There is a limit to the safe working height at which a worker can access the building work from ground level. Therefore some form of temporary support is required to provide a safe and convenient working surface. This is known as scaffold or scaffolding. Scaffolding is used on new-build projects and for work to existing structures, including maintenance and repair work. The temporary structure needs to be structurally safe yet also capable of rapid erection, disassembly and reuse.
- Provide a safe working horizontal platform
- Provide safe horizontal and vertical access to buildings
Scaffold may be owned and maintained by a contractor, although it is more common for the scaffolding to be hired from a scaffolding subcontractor as and when required. Temporary structures must be designed to suit their purpose by a competent person (e.g. certified structural engineer)
A competent person must inspect the whole of the scaffolding and associated temporary supports, including the tying in and sections that are welded, bolted and fabricated off site, prior to use. The inspection must be recorded in the site log. Subsequently the structure must be checked on a regular basis to ensure it remains safe throughout its use. Scaffolding and temporary works should always be checked before use following extreme weather conditions,e.g. strong winds.
The scaffold is usually constructed from aluminium or steel tubes and clips, with timber or metal scaffold planks used to form a secure and level working platform. Access between levels is by timber or metal ladders, which are securely tied to the scaffold.Other common components are scaffold boards and edge protection.
A standard scaffold board is 225mm wide by 38mm thick with a maximum span of 1.5m. The board is made from sawn softwood. Lightweight metal scaffold boards are used in some systems. Greater spans can be achieved by using thicker boards; the distances between transoms on which the scaffolding boards span must not exceed the maximum span allowed for each board. Each board must be closely butted together so that there is no chance of the board slipping off the supporting tubes. Each board must overhang the ledger by 50mm, but the overhang must not exceed four times the thickness of each scaffolding board.
Scaffolding boards are butted together to make a working platform; the minimum working platform depth is three boards. When materials are loaded onto the platform, the clear passage for workers should be at least 430mm. If the materials are to be manoeuvred on the scaffold, a distance of 600mm clear pedestrian passage must be maintained at all times. When laying bricks, the scaffold platform should be at least five boards wide (1150mm). Hop-up brackets may be used to increase the working height of the lift and to increase the working width of the scaffolding platform. When using hop-up brackets, care must be taken not to overload the scaffold. The cantilevered bracket induces bending moments in the standards.
Toe boards must be used at the end of the scaffolding to ensure that materials and tools do not fall off the scaffold. The toe boards must be a minimum height of 150mm. The boards also prevent the possibility of people slipping off the edge of the platform. Toe boards may be removed to allow access for materials and workers, but must be replaced immediately afterwards.
Safety for pedestrians
When scaffolding is positioned in areas accessed by pedestrians it is necessary to use high visibility warning tape and lighting. Lagging and padding can be placed around the standards to reduce the risk of injury from accidental impact with the scaffold. Close boarding, netting and sheeting must be used to prevent objects from falling onto anyone below. While the scaffolding is being erected it will be necessary to use a physical barrier to protect the area. Warning signs should be used to prevent access by unauthorised persons and signage should be positioned at access points.
Putlog scaffolds are erected as the external wall is constructed. The scaffolding uses the external wall as part of the support system . Standards and ledgers are tied to the putlogs. Each putlog has one flat end that rests on the bed or perpendicular joints in the brick or blockwork.
The blade end of the putlog is usually placed horizontally and inserted fully into the brickwork joint, ensuring a full bearing is achieved. Where putlog scaffolds are used on refurbishment work, joints may be raked out to insert the blade end. In such works the blade may also be placed vertically. Where the putlog scaffold is used in new works, the putlog is placed on the wall at the required lift height and the wall is constructed around the blade end of the putlog. While the system uses less scaffolding and is less expensive than independent scaffolding, it is essential that the erection of the scaffold is coordinated with the sequencing of brickwork. The scaffold lifts must progress at the same speed as the masonry work. Health and safety requirements call for competent and certified scaffolding erectors to construct and alter scaffolding; thus this system is not used as much as it used to be. At one time, bricklaying gangs would have a labourer who could also erect the scaffold as the brickwork progressed. With good scheduling and coordination of brickwork and scaffold lifts, the system can still prove economical.Zigzag (sway) bracing is applied diagonally to the face of the scaffold, tying the ledgers and standards together. Plan bracing and ledger bracing should be used where specified.
These scaffolds are erected ‘independently’ of the building structure, unlike putlog scaffolds, and are tied to the structure through window openings. Ties are required to ensure horizontal stability is maintained.
Independent scaffolds are constructed from two parallel rows of standards tied by transoms, which bridge the width of the scaffold, and ledgers, which run along the length of the scaffold. A space is usually maintained between the scaffold and the building to allow the masonry to progress unhindered by the scaffold. The gap allows the brickwork to be checked for plumb and also helps to reduce damage to the brickwork caused by mortar snots splashing off the scaffold and onto the wall. On long stretches of scaffold, continuous diagonal tubes can be used to run from the top to the bottom of the scaffold structure. These act as façade bracing. Bracing is used to resist horizontal loads and to stiffen the structure. The bracing prevents distortion to the rectangular grids. Sway (zigzag) bracing may be applied diagonally to the face of the scaffold, tying the ledgers and standards together. Lateral bracing is also applied across the ledgers.
Proprietary scaffold systems
Proprietary systems are another type of independent scaffold. Proprietary systems rely on the same principles as independent scaffold but use standard lengths for ledgers, transoms and standards, all of which are capable of being clipped together and dismantled easily and quickly.
The standards often come with spigot ends, which allow the next standard to be located over the locating spigot very quickly. The jointing systems vary depending on the manufacturer. Proprietary systems rely on ledgers and transoms having a locating lug or bracket fixed to each end; these ends can be quickly dropped into the clips, sockets or cups, which are fixed at regular intervals on the standards. Components, such as ledgers and transoms, are designed so that they can be interchangeable.
For small scaffolds, bracing may not be required across the width of the scaffold since the frame is very rigid. Bracing must, however, run across the bays in accordance with the manufacturer’s instructions. Where loads are increased or hop-ups are used, additional bracing is required. Each proprietary system varies with manufacturer and system, and the manufacturer’s instructions must be carefully followed to ensure that the scaffold is erected safely.
To aid the flow of work, ‘hop-up’ scaffolding units can be used to increase the height that the workforce can access at each lift. These can be used between the standards or can be used between the internal standard and wall, providing a platform that is closer to the area of work and not reducing the width of the standard platform.
Proprietary scaffolds have the benefit of rapid erection and disassembly; however, their use is limited to relatively standard operations due to the size of the components. Where loads are known to be considerable and the scaffolding arrangement is complicated due to specific project layout/geometry, then traditional scaffolding designed by a structural engineer may provide a more flexible and appropriate solution.