Description

Balanced cantilever bridges behave differently during erection than in their completed, serviceable state, and the erection sequence is not fixed by the designer alone — it follows the contractor's equipment and site constraints. When that sequence changes, the design has to be rechecked. This session works through a three-span, two-track viaduct with spans of 24.8 m, 40.576 m and 39.6 m, two intermediate piers carrying six cantilever segments each, modeled in MIDAS CIVIL NX with pile springs for soil-structure interaction, and uses construction stage analysis to test how sensitive the structure is to the order in which it is built.

Two checks are run on the cantilever stage. Segment out of balance covers the case where only one lifter is serviceable and a segment is erected without its pair, loading the pier out of balance about both its longitudinal and transverse axes. Accidental segment drop represents the sudden loss of equilibrium by replacing the lost segment with an upward concentrated load, with an adjustment factor agreed between designer and owner (taken as 1.0 here). Both are read back through Beam Diagrams on CS: Summation to give pier column and pile foundation design forces.

The second half compares stitching and stressing sequences with tendon layout, geometry and loading held constant, tracking CS: Tendon Secondary only. Stressing one track before the other splits the tendon secondary moment between the two tracks, while stressing both simultaneously leaves them almost identical — the pier crosshead transmits force from one track to the other, and stressing one side first leaves that transfer unbalanced. Stitching the end spans before the mid span moves the concentration from one intermediate pier to the other and roughly doubles the governing tendon secondary moment at that pier, an effect driven by the short 24.8 m end span attracting force because it is stiffer.