There are many different kinds of bridges. You have probably crossed several of them. Each type of bridge offers specific advantages, and comes with different limitations.
The Golden Gate Bridge, which spans the one-mile strait between San Fransisco Bay and the Pacific Ocean, is suspension bridge. Its deck (the load-bearing portion that supports the roadway) is held up by vertical suspenders that attach to large suspension cables.
Suspension bridges are capable of spanning great distances and can be built high above ground or water level. They are also very flexible and can withstand some movement during high winds or earthquakes. However, they require very strong foundations and cannot carry as heavy loads as other bridge types because the deck has to be relatively light.
Arch bridges like The Richmond Bridge, in Richmond, Australia, above, are some of the oldest operational bridges in the world. Arch bridges work by transferring part of weight of the bridge and its load horizontally into abutments – places where the bridge rests or make contact with the ground.
Arch bridges are incredibly sturdy - and even get stronger over time if they are well maintained. They can be built with many different kinds of materials and can withstand much heavier loads than some other bridge types. A single arch can only span a limited distance, so longer bridges require multiple arches. This makes arch bridges more time-consuming, and often much more expensive to build.
This railroad bridge in Croatia is an example of a truss bridge. This particular design is a variation of the Baltimore Truss, which is common in railroad bridges.
This bridge in Lockport, New York, is a deck-truss bridge. In this design, the deck rests on top of the truss rather than inside it.
The Forth Bridge in Scotland is a beautiful example of a cantilever truss bridge. Cantilevers are only supported on one side and, in large spans, are often built in pairs. This design can be built outward from set points rather than from end to end.
A truss is a structure of connected (usually straight) pieces that acts as a one piece. Trusses can dramatically increase the overall strength of a structure without adding a lot of weight or material. They can also can be built in one place, then transported to somewhere else for assembly. Truss bridges are often made from steel or other metal - but can just as easily be constructed out of wood or other material.
There are many more variations on the truss bridge. Once you understand how trusses work, you'll start to see them all around you: on bridges, in buildings, supporting towers, even in the cranes that are used to build other structures.
As an engineer, you’ll want to consider how long your bridge needs to be to connect two places (it’s “span”), how heavy of a load it needs to hold (in our case: pennies), and meeting budgetary restrictions (limited materials) when designing and constructing your bridge. Pro tip for your design: when working with toothpicks and marshmallows, triangular shapes will provide the best bridge strength and stability.
If you enjoyed this activity – take it further with more design challenges!
Thought challenge: What materials would you use if you needed to design a bridge to last more than 100 years?
The goal of this activity is to get creative and practice engineering design! If you don’t have the exact materials on hand – check around to see if there are other materials that you can use instead. A few alternative building supplies you might consider:
In place of toothpicks: try pencils, straws, or popsicle sticks
In place of marshmallows: try Play-Doh, tape, craft glue, homemade modeling clay.
If you use glue or clay, you’ll need to give it plenty of time to dry before testing the strength or earthquake resistance of your bridge. Also, toothpick bridges made with glue are much stronger than those made with marshmallows, so try testing the strength by placing large textbooks or cans of soup on top instead of pennies.
In place of pennies: try other small coins, bottle caps, individually wrapped pieces of candy...
For propping up your bridge: try textbooks or shoe boxes, photo boxes, tissue boxes, or cereal boxes of the same size with something inside of them to keep them from sliding
Photo credits:
Image of The Golden Gate Bridge by Stephen Kapinos, found on freeimages.com
Image of The Richmond Bridge by JJ Harrison, found on Wikipedia
Image of Railway bridge over the Mura river in Croatia by Silverije, found on Wikipedia
Image of Deck truss bridge in New York by Leonard G, found on Wikipedia
Image of Forth Bridge by Andrew Shiva found on Wikipedia, shared via
Design a bridge out of a limited number of toothpicks and marshmallows that is strong enough to support at least 10 pennies in a small cup. Your bridge must also meet the following requirements:
If you don’t have the exact materials on hand – look around to see what other materials you can use instead. Check the bottom of the page for suggested substitute materials.
Always check with a parent or guardian before you start a project and make sure it is okay to use materials you find around the house.
