Fabricated hung scaffolds

Working with fabricated hung scaffolds? Find out about their construction, compliance and safety factors.

Design

Fabricated hung scaffolds are purpose-designed temporary structures. They are anchored to a permanent structure to support a working platform.

Unlike suspended scaffolds, they are not able to be raised or lowered while in use.

They are usually installed as a static structure. But they can sometimes be hung from girder trolleys or mobile suspension rigs so they can be horizontally travelled.

Fabricated hung scaffolds are usually constructed from structural steel, aluminium or timber components. Typical working platforms include:

• checkerplate
• gridmesh
• timber flooring.

Uses

Fabricated hung scaffolding is used when building large structures, such as power stations. These scaffolds provide platforms for steel erection and consolidation. They also provide temporary access ways between parts of the unfinished structure.

Fabricated hung scaffolding is also used during the construction of oil and gas rigs, or slung under bridges for maintenance and repair work.

There are two types of fabricated hung scaffolds:

• those fixed to structural members on the ground before lifting, and
• those independently lifted into position using cranes, winches or chain blocks.

Certification

The installation and dismantling of fabricated hung scaffolds must be carried out or directly supervised by a person holding one of these certificates:

• Advanced Rigging certificate
• Advanced Scaffolding certificate.

Hung scaffolds which are not fully fabricated, such as those constructed from tubes and couplers, need the Advanced Scaffolding certificate.

Design compliance

Fabricated hung scaffolds should comply with the minimum requirements of AS 1576.1 Scaffolding-General requirements.

A competent person (like a structural engineer) should verify this in writing. They should verify the compliance of:

• the design of the fabricated hung scaffolds, and
• the strength and condition of the supporting structure.

Their written verification should specify:

• the duty classification of the working platform, and
• the method(s) of anchorage to the supporting structure.

Before installing the scaffold, make sure you have sighted the written verification and carefully read any of the limitations and conditions attached.

Duty classifications

Fabricated hung scaffolds are classified in the following four ways:

• Light Duty
  Maximum allowable live load: 225kg per platform per bay
  Minimum platform width: 450mm
   
• Medium Duty
  Maximum allowable live load: 450kg per platform per bay
  Minimum platform width: 900mm
   
• Heavy Duty
  Maximum allowable live load: 675kg per platform per bay
  Minimum platform width: 1m
• Special Duty
  Maximum allowable live load: as specified in the design verification (but greater than heavy duty)
  Minimum platform width: as specified (but at least 1m).

Platforms used for through access of persons and materials must be designed to at least heavy duty specifications. But the platform width can be reduced to 450mm (for persons and hand-tools only) or 675mm (for general materials movement).

Platform construction

The platform of a fabricated hung scaffold must be closely decked with an even, slip-resistant surface with no trip hazards. The platform components must be secured to prevent uplift or dislodgment.

The platform should be horizontal. In some cases, the scaffold may be designed to have a sloping platform (such as for continuous access under sloping structural beams).

Slope

The maximum allowable slope on a working platform is 7° (1:8).

For access platforms, the slope may be increased to 20° (1:3). This can be done if the full width of the platform surface is cleated to prevent people from slipping. Cleats should be 25mm thick by 50 mm wide and should be secured at intervals of 450mm.

Edges

Edge protection is required to be risk assessed at the open sides and ends of all platforms from which a person or object could fall. Edge protection includes guardrailing and toeboards.

Guardrailing must be constructed from rigid components. Fibre rope, chain and steel wire rope is not permitted. The height of the guardrail must be not less than 900mm and not more than 1100mm above the platform surface.

Toeboards may be timber or metal kickplates. They must extend at least 150mm above the platform surface. Any gap between a toeboard and platform must not exceed 10mm.

The gap between the guardrail and the toeboard must be protected by a midrail or infill (like meshed screens or construction-grade plywood sheets).

Where a midrail is used and material stacked near the platform edge extends past the toeboard, additional infill (such as extra toeboards fixed above the existing toeboards) must be fitted. This is to prevent the possibility of any material being knocked over the platform edge.

Access to the working platform

Safe access must be provided to the working platform. Where direct access at the same level from the existing structure is not available, access such as ladders, stairways or ramps should be used.

Portable ladders used for access should be single ladders. Extension ladders are not suitable as access ladders. Ladders must be industrial grade. Domestic grade ladders are not to be used.

Access ladders should be fixed at a slope not less than 6:1 and not more than 4:1. They must be secured against movement in any direction. They must extend above landings by at least 900mm.

Scaffold anchorage

A fabricated hung scaffold can be fixed to the supporting structure in different ways, such as:

• by using integral rigid hanging supports yoked around or bolted to the overhead structure, or
• by hanging the scaffold with lifting slings and shackles.

Hanging supports can be vertical or angled, depending on the design and location of the support points.

Safety factors

The safety factors for slings supporting scaffolds are higher than the safety factors for general rigging.

The maximum load supported by a flexible steel wire rope must not exceed 1/6 of its minimum guaranteed breaking strain (compared to one fifth for general rigging).

This means the normal rigging rule used to calculate allowable load on a flexible steel wire rope cannot be used. Instead of 8 x diameter squared, use 7.5 x diameter squared.

The maximum load on a chain supporting a scaffold also cannot exceed 1/6 of the breaking strain.

Any sling or hanging member which directly supports the scaffold is performing the function of a scaffolding standard.

In calculating the maximum load on a standard, you need to include:

• the portion of the scaffold's self-weight supported by the standard (the dead load), and
• a portion of the maximum permissible duty classification (the live load).

To allow for uneven platform loading, it is always assumed that a standard is supporting at least one third of the duty live load on each platform and in each bay it serves.

For example, an intermediate standard in a run of heavy duty scaffold one bay in width, supports two longitudinally adjacent bays. If this scaffold has a single full-length working platform, the intermediate standard is assumed to be supporting a live load of 675 kg x 1/3 x 2 bays = 450kg.

Scaffolds hung from girder trolleys

Where the scaffold is to be hung from girder trolleys to provide horizontal travel, make sure the assembly of trolleys has been longitudinally and transversely fixed together with rigid tie bars and plan bracing. This is so that the hanging standards remain vertical and the trolleys do not move out of alignment with each other.

Make sure the open ends of the girders supporting the trolleys have been fitted with end stops. This is to prevent the trolleys running off the ends. Trolleys must have a working load limit (WLL) greater than the total loads they need to support.