Bridges, especially the kinds designed and built by human beings, have been a source of endless wonder and fascination ever since people first found ways to construct them. Perhaps you and your friends played by small streams as youngsters and instinctively tried building small makeshift bridges out of available sticks and logs. If so, you can perhaps imagine the basic way your long-ago ancestors approached the same task but out of need rather than recreation and on a larger scale.
In the modern world, of course, enormous bridges carrying more than a dozen lanes of traffic and featuring multiple road decks are the norm in major cities that include sizable rivers or connect across small strips of ocean or lake water to connect to another land mass. These elegant behemoths come in a variety of types, with the design of each chosen to suit the specific land layout and transportation needs of the settlement in question.
Bridges are a time-tested and time-honored type of school science-fair or science-class project. If you want to impress your mentors and peers, you should choose a design that is especially strong. Some kinds of bridges and their subclasses lend themselves to this task more readily than others.
Types of Bridges
There are five main types of bridges, which vary primarily in their basic shape and therefore in terms of how the stress they experience – from both their own considerable weight and that of their traffic – is distributed. Chances are good if you live in a major city that you have made use of some or perhaps all of these bridge types, probably without being able to name more than one of them.
Beam Bridge: This is the simplest kind of bridge, requiring just a straight span, which bears all of the tension ("pulling" stress) of the bridge load, and piers, which bear the compression forces. These are cheap and easier to build than other bridges, but they're poorly suited for large waterways because their individual spans between piers cannot be very long. Also, while inexpensive, they are considered aesthetically charmless in most settings.
Arch Bridge: This type of bridge dates back to ancient Roman times. It is favored for its beauty, but in many cases the land to be bridged doesn't allow for the broad contact base of this type of bridge. These can reach impressive sizes. An arch bridge over the New River in West Virginia built in 1978 has a single strong span exceeding 3,000 feet.
Truss Bridge: There are actually at least four different individual truss bridge designs, with the number depending on the source consulted. These, like beam bridges, are inexpensive and easier to built than most types of bridges, but they are unattractive to look at and poorly suited for waterways for the same reason beam bridges are – a short distance between piers.
Suspension Bridge: If you were able to name a single type of bridge before delving into this article, it's most likely the suspension bridge, revered by the world for its grandeur and by engineers for its mathematical and physical cleverness. The Brooklyn Bridge and other suspension bridges have cables running along the deck and anchored to land at each end to offset tension; two (usually) giant piers called towers absorb the compression, and the deck is suspended by a set of vertical cables from the other set.
Cable-Stayed Bridge: The Zakim Bridge in Boston is an alluring example of this type of bridge. Like a suspension bridge, it makes use of cables and individual spans can be enormously long, in excess of one mile (1.6 km). The cables, however, run from the tops of the towers to the road deck instead of to each end of the bridge.
Building Model Bridges
You can use different materials for different styles of scaled-down bridges. Suggestions are given below for using ordinary playing cards, but feel free to improvise as long as your bridge meets the definition for its kind.
For all bridge types, you could use toilet-paper or paper-towel rolls or something else that is inexpensive and easy to acquire. You'll also want a good supply of scotch tape, scissors, a stapler, string, a ruler and perhaps a hole punch.
Beam Bridge Model: Fold your cards along right angles to make the piers and tape the ends of other cards carefully together to make the beam (deck). Note how placing the junctions at different points with respect to the piers below affects what happens when you "test" your bridge strength by depressing it carefully with a finger. Where should the junctions be, and what happens when you use too few of them?
Arch Bridge Model: This one takes some patience and a lot of tape, but refer to a photo of an existing arch bridge for guidance. You can make individual "bricks" by folding individual cards into little boxes and taping them closed. Make sure you include abutments to counteract the considerable laterally directed forces at the bottom ends of the bridge.
Truss Bridge Model: Below, you will learn of the various types of truss bridges, but the important aspect is the stability of the triangle as a geometric construct. Note that if you build a "box truss" with square "spans" instead of interlocking triangles, the structure is less stable.
Suspension Bridge Model: This is difficult to pull off alone unless you have a lot of time and patience. It is easiest to build the towers and deck first and then play with adjustments to the cable length and tension to see what "feels" the strongest. Imagine being the project manager of the first-ever suspension bridge!
Cable-Stayed Bridge Model: Again, this is quite an elaborate model choice, but offers a good chance to play with the ideal length of the cables descending from the tower tops. What do you think would happen if a bridge of this type were made with asymmetrical cable sizes between the two towers?
Truss Bridge Designs
There are a number of different styles of truss bridges in use. You can make truss bridges more easily using popsicle sticks than using playing cards; just have wood glue available.
Viewed from the side, a truss looks like a trapezoid – a rectangle with symmetrically sloping ends rather than parallel ones. The parts of this four-sided frame are the top and bottom chords and the ends are called posts. The inside pieces of the truss are called members.
- Warren: This model uses members that form equilateral triangles, but there are no vertical members.
- Pratt: This variety has a series of skinny rectangles containing inward-leaning members to form triangle pairs in the individual units.
- Howe: This is very similar to the Pratt, but in the middle, the "leaning" members meet at the top chord instead of at the bottom chord. It is better than the Pratt in terms of handling compression, all else being the same, but costlier.
- K truss: As its name suggests, this complex truss design includes units in the shape of the letter K and its mirror image.
Which truss bridge is the strongest? That depends on materials and other factors beside basic design, in the same way some bicyclists can travel faster than a poor driver in a bum automobile. But either way, a lot of strong work has gone into all of these bridges and their designs over time.
- West Kentucky Community and Technical College: Model Truss Bridge Design
- ScienceProjects.org: Designing a Strong Bridge
- National Building Museum: Bridge Basics
- Scientific American: Popsicle Stick Trusses: What Shape Is Strongest?
- U.S. Bureau of Reclamation: Design and Build Your Model Bridge
- Garrett's Bridges: Truss Design