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)– 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

Suspension Bridge

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