--- title: "Bridge site factors and components with sketches" subject: "Transportation Engineering II" chapter: 5 key: "semester_7/transportation_engineering_ii:bridge-site-factors-components-sketches" source: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/bridge-site-factors-components-sketches site: BCT Engineering --- # Bridge site factors and components with sketches Transportation Engineering II · Chapter 5: Introduction to Bridge and Tunnel Engineering - Asked in: 2076 Chaitra, 8 marks - Video: [BRIDGE ENGINEERING | LECTURE 1 | SELECTION OF SITE FOR BRIDGE | MPSC MES MAINS | SSC | CCC](https://www.youtube.com/watch?v=_RI1sbESERA) — Study CIVIL ENGINEERING - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/transportation_engineering_ii/bridge-site-factors-components-sketches - Whole chapter: https://www.bctengineer.com/md/semester_7/transportation_engineering_ii/chapter-5 ## Question Discuss the factors which affect the bridge site selection. List the essential components of bridge with sketches. ## Short answer Key site factors: stable straight river reach, adequate waterway and flood data, firm foundations below scour depth, square crossing with straight approaches, navigation clearance, economy, geology, and environmental impact. Essential bridge components: superstructure (deck, girders, parapets, bearings) and substructure (piers, abutments, foundations), plus approaches and wing walls. ## Answer #### 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. ```text 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. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which river reach is preferred for a bridge site? A. A meandering reach with bends B. A confluence of two streams C. A straight, stable reach with a well-defined channel D. A braided, aggrading reach Answer: C. A straight stable reach gives predictable flow; bends, confluences and braided or aggrading reaches should be avoided. (Key point: Stream behaviour) 2. How should the bridge be placed relative to the river flow? A. Square (normal) to the flow B. Heavily skewed to the flow C. Parallel to the flow D. Angle does not matter Answer: A. A bridge normal to the flow needs the shortest length and the least obstruction and scour problems. (Key point: Alignment to flow) 3. Which foundation condition is desirable at a bridge site? A. Soft liquefiable soil at shallow depth B. Firm strata or rock at reasonable depth with affordable scour depth C. Landslide-prone slopes D. Very deep soft clay Answer: B. Firm strata at reasonable depth and manageable scour depth (Lacey's regime theory) keep the foundation safe and economical. (Key point: Foundation and scour) 4. Which set belongs to the substructure of a bridge? A. Deck slab, girders, parapets B. Piers, abutments, foundations C. Railings, kerbs, expansion joints D. Wearing coat and drainage spouts Answer: B. The substructure comprises bearings, piers, abutments and foundations that transfer loads to the ground. 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