Scaffolding
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- This article is not about instructional scaffolding.
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Scaffolding is a temporary framework used to support people and material in the construction or repair of buildings and other large structures. It is usually a modular system of metal pipes (termed tubes in Britain), although it can be made out of other materials. Bamboo is still used frequently in Asia.
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British scaffolding
The following description is for 'classic' rather than system scaffolding. All scaffolds are subject to certain minimum requirements set out in British Standard (BS) 5973 - the Code of Practice for Access and Working Scafolds and Special Scafold Structures in Steel. The Provision and Use of Work Equipment Regulations (1993) also applies.
Materials
The basic materials are tubes, couplers and boards.
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Tubes are either steel or aluminium. If steel they are either 'black' or galvanised. The tubes come in a variety of lenghts and a standard diameter of 48.3 mm. The chief difference between the two type of tubes is the greater lightness of aluminium tubes (1.7 kg/m as opposed to 4.4 kg/m) and also a greater flexibility and so less resistance to force. Tubes are generally bought in 6.3 m lengths and can then be cut down to certain typical sizes.
Boards provide a working surface for users of the scaffold. They are seasoned wood and come in three thicknesses (38 mm (usual), 50 mm and 63 mm) are a standard width (225 mm) and are a maximum of 3.9 m long. The board ends are protected by metal plates called hoop irons or sometimes nail plates. As well as wood steel or aluminium decking is used or laminate boards. As well as boards for the working platform there are sole boards which are placed beneath the scaffolding if the surface is soft or otherwise suspect, although ordinary boards can be used, and meet the minimum requirements, they can be too long and tougher boards can be necessary.
Couplers are the fittings which hold the tubes together The most common are called scaffold couplers, there are three basic types: right-angle couplers, putlog couplers and swivel couplers. To join tubes end-to-end joint pins (also called spigots) or sleeve couplers are used, or both together.
Other common materials lnclude base plates, ladders, ropes, anchor ties, reveal ties, gin wheels, sheeting etc. etc.
Despite the metric measurements given many scaffolders measure tubes and boards in imperial units. With tubes from 21 feet down and boards from 13 ft down.
Basic scaffolding
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The key elements of a scaffold are standards, ledgers and transoms. The standards, also called uprights, are the vertical tubes that transfer the entire mass of the structure to the ground where they rest on a square base plate to spread the load. The base plate has a shank in its centre to hold the tube and is sometimes pinned to a sole board. Ledgers are horizontal tubes which connect between the standards. Transoms rest upon the ledgers at right angles. Main transoms are placed next to the standards, they hold the standards in place and provide support for boards; intermediate transoms are those placed between the main transoms to provide extra support for boards.
As well as the tubes at right angles there are cross braces to increase rigidity, these are placed diagonally from ledger to ledger, next to the standards to which they are fitted. If the braces are fitted to the ledgers they are called ledger braces. To limit sway a facade brace is fitted to the face of the scaffold every 30 metres or so at an angle of 35°-55° running right from the base to the top of the scaffold and fixed at every level.
Of the couplers previously mentioned, right-angle couplers join ledgers or transoms to standards, putlog couplers join transoms to ledgers and swivel couplers are to connect tubes at any other angle. The actual joints are staggered to avoid occurring at the same level in neighbouring standards.
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The spacing of the basic elements in the scaffold are fairly standard. For a general purpose scaffold the maximum bay length is 2.1 m, for heavier work the bay size is reduced to 2 or even 1.8 m while for inspection a bay width of up to 2.7 m is allowed.
The scaffolding width is determined by the width of the boards, the minimum width allowed is 600 mm but a more typical four-board scaffold would be 870 mm wide from standard to standard. More heavy duty scaffolding can require 5, 6 or even up to 8 boards width. Often an inside board is added to reduce the gap between the inner standard and the structure.
The lift height, the spacing between ledgers is 2 m, although the base lift can be up to 2.7 m. The diagram above also shows a kicker lift, which is just 150 mm or so above the ground.
Transom spacing is determined by the thickness of the boards supported, 38 mm boards require a transom spacing of no more than 1.5 m while a 50 mm board can stand a transom spacing of 2.6 m and 63 mm boards can have a maximum span of 3.25 m. The minimum overhang for all boards is 50 mm.
Foundations
Good foundations are essential. Often scaffold framworks will require more than simple base plates to safely carry and spread the load. Scaffolding can be used without base plates on concrete or similar hard surfaces, although base plates are always recommended. For surfaces like pavements or tarmac base plates are necessary. For softer or more doubtful surfaces sole boards must be used, beneath a single standard a sole board should be at least 1,000 cm² with no dimension less than 220 mm, the thickness must be at least 350 mm. For heavier duty scaffold much more substantial baulks set in concrete can be required. On uneven ground steps must be cut for the base plates, a minimum step size of around 450 mm is recommended.
A working platform requires certain other elements to be safe. They must be close-boarded, have double guard rails and toe and stop boards. Safe and secure access must also be provided.
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Ties
Scaffolds are only rarely independent structures. To ensure a constant and correct space between the structure and the scaffold ties are used to link the two. General practice is to attach a tie every 6 m on alternate lifts. The ties are coupled to the scaffold as close to the junction of standard and ledger (node point) as possible. As many ties as possible are positive or two-way rather than simple friction or one-way.
Due to the different nature of structures there are a variety of different ties to take advantage of the opportunities.
Through ties are put through structure openings such as windows. A vertical inside tube crossing the opening is attached to the scaffold by a transom and a crossing horizontal tube on the outside called a bridle tube. The gaps between the tubes and the structure surfaces are packed or wedged with timber sections to ensure a solid fit.
Box ties are used to attach the scaffold to suitable pillars or comparable features. Two additionl transoms are put across from the lift on each side of the feature and are joined on both sides with shorter tubes called tie tubes. When a complete box tie is impossible a l-shaped lip tie can be used to hook the scaffold to the structure, to limit inward movement an additional transom, a butt transom, is place hard against the outside face of the structure.
Sometimes it is possible to use anchor ties (also called bolt ties), these are ties fitted into holes drilled in the structure. A common type is a ring bolt with an expanding wedge which is then tied to a node point.
The least 'invasive' tie is a reveal tie. These use an opening in the structure but use a tube wedged horizontally in the opening. The reveal tube is usually held in place by a reveal screw pin (an adjustable threaded bar) and protective packing at either end. A transom tie tube links the reveal tube to the scaffold. Reveal ties are not well regarded, they rely solely on friction and need regular checking so it is not recommended that more than half of all ties be reveal ties.
If it is not possible to use a safe number of ties rakers can be used. These are single tubes attached to a ledger extending out from the scaffold at an angle of less than 75° and securely founded. A transom at the base then completes a triangle back to the base of the main scaffold.
Putlog scaffold
As well as putlog couplers there are also putlog tubes, these have a flattened end or have been fitted with a blade. This feature allows the end of the tube to be within or rest upon the brickwork of the structure. They can be called a bricklayer's scaffold and as such consist only of a single row of standards with a single ledger, the putlogs are transoms - attached to the ledger at one end but integrated into the bricks at the other. Spacing is as general purpose scaffold and ties are still required.