Bridge site factors and components with sketches

Factors affecting bridge site selection

A bridge site is chosen so that the bridge is safe and economical, and serves both the road alignment and the river with the least disturbance. Factors to consider (IRC:5 / IRC:SP:13 and Nepal DoR bridge practice):

1. Stream/river behaviour: a straight, stable reach with a well-defined, non-meandering channel; avoid bends, confluences, and braided and aggrading reaches.
2. Waterway and flood data: a narrow, stable crossing with high banks; adequate waterway for the design flood (usually a 100-year return period for major bridges as per IRC:5 and DoR, 50-year for small bridges and culverts as per IRC:SP:13); reliable HFL, discharge and velocity records.
3. Foundation conditions: firm strata or rock at a reasonable depth; the foundation must be taken below the scour depth (Lacey's regime theory, IRC:78) at an affordable cost; avoid soft, liquefiable or landslide-prone soils.
4. Approaches and alignment: the bridge should be square (normal) to the river flow; approach roads should be straight, of limited height and not on unstable slopes. For major bridges the road alignment is adjusted to suit the best bridge site, while for minor bridges and culverts the bridge follows the road alignment.
5. Navigation and clearance: vertical and horizontal clearance for boats, floating logs or boulders; vertical clearance between HFL and the soffit of 150 mm to 1.5 m depending on the discharge (IRC:5), with more in Nepal's boulder-carrying rivers.
6. Economy: total cost of bridge, approaches and river training works; nearness of construction materials and labour, and access for equipment.
7. Geology and seismicity: avoid active faults and steep gorges with rockfall; follow the seismic provisions of NBC 105 and IRC:6.
8. Existing structures and utilities: distance from other bridges, dams and weirs (backwater/draw-down effects), settlements, and land acquisition.
9. Environmental and social aspects: minimum afflux, no damage to property upstream, aesthetics, and environmental clearance requirements.
10. Construction convenience: space for temporary works and diversion, and a low-flow period for working.

Essential components of a bridge

A bridge has a superstructure (above the bearings) and a substructure (bearings, piers, abutments, 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: carries the traffic loads (IRC Class A/70R/AA loading as per IRC:6) to the girders.
• Girders/trusses/arches: main load-carrying members that span between supports.
• Cross girders/diaphragms: give lateral stability and distribute load.
• Parapets, railings, kerbs, footpaths: 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.