Bridge & Viaduct
Piers

A bridge is the general term for any structure that carries a route over an obstacle such as a river, canal, road or railway. A viaduct is a type of bridge: it crosses a valley or a wide, deep stretch of ground on a series of piers with many spans between them. In short, every viaduct is a bridge, but not every bridge is a viaduct.

Bridges usually cross water or transport obstacles with one or a few spans, whereas viaducts carry a route across a long distance to overcome changes in level and are common on highway and railway projects. Because viaducts have many tall piers, slipform, climbing formwork and post-tensioning are key methods in pier construction.

Slipforming is a suitable method for tall piers with a continuous cross-section. As the formwork rises hydraulically, concrete is placed without interruption, which reduces cold joints and makes verticality easier to monitor. The time savings are significant on pylons and tall pier shafts. Embedded anchorages and post-tensioning ducts are included in the drawings before the rise begins. The decision depends on height, changes in cross-section and space constraints on site.

Wind funnelled along a river bed or valley puts pier formwork under strain. Temporary bracing and working platforms are selected to suit these loads, and the slipform rate of rise is tied to wind limits.

Climbing formwork may be preferred for lower piers, piers with plain walls, or piers whose cross-section changes frequently. At each stage the formwork is struck and re-erected at the next level, and the concrete rises in lifts. Crane access and the anchor plan are at the heart of OHS planning. The choice between climbing formwork and slipform is made by comparing geometry and programme, and both methods can be discussed during the site survey.

Climbing formwork anchor holes affect the appearance of the wall. The patching mortar is defined in the specification, and on exposed piers this detail becomes an aesthetic requirement.

Tendon ducts and anchorage pockets must be shown on the drawings before any concrete is placed. When the slipform is rising, these voids are formed at every level without exception. Stressing is applied in stages once the shaft has gained sufficient strength, and records include elongation and tension values. A misplaced duct undermines the pier's structural model.

If the duct profile is wrong, the tendon will either bind or sit loose. The slipform level and the duct level must match to the millimetre.

Measurements are taken at every lift using a plumb line, total station or equivalent method. Since wind and sunlight can distort readings, the timing of each measurement is also planned. Small deviations are corrected by adjusting the formwork; if they are allowed to accumulate, the pier drifts outside its design section. All records go into the handover file. Verticality is the visible proof of slipform discipline.

Sunlight heats one face and causes it to expand, skewing the reading. Early-morning or shaded measurements are therefore preferred, and the report always states the time of each reading.