Discover Engineering ENGR 096 Bridges Discover Engineering ENGR 096

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)

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

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

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

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

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

Trusses

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!

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

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

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

Suspension Bridge

Suspension Bridge Have supporting truss system underneath

A classic suspension bridge in New York City

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

Cable-Stayed Bridge

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

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

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

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.

Example of how to load your bridge