OHS Code Explanation Guide

Published Date: July 01, 2009
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Part 23 Scaffolds and Temporary Work Platforms

Section 324 Design

Subsections 324(1)(a) and 324(1)(b)

Tie-ins anchor a scaffold to the structure it serves, preventing the scaffold from falling into or away from the structure. Tie-ins also improve a scaffold’s lateral stability by bracing the structure. Figure 23.2 shows several of the many types of tie-ins that can be used. A reveal tie is considered to be a non-positive tie-in as it depends on friction for its holding power. A box tie is a positive tie-in because it encircles an immovable portion of the structure. Anchor bolt ties are yet another alternative.

A particular scaffold or load may require additional tie-ins. The 4.6 metre vertical and 6.4 metre horizontal intervals stated in the subsection are the minimum distances at which tie-ins must be placed. Tie-ins must never be placed at intervals greater than these minimum distances.

In some situations there may be an advantage to using tie-ins in combination with outriggers (the use of outriggers with free-standing scaffolds is discussed in section 334). When used in combination, outriggers can stabilize the scaffold up to a maximum height equal to 3 times the scaffold’s smallest base dimension. Beyond that height, tie-ins must be used as described in this section.

Subsection 324(1)(c)

Hoarding refers to tarps or other materials used to cover a scaffold. When hoarding is used, the stress on the ties stabilizing the scaffold increases due to wind loading. As a result, the number of tie-ins used must also increase. Rather than the 4.6 metre vertical and 6.4 metre horizontal intervals required for scaffolds that are not hoarded, hoarded scaffolds require tie-ins at 3 metre vertical and 3 metre horizontal intervals. Tie-ins on hoarded scaffolds must never be placed at intervals greater than these minimum distances.

Subsection 324(1)(d)

As required by Clause 7.2.3.1 of CSA Standard S269.2-M87 (R1998), Access Scaffolding for Construction Purposes, vertical load-carrying members must be erected and maintained within the following limits:

(a) not more than 12 millimetres (0.47 inches ) out of plumb in 3 metres (9.8 feet);
(b) not more than 19 millimetres (0.75 inches) out of plumb in 6 metres (20 feet); or
(c) not more than 38 millimetres (1.5 inches) in the height of the structure.

Departures from plumb must be corrected by adjusting the devices provided for this purpose, e.g. wedges, jackscrews, etc.

Devices such as base plates and jackscrews effectively disperse loads from scaffold vertical members to the scaffold foundation. A vertical member cannot rest directly on a mud sill, board or block of wood without an intervening load dispersing device. The compressive forces created at the end of the vertical member can easily exceed the strength of the sill, board or block, damaging it and making the scaffold unstable.

Baseplates and mudsills

A scaffold transmits its load through its legs to its baseplates and mudsills, and them onto the foundation. By using baseplates and mudsills to control load distribution, workers erecting the scaffold can significantly decrease the likelihood of foundation failure.

Figure 23.2 Examples of typical tie-ins

The importance of baseplates and mudsills is even more dramatic if the leg load transmitted to a foundation without them is considered. For example, consider a light duty scaffold one tier high supporting 122 kilograms/square metre (25 pounds/square foot). Assume a total surface area of 3.7 square metres (40 square feet) between its standards. This scaffold has a maximum intended load of 454 kilograms (1000 pounds) live load. Include an estimated 227 kilograms (500 pounds) for the scaffold dead load. The total leg load is therefore 681 kilograms (1500 pounds). Using the safety ratio of 4 times the intended load means that the foundation must support 2722 kilograms (6000 pounds).

If the load is level, the 2722 kilograms (6000 pounds) load is distributed evenly through the legs to the foundation. Each leg receives 681 kilograms (1500 pounds) of the load. This load is concentrated on the extremely small surface area of the scaffold leg as shown in Figure 23.3.

Figure 23.3 Loading and cross-sectional area of the leg at the scaffold baseplate

On a scaffold leg area of 25 square millimetres (1 square inch), the compressive force for a 681 kilogram (1500 pound) load is 1,054,656.5 kilograms/square metre (216000 pounds/square foot). This concentrated weight will drive the leg into any type of soil, punch it through asphalt surfaces, and even shatter wood, concrete, or stone foundations.

As weight is transferred from the small surface areas of the legs to the larger surface areas of baseplates or mudsills, the load per square unit of area decreases significantly (see Figure 23.4). For example, a 4536 kilogram (10000 pound) load on a 0.09 square metre (1 square foot) baseplate transmits 48,827 kilograms per square metres (10000 pounds per square foot) to the foundation. A 0.09 metre x 1.2 metre (1 foot x 4 foot) mudsill under the baseplate reduces the load even further to 26,911 kilograms/square metre (2500 pounds/square foot). Many soils can support a load of that weight.

Figure 23.4 Use of baseplate and sills reduces foundation loading

Baseplates

Baseplates help distribute concentrated leg loads over a larger area. They also connect scaffold standards and mudsills. Baseplates attach to scaffold legs with pins or locking devices. Workers erecting scaffolds often put screwjacks between the scaffold legs and baseplates to allow the scaffold to be leveled (see Figure 23.5). Baseplates usually contain predrilled nail holes for attaching the plates to a mudsill.

A baseplate measuring 150 millimetres by 150 millimetres provides approximately 0.023 square metres (36 square inches) of load distribution area. The load distribution area of a typical scaffold leg is approximately 25 square millimetres (1 square inch). Therefore the baseplate reduces leg load force on the foundation by a factor of 36 by distributing the load over a much larger area. A 0.04 square metre (64 square inch) baseplate reduces the force on the foundation by a factor of 64.

Figure 23.5 Baseplates help distribute the leg load

Mudsills

Normally, baseplates alone are inadequate for load distribution. Good erection practice often includes a timber mudsill under the baseplate. Mudsills serve two purposes:

(1) They provide a friction surface – baseplates are smooth metal and can easily slip. A timber mudsill has more texture. It does not allow the baseplate to slip as easily. Mudsills also have more surface area than baseplates which means they have more contact with the surface they rest on.

(2) They distribute loads over a larger foundation area – because mudsills have more surface area than baseplates, mudsills distribute any load placed on them over a larger area of the foundation.

Mudsills are usually made of wood and come in many sizes. Workers erecting a scaffold should choose a size according to the load and the foundation strength required. For typical scaffold work under normal conditions, a 50 millimetre x 250 millimetre (2 inch x 10 inch) wood mudsill is adequate. Table 23.1 suggests the type of mudsills that should be used under various ground conditions.

Table 23.1 Sample mudsills

Subsection 324(2)

Ropes or wire ropes used in scaffolding may be exposed to potentially damaging processes such as welding operations or the cleaning of masonry surfaces with acid solutions. Where this is the case, the ropes must be made of heat or chemical resistant materials.

Subsection 324(3)

Unpainted, dressed lumber is specified so that it can be inspected visually for defects such as cracks, large knots or faults.

Subsection 324(4)

This subsection presents tie-in requirements specific to hoarded masonry walk-through scaffold frames. These scaffold frames are approximately 2.1 metres by 2.1 metres in size. For an erected masonry scaffold frame to maintain its rigidity, tie-ins should be connected to both sides of a frame as close as practicable to horizontal frame members. Restricting the tie-in points to the 3 metre spacings required by subsection 324(1)(c) places the tie-ins at less than desirable locations that can reduce the rigidity of the erected masonry scaffold and can restrict the movement of workers and materials on the scaffold.

Subsection 324(4) requires a vertical and horizontal tie-in for each 9 square metres of hoarding surface area (3 metre horizontal x 3 metre vertical interval = 9 square metres), regardless of the type of scaffold frame being used. This subsection maintains the 9 square metre surface area requirement while allowing the vertical tie-in spacing distance to vary within the range of 2 metres to 3 metres to better suit the dimensions of a masonry walk-through scaffold.

Subsection 324(4) requires that the product of the vertical tie-in spacing distance and the horizontal tie-in spacing distance equal 9 square metres. For example,

(a) with a vertical tie-in spacing of 2 metres, the horizontal tie-in spacing must be no more than 4.5 metres (2 x 4.5 = 9),
(b) with a vertical tie-in spacing of 2.5 metres, the horizontal tie-in spacing must be no more than 3.6 metres (2.5 x 3.6 = 9), or
(c) with a vertical spacing of 3 metres, the horizontal tie-in spacing must be no more than 3 metres (3 x 3 = 9).

Horizontal tie-ins will most likely be placed at every second frame [a horizontal distance of 4.2 metres (2 x 2.1 metres)], resulting in vertical tie-ins being spaced at 2.1 metres intervals.

Subsection 324(5)

As powered mobile equipment and vehicles move about on a work site, they can unintentionally contact unprotected scaffolding and temporary work platforms, damaging these structures and possibly injuring workers. This subsection requires that employers take reasonable measures to protect scaffolding or temporary work platforms from being contacted. This might be achieved through selective placement of the structures to eliminate the potential for contact, or erecting or placing barriers that direct equipment and vehicles away from the structures.