PASMA Stability and Outriggers 1 — Questions and Answers
Question 1: What is the maximum height-to-base ratio for a mobile tower scaffold used outdoors?
- 2:1
- 3:1 (Correct answer)
- 3.5:1
- 4:1
Correct answer: 3:1
For outdoor use, the maximum height-to-base ratio is 3:1, meaning the tower height must not exceed three times the narrowest base dimension. This accounts for the additional wind loading that occurs outdoors.
Question 2: What is the maximum height-to-base ratio for a mobile tower scaffold used indoors?
- 2:1
- 3:1
- 3.5:1 (Correct answer)
- 5:1
Correct answer: 3.5:1
For indoor use, the maximum height-to-base ratio is 3.5:1. The higher ratio is permitted because indoor towers are sheltered from wind. However, doors, openings, and internal air movement should still be considered.
Question 3: How do outriggers improve the stability of a tower scaffold?
- They add weight to the top of the tower
- They increase the effective base size, thereby improving the height-to-base ratio (Correct answer)
- They make the tower easier to move
- They reduce the overall height of the tower
Correct answer: They increase the effective base size, thereby improving the height-to-base ratio
Outriggers extend the effective base footprint of the tower. By increasing the base size, they improve the height-to-base ratio, allowing the tower to be used at greater heights while maintaining stability against overturning.
Question 4: What must outriggers rest on to be effective?
- Loose gravel
- Firm, level ground with base plates to spread the load (Correct answer)
- The tower's own castor wheels
- Nothing — they can be suspended in the air
Correct answer: Firm, level ground with base plates to spread the load
Outriggers must make firm contact with the ground through base plates that spread the load. If the ground is soft, sole boards may be needed under the base plates. An outrigger resting on soft ground can sink, rendering it ineffective.
Question 5: What is the main force that causes a tower scaffold to overturn?
- Gravity pulling it straight down
- A horizontal force (such as wind or pushing) that creates a turning moment about the base edge (Correct answer)
- The weight of the components
- Magnetic forces
Correct answer: A horizontal force (such as wind or pushing) that creates a turning moment about the base edge
Overturning occurs when a horizontal force (wind, operatives leaning out, impact) creates a turning moment (torque) about the base edge that exceeds the restoring moment from the tower's own weight. The wider the base, the greater the restoring moment.
Question 6: What effect does working at the edge of a platform have on tower stability?
- No effect
- It moves the centre of gravity towards the edge, increasing the risk of overturning (Correct answer)
- It improves stability by distributing weight evenly
- It only affects stability above 10 metres
Correct answer: It moves the centre of gravity towards the edge, increasing the risk of overturning
When an operative works at the platform edge (e.g., leaning out to reach something), their weight shifts the combined centre of gravity towards that edge. This reduces the restoring moment on that side and increases the overturning moment.
PASMA Stability and Outriggers 1
What is the maximum height-to-base ratio for a mobile tower scaffold used outdoors?