bridges
suspension bridge
2,000-7,000ft, truss system prevents bending & twisting, spreads out weight, allow things underneath, light, strong, long, most expensive
arch bridge
200-500 ft, natural strength, tricky to build (unstable until 2 spans meet, good for deep gorges
beam bridge
250 ft, cheapest, easiest to build, further span= weaker & more bend
cable-stayed bridge
500-2,800ft, fast build, less cables, allow things underneath, still expensive
how does arch handle tension and compression
Arch bridges push the force along the arch (compression) and the abutments on the side push back on the structure (tension)
newton's 3rd law
Compression and tension are equal and opposite forces that must balance for the structure to stay up. Also, as the load pushes down on the bridge, the bridge pushes back on the load.
what factors influence bridge design chosen by engineers
Distance, weather patterns, natural formations and the amount of traffic
what are the 2 balanced forces
Tension and compression must be balanced. If unbalanced, compression must be greater because it holds the bridge together. If tension is greater, the bridge will collapse in the middle. Also, load and support forces must be balanced OR support must be greater than load. If load force is greater, the bridge will collapse.
advantage of truss system (especially of beam bridge)
adds rigidity to the pliable beam bridge increasing its ability to dissipate tension & compression and puts force on top and bottom leaving middle with no load
1,200 ft span across deep river gorge
arch bridge, natural strength, no need for supports, won't damage rock
why is size limited on truss bridge
as distance increases, size does too. after too much distance, the weight is so large that the truss can't support it
300ft span across narrow waterway
beam bridge with truss, short span, low cost for unimportant area, needs extra support
3,000 ft across busy shipping channel
cable-stayed bridge, medium length, boats go under, not as expensive to replace damages
2 ways to deal with reducing buckling and snapping
dissipate: to spread out force over a greater area no one spot has all the force (truss) transfer: to move force from one area of weakness to an area of strength (suspension)
truss bridge
distances vary, bars rarely bend, good for further distances than beam, spread weight to all triangles, distance limited with weight
compression
force that acts to compress or shorten the thing that it's acting on
tension
force that acts to expand or lengthen the thing that it's acting on
why is a triangle shape used on truss bridges
rigid structure, transfers load from a single point to a larger area
torsion
rotational or twisting force, suspension bridges are more vulnerable, natural arch and truss have eliminated problem, test models in wind tunnels first
what is the main feature that controls the type of bridge
span
4,200ft span across ocean w/ boats underneath
suspension bridge, long span, allow boats underneath
difference between cable-stayed and suspension
suspension= M shape, 4 anchors, 2 towers cable stayed= A shape, 1 tower, doesn't nee 4 anchors, cables up from road to single tower and secured
differences between tension & compression
tension: expands/ lengthens compression: compress/ shorten
how do tension and compression have to do with buckling and snapping
too much compression: buckling too much tension: snapping
what are the roles of the cables
transfer compression to towers to dissipate into ground,recipients of tension, stretched from anchor to anchor (anchors also have tension)
bridge materials
used to be made of wood (would rot) and rope (can snap), steel and concrete used now, no perfect material, pick the material that is not too heavy but holds up in weather and heavy loads (steel is lighter than iron and cheaper than wood)
buckling
what happens when the force of compression overcomes an object's ability to handle compression
snapping
what happens when the force of tension overcomes an object's ability to handle tension