Classification of bridges by structure, material, loading

Classification of bridges

By structure (static system)

• Simply supported, continuous and cantilever bridges.
• Arch, rigid frame, truss, girder, suspension and cable-stayed bridges.
• Beam/slab and box girder bridges.

By material

• Timber bridges: temporary or light bridges.
• Masonry (stone/brick): arches and piers of short spans.
• Reinforced concrete (RCC): slab and T-beam bridges.
• Prestressed concrete (PSC): long girders and box sections.
• Steel: truss, plate girder and Bailey bridges; composite bridges (steel girders with a concrete deck).
• Others: FRP, and bamboo/rope (e.g. suspension trail bridges in rural Nepal).

By loading (IRC:6 / Nepal Bridge Standards)

• Highway bridge (carries road vehicles: IRC Class AA, 70R, A, B).
• Railway bridge (railway loading).
• Footbridge (pedestrians and light loads, e.g. a 5 kN/m² footway load).
• Pipeline, aqueduct, viaduct and combined road-rail bridges.
• Class A is the standard live load; Class 70R/AA are for heavy tracked and wheeled vehicles on national highways; Class B is for temporary bridges.

Sketch of bridge with components

A bridge has a superstructure (above the bearings) and a substructure (bearings, piers, abutments and foundations), plus approaches and protective works.

 Approach   Parapet/Railing    Deck slab   Approach
 slab        ________________________      slab
 ____   ____|_____|_____|_____|_____|____   ____
      \  |  [girder][girder][girder]    |  /
 Wing  \ |Bearing        Bearing  |  / Wing
 wall   |Abutment  [ PIER ]   Abutment| wall
        |   |         |  |          |
 HFL ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 Scour level - - - - - - - - - - - - - - - -
     [Footing]    [Pile/Well]    [Footing]

Superstructure

• Deck slab and wearing coat: carry the traffic loads (IRC Class A/70R/AA loading as per IRC:6) to the girders.
• Girders/trusses/arches: the main load-carrying members that span between supports.
• Cross girders/diaphragms: give lateral stability and distribute the load.
• Parapets, railings, kerbs and footpaths: provide safety for vehicles and pedestrians.
• Expansion joints and drainage spouts: allow thermal movement and remove surface water.

Bearings: transmit reactions from the superstructure to the substructure and permit rotation and movement (elastomeric, roller and rocker types).

Substructure

• Abutments: end supports that also retain the approach fill, with wing walls and return walls that hold back the earth.
• Piers: intermediate supports carrying the vertical load, with a pier cap and bed block.
• Foundations: open (spread) footings, piles, wells or caissons that carry the loads to sound strata below the scour depth.

Others: approach embankment and approach slab, protective works (guide bunds, spurs, apron, pitching) and river training works. The design HFL, LWL and scour level are marked on the section.