Stability and Outriggers Flashcards

What is the maximum height-to-base ratio for a mobile tower scaffold used outdoors?

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.

What is the maximum height-to-base ratio for a mobile tower scaffold used indoors?

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.

How do outriggers improve the stability of a tower scaffold?

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.

What must outriggers rest on to be effective?

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.

What is the main force that causes a tower scaffold to overturn?

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.

What effect does working at the edge of a platform have on tower stability?

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.