Last reviewed 30 Sept 2026 · 6 min read
Why the construction sequence matters
In a bridge built in stages — cantilever segments, span-by-span erection, incremental launching, a deck cast in several pours, cable-stayed erection — the structure at each stage is different from the finished one, carries different loads, and gets its stresses at different times. Because concrete creeps and shrinks and prestress relaxes, the state of stress in the finished bridge depends on the whole history. A single analysis of the completed structure with all loads applied at once can be seriously wrong.
Construction-stage analysis follows the structure through its history: each stage adds or removes elements, loads and supports, and the analysis carries forward the stresses and time-dependent effects.
Building the staged model
- Element activation — new segments, girders, slab, cross-girders, cables are added with their own age (time of casting).
- Element deactivation — falsework, temporary props, launching nose, form travellers, cable clamps are removed.
- Boundary conditions — supports appear (a permanent bearing) or disappear (temporary support); a closure joins two cantilevers into a continuous girder.
- Loads — self-weight of the new segment, wet-concrete weight, form-traveller weight, prestress from the tendons as they are stressed, cable forces, construction loads, wind on the exposed structure.
- Time steps — the interval between stages (curing time, waiting days), during which creep, shrinkage and relaxation act.
Each stage has a duration; the analysis computes the response at the beginning and end of the stage, including the time-dependent changes within it.
Time-dependent behaviour of concrete
- Creep — increase of strain under sustained stress; modelled by a creep coefficient that depends on the age at loading , the age , the humidity, the member size and the concrete grade (models in the codes: CEB-FIP / IRC:112 / Eurocode).
- Shrinkage — drying and autogenous shrinkage strain; also a function of time, environment and size.
- Modulus growth — increases with age; segments loaded at early age creep more.
- Prestress losses — friction and slip (immediate), plus creep, shrinkage and relaxation (time-dependent) are calculated inside the analysis or supplied.
- Temperature — daily and seasonal effects, especially for long, thin decks.
For time integration the software often uses the age-adjusted effective modulus method (AAEM), which turns the creep problem into a series of elastic problems with a modified modulus ( = ageing coefficient ≈ 0.8) and an "initial-strain" load.
Redistribution when the structural system changes
If a structure is first built as a statically determinate system (two cantilevers, or simply supported spans) and later made continuous (indeterminate), sustained loads have already deformed the first system. Over time creep tries to move the bending moments towards the values of the final continuous system — the structure "remembers" the earlier shape but creeps toward the later one.
= moment in the first (cantilever) system under the load; = moment the same load would produce in the final (continuous) system; = creep coefficient. As the moment tends to ; for typical about 86 % of the redistribution has taken place.
That is why continuity tendons and the closure sequence are designed with creep in mind; the secondary moments from prestress and the settlement of falsework also matter.
A cantilevered girder under dead load has a support moment = −6000 kN·m in the cantilever stage. After closure the girder is continuous and the same dead load would give = −3500 kN·m. Creep coefficient = 1.5.
The support moment falls by about 1943 kN·m and mid-span moment rises correspondingly — the redistribution must be covered by the tendons and the section.