Basic Scaffolding Construction: An Overview of British Standard BS 5973-1993
Introduction, Basic Definitions, Major Components
What is Scaffolding?
Scaffolding is a basic temporary structure embracing an existing or future possible structure per design that aids the construction worker in having safe and ergonomic access to intended job and broadly used in industries and private properties, to some extent.
For building and use of a Scaffold, optimum protection, supervision etc. are required as failure to do so may cause injury leading to fatality. Incidents also contribute to loss of work hour, resources, human lives, effect to environment and countless other aspects of our personal and professional lives.
There are a number of Standards that could have considered for Scaffold construction, for instance, the US OSHA standard, British Standard, Australian Standard etc. Business and Personal choice and implementation of standard depend primarily on compliance with local legislation and of course as set by an individual standard.
British Standard for building a Scaffold offers a definitive exposure to the relevant construction and safety aspect right from the beginning of construction and day to day inspection that fosters safe work practice and maintenance of the temporary structure built to meet business or personal needs.
Considering the most potential hazards associated with building, maintenance and working on Scaffolding, the common issues observed are:
a. Removing boards from working platform without proper communication and meeting the standard requirement
b. Taking bracing out at the platform levels
c. Removing hand rails and toe boards
d. Taking ladders away from their intended places and
e. Failing to stack material properly
Definitions
The basic definitions related to Scaffolding are as follows:
Standard: An upright component supporting tube bearing on the ground.
Ledger: An horizontal component / tube that supports transom.
Transom: A tube fixed across two or more components that balances out the lateral imbalanced force and provides stability.
Sway Brace: Restricts left to right movement of Scaffolding considering the front view of the structure.
Base Plate: Used to distribute the load from standards to ground.
Sole Plate / boards: A suitable piece of timber to spread the load.
Foot lift: A lift is erected near the ground to reinforce the base of the scaffold and allow clearance of that base from ground level.
Scaffold bay: The spans of upright members / Standards along the Front View of Scaffold. A rule of thumb in load calculation to follow is "Smaller the bay, larger the Safe Working Load (SWL) of that Scaffold."
Height: The vertical distance considering the base and topmost assembly of the Ledger and Transoms.
Width: The width of a scaffold measured at right angles to the ledgers from the center of the standards. Another measure to calculate the width is by calculating the number of boards in between two adjunct Standards.
Length: The length of a Scaffold between to standards sitting at either ends of a Scaffold structure.
Lift Height: The elevation length between main horizontal members that are called Ledgers.
Puncheon: A Standard that is not supported from the ground.
Spur: An inclining bearing tube used for structure reinforcement from rickety or wobbly movement along longitudinal or lateral direction.
Needle: Needle is one horizontal cantilever tube.
Butt tube: Any bearing tube of comparatively short in length.
Scarf: The length of an Upright / Standard above the working lift.
Lapping: Lapping means joining standards or ledgers that reinforces the structure.
Decking out: It means boarding out of working platform.
Sheeting out: Horizontal or vertical component fixed on inclined sheets of material such as corrugated or plastic sheet attached to the scaffold to provide protection.
Toe board: A barricade installed at the platform ground level next to "toe" preventing inadvertent dropping off tools, objects, small debris from construction work to lower platform and grade.
Guardrail: Guardrail is an engineered control for falling off of height and has got set vertical distances. Along the horizontal direction of a structure, there is usually a couple of Guardrails, namely, top and mid guard rail. There is also stop end guard rail and drop bars in place.
Scaffold tie: This is another reinforcement process used by attaching the scaffold to an existing structure / building for stability. A tie that is fixed to a building is called the Physical Tie.
Reveal tie: Tube wedged in to the opening in the building.
Foot tie: A tube added to secure the base of the standard.
Scaffold Tubes: Tubes should be manufactured and tested in accordance with BS 1139 Part 1 Specification for tubes for use in Scaffold construction.
The array of Scaffolding tubes follows the following:
a. Black steel
b. Galvanized steel
c. Aluminum alloy tubes
The specifics of Aluminum Tubes are as follows:
Outside diameter 48.3 mm
Nominal wall thickness 4.5 mm
Weight 1.7 kg / m
Heat Treatment: Aluminum tubes are tempered and subject to weakening of further heat treatment.
Apart from the Aluminum tubes, the Black and Galvanized steel tubes are both similar in dimensions. However, Galvanized ones are preferred over the other due to being corrosion resistant and protected from weathering effect.
The specifics of Galvanized Tubes are as follows:
Outside diameter 48.3 mm
Nominal wall thickness 4 mm
Weight 4.37 kg / m
Length 6.4 m
A precaution that should be followed while choosing the construction material is the different type of tubes should not be mixed together because of their difference in Material properties like Yield Stress, Elasticity, Fatigue, Bulk Modulus etc.
The end of the tube should be square to the tube axis. If it's needed to be cut, the cut surface containing sharp edge should be trimmed to fillet; otherwise it generates fair chance of getting cut, lacerated or worse during construction and use of the structure.
Before use, all the tubes must be inspected for splits, flaws, corrosion etc. The marginal value for weight for such scaffold tube is 3.75 kg / m beyond which, the tube must be scrapped of the bunch.
BS 1139 Part 1 dictates that tube should not deviate by more than 1/600 of its length.
Scaffold Fittings: Scaffold fittings can be Drop forged, sprung steel or Pressed steel. The standard followed for Scaffold Fittings is BS 1139 Part 2 Specification for couplers and fittings for use in tubular scaffolding.
Mass: For steel coupler, it ranges from 1 kg to 2.5 kg.
Precaution: Couplers and fittings must be checked for rust, thread damage, wear, tear and distortion.
Types:
Right Angle Coupler: Used for connecting tubes at right angles, for instance, standard to ledger connection.
Specification: Pressed steel is used and Safe Working Load is 6.25 kilo newton.
Swivel Coupler: Used for connecting tubes at any angle of desire as it has greater degree of freedom than the earlier kind. This could be used for a brace to a Standard.
Specification: Pressed steel is used Safe Working Load is 6.25 kilo newton.
Sleeve Coupler: End to end connection in between tubes is built by using this external fitting and works practically as a metal sleeve.
Specification: Pressed steel is used and Safe Working Load is 3.1 kilo newton.
Putlog Coupler / Single Clamp: This type of coupler is used of either of the below mentioned purposes:
a. Fixing a putlog
b. Fixing a transom to a ledger and
c. Connect a tube used as a guardrail
Specification: Pressed steel is used and Safe Working Load is 0.5 kilo newton.
Girder Coupler / Beam Clamp: This type of coupler is used with I-beam in a structure or the similar kind and they are used in pairs.
Specification: Drop forged steel is used and Safe Working Load is 30 kilo newton.
Base Plate: This is a square shaped ground rested piece of fitting in Scaffolding that:
a. Distributes the load from the standard to ground
b. Restricts lateral movement or swaying and
c. Helps preventing damaging the tube material
Specification: Pressed steel is used and Dimensions are 150 mm x 150 mm x 7 mm. There is one vertical pin at the center with Outer Diameter 50.8 mm.
There is one special type of Base Plate apart from the basic form of this fitting, namely the Adjustable Base Plate allowing adjustment of the Standards to fit the varying floor level.
Sole Plate: Sole plates are used together with the base plate to ensure adequate load transfer to the ground for designed balance of Scaffold nodes.
Specification:
Material Pressed steel
Dimension (Firm Ground) 500 mm x 225 mm x 35 mm
Dimension (Soft Ground) 765 mm x 225 mm x 35 mm
Dimension (Two Standard Scaffold) 1550 mm x 225 mm x 35 mm
There is one special type of Base Plate apart from the basic form of this fitting, namely the Adjustable Base Plate allowing adjustment of the Standards to fit the varying floor level.
Scaffold Boards (Timber):
According to BS 2482- 1981 specification for Timber Scaffold Boards, it is deemed necessary to check for the condition before each use and color code accordingly.
The expected dimension of each piece should be as follows:
Material Sawn Softwood
Dimension 3000/2500/2000/1500/1000 mm x 200 mm x 38 mm
Vertical pin at center Outer diameter 50.8 mm
Precaution:
Legibly marked timber piece with BS 2482 Accompanied by Supplier Trade Mark / Identification, Machine (M) / Visual (V) gradation symbol, supporting span length followed by denotation MAX are required to be in place before use. The boards should also be of fire retardant application.
Relation of thickness to Span, Overhang and Pressure reference table:
Dimension (Firm Ground) Dimension (Firm Ground) Dimension (Firm Ground) Dimension (Firm Ground) Dimension (Firm Ground)
38 mm 1.5 m 50 mm 150 mm 24 kg / m2
50 mm 2.6 m 50 mm 200 mm 33 kg / m2
63 mm 3.25 m 50 mm 250 mm 41 kg / m2