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Bridges. Introduction to design . Bridge s are the KEY ELEMENTS in a Transportation System. All bridges must contend with two important forces: Compression and Tension. Compression is a pushing together force. ( compressional stress) Tension is a pulling apart force.
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Bridges Introduction to design
All bridges must contend with two important forces: Compression and Tension Compression is a pushing together force. (compressional stress) Tension is a pulling apart force. (Tensional stress)
Tension and Compression
Tension and Compression Buckling: compressive forces overcome the structure. Snapping: tensile forces overcome the structure. Best way to deal with these forces: Dissipate or Transfer
Tension and Compression Dissipate: allowing the force to be spread out more evenly over a greater area. Transfer: moving force from an area of weakness to an area of strength.
Statics • The study of structural design • “Balancing forces so structures stay up” Assumptions of statics are: • Materials do not stretch • Pivots support tension and compression only • Cables support tension only
Five main types of Bridges Beam Truss Arch Suspension Cable-stayed
Beam Bridge(Girder bridge) • Simplest and most inexpensive kind of bridge. • A horizontal beam supported at each end by piers where the weight of the beam pushes straight down on the piers. • For short and medium spans. Basic Beam Bridge
Bending Beams • You’ve seen materials bend: how does bending happen? • Look at the following diagram. Think how it will bend. Where would there be tension? Compression? No forces?
Bending Beams • Top is in tension • Bottom is in compression • Middle has no stress • The mass in the middle of the beam is wasted • It contributes nothing to stop bending • It adds weight which contributes to bending
Better Shape for a Beam • An I-Beam is designed to have the most material at the top and the bottom
ArchBridge • One of the oldest types of bridges. • Weight of bridge istransferred outward along the curve of the arch to the supports (abutments) at each end. • Abutmentscarry the load and keep the ends of the bridge from spreading out.
Trusses A structure of long slender members joined at their end points.
Roofing Trusses • Examples of trusses used to build roofs. • The more members, the more strength • The more members, the greater the expense,
Truss Bridge • Bridges are an example of trusses being used to hold enormous masses.
Truss Bridge • The truss shown here with a load has • Top under compression • Bottom under tension 10N F1 F3
What’s the Angle • Question: What angle disperses the weight most evenly? • Answer: 45 degrees
What’s the Angle • The closer the beam gets to 900, the more force is put into compressing the beam along its long axis. • The closer the beam gets to 00, the more force is put into compressing the beam along its short axis. • Keep this in mind when building. For more info on trusses, click here to get to MAKE magazine description
Suspension Bridge • Aesthetic, light, and strong but tend to be the most expensive to build. • Can span distances from 2,000 to 7,000 feet -- far longer than any other kind of bridge. • Suspendsthe roadway fromflexible main cables, which extend from one end of the bridge to the other. • Main cables rest on top of high towers and are secured at each end by anchorages.
Suspension Bridge • Most of the weight of the bridge is carried by the cables to the anchorages, which are imbedded in either solid rock or massive concrete blocks. • Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free.
Cable-stayed Bridge • Look similar to suspensions bridges -- both have roadways that hang from cables and both have towers. • The difference lies in how the cables are connected to the towers. • Suspension bridges: cables ride freely across the towers transmitting the load to the anchorages at either end. • Cable-stayed bridges: cables are attached to the towers, which alone bear the load.
Cable-stayed Bridge • Cables can be attached to the roadway in a variety of ways. • Radial pattern: cables extend from several points on the road to a single point at the top of the tower. • Parallel pattern: cables are attached at different heights along the tower, running parallel to one other.
Bridge Building Competition, Round 2 Points: 1 point for each gram the bridge holds without collapse. 10 points for each cm of length beyond 28 until 35. 20 points for each cm of length beyond 35 until 45. Three winners: Largest load supported Longest Span Most cost efficient Bridge (points/cost)
Note: Chopsticks are not included in round 2 of competition.
Be Careful • When you design, remember that the force of the load gets transmitted to the joints (where the beams come together). • Make sure your joints are strong!
Experiment • Test materials until failure: paper, aluminum foil • Clamp foil materials so the entire piece takes the stress, then add weight until the piece fails • Observe the edge, and try to determine whether the failure is ductile or brittle • Question: When you rip off a piece of aluminum foil how exactly do you do it? Can you break a piece by hand?
