Bridges

1. Bridges

2. What is a bridge? • A bridge is a structure that allows people or vehicles to cross an obstacle • is a structure built to span a valley, road, body of water, or other physical obstacle, for the purpose of providing passage over the obstacle

3. Types of bridges • Beam bridges • Truss bridges • Arch bridges • Suspension bridges • Cable Stayed bridges

4. Beam bridges • Beam: continuously spanned deck that is connected by piers, rigid, usually horizontal, structural element • Piers (a vertical supporting structure) • Support the load • Load (weight distribution ) travels downward • Uses pre-stressed concrete (steel rods embedded through concrete) • Concrete: strong to resist compression • Steel rods: strong to resist tension

5. Beam Bridges Advantages Disadvantages • Simple to install • Can withstand effects of thermal expansion • Supports are simple vertical piers • Not expensive to build • Can rarely span more than 250 ft • Strength of beam depends largely on spacing of the piers • Not suitable where you need good clearance

6. Newton’s Third law: • action: deck (supported roadway) pushing on piers • reaction: piers pushing back

7. Examples • Lake Ponchartrain Causeway, Louisiana • Longest bridge in the world • 24 miles long

8. Truss bridges • A type of beam bridge • Tension and compression through trusses

9. Truss Bridges Advantages Disadvantages • Adds rigidity to existing beam • Compression and tension are spread evenly through trusses- dissipates evenly • Can support heavy weights- train • Can span (the distance a beam extends) longer distance than beam bridge • Cannot handle bending forces • Large amounts of materials needed • Best suited for straight alignments

10. Newton’s Third law: • action: truss pushing on piers • reaction: piers pushing on trusses

11. Examples • Forth of Firth: Scotland

12. Arch bridge • Steel and pre-stressed concrete • Very little tension force • Compression lines • Abutments (outermost end supports)

13. Arch Bridges Advantages Disadvantages • Has great natural strength • Shape of the arch naturally dissipates (spreads evenly) the load from the center of deck to the abutments • Abutments support the load and prevent the bridge from spreading out • Spans can go distances of 800 – 1000 ft longer than beam bridge • The greater the degree of curvature the greater the tension • Expensive to build

14. Newton’s Third law: • action: deck load pushes down on sides of arch • reaction: abutments and side wall push back

15. Examples • Roman civilization made extensive use of Arch bridges

16. suspension bridges • Towers with cables attached • Support and hold up cables on the bridge • Deck (supported roadway on a bridge) • Stiffens and reduces tendency to sway • Cable • Made from steel wire to withstand great tension • Anchored in place by a supported truss system • Load pulls on cables and suspenders • Compression goes through deck and transfers to the towers

17. suspension Bridges Advantages Disadvantages • Can span longest distances of all bridges • 2000-7000 ft • Very strong in tension forces • Costly to build • Susceptible to torsion (twisting) • Wind: bridge can sway

18. Newton’s Third law: • action: deck pulls down on cables and suspension • reaction: cables and suspension pull back up

19. Examples • Golden Gate Bridge, San Francisco, CA

20. Cable stayed bridges • Constructed out of pre-cast concrete section • Fast to construct • Cables are secured to the towers and bear the load of the bridge • For medium length spans: becoming the bridge of choice

21. Cable stayed Bridges Advantages Disadvantages • Cost effective: less than suspension bridge • Good for medium length spans • 2800ft - 5000 ft • Require less cables than suspension bridges • Aesthetic look • Does not require 2 towers and 4 anchorages like suspension • Lack stiffness like suspension bridge • Not generally used for railroad crossing

22. Newton’s Third law: • action: towers pull down on cables • reaction: cables pull back up

23. Examples