SECTION 6: PARTICULAR SCAFFOLDS AND SCAFFOLDING STRUCTURES - Continued

6.5 Cantilevered scaffolds

A cantilevered scaffold is an independent tied standing scaffold constructed in accordance with the BPG but erected from cantilevered beams (RSJ, UB, soldiers, trusses, etc) out from a building or structure. Cantilevered scaffolds using a beam type arrangement (as seen in figure 68) must be designed by a chartered engineer.

Needles should be secured by through bolting (dividag bars), anchoring, or propping between the needle and the floor above. Where possible the inboard part of the needle should be at least 3 times the outboard length. The base of the scaffold should be tied to the needle as close as practical to the locating Uhead jack.

All practicable steps need to be taken to protect the area below the cantilever during the erection and dismantling process. Additional precautions such as full planking and plying the base lift of the scaffold, kickboards and screening should be used to prevent the dislodgement of materials from the working platforms.

Figure 68: Cantilevered scaffold using a beam arrangement

Figure 69 shows a scaffold erected out a window opening of a building. A propping tower is erected between two concrete floors with a needle transom (tube) extending through the opening. The cantilever effect of the weight of the intended scaffold is transferred to the lower window opening as shown.

Figure 69: Cantilevered scaffold - use of propping tower for weight transference

Figure 70 shows a proprietary mobile tower with cantilevered working platform. The structure is stabilised by use of a weighted raker arrangement at the base.

Figure 70: Mobile tower with cantilevered working platform

6.6 Heavy duty gantries

Heavy-duty loading gantries or pedestrian gantries should be designed by a chartered engineer and require the following basic components:

• Primary beams housed in Uhead jacks to transfer load directly down the standard.
• Primary beams connected to the Uhead jacks.
• Secondary beams at no more than 600mm centres.
• Secondary beams connected to primary beams.
• Top surface either plied, planked or both - can use formwork panels.
• Adequate longitudinal bracing.
• Adequate transverse bracing or a rigid tie system to a structure.
• Adequate edge protection (toeboard, guardrails, brick guards or screening).
• Adequate traffic protection for the gantry (water filled barriers, etc).
• Adequate distance from any roadway.

Check with local authorities for regulations regarding distances from roadways, footpath permits and minimum width and clearance requirements for pedestrians.

Figure 71: Heavy-duty gantry erected from proprietary scaffolding

6.7 Vessel scaffolds

The term vessel scaffolding is generically applied to all scaffolding that encompasses the entire face of a vessel (e.g. a tank or chimney). Tank scaffolding is commonly used as a particular term to describe vessel scaffolding around a tank. Figures 72 and 73 show a typical plan view and isometric view of a completed vessel scaffold.

Figure 72: Vessel scaffold - plan view

Figure 73: Vessel scaffold - isometric view

6.8 Hanging scaffolds

A hanging scaffold is a scaffold hung from a structure that is static in the vertical plane; it can not be raised or lowered by any means. Hanging scaffolds can be hung from tubes, wire ropes, ropes or chains etc.

Hanging scaffolds are classified as a special duty scaffold and should be designed and notified as such.

Hanging scaffold foundations are opposite to that of a standing scaffold. Particular attention must be paid to whether the structure the hanging scaffold is to be attached to is able to support the hanging scaffold and its intended loads.

6.8.1 Tube and fitting hanging scaffold

Hanging scaffolds must have additional check clips immediately above the suspension point and immediately below the bottom ledger. Generally a 'T bar' type arrangement should be used approximately 1.0m below the bottom ledger for the scaffolder to stand or sit on while the base lift of the scaffold is erected and dismantled. The T bar should have a check clip under it. Using a double coupler with the gate or flap open is not safe. The double coupler is not designed to take a persons weight and it has been known for the gate or flap to shear off the pin.

Beam clamps used to secure tubes to a beam, rolled steel joist (RSJ), universal beam (UB), etc should, where possible, be used in tandem on either side of the beam and ideally create a box tie around the beam.

It is possible to erect a hanging scaffold lower than the longest standard. In a tube and fitting hanging scaffold joiners must never be used to lengthen a standard. However additional standards may be attached to a minimum of two horizontal ledgers or guardrails within 300mm from the standard. Check clips are used above the ledger and below the ledger supporting the standard.

In system type scaffold the standard must be either joined with high tensile bolts or pins and/or spliced together with tube and fittings. A splice arrangement to connect two standards in tension should have at least two parallel couplers on either side of the standards join connecting each standard to the tube.

Steel Wire Rope (without eyes) used as a hanging scaffold should be fixed to a shackle by a wedge type socket together with a wire rope grip (bull dog clip) fixed to the rope tail. Alternatively, a thimble eye splice can be made using a double base clamp or three fist grip rope clamps. Wire rope grips (bull dog clip) should not be used to secure wire ropes that support persons or substantial materials. Wire rope grips must be correctly applied in order that the stresses within the terminal are evenly distributed. Always fit the grips the same way round, with the bridge on the loaded or long part of the rope and the U-bolt on the short part. On any wire rope a minimum of three grips must be used and spaced at a distance, centre to centre of grips, of six rope diameters.

Steel wire ropes or chains secured around the sharp edges of beams should be protected from damage by beam chaffers. Also ledgers supported from the eyes of slings or from shackles should be provided with a coupler on either side of each support point to prevent the ledger from sliding. Shackle pins must be moused to prevent unwinding.

Figure 74 shows a hanging tube and fitting scaffold supported from a beam. As here, always ensure that the putlog or transom above the steel beam is connected with beam clamps or box the beam with tube to prevent movement.

While check clips are placed on the standards above the top putlog or transom, the scaffold must also have check clips below the bottom putlog or transom and below the T bar from which the scaffolder erects the scaffold.

Figure 74: Above: Hanging tube and fitting scaffold hung from a beam

6.9 Screening or containment sheeting

Screening is used for both safety and environmental purposes. Where work is carried out close to pedestrian or vehicular access, scaffolds that are fully screened can minimise both the risk to the public and the area lost to public access. Note that toeboards must still be fitted to screened scaffolds

When selecting screening consider the following:

• What degree of protection is required?
• Is the containment of dust a requirement?
• What chemicals are to be used from the scaffold?
• The degree of flammability of the screening.
• The ventilation of the contained space (porosity).
• The need for light transmission.
• The size of sheets or screen sections.
• The requirements for fixing the screening.

Scaffolds fitted with screening have increased environmental loads (wind loads, rain loads, etc) and increase the dead load of the scaffold. Consequently additional ties are required for screened scaffolds. It is recommended that significant scaffolds or those exposed to extreme weather conditions be designed by a chartered engineer.

The design of a screened scaffold should consider the:

• Weight of the screening.
• Environmental load on the scaffold and the supporting structure.
• Position of the screened scaffold in relation to other structures.
• Fixing in respect to the strength of the screening.
• Ability of the fixing to fail if experiencing forces (especially wind) beyond the capacity of the ties or screening to withstand such forces. A "fail capacity" will prevent the scaffold or structure being damaged.

When fixing screening:

• Only fix screening from fully decked and guardrailed platforms.
• Screening should be fitted to the outside of the scaffold unless specified.
• Flush the outside of the scaffold to prevent tubes etc from protruding.
• Screening should be continuous either by using sufficient overlap (preferable) or by carefully butt joining the screening.
• Always secure the top edge of the screening before fixing the bottom edge.
• Use a tag line in windy conditions to control the screening during fixing.
• Keep the screening sufficiently taut.
• Screening should be lapped under from the top for containment and lapped over from the top for protection.
• Always screen the ends of the scaffold as close as practical to the building or structure to prevent the wind getting behind the screening.
• Screening blown into a scaffold has the scaffold as a framework to support it. Screening blown away from the scaffolding framework only has the ties to support it.

The following are some common materials used for screening or containment:

• Shade cloth.
• Monoflex.
• Shrink wrap.
• Plastic.

Hessian must not be used as screening as it is not fire retardant.

While there are different varieties of shade cloth, wind is able to pass through it depending on the screening's porosity. This not only reduces the wind loading on the scaffold and structure it also allows ventilation and light.

Products such as monoflex, shrink wrap and plastic almost completely contain the scaffold. They can therefore be fixed to create a negative air situation or special containment for asbestos removal. However they greatly increase the wind load and pressure on the scaffold and structure. Ventilation is a problem with these products.

When using proprietary scaffolding that does not have a positive joint between the vertical standards in conjunction with screening, it is recommended that joints be spliced or that additional bracing be provided across the joins to prevent uplift of the join.

6.9.1 Industrial safety nets

Industrial safety nets are sometimes used as an effective means of fall protection for people working at height, particularly where it is not practical to provide scaffolds or temporary guardrailing. They are generally manufactured from synthetic fibre knotted lines with a mesh size of about 100mm. These lines are attached to perimeter cords. Safety nets are usually available in 3.0m x 4.0m sizes or larger.

Each net will have a label that will state the maximum fall distance for which the net has been designed. This is usually between 1.0m and 6.0m.

It is generally accepted that securing a safety net to a building, structure or scaffold requires input from a chartered engineer, as the impact loads applied to a safety net require the supporting structure or scaffold to support these loads.

Refer to BS 3913 for connection methods, possible impact loads etc.

Figure 75 shows a plan view of a safety net cantilevered between two floor slabs. Figure 76 is a section view of the same structure.

Figure 75: Plan view of safety net cantilevered between two floor slabs

Figure 76: Section view of safety net cantilevered between two floor slabs

Figure 77: Typical arrangement of outrigger perimeter net

Figure 78: Typical net and its supporting framework

6.10 Mechanical lifting appliances on a scaffold

Hoists, winches and other lifting appliances may be mounted on scaffolding only if the scaffold framework is adequate in strength, or is specially strengthened and tied back, to take the imposed loads, both dead loads and live loads, to a maximum of 250kg.

The strengthening of the scaffold must be calculated as a minimum of at least 2 times the lifting capacity of the appliance.

Where the lifting capacity exceeds 250kg the scaffold must be strengthened in accordance with requirements imposed by a chartered engineer.

Figures 79 and 80 show typical scaffold supported materials or appliance hoists.

Figure 79: Mechanical materials hoist supported by a scaffold – side view

Figure 80: Mechanical material hoist supported by a scaffold - front view

6.10.1 Gin wheels

Gin wheels are used as lifting appliances in conjunction with ropes. They come in a variety of designs such as hook or ring types and are attached to a scaffold or structure to raise and lower a load. The maximum load permitted to be raised or lowered from a gin wheel is 50kg. The scaffold must be made stable enough to compensate for the weight of the intended loads by way of additional ties to the building or rakers.

The rope used for a gin wheel should preferably be a natural fibre rope (manila) with a minimum diameter of 16mm. Smaller diameter ropes can be spliced into this main rope to provide a more user friendly diameter rope to tie onto equipment. It is common to end splice the main gin wheel rope to form an endless rope with two smaller diameter ropes spliced into the main rope for attaching equipment.

Figure 81: Gin wheel less than 600mm from standard (ring attachment)

If a scaffold tube is cantilevered from a scaffold, including to support a gin wheel, it must be attached to standards using double couplers (R/A). The gin wheel must not be connected to this tube any more than 600mm from the outer standard unless supported by a brace (see figure 81). Here a gin wheel with a ring attachment goes over a scaffold tube but is locked in place with two scaffold fittings. If the gin wheel is less than 600mm from the standard then the tube does not need to be supported. Note that unsupported tube (putlog) must be connected to two standards (excluded for clarity in figure 81).

Figure 82 shows a gin wheel more than 600mm from the standard. The tube supporting the gin wheel must be braced as shown. Be aware of the overturning pressure this will place on the scaffold. Hence an additional tie is included as shown.

Figure 82: Gin wheel more than 600mm from standard

Figure 83: Gin wheel less than 600mm from standard (lashed)

Figure 83 shows a gin wheel with a hook arrangement lashed in position. The unsupported tube (putlog) must be connected to two standards. This is excluded for clarity in figure 83.

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