PASMA Stability and Outriggers 2 — Questions and Answers
Question 1: What is the 'overturning moment' of a tower scaffold?
- The time it takes to dismantle the tower
- The tendency of the tower to tip over, calculated as the horizontal force multiplied by the height at which it acts (Correct answer)
- The weight of the tower
- The time between inspections
Correct answer: The tendency of the tower to tip over, calculated as the horizontal force multiplied by the height at which it acts
The overturning moment is the product of a horizontal force (e.g., wind) and the distance from the point of application to the tipping point (base edge). The taller the tower and greater the force, the larger the overturning moment and the greater the risk of toppling.
Question 2: What is the 'restoring moment' and why is it important?
- A moment of rest during the work shift
- The counteracting force created by the tower's own weight about the opposite base edge that resists overturning (Correct answer)
- The time to restore a damaged tower
- A break taken after an accident
Correct answer: The counteracting force created by the tower's own weight about the opposite base edge that resists overturning
The restoring moment is the product of the tower's weight and the distance from the centre of gravity to the tipping edge. It must exceed the overturning moment for the tower to remain stable. A wider base and heavier tower increase the restoring moment.
Question 3: When must stabilisers or outriggers be fitted to a tower scaffold?
- Only when the tower is above 20 metres
- Whenever the manufacturer's instructions require them, typically when the tower exceeds the freestanding height-to-base ratio (Correct answer)
- Only when requested by the client
- Stabilisers are never mandatory
Correct answer: Whenever the manufacturer's instructions require them, typically when the tower exceeds the freestanding height-to-base ratio
Stabilisers or outriggers must be fitted whenever the manufacturer's instruction manual specifies them, which is typically when the tower height exceeds the freestanding height-to-base ratio limits. The manufacturer's guidance is definitive.
Question 4: What is the effect of wind on an unattended tower scaffold left fully erected?
- Wind has no effect on metal towers
- Wind can create sufficient force to overturn the tower, especially if sheeting or boards increase the wind-catching area (Correct answer)
- Wind only affects wooden towers
- Wind can only move the tower if the brakes are released
Correct answer: Wind can create sufficient force to overturn the tower, especially if sheeting or boards increase the wind-catching area
An unattended tower presents a large surface area to the wind. The force can be enough to overturn it, particularly if any sheeting, banners, or unfixed materials are attached, as these act like sails. The tower should be lowered, dismantled, or tied off if left unattended in exposed conditions.
Question 5: How does adding materials and tools to the platform affect tower stability?
- It has no effect
- It raises the centre of gravity and increases the overturning moment, reducing stability (Correct answer)
- It always improves stability by adding weight
- It only matters if the materials are flammable
Correct answer: It raises the centre of gravity and increases the overturning moment, reducing stability
Loading the platform with materials raises the combined centre of gravity of the tower system. A higher centre of gravity increases the overturning moment for any given horizontal force, making the tower less stable. This is why safe working loads must be observed.
Question 6: What is a 'ballast' in relation to tower scaffold stability?
- A type of scaffold fitting
- Additional weight placed at the base of the tower to lower the centre of gravity and increase stability (Correct answer)
- A brand name for outriggers
- A type of castor wheel
Correct answer: Additional weight placed at the base of the tower to lower the centre of gravity and increase stability
Ballast is weight added to the base of the tower (typically kentledge blocks or sandbags) to lower the overall centre of gravity and increase the restoring moment. It must only be used as specified by the manufacturer and must be securely fixed.
PASMA Stability and Outriggers 2
What is the 'overturning moment' of a tower scaffold?