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Consider the challenges in designing a multi$-$span bridge in an area with significant seismic activity. Discuss the theoretical considerations that must be taken into account when planning the bridge$$s structural system, foundation design, and material selection. Specifically, address the principles of seismic load analysis, the importance of ductility and energy dissipation in the structural design, and how soil$-$structure interaction can affect the overall stability of the bridge. Additionally, explain the role of redundancy and robustness in ensuring the bridge can withstand not only the primary earthquake event but also potential aftershocks. How do modern codes and standards guide engineers in incorporating these elements into their designs? Lastly, consider the trade$-$offs between different construction materials and techniques in terms of cost$,$ performance, and long$-$term durability in a seismic$-$prone region.