Why are cantilever bridges built outward from towers or piers?

Cantilever bridges are built outward from towers or piers primarily to facilitate efficient construction over large spans and heavy loads, often without the need for extensive temporary support structures known as falsework.123

The process typically involves the following structural and mechanical advantages:

• Balanced Construction: By building outward from central towers, the structure remains balanced. In steel cantilever bridges, the frame is extended from the towers toward both the center of the span and the abutments. This method allows each new section to be supported by the previously completed portion.23
• Load Management: The towers act as critical support points that transfer forces to the foundations. The inner towers carry compression forces to the foundation, while outer towers manage tension forces through anchorages at the shore.42
• Efficiency: For prestressed concrete bridges, the cantilever method allows segments to be cast and prestressed onto the existing structure, eliminating the need for ground-based support during the building process.3
• Historical Precedent: Early wooden cantilever designs in Asia used crisscrossed logs jutting out from piers to allow water to pass through, which reduced resistance to floods compared to solid foundations.52