Scaffolding in New Zealand - Best Practice Guideline for
SECTION 4: TUBE AND FITTING SCAFFOLDING - Continued
4.5 Scaffolding ties
A tie is the attachment by which scaffolding is connected to a structure. A tie that is not capable of transmitting tension and compression must be used in conjunction with a spreader or putlog extension. They must be placed close to each other and be adequately held in position.
4.5.1 Requirements for scaffolding ties
Ties are necessary to prevent inward or outward movement of the scaffold and to assist the standards to act as load-carrying members. To facilitate stability ties must be:
- Fixed to the standard as close as practical to the ledger intersection. If ties can't be fixed to the standard they can be fixed across ledgers with double couplers.
- Capable of safely withstanding a horizontal force equivalent to 10 percent of the weight of the scaffold plus the full live load on the area of scaffolding in the vicinity of each tie, and the environmental load.
- Capable of supporting all loads and forces that bear on it. The safe working load on a tie must not exceed one quarter of the breaking load of the tie and anchorage. Also the structure you are tying to must be capable of supporting the loadings placed on it.
- Used with spreaders or extended putlogs to the building or structure where they are of a type that is not capable of transmitting tension and compression, such as wire ties.
Wire ties using 3.14mm black tie wire have traditionally been the accepted tie method. However, where possible, these ties should be replaced with a rigid tie.
4.5.1.1 Tie spacings
It is recommended that ties be spaced at a maximum of 4.8m in the horizontal plane and 4.2m in the vertical. Ideally alternate the ties at every second lift and set of standards. Tie spacings can be increased if required but additional strengthening of the scaffold may be needed. For example, plan bracing a lift at the tie height would mean you could increase the tie spacings to 9.6m in the horizontal. The recommended spacings can also be increased by using raking tubes or raker bays (bolster bays) at the base of the scaffold to increase its base width.
Spacing distances may differ for several reasons. Some examples include:
- Use of screen mesh or other material which adds a wind loading.
- Other environmental loads.
- Lifting appliances attached to a scaffold.
- Through load transfer to the scaffold base, lower standards supporting high dead and/or live loads.
- Use of plan bracing,
- Use of raker bracing to the ground or other substantive support surface.
- Tie systems designed to support proprietary scaffolds.
In most instances these tie spacings are determined by design.
4.5.2 Rigid ties
A rigid tie is a scaffold tube e.g. a prop inner with a baseplate, with the same dimensions and specifications as a standard baseplate, fixed to the end. The tube should be long enough to be fixed to standards and ledgers and butt the building or structure, but can be used from one standard or ledger if necessary. The baseplate has two or four holes of approximately 12mm to 16mm diameter drilled into the plate. The holes are marked through the plate onto the structure or building and the rigid tie slid back through the double couplers to enable the holes to be drilled. The rigid tie is slid back into position and an approved anchor is inserted and then tightened over the baseplate. All ties should be connected to the scaffold with double couplers. Only use load-bearing couplers (double couplers) to attach ties to scaffolds. A rigid tie assembly is shown at figure 20.
Commonly used anchors for rigid ties include dyna bolts, tru bolts, boa coils and chemset bolts. Always follow the manufacturer's specifications and design loads permitted on anchors.
Figure 20: Rigid tie assembly bolted to a wall[3]
4.5.3 Other types of ties
A variety of other tie types are shown in the figures below.
Figure 23: Standard reveal tie
Note that in the reveal tie shown in figure 23 a basejack tube and baseplate have been used
Wire ties
Figure 24: Wire tie system
Figure 24 shows a wire tie system waiting for the wire to be twisted. This will place the wire in tension and the putlog tube butted against the wall in compression. It is recommended that the tube butting the wall be no further than 300mm from the tie assembly.
Loading on wire ties
Tie wire, double 3.14mm "black" wire twisted 2 turns: SWL 160kg or 1.6kN
Tie wire, double 3.14mm "galvanised twisted 2 turns: SWL 160kg or 1.6kN
Tie wire, double 3.14mm "galvanised twisted 3 turns: SWL 250kg or 2.5kN
Tie wire, double 3.14mm "galvanised twisted 4 turns: SWL 300kg or 3.0kN
AS/NZS 4576 requires that each tie assembly have a minimum tensile and compressive working load capacity of 6.0kN (610kg). It is therefore recommended that additional wire ties be used to gain the 6.0kN (610kg) safe working load capacity or, preferably, that wire ties be replaced with rigid ties.
4.6 Bracing
All scaffold structures should be braced in at least two directions or planes. All bracing should commence as close as practicable to the ground and intersect with the scaffold node points if possible.
4.6.1 Longitudinal bracing or face bracing
Longitudinal bracing or face bracing is usually fixed to the outside of the scaffold to prevent the horizontal movement of the scaffold, also termed "sway brace".
Figures 25 - 28 show different configurations of longitudinal bracing that runs parallel to the work face. Longitudinal bracing can be attached to putlogs or transoms using double or R/A couplers. This is the most widely used method, but on a standard 2.4m long bay x 2.0m high lift the angle of the longitudinal brace is less than 45 degrees to the horizontal. Therefore, alternatively, longitudinal bracing can be attached to the standards directly by using swivel couplers allowing the longitudinal brace to be positioned as close as practicable to 45 degrees (> 40 degrees and < 50 degrees).
Longitudinal bracing should be placed as close as practicable to the base of the scaffold - run the longitudinal brace to the ground if possible. This will add to the stability of the scaffold and reduce any horizontal movement below the longitudinal brace. Generally longitudinal bracing, either dogleg or parallel, is connected from standard to standard in one scaffold bay.
For tube and fitting scaffolds longitudinal bracing should be spaced a maximum of every sixth set of standards (maximum spacing 14.4m) and run to the top working platform where possible. For proprietary scaffolds longitudinal bracing should follow the manufacturer's specifications.
Figure 25: Spacing for longitudinal bracing
Figure 26: Dog leg longitudinal bracing
Figure 27: Diamond longitudinal bracing
Figure 28: Parallel longitudinal bracing
4.6.2 Plan bracing
Plan bracing is used to stabilise a scaffold in the horizontal plane. Attaching braces across standards as in figure 29 and using double couplers (R/A couplers) follows the same principle as transverse and longitudinal bracing.
On such scaffolds as a tall circular vessel exterior scaffold (chimney scaffold), plan bracing can be used to prevent the rotation of the scaffold when only butt ties can be used against the vessel. Further, when tie spacings cannot be adhered to plan bracing can be used to reduce the number of ties required.
Figure 29: Principles of plan bracing
4.6.3 Transverse bracing
Ideally, transverse bracing should be attached from the underside of the ledger above the lift with a double coupler (R/A coupler) to above the ledger at the platform height. Alternatively, transverse bracing can be attached from the ledger above the lift with a double coupler to the standard with a swivel coupler. This may be necessary to clear kickboards for example. Transverse bracing may also be attached from standard to standard with swivel couplers.
For tube and fitting scaffolds it is recommended that transverse bracing be placed a maximum of every fourth set of standards and run to the top working platform where possible. The frequency of transverse bracing will also be dependent on the tie spacings.
4.6.4 Dog leg bracing
This is the most commonly used form of transverse bracing. When it is impractical to fit ties at vertical spacings, dogleg bracings should be used. Temporary ties may be required to ensure the stability of the scaffold during erection and dismantling. When using dogleg bracing ensure ties are capable of carrying the substantial loads required of them.
Figure 30: Dog leg bracing
4.6.5 Parallel bracing
When transverse bracing is established diagonally between inner and outer standards on multiple levels it forms a parallel brace, as seen in figure 31.
Ideally get the transverse brace at the base of the scaffold as close as possible to the ground. The stability of the base of the scaffold will be greatly improved if the brace can be either:
- Butted to the base of the wall you are scaffolding; or
- Forced into the surface you are scaffolding from.
It is acceptable on light duty scaffold to connect the lower section of the brace to the standard by using a short butt tube off the standard with double couplers. Then connect the brace with a double coupler as close as practicable to the standard in the event of having no swivels.
Figure 31: Parallel bracing (shown in two directions)
Footnotes
[3] This is the preferred tie method in most circumstances