frizzflowerUrban and Civil

Nov 29, 2013 (3 years and 8 months ago)



Discover Engineering

ENGR 096


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)


Beam bridge: spans up to 200 feet

Arch bridge: 1000 feet

Suspension bridge: 7000 feet

Difference comes from compression and

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



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

Works for trusses too!

Arch Bridge

Semicircular with abutments on each end

Arch diverts weight from deck to abutments

Compression: always under compression (no

Arch Bridges

Does not need additional supports or cables

Arches made of stone don’t even need

Suspension Bridge

Cables, ropes, chains suspend the deck from towers

Towers support majority of the weight


Pushes down on suspension bridge’s deck

Cables transfer compression to towers


Cables running between two anchorages under tension

Suspension Bridge

Suspension Bridge

Have supporting truss system


Suspension Bridge

A classic suspension bridge in New York City

Suspension Bridges

Two types:

Suspension (curved cables)

stayed (straight cables, no anchorages

Stayed Bridge

Other Forces

Torsion (twisting force)

Eliminated in beam and arch bridges

Critical in suspension bridges

High winds

Minimized by deck
stiffening trusses


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


Designed to interrupt resonant waves

Overlapping plates create friction to offset frequency of


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


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

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