaffolds will vary; don’t expect all scaffolds to be the same.  The work activity will dictate the actual design and construction of the scaffold.  Therefore it is important that a qualified person designs the scaffold, as is expected by the standards and good construction practice.  The live load that will be applied to the scaffold is the significant factor in the final design and configuration of all scaffolds and particularly in scaffolds that have modified bracing systems.  Past practice and experience don’t necessarily determine future success.  Remember, just because the scaffold didn’t fall over the last time you built it doesn’t mean the scaffold is safe or in compliance with the standards and good construction practice.  Maybe you were just plain lucky!  Think before you remove that crossbrace.  Is it okay or is this brace the critical one.  If you don’t know, don’t remove it.  Contact that qualified person for the answer.  The qualified person really can give you the wide open spaces.Remove the crossbrace; it’s in the way! If the scaffold doesn’t fall over, apparently the brace isn’t needed! Those people who put in all the braces don’t know how to build scaffolds! Before you get too excited, the foregoing statements are not true—please keep reading!

 

Scaffolds are temporary elevated platforms that provide access to work surfaces. Ideally, that access should be clear of obstructions so the workers’ progress is not blocked. In other words, the workers desire, and many times, require wide open spaces to work.

 

What does this mean for the various types of scaffolds that are used for a variety of work activities and circumstances? What effect do the components and configuration of those components have on worker access? For example, what effect do the components of a suspended scaffold have on access? Typically, suspended scaffolds rarely have any obstructions between the suspension ropes. Suspended scaffold suspension ropes normally do not create a problem since the suspension ropes are next to or behind the worker. Supported scaffolds, on the other hand, present a different situation. Vertical legs are placed at frequent intervals that can easily interfere with the work activity. In the case of a frame scaffold, the legs are commonly spaced no more than 10 feet apart and connected by crossbraces. These legs and braces can present an obstacle to the worker and can easily impede the work. Fortunately, side brackets can solve this dilemma by placing the worker in front of the scaffold leg, (towards the work surface,) allowing an open space that runs the full length of the scaffold. Tube and coupler scaffolds and systems scaffolds can also use side brackets or they can be configured in such a way that the diagonal bracing is minimized. Recently introduced modular scaffolds, such as Waco Exprescaff™, Patent Sprint™, and Safway SL Frame System™, solve the problem of intrusive crossbracing by designing out the traditional cross bracing.

 

One solution to the crossbrace problem with frame scaffolds has been used primarily in the western United States for many years. The front crossbraces are replaced with a horizontal brace installed on the top frame ledger, adjacent to the inside legs. Scaffolds configured in this way, with only horizontal braces, are a common sight where stucco, plastering and other similar work is being performed, since the workers require an open space to install the backing and the various coats of finish in an uninterrupted sequence. Is this safe? Is it similar to the new modular scaffolds? Is this practice in compliance with applicable standards? The simple answer is: It depends.

 

All supported scaffolds, that is, scaffolds built from the ground up, respond to the same engineering principles. Consequently, proper and sufficient bracing is mandatory no matter what kind of supported scaffold is being used. Removing crossbraces from a frame scaffold without replacing them with another form of bracing is asking for trouble.

Frame scaffolds require crossbraces for stability and strength. Removing critical crossbraces can result in substantially decreased capacity and dangerous instability. However, the capacity and stability of the scaffold can be reinstated by substituting alternate forms of bracing. These alternate forms of bracing can be; ties to the structure; horizontal bracing; and diagonal bracing. In addition, increased leg diameter can reduce the frequency of the bracing. For example, the new modular scaffolds utilize diagonal bracing on the outside of the scaffold at specified intervals, larger diameter legs, and hook plank that becomes a part of the bracing system. Additionally, certain modular systems utilize the guardrail system as a portion of the bracing system. Can frame scaffolds be configured the same way? Probably not. However, selective crossbrace removal and more frequent, specifically designed ties, can result in a scaffold that will be adequate for the intended purpose.

 

What is a properly braced scaffold and who determines the design? A qualified person does. This individual, who has the experience, education, training, and expertise, knows what to do when crossbraces are removed and what is required to provide sufficient support and stability. Typically, these scaffolds will have a configuration that has at least the following:

 

• Tension and compression ties to the structure every 20 feet horizontally and every other tier vertically.

• Tension and compression ties at the ends of the scaffold. These ties will be at 45 degrees to the structure to provide added stiffness to the legs.

• Crossbraces every tier vertically on the end bays.

• Crossbraces in every third, fourth or fifth bay horizontally, depending on the circumstances.

• Additional leg stiffeners and/or crossbraces in the scaffold, depending on the height of the scaffold and the type of scaffold equipment being used.

• Crossbraces on the outside of the frame every tier vertically on the end bays and every other bay in between. (Alternatively, the scaffold will be continually crossbraced.)

 

Scaffolds will vary; don’t expect all scaffolds to be the same. The work activity will dictate the actual design and construction of the scaffold. Therefore it is important that a qualified person designs the scaffold, as is expected by the standards and good construction practice. The live load that will be applied to the scaffold is the significant factor in the final design and configuration of all scaffolds and particularly in scaffolds that have modified bracing systems. Past practice and experience don’t necessarily determine future success. Remember, just because the scaffold didn’t fall over the last time you built it doesn’t mean the scaffold is safe or in compliance with the standards and good construction practice. Maybe you were just plain lucky! Think before you remove that crossbrace. Is it okay or is this brace the critical one. If you don’t know, don’t remove it. Contact that qualified person for the answer. The qualified person really can give you the wide open spaces.

So That’s Why…..

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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 are 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. Don’t mess with Mr. Euler’s formula!