What are the key principles underlying the design and construction of suspension bridges, and how do they differ from those of other bridge types? Discuss the structural components involved, including the main cables, towers, and deck, and analyze how these elements interact to distribute loads and ensure stability. Furthermore, delve into the factors influencing the selection of materials for such bridges, considering aspects like durability, cost$-$effectiveness, and environmental impact. Finally, explore the challenges associated with maintaining and repairing suspension bridges over their lifespan, particularly in terms of managing corrosion, fatigue, and structural integrity.