Concrete and steel bridges
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Concrete and steel bridges
- 3. 3 Advantages of Steel • High tensile and compressive strength • High strength to weight ratio – low self weight & less loading on substructure • Uniformity • High quality material • Speed of construction • Versatility of construction • Elasticity – follows Hooke’s Law very accurately • Ductility – can withstand high deformations before failure ➔ gives a warning before failure • Modifications can be made easily – strengthening of beams
- 4. 4 4 Design Stress Strain Curve for Concrete – BS8110
- 5. 55 Design Stress Strain Curve for Reinforcement – BS8110
- 6. 6 Disadvantages of Steel • Maintenance cost is high – corrodes when exposed to air, water and humidity • Fire proofing cost – strength reduces at high temperatures • Susceptible to buckling – slender structures • Fatigue – strength reduces due to cyclic loading
- 7. 7 Types of Steel Bridges • Beam / Girder • Truss • Plate girder • Box girder • Cantilever • Arch • Cable stayed • Suspension • Movable • Temporary / emergency
- 8. 8 Beam / Girder Bridges
- 9. 9 Beam / Girder Bridges (Contd.) • Rolled I sections are used as the main support system • Other members – cross girders, stringers, deck, bracings • Two beam/girder system or multiple beam/girder system with cross beams (cross girders)
- 10. 10 Truss Bridges
- 12. 12 C
- 13. 13 Truss Bridges (Contd.) • Spans up to 200m possible • Structurally efficient • Reduced self weight • Several types of truss arrangements possible – Warren, Modified Warren, Pratt, Fink, Howe etc.
- 14. 14 Truss Bridges (Contd.) Components of a truss bridge
- 15. Warren Truss Pratt Truss Equal length members Easy fabrication economy Longer members in tension Slender diagonals are possible Efficient structure Truss Bridges (Contd.)
- 18. 18 Box Girder Bridges (Contd.) • Very high torsional rigidity • Effective use of construction materials • Longer spans possible • Complex execution
- 20. 20 Cantilever Bridges (Contd.) • Permits a long clear span for navigation • Main structural elements are anchor arm, cantilever arm and suspended span
- 21. 21 Arch Bridges
- 23. 23 Arch Bridges (Contd.) • Structural action similar to old masonry arches – bending moments much lower than in typical bridges • Deck support – Hangers or struts
- 25. 25 Cable Stayed Bridges • Spans : 200 – 500m • Main structural elements – Cables, towers (pylons) and deck
- 27. 27 Suspension Bridges (Contd.) • Spans exceeding 600m • Main structural elements – Flexible cables, towers (pylons), anchorages, hangers, deck • Light and strong • Very expensive
- 30. 30 Movable Bridges (Contd.) • Permits passage for navigation by moving / swinging the bridge deck • When it is either impractical or too expensive to provide a bridge with sufficient vertical clearance • Moving action – Lift/sink or swing
- 31. 31 Temporary / Emergency Bridges
- 32. 32 Temporary / Emergency Bridges (Contd.)
- 33. 33 Temporary / Emergency Bridges (Contd.) • Erected within a very short period (1 day) • No permanent abutments / piers • Specially provided during wars and after natural disasters
- 35. 35 Advantages of Concrete • Ingredients of concrete are easily available. • Unlike natural stones Concrete is free from defects and flaws (faults, joints, stratifications etc.). • Concrete can be manufactured to desired strength with economy. • The durability of concrete is very high. • It can be cast to any desired shape. • Casting of concrete can be done at work site which makes it economical. • Maintenance cost of concrete is almost negligible.
- 36. 36 Disadvantages of Concrete • Compared to other binding materials, the tensile strength of concreter is relatively low. • Concrete is less ductile. • The weight is high compared to its strength.
- 37. 37 RC Bridges • Suitable for small and medium spans • Special requirements – restriction on provision of formwork, construction period • Most of highway bridges are RC • Several types available • Slab • Beam • Box girder • Continuous girder • Balanced cantilever • Arch
- 38. 38 Slab Bridges
- 39. 39 Slab Bridges RC Slab serves as the main superstructure member, requiring no other beams or girders to carry the loads. Not suitable for larger spans since the slab becomes thicker increasing the dead weight. • Economical • Simple design • Solid / hollow slabs
- 40. 40 Beam Bridge
- 41. 41 Beam Bridge RC beams serve as the main superstructure member. Widely used due to simplicity, economy, wide availability of the standard sections, and speed of erection. Precast beams are placed on the supporting piers or abutments, usually on rubber bearings which are maintenance free. • I beams are used • A deck is constructed on top of beams spanning between the beams
- 42. 42 Box Girder Bridges For spans greater than around 45 metres, pre- stressed concrete box girders are the most common method of concrete bridge construction. The main spans are hollow and the shape of the 'box' will vary from bridge to bridge and along the span, being deeper in cross-section at the abutments and piers and shallower at mid span. • Incrementally launched • Span by span • Balanced cantilever
- 44. 44 Bridge Design
- 45. 45 Loads on Bridges (BS Code)
- 46. 46 Loads on Bridges (Eurocode)
- 52. 52 Stresses in a Plate Girder
- 53. 53 Stresses in a Box Girder
- 54. 54 Bending of Box Girders