bridges discover engineering engr 096. bridges three main types of bridges: – beam bridge – arch...
TRANSCRIPT
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
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 Bridges
Two types:– Suspension (curved cables)– Cable-stayed (straight cables, no anchorages
required)
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.