Last reviewed 30 Sept 2026 · 6 min read
Substructure — what it does
The substructure carries the reactions from the bearings to the foundations and retains the approach embankment. It has to resist vertical loads from the superstructure and its own weight, horizontal loads (braking, wind, water current, seismic, earth pressure) and — in rivers — the effect of scour and debris or vessel impact.
Piers
A pier is an intermediate support. Its shape reduces the resistance to flow and gives stable proportions.
| Type | Features |
|---|---|
| Solid (wall) pier | Rectangular or with round/pointed nose; masonry or concrete; short bridges and heavy loads |
| Trestle or column pier | Single or multiple columns with a cap beam; economic and common for viaducts |
| Hammerhead pier | Single column with a wide cantilever cap; used for flyovers and metro viaducts to save space |
| Cellular / hollow pier | Box section; for tall piers, reduces weight |
| Pile-bent pier | Piles extended above ground with a cap; small bridges |
| Well (cylinder) pier | Well foundation continued to the cap; river bridges |
Nose shape: a semicircular or cut-and-ease-water nose reduces the water force and the afflux; the pier is aligned with the flow. Piers at a skew or in a bend are given a slightly larger nose and larger scour allowances.
Forces on piers
- Vertical: dead load from the superstructure, live load reaction with impact, self-weight of the pier and buoyancy.
- Longitudinal: braking or tractive force transmitted through bearings, friction at the bearings, and the water current force.
- Transverse: wind on the superstructure and the pier, centrifugal force on curved bridges, water current at an angle, seismic force.
- Others: collision of vessels or debris, earth pressure (for stubby piers or on slopes) and differential settlement.
Height and stability — a tall slender pier is checked for buckling (slenderness), second-order effects (–) and dynamic behaviour. Solid piers are checked for overturning (factor of safety ≥ 2 usually), sliding (≥ 1.5), and that no tension develops in the base (or that the bearing pressure is within limits).
Pier cap and bed block
The pier cap (or pier head) distributes the girder reactions into the shaft; it is designed as a beam/cantilever, often as a deep beam with strut-and-tie. Bed blocks and pedestals under bearings spread the concentrated bearing load and keep it clear of drainage; reinforcement (hoop and mesh) resists bursting.