1. Discover Engineering ENGR 096
Bridges
Discover Engineering
ENGR 096

2. Bridges Three main types of bridges:
Beam bridge
Arch bridge
Suspension bridge
Difference between the three is the distance crossed in single span
Span: distance between two bridge supports (columns, towers, wall of canyon)

3. Bridges Beam bridge: spans up to 200 feet Arch bridge: 1000 feet
Suspension bridge: 7000 feet
Difference comes from compression and tension

4. Bridge Forces Compression (squeeze force) Tension (pull force)
Too much compression (buckling)
Tension (pull force)
Too much tension (snapping)

5. Bridge Forces Dissipation (spread out over greater area)
Arch bridge
Transfer (move force from area of weakness to area of strength)
Suspension bridge

6. The Beam Bridge Rigid horizontal structure resting on two piers
Weight of bridge and load supported by piers

7. The Beam Bridge Usually concrete or steel beams
Taller beams can span longer distances (more material to dissipate tension)
Tall beams are supported with a truss (adds rigidity to existing beam)
Limited in size

8. Trusses

9. I-Beam Top of beam experiences most compression
Bottom of beam experiences most tension
Middle of beam experiences very little compression or tension
Best design is beam with more material on top and bottom than the middle (I-beams)
Works for trusses too!

10. Arch Bridge Semicircular with abutments on each end
Arch diverts weight from deck to abutments
Compression: always under compression (no tension)

11. Arch Bridges Does not need additional supports or cables
Arches made of stone don’t even need mortar

12. Suspension Bridge Cables, ropes, chains suspend the deck from towers
Towers support majority of the weight
Compression
Pushes down on suspension bridge’s deck
Cables transfer compression to towers
Tension
Cables running between two anchorages under tension

13. Suspension Bridge

14. Suspension Bridge
Have supporting truss system underneath

15. A classic suspension bridge in New York City

16. Suspension Bridges Two types: Suspension (curved cables)
Cable-stayed (straight cables, no anchorages required)

17. Cable-Stayed Bridge

18. Other Forces Torsion (twisting force)
Eliminated in beam and arch bridges
Critical in suspension bridges
High winds
Minimized by deck-stiffening trusses

19. Resonance
A vibration in something caused by external force that is in harmony with natural vibration
Similar to making constant waves in a swimming pool or maintaining one’s oscillation on a swing
Check out what resonance did to this bridge in Washington state back in 1940 (YouTube Tacome Narrows Bridge link)
Dampeners:
Designed to interrupt resonant waves
Overlapping plates create friction to offset frequency of waves

20. Weather Hardest to combat
Rain, ice, wind, and salt can bring a bridge down
Design progression: iron replaced wood, steel replaced iron
Each new design addresses some past failure
Preventative maintenance

21. Lab
Build a bridge entirely out of uncooked spaghetti pasta and glue. Your bridge is to span a distance of 8 inches and withstand the most amount of weight as possible
Record the weight of your bridge.
Place your bridge on two piers spaced 8 inches apart and find the maximum load that your bridge can support
Record the final weight that your bridge was able to support. Find your load to weight ratio (Load divided by weight of bridge).
Turn in your ratio and a photo/video of your bridge in action to the Discussion Board by Thursday, November 13.
Remember to use knowledge learned from the lecture. Beam and suspension bridges work the best for this project. Hint: use a truss system.

22. Example of how to load your bridge