Bridging the Gap: Building Bridges 101, It Is Time to Get To Work

By: Gustavo A. Sandoval

Types of Bridges

Beam Bridge – constructed to span short distances because of its horizontal beam supported at each end by piers.

Beam Bridge

Truss Bridges

Truss Bridge – constructed with straight, steel bars in series of triangles. Rigid arms extend from both piers, diagonal steel tubes, projecting from the top and bottom of each pier, hold the arms in place. The arms that project toward the middle are supported on one side.

Truss Bridge

Arch Bridge

An arch bridge is a bridge with abutments at each end shaped as a curved arch. Arch bridges work by transferring the weight of the bridge and its loads partially into a horizontal thrust restrained by the abutments at either side.

Arch Bridge

Suspension Bridge

A suspension bridge is a type of bridge in which the deck (the load-bearing portion) is hung below suspension cables on vertical suspenders. The force on the roadway is transferred into compression on the towers which support most of the bridge's weight.

Suspension Bridge

Forces on a Bridge

Compression Tension Torsion Shear

Building your own Bridge

First you must decide on the shape and overall design of the bridge just as an engineer does.

Building your own Bridge

Sketch out the shape of the bridge to full size on a piece of paper. The sketch will tell you where each spaghetti noodle will go when you start gluing them together. The sketch will include the top and bottom view of the bridge and the cross bracing in between the top and bottom.

Building your own Bridge

Glue the cross bracing between the top and bottom cords.

You have now completed only one truss. This has to be repeated for the second truss. This picture shows two completed trusses.

Building your own Bridge

You will need to put some supports across the bottom cords of the two trusses to support the bridge deck, these are called deck beams. Making sure that the trusses are no more than 2 ¾ inches apart. The next step is to add the bridge deck.

Completed Bridge

Building your own Bridge

• Let the bridge sit 15 minutes before testing the bridge.

• Weigh the bridge before testing and record this number.

• Place the bridge on the testing frame.

Building your own Bridge

• Once you have place the bridge on the testing frame you made weight to the bridge. You can use steel beams make sure they are the same length and weight.

• Once the bridge breaks you can test the efficiency of the bridge by using the following equation efficiency = total weight held/weight of bridge

Glossary

Beam a horizontal structure supporting vertical loads by resting bending

Arch typically curved structural member spanning an opening and serving as a support

Force any action that tends to maintain or alter the position of a structure

Compression is a pushing force

Tension is a stretching force that pulls on materials

Torsion is a twisting force

Span is the horizontal space between two support of a structure

Structural efficiency is the ratio of load carried to bridge mass

Shear is sliding force

Resources

http://bridgecontest.usma.edu/download.htm http://www.pbs.org/wgbh/buildingbig/bridge/b

asics.html http://www.iit.edu/~hsbridge