1. Bridges
Introduction
to design

2. Bridges are the KEY ELEMENTS in a Transportation System

3. 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)

4. Tension and
Compression

5. 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

6. 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.

7. 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

8. Five main types of Bridges
Beam
Truss
Arch 
Suspension
Cable-stayed

9. 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

10. 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?

11. 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

12. Better Shape for a Beam
An I-Beam is designed to have the most material at the top and the bottom

13. Beam Bridge

14. Beam Bridge

15. Beam Bridge

16. Beam Bridge

17. 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.

18. Arch Bridge

19. Arch Bridge

20. Arch Bridge

21. Arch Bridge

22. Trusses
A structure of long slender members joined at their end points.

23. Roofing Trusses Examples of trusses used to build roofs.
The more members, the more strength
The more members, the greater the expense,

24. Bridge Trusses

25. Truss Bridge
Bridges are an example of trusses being used to hold enormous masses.

26. Truss Bridge The truss shown here with a load has
Top under compression
Bottom under tension
10N F1 F3

27. What’s the Angle Question: What angle disperses the weight
most evenly?
Answer: 45 degrees

28. 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

29. Truss Bridge

30. Truss Bridge

31. Truss Bridge

32. Truss Bridge

33. Truss Bridge

34. Truss Bridge

35. 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.

36. 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.

37. Suspension Bridge

38. Suspension Bridge

39. Suspension Bridge

40. Suspension Bridge

41. 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.

42. 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.

43. Cable-stayed Bridge

44. 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)

45. Note: Chopsticks are not included
in round 2 of competition.

46. 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!

47. 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?