Segmental Bridge Construction: Balanced Cantilever and Span-by-Span Methods
Building a long-span concrete bridge deck usually means either supporting the whole span on falsework from below while it's cast, which is impossible over a deep valley, a busy highway, or open water, or building it in short pieces that don't need continuous support at all. Segmental construction is the second approach: the deck is broken into short segments, typically a few meters long, each one cast or precast and then post-tensioned to the segments already in place, letting the bridge grow outward without ever needing continuous falsework spanning the full length.
Balanced Cantilever Construction Grows in Both Directions at Once
In balanced cantilever construction, segments are added symmetrically on both sides of a pier simultaneously, so the cantilevered structure stays roughly in equilibrium about the pier throughout construction rather than tipping under the weight of an unbalanced overhang. Each new segment is stressed to the previous one with post-tensioning tendons running through the top of the deck section, the region in tension for a cantilever under its own weight during construction, before the next pair of segments is added, and this cycle, cast or place, stress, advance, repeats outward from each pier toward mid-span or toward the adjacent pier's advancing cantilever.
Because the structure is a genuine cantilever until the final closure segment locks two advancing cantilevers together at midspan, the construction sequence itself creates a load case, and a stress state, that the finished, continuous bridge will never see again once it's complete, a temporary condition every bit as demanding to check as the formwork loads discussed in formwork and shoring design, except here the "temporary structure" is the permanent bridge itself, not a form to be stripped away later.
The closure pour, where two advancing cantilevers finally meet at midspan, has to account for how much each cantilever tip has crept, shrunk, and deflected under sustained load by the time the segments meet, since a closure gap sized only for the as-built geometry on the day of casting can end up under or over-length by the time creep and thermal movement are accounted for, a scheduling and geometry control challenge unique to this construction method.
Span-by-Span Construction Trades Cantilever Stress for Speed
Span-by-span construction instead assembles an entire span's worth of segments on a temporary supporting truss or gantry that spans between two piers, post-tensions the full span together while it's fully supported, and then advances the gantry to the next span once the completed span can support itself. Because the segments are fully supported during assembly rather than cantilevering out into open air, span-by-span construction avoids the temporary cantilever stress state that governs balanced cantilever design, generally allowing faster repetitive production once the gantry and segment casting cycle are established, at the cost of the specialized launching gantry equipment itself.
Precast segments for span-by-span work are typically cast off-site in a casting yard using match-casting, where each new segment is cast directly against the face of the previously cast segment so the two mate perfectly when later assembled in the field, eliminating the wet joint and its cure time from the erection sequence, a production approach conceptually similar to the factory-controlled quality behind precast concrete connections used in building construction, just applied at bridge-segment scale.
Post-Tensioning Ties the Segments Into One Continuous Structure
Regardless of erection method, the segments only become a working bridge once post-tensioning tendons, run either internally through ducts cast into the segments or externally along the box girder's interior void, are stressed and grouted, locking the individual pieces into continuous compression across every joint. This is the same load-balancing principle covered in post-tensioned concrete slabs, applied here at the scale of an entire bridge span rather than a building floor, and it's what allows a bridge assembled from short, individually modest segments to behave structurally as one long, continuous girder once construction is complete.
The American Segmental Bridge Institute and the Federal Highway Administration both publish detailed design and construction guidance covering balanced cantilever and span-by-span methods, tendon layout, and construction-stage analysis requirements, maintained by the American Segmental Bridge Institute.