Scaffolding in New Zealand - Best Practice Guideline for

Previous Section | Table of Contents | Next Section

SECTION 6: PARTICULAR SCAFFOLDS AND SCAFFOLDING STRUCTURES

6.1 Mobile scaffolds

A mobile scaffold is a type of free standing scaffold supported on wheels, castors or other devices for ease of movement on a firm level supporting structure. Mobile scaffolds can be erected from a variety of components or systems, including:

Aluminium prefabricated systems.
Fibreglass prefabricated systems (non conductive).
Steel frame scaffolds (H frame).
System or modular steel scaffolds.
Tube and fitting scaffolds, etc.

Important points

Some important points to remember when dealing with mobile scaffolds include:

They should be erected, maintained, altered and dismantled by a certificated scaffolder of the correct class if they are more than 5 metres in height.
They should be erected, maintained, altered and dismantled by a competent person if less than 5 metres in height.
They should be erected to the manufacturer's instructions.
They should be used on a sound, clear and level surface.
Ensure working platforms are decked to the full width and adequately restrained against displacement.
Ensure top and mid guardrails are erected on platforms.
Erect platform toeboards to prevent items on the platform falling on people below.
Provide safe means of access to work platforms.
Where the platform incorporates a hatch door, ensure the hatch is closed except during access or egress.

6.1.1 Stability

All freestanding scaffolds, whether mobile or static, need to be stabilised against overturning forces. Mobile scaffolds in particular are prone to tip over during use for a variety of reasons, including:

Sudden stops while being moved. This is a key reason why they must not be ridden while being moved.
Height to the top most platform is greater than 3 times the minimum base dimension.
People standing at or near the edge of the platform in conjunction with a sudden movement or action. This creates a temporary high point loading.
Capacity being based on a distributed load, not a point load at the edge. This means overturning can occur even when the design load capacity of the platform is not exceeded.
The narrower and lighter the scaffold, the more likely that it will tip over.

General principles to improve stability

The height of the top working platform must be no more than three times the minimum base dimension of the structure for scaffolds over 1.8 metres in height.
The height of the top working platform must be no more than two times the minimum base dimension of the structure for scaffolds under 1.8 metres in height.

These base to height ratios are only acceptable under normal weather conditions. Additional precautions must be taken if adverse weather conditions are expected. Further, it is accepted practice to use rakers, outriggers, or larger base frames to increase the minimum base dimension.

Mobile scaffolds generally need to be braced on all sides. All mobiles must also be plan braced to prevent the tower from deforming in shape (parallelogram), except in the case of a mobile aluminium framed scaffold where the lowest fully decked platform is below 3 metres high.

Specific recommendations to improve stability

Position the scaffold as close as possible to the area being worked on.
Apply the castor brakes while the scaffold is in use.
Don't ride the scaffold while it is being moved.
Maintain the height to width ratio.
Use outrigger bracing.
Add weight to the scaffold base to improve stability.
Establish with the manufacturer exactly what the established safe working load relates to.
Where possible use alternative more stable equipment, such as a scissor hoist.

6.1.2 Non adjustable castors

Castors allow a scaffold to be moved or be mobile. Castors must:

Be able to support the dead and live loads imposed upon the scaffold.
Be a minimum of 125mm in diameter and be secured to the standard. This can be done by using an expanding internal spigot that fits into the standard or by using a locking device on a sleeve type external fitting over the standard. This is to prevent a castor from falling out of the standard when wheeled over uneven surfaces.
Have the SWL (Safe Working Load) clearly shown on them.
Have some kind of braking or locking system. This can come in a range of types such as screw and compression devices on the actual castor itself.
Have a minimum 150mm spigot or pintle length (internal spigot length or external socket length) in or over the standard.

6.1.3 Adjustable castors

Adjustable castors have the same requirements as a normal castor but can be adjusted vertically by way of a threaded spigot or stem with a positioning nut. The threaded extension, while maintaining a minimum of 150mm spigot or pintle length in or over the standard, must not exceed 600mm adjustment. Castors can also be used horizontally as a running device to keep a mobile scaffold off a surface.

It is recommended that the threaded extension, while maintaining a minimum 150mm spigot or pintle length in or over the standard, should have a maximum extension that does not exceed half of the total extension. For example a castor with 500mm extension should preferably be kept to a maximum of 250mm extension, but can be used out to 350mm extension. Adjustable castors do not need to be secured to the standard if the spigot or pintle length is greater than 300mm.

As stated earlier, in the following diagrams kickboards are omitted for clarity.

Figure 47: Single deck width mobile with outriggers

Figure 48: Double deck width mobile

Figure 49: Five deck wide mobile

Figure 50: Double deck width mobile. Base extended with rakers

Figure 51: Two double deck mobiles with proprietary bridge (span deck)

Figure 52: Tube and fitting mobile tower

Figure 53: H Frame mobile tower

Figure 54: System mobile tower

Note: Only the top lift is a working platform

6.2 Rakers and outriggers

Raking tubes, rakers or outriggers are attached to a scaffold to increase its base width thereby aiding the stability of the scaffold.

Raking tubes must be braced. For tube and fitting scaffolding, ideally attach the tube brace to the standards of the scaffold or to ledgers or guardrails that are connected with double couplers (R/A). Do not attach the raking tube more than 300mm from the standard.

For system or modular type scaffolding, ideally attach the tube brace to the standards of the scaffold or to ledgers or guardrails. Ledgers or guardrails must be locked into place by using a check clip above the joins or pins of the ledgers and guardrails. Do not attach the raking tube more than 300mm from the standard.

The tube brace strengthens the raking tube and prevents it from bending or spreading, see figure 55. Where possible this brace should be above head height. Where the raking tube connects to the ground it is advisable to use a soleboard and baseplate to distribute the point load of the tube. When erecting rakers on soft ground where a soleboard under the raker is not suitable, it is advisable to drive the raking tube into the ground to find a solid base. If this is required a horizontal tube must be connected perpendicular to the raker at ground level using a load bearing fitting. This prevents the raker from being forced further into the ground. Alternatively a tube can be driven into the ground and the raker attached to this tube as close to the ground as possible.

Running a horizontal tube along the raking tube connected with double couplers, then connecting the tube brace to the horizontal tube within 300mm of the raking tube is acceptable.

Figure 55: Raker with tube brace

An additional tube can be positioned at the lower working platform height, or as low as possible, for additional stability. Raking tubes may need to be braced in two directions for additional strength.

6.3 Scaffolds for roof edge protection (guardrailing)

This section deals with the provision of roof edge protection using standard scaffolding.

The width of the soffit and the position of the scaffold in relation to the building will determine the method used in order to utilise the outside and inside standards of a scaffold to support the guardrails. A dogleg brace (transverse brace) can be used to stabilise the extended standard. Alternatively, where the roofline, if continued, intersects the inside or outside standard it is recommended that a guardrail be placed within + or - 200mm from this intersection point (see figure 56).

Figure 56: Use of scaffolding for roof protection

6.3.1 Roof pitch less than 25 degrees

A scaffold platform may be positioned to provide roof edge protection. Figure 57 shows the location of the scaffold platform when scaffolding is used to provide guardrailing for roof work and the roof pitch is less than or equal to 25 degrees.

To minimise the risk of serious injury from falling onto a scaffold platform from the edge of the roof locate the platform as near to the gutter line as possible and no more than 1.0m below the lower edge of the roof surface. Guardrails should be spaced at approximately 500mm centres up from the deck with the top guardrail a minimum of 900mm above where the line of the roof pitch intersects the outside standard, as shown in figure 57.

Ensure the scaffold is secured to the building to prevent overturning should someone fall from the roof and strike the guardrails. This can be achieved by tying to the building, using raker bays or raking tubes, or by widening the base of the scaffold.

Figure 57: Location of scaffold platform for edge protection (roof pitch 25° or less)

6.3.2 Roof pitch greater than 25 degrees

When a roof slope exceeds 25 degrees the potential for sliding down the roof becomes greater. If a scaffold platform is used to provide guardrailing in this situation it is recommended that the working platform be placed as close as practical to the edge of the roof + or - 200mm. This can also be achieved by using a hop up bracket from the outside standard at the roof edge line. The guardrailing should be fitted as previously shown. It is recommended that a gap of no more than 100mm be left between the deck and the finished gutter. If this cannot be achieved position the deck as close as practical to the gutter, but it must not exceed 200mm from the gutter line to the leading edge of the plank (see figure 58).

Figure 58: Location of scaffold platform for edge protection (roof pitch greater than 25°)

6.3.3 Roof edge protection for gable ends

Edge protection must be provided as close as practicable to the gable ends with the scaffold no more than 300mm from the structure. The midrail should be approximately 500mm above the line of the gable (slope of the roof) and the top guardrail 1000mm above the line of the gable.

Figure 59: Edge protection at gable ends
Access omitted for clarity.

6.4 Scaffolding over verandahs, gantries or roofs

Scaffolding over verandahs, gantries or roofs can be potentially hazardous due to a variety of different hazards associated with scaffolding on, through or over a structure.

In these circumstances, once you have determined the requirements of the scaffold the following shows some of the options available to you:

Scaffold directly through the verandah e.g. standards to pass through the verandah.
Backprop the verandah directly below each standard. This transfers the scaffold weight through the verandah to the backpropping directly below each standard.
Backprop the verandah when the standards and backpropping are not directly in line. This may require using a beam system on top of the backpropping under the verandah and a similar beam system on top of the verandah below the standards. This method is used to transfer the load of the standard along the beam system when the backpropping has to be offset.
Scaffold directly on the verandah. An engineer must certify that the verandah can support the imposed loads of the scaffold and the DOL may request to inspect the engineer's certificate.
Erect a heavy-duty gantry over the verandah and scaffold from the gantry.
Cantilever out a window or opening above the verandah to support the scaffold.
Hang a scaffold from the parapet to form a hanging scaffold.
Suspend a scaffold (swinging stage) or (boatswain's chair).

It is advisable to get an engineer's certificate on any backpropping required other than backpropping directly below the standards when the verandah supports are clearly shown. It is recommended that all scaffolds erected on verandahs, gantries or roofs be checked by a chartered engineer, thereby transferring the liability to the engineer to state that the verandah, gantry or roof can support the imposed loads.

Figure 60: Scaffold through verandah

Figure 61: Backprop directly below standards

With backpropping, either an engineer will certify the position and spacing of the verandah support to ensure that the weight of the standards is transferred directly onto the back propping or you will have to expose the verandah supports. Do not assume that the supports within the verandah that are best to be used to transfer weight run perpendicular to the building.

Figure 62 shows one leg of the scaffold going through the verandah. The inside leg of the scaffold is supported by the verandah or load is transferred to the tie. An engineers certificate would be required to ensure the verandah could support the load of the inside standard. It would not be required if you could ensure that the outside standard and the tie arrangement could support the intended load.

Figure 62: Outside standard through verandah

Figure 63 shows scaffold erected on a verandah with only one standard directly below backpropped. An engineer's certificate would be required if you could not guarantee that the outside standard and the tie could support the inside standard weight.

Figure 63: Outside standard backpropped

Figure 64 shows a scaffold erected above the verandah with the inside leg of the scaffold directly backpropped below but with the outside of the verandah only supported from below. An engineer's certificate would be required unless you have exposed the verandah supports and ensured that the weight of the scaffold can be transferred through the verandah to the outside prop.

Figure 64: Inside standard directly backpropped, outside of verandah supported

Figure 65 shows a scaffold erected above a verandah with only the outside of the verandah backpropped. An engineer's certificate would be required.

Figure 65: Outside of verandah backpropped

Figure 66 shows a heavy duty steel gantry erected on the footpath with the scaffold erected above. The steel gantry would have to be designed by a chartered engineer.

Figure 66: Gantry supports verandah

Figure 67 shows a scaffold erected directly on a verandah. In this circumstance verandahs must be checked by a chartered engineer.

Figure 67: Scaffold directly on Verandah

6.4.1 Supporting structures

The scaffold needs to be designed to ensure that the load placed on the supporting structure under the most adverse circumstances does not adversely affect its structural integrity or the serviceability of the scaffold during the period of its service. Where necessary, the supporting structure needs to be strengthened to ensure it has sufficient strength and rigidity to accommodate the imposed loads. Provision may be required to prevent damage to the supporting structure.

Where soleplates are used they have to be designed to have sufficient strength and rigidity to distribute the load to the supporting structure.