This arrangement primarily stems from three key considerations:
(i) The inherent static indeterminacy of the permanent structure. If the initial construction phase solely focuses on the first span (length L) using a span-by-span approach, the partially completed bridge would experience a larger mid-span sagging moment than the final two-span structure. To counteract this, a 0.25L bridge segment is extended beyond the second pier, providing a counterbalancing moment and mitigating the excessive mid-span moment in the interim.
(ii) The strategic positioning of the 1.25L extension from the first pier. This location approximates the point of contraflexure (assuming a uniformly loaded two-span bridge), where bending moments are near zero in the future loaded state. Consequently, designing the construction joint at this point minimizes any adverse effects on the bridge’s structural performance.
(iii) The practical implications for prestressed bridges. Prestressing operations are conducted after the initial bridge segment is completed. However, carrying out this work at the heavily reinforced first pier is less desirable compared to the 1.25L point, which offers more space to facilitate the prestressing process.
In essence, this approach in span-by-span construction is a well-considered strategy that addresses structural, design, and practical aspects, ensuring the efficient and safe construction of bridges.