In post-tensioned segmental bridges, which sequence correctly describes the typical construction and its advantages?

• A. Segments cast in sequence; tendons threaded and post-tensioned after concrete gains strength, locking segments together; advantages include longer spans, controlled cracking, and reduced self-weight.
• B. Segments cast in parallel; tendons pre-tensioned before concrete gains strength, which locks segments and reduces airflow.
• C. Segments cast individually after erection; no post-tensioning; advantages: simpler construction.
• D. Segments cast in sequence; tendons never post-tensioned; advantages: higher self-weight.

Answer: (A)

In post-tensioned segmental bridges, the segments are built in sequence to form a continuous girder, and tendons are threaded through ducts after the concrete has gained enough strength, then tensioned to lock the segments together and compress the concrete. This post-tensioning creates a compressive force within the member, which improves its load-carrying capacity and keeps cracks tight. The sequential casting along the span, combined with post-tensioning, results in a strong, continuous structure that behaves like a single unit rather than separate pieces.

This approach enables longer spans because the post-tensioning increases bending capacity and stiffness without adding excessive weight. The induced compression helps control cracking under service loads, improving durability and crack width management. The use of hollow or box-section segments, made efficient by post-tensioning, also reduces the dead load, contributing to the ability to span longer distances with fewer supports.

Other sequences described in the alternatives don’t align with how this method delivers its benefits. Pre-tensioning before concrete gains strength is characteristic of some precast element methods, not the typical in-situ post-tensioned segmental construction; and scenarios with no post-tensioning or with higher self-weight miss the essential advantages of compressive reinforcement, continuous behavior, and weight efficiency that define this construction.