Engineering Bridges

The same force phenomena that act on skyscrapers act on bridges, but in different ways. Bridges solve the problem of how to travel from one shore to another with goods and/or people. In this lesson, students engineer a bridge prototype that resists the phenomena of tension and compression forces. 

• Like skyscrapers, bridges have to be designed to withstand all of the forces that act on it. 
• Different bridge designs have different strategies to resist tension and compression and span different distances. Engineers need to think about many factors when deciding on a bridge design.
• A part of all bridges will have to be unsupported from below. The distance a bridge crosses unsupported from below is called its span. The bridge has to be designed so that the span is strong enough to withstand all of the weight on top of it.

• How are bridges similar to skyscrapers?
• What are some of the challenges facing engineers who design bridges?
• How do compression and tension act on bridges?
• What problem do suspension bridges solve?
• When would a beam bridge be a good choice for a bridge?
• When would a truss bridge be a better choice for a bridge than a beam bridge or an arch bridge?

Misconception: The forces that act on objects are different from the forces that act on living things.

Fact: The same forces act on objects and on living things.

Misconception: If an object is at rest, no forces are acting on it.

Fact: Forces act on everything at all times.

Misconception: The shape of a structure doesn’t affect its ability to withstand forces

Fact: Some structures are better able to withstand the different forces that act on it than other structures.

Arch Bridge : a bridge with a roadway held up by a semi- circle curve unsupported by piers

Beam Bridge : a short bridge with a roadway supported by columns called piers

Bridge : a structure that allows humans to cross over obstacles such as valleys or bodies of water

Cable : a very strong piece of rope made of steel wire

Cable-Stayed Bridge : a bridge in which the weight of the deck is supported by a number of cables running directly to one or more towers

Span : the distance a bridge crosses unsupported from below

Suspension Bridge : a bridge with long cables that hold up the roadway

Truss Bridge : a modified beam bridge made with many diagonal cross-supports (trusses)

Designing Structures that Benefit Society

When Miguel Rosales was in middle and high school, his favorite class was art. He now uses art in his job because Miguel is an engineer and architect who designs bridges.

Miguel enjoys designing bridges because they have such an important role in society. A bridge is a structure that allows humans to cross obstacles such as valleys or bodies of water. The span is the distance a bridge crosses unsupported from below.

Designing Boston’s Zakim Bridge

Miguel was the lead architect and designer for the Leonard P. Zakim Bunker Hill Bridge. This bridge crosses the Charles River in Boston, Massachusetts.

This bridge took 10 years to complete. Miguel worked hard on the bridge’s design. He wanted it to fit into its surroundings. Before construction on the bridge began, Miguel made many 3-D drawings and models. He also made computer drawings so he could see how the bridge would appear when it was complete.

Like skyscrapers, bridges have to balance all of the forces acting on them. A well-built bridge doesn’t fail because it is made of shapes and materials that balance these forces.

Design of Bridges

Compression happens on the deck or roadway of a bridge. The weight of people walking or cars driving puts downward pressure on the structure. If a bridge experiences too great a load without enough upward support, it will buckle or break. Tension happens as traffic on the roadway causes the underside of the bridge to become longer. If a bridge experiences too much tension on the bottom without enough support, it will snap.

Different bridge designs handle the forces that act on the bridge in different ways, such as by transferring force from an area of weakness to an area of strength.

Zakim Bridge Design

The Zakim Bridge is a kind of cable-stayed bridge. This is a bridge design that includes one or more towers with cables that run directly from the tower to the roadway. Cables are very strong pieces of rope made of steel wire. The cables running directly from the tower to the deck form the distinctive fan-like pattern you see in the Zakim Bridge.

Cable-stay bridges are similar to another bridge design called a suspension bridge. Engineers design cable-stayed bridges or suspension bridges when they need to span long distances. To suspend means to hang in the air.

In this lesson, students solve the problem of replacing old automobile bridges by engineering their own bridge prototypes that resists tension and compression forces. Through this engineering design activity, students evaluate how engineers use scientific concepts and knowledge to design technologies that solve problems.