The abutment ends the bridge and starts the road. It holds the last span, retains the approach fill, and, through the joint and bearings at its seat, absorbs the movement of the whole deck. Seat-type abutments with a backwall and wingwalls are the traditional answer; stub abutments on piles sit at the top of a fill slope; mechanically stabilised earth (MSE) walls carry the fill while a separate footing carries the bridge.

seat-typeintegralstub on MSE wall

The trend of the last thirty years is the integral abutment: no joint, no bearings; the girders are cast into the abutment and the whole structure moves as one, the piles flexing with it. It removes the two components that cause most maintenance, at the cost of a deck length limit (roughly 100–200 m depending on climate and material) and careful design of the approach slab. Semi-integral versions keep bearings but drop the joint.

Loads: earth pressure (active, at-rest or passive depending on how the wall moves), live-load surcharge, the bridge reactions, and during earthquakes the inertia of the backfill. Stability is checked for sliding, overturning and bearing.

A pier is chosen for its site as much as its load. Multi-column bents suit wide decks on land; single hammerhead piers keep a small footprint in medians and water; wall piers resist stream flow and ship impact; tall hollow piers carry viaducts across valleys. The cap distributes the bearing reactions to the columns and is designed as a deep beam or by strut-and-tie.

multi-column benthammerheadwall pierhollow pier

Columns are compression members under bending in two directions, with slenderness effects that become significant above height-to-width ratios around 10, and with ductility requirements in seismic zones that dictate spiral or hoop confinement, lap-splice locations, and the amount of longitudinal steel. Interaction diagrams, axial load against moment, are the design tool, and the site's pier tool draws them from real bar layouts.

Vehicle collision on piers next to roadways and vessel collision on piers in navigable water are extreme-event checks that often govern pier dimensions and require protection systems rather than strength.

Where competent soil or rock is within a few metres, a spread footing is the cheapest foundation there is: a concrete pad wide enough that the bearing pressure is below what the ground can carry, and stiff enough to spread the column load. The checks are bearing resistance (from the soil's shear strength, with φ around 0.45–0.55), settlement at service load (usually the governing check on soil), eccentricity (the resultant must stay within the middle of the footing under strength loads), and sliding.

Footings on erodible soil in rivers are the classic scour victim; AASHTO requires the footing bottom below the check-flood scour depth. Structural design of the footing itself is one-way and two-way shear and bending as a cantilever from the column face.

When the good ground is deep, the load goes down to it through driven piles, steel H-sections, pipe piles, precast prestressed concrete, timber, or drilled shafts (bored piles), cast in a hole that may be 1–3 m in diameter and 30–80 m deep. Driven piles are cheap and their capacity is confirmed as they are driven (by dynamic testing and driving formulas); shafts carry huge loads on a single element, suit noise-sensitive sites and large lateral loads, and are the foundation of most long-span towers.

driven pile group: side friction + end bearingdrilled shaft with reinforcing cage

Axial capacity is side friction plus end bearing, estimated from soil strength by the α (total stress, clay) and β (effective stress, sand) methods or from rock strength for sockets, then verified by static or dynamic load tests. Lateral capacity governs piers under seismic or ship-impact loads and is analysed with p–y curves. Group effects reduce capacity when piles are close, and negative skin friction (downdrag) adds load when the surrounding soil settles.

Moving water removes the bed around a pier. Three mechanisms add up: long-term degradation of the channel; contraction scour where the bridge narrows the flow; and local scour, the horseshoe vortex that digs a hole at the pier itself. Local scour depth scales with pier width and with flow depth and velocity; HEC-18 gives the equations, and the site's tool applies them.

Scour is the leading cause of bridge collapse in the United States and among the leading causes everywhere. The Schoharie Creek bridge (New York, 1987) fell when a flood undermined a spread footing that inspectors could not see; the responses were the National Scour Program, scour-critical ratings, and the rule that foundations are designed for the 100-year scour and checked for the 500-year. Countermeasures are riprap and collars for existing bridges and deep foundations for new ones.

Bridges survive earthquakes by design of the substructure. The philosophy of capacity design: decide where the structure will yield (the column ends), make those regions ductile with closely spaced confinement, and make everything else, cap, footing, joint, shear, stronger than the overstrength moment the hinge can deliver, so that the chosen hinge forms and nothing brittle fails first.

Then keep the deck on: seat widths sized for the displacement demand plus a margin, restrainers and shear keys where needed, and joints and bearings that can move or fail without dropping the span. Isolation bearings (lead-rubber, friction pendulum) lengthen the period and cut the demand, and are common on retrofits and on important bridges. Liquefaction of loose saturated sand is the foundation hazard: it removes the ground's support and can move whole abutments, and is addressed by ground improvement or by designing piles to survive it.