1. To eliminate the need for bearings and joints at the pier
2. To simplify deck cambering and alignment during cantilever erection
3. To improve fit-up tolerance and manage construction variability
4. To reduce the number of piers required over water

Explanation

Explanation: In balanced cantilever bridges like the Firth of Forth, main spans are constructed outward from piers using cantilever arms. Rather than joining cantilever tips directly, a short-suspended span is inserted between them. While this does modestly reduce bending moments at the cantilever tips and simplifies erection sequencing, its primary purpose in the Firth of Forth Bridge is to accommodate the effects of construction tolerances and differential deflections between cantilever arms. By introducing a relatively short-suspended span, engineers can make final fit-up adjustments more easily than attempting to match two massive cantilever arms tip-to-tip with high precision. This makes the structure more constructible and resilient to alignment or camber variability during staged erection.

Learning Resources:
• “Bridge Engineering Handbook” – Balanced cantilever segmental construction methods
• Case Study: Firth of Forth Bridge (Scotland) – Historic cantilever with suspended spans
• Mola Visualization: Use the Mola Kit to construct cantilever arms extending from adjacent piers. Leave a small gap between the tips and insert a short-suspended segment to connect them. This configuration allows you to visualize how a suspended span accommodates small alignment differences and construction tolerances. Try joining the cantilever arms directly without the suspended segment and observe how sensitive the fit-up becomes.