Construction Technology - SCAFFOLDING (part 1)

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SCAFFOLDING

A scaffold is a temporary structure from which persons can gain access to a place of work in order to carry out building operations. It includes any working platforms, ladders and guard rails. Basically there are two forms of scaffolding:

• putlog scaffolds;
  
• independent scaffolds.
  

PUTLOG SCAFFOLDS

This form of scaffolding consists of a single row of uprights or standards set away from the wall at a distance that will accommodate the required width of the working platform. The standards are joined together with horizontal members called ledgers and are tied to the building with cross-members called putlogs.

The scaffold is erected as the building rises, and is used mostly for buildings of traditional brick construction (see Fig. 1.5.1).

INDEPENDENT SCAFFOLDS

An independent scaffold has two rows of standards, which are tied by cross-members called transoms. This form of scaffold does not rely upon the building for support and is therefore suitable for use in conjunction with framed structures (see Fig. 1.5.2).

Every scaffold should be securely tied to the building at intervals of approximately 3.600 m vertically and 6.000 m horizontally. This can be achieved by using a horizontal tube called a bridle bearing on the inside of the wall and across a window opening with cross-members connected to it (see Fig. 1.5.1); alternatively a tube with a reveal pin in the opening can provide a connection point for the cross-members (see Fig. 1.5.2). If suitable openings are not available then the scaffold should be strutted from the ground using raking tubes inclined towards the building.

MATERIALS

Scaffolding can be of:

• tubular steel;
  
• tubular aluminium alloy;
  
• timber.
  

TUBULAR STEEL

British Standard 1139 gives recommendations for both welded and seamless steel tubes of 48 mm outside diameter with a nominal 38 mm bore diameter. Steel tubes can be obtained galvanised (to guard against corrosion); ungalvanised tubes will require special care such as painting, varnishing or an oil bath after use. Steel tubes are nearly three times heavier than comparable aluminium alloy tubes, but are far stronger, and as their deflection is approximately one-third that of aluminium alloy

tubes, longer spans can be used.

ALUMINIUM ALLOY

Seamless tubes of aluminium alloy with a 48 mm outside diameter are specified in BS 1139 for metal scaffolding. No protective treatment is required unless they are to be used in contact with materials such as damp lime, wet cement or seawater, which can cause corrosion of the aluminium alloy tubes. A suitable protective treatment would be to coat the tubes with bitumastic paint before use.

TIMBER

The use of timber as a temporary structure in the form of a scaffold is now rarely encountered in the UK, although it is extensively used in the developing world.

The timber used is structural-quality softwood in either putlog or independent format. The members are lashed together with wire or rope instead of the coupling fittings used with metal scaffolds.

SCAFFOLD BOARDS

These are usually boards of softwood timber, complying with the recommendations of BS 2482, used to form the working platform at the required level. They should be formed out of specified softwoods of 225 mm × 38 mm section and not exceeding 4.800 m in length. To prevent the ends from splitting they should be end bound with not less than 25 mm wide × 0.9 mm galvanised hoop iron extending at least

150 mm along each edge and fixed with a minimum of two fixings to each end. The strength of the boards should be such that they can support a uniformly distributed load of 6.7 kN/m2 when supported at 1.200 m centres