Bridge Anatomy: Substructure vs. Superstructure Explained

Every bridge comprises two fundamental structural systems: the substructure (below bearings) and the superstructure (above bearings). Together, they transfer loads from traffic and environment to the ground.

1. Substructure — The Foundation System

Transfers all loads to the earth.

Components

• Foundations: Interface with ground.
  • Shallow: Spread footings, mats (good soil, low loads).
  • Deep: Driven piles (steel H, concrete, timber), drilled shafts (caissons), micropiles (poor soil, high loads, scour risk).
• Piers (Bents): Vertical columns rising from foundations to support superstructure.
  • Materials: Reinforced concrete (most common), steel, stone (historic), composite.
  • Shapes: Hammerhead, multi-column, wall-type, pile bents.
  • Functions: Carry vertical loads; resist lateral (wind, seismic, water, impact); define span lengths.
• Abutments: End supports retaining approach embankment.
  • Retain backfill (earth pressure).
  • Support superstructure end (bearing seat).
  • Protect approach from erosion.
  • Types: Gravity, semi-gravity, cantilever, spill-through, integral (jointless).
• Wing Walls: Extend from abutments to retain side slopes.

2. Superstructure — The Span System

Carries traffic across the obstacle.

Components

• Bridge Deck: Riding surface.
  • Structural: Concrete slab (cast-in-place, precast), steel orthotropic deck, timber, FRP.
  • Wearing surface: Asphalt, concrete overlay, polymer.
  • Drainage, barriers, lighting, utilities embedded.
• Primary Load-Carrying Members (Span Elements):
  • Girders/Beams: I-girders (steel, prestressed concrete), box girders, bulb-tees. Spaced 1.5–3 m.
  • Trusses: Triangulated frames (Warren, Pratt, K, Vierendeel). Efficient for long spans (100–500+ m). Through-deck or deck truss.
  • Arches: Compression ribs (concrete, steel). Span 100–500 m. True arch vs. tied-arch (bowstring).
  • Cables: Suspension: Main cables + suspenders (1,000–2,000+ m). Cable-stayed: Fans/harps from towers (200–1,100 m).
• Secondary Members: Cross-frames, diaphragms, lateral bracing — distribute loads, stabilize girders.
• Bearings: Critical interface — allow translation/rotation (elastomeric, pot, spherical, sliding) while transferring loads to substructure.
• Expansion Joints: Accommodate thermal movement at abutments/piers.

Integration

Substructure + Superstructure = Structural System. Design is iterative: superstructure loads dictate substructure sizing; foundation conditions constrain superstructure choices. Modern codes (AASHTO LRFD, Eurocodes) require seismic, fatigue, scour, redundancy checks across the entire system.