1. To support the self-weight of the cables
2. To counteract the torsion induced by wind loads
3. To resist flexural effects from live loads applied on only one span
4. To eliminate the need for cable anchorage at the deck ends

Explanation

Explanation: Even in a symmetrical cable-stayed bridge, the towers must resist bending moments because transient live loads (like vehicles) are often not balanced across both spans. When one span carries more load than the other, an unbalanced cable force is transferred to the tower, generating significant flexural demand. The tower must therefore be designed as a moment-resisting structure to prevent excessive deflection or instability.

Contrast with Classical Arrangement: In classical cable-stayed bridges with a shorter side span and longer main span, the tower is stabilized not by resisting flexural moments, but by the backstays (cables anchored into the side span) which counteract unbalanced forces from transient live loads on the main span. These backstays pull on the top of the tower, providing rotational equilibrium. This often results in uplift at the far end of the side span, which must be resisted at the anchorage or abutment. In such systems, the tower can be pinned at the base, as flexural demands are redirected into axial forces in the stays and deck.

Learning Resources:
• “Cable-Stayed Bridges: Theory and Design,” Gimsing & Georgakis – Tower stiffness and moment design
• Case Study: Zakim Bunker Hill Bridge (Boston) – Tower bending under asymmetric loading
• Case Study: Sunshine Skyway Bridge (Florida) – Classical side span + main span balance
• Mola Visualization: Build a symmetrical cable-stayed bridge using the Mola Structural Kit. Apply vertical load to one span only. Observe how the unbalanced load induces rotation or bending in the tower. Then rebuild the model with a short side span and longer main span and observe how the system balances through axial deck forces rather than tower flexure.