Bridges Introduction to design

Bridges are the KEY ELEMENTS in a Transportation System

Compression and Tension 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.

The study of structural design 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?

The mass in the middle of the beam is wasted 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

Beam Bridge

Beam Bridge

Beam Bridge

Beam Bridge

Arch Bridge One of the oldest types of bridges. Weight of bridge is transferred outward along the curve of the arch to the supports (abutments) at each end. Abutments carry the load and keep the ends of the bridge from spreading out.

Arch Bridge

Arch Bridge

Arch Bridge

Arch Bridge

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,

Bridge Trusses

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

Truss Bridge

Truss Bridge

Truss Bridge

Truss Bridge

Truss Bridge

Truss Bridge

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. Suspends the roadway from flexible 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.

Suspension Bridge

Suspension Bridge

Suspension Bridge

Suspension Bridge

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.

Cable-stayed Bridge

Points: Three winners: Bridge Building Competition, Round 2 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?