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Running Time: 5m 55s
Size: 8.2 MB

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Have you ever wondered how different bridge designs manage the forces of tension and compression? In this video segment adapted from Design Squad—a PBS TV series featuring high school contestants tackling engineering challenges—a team of students competes in a bridge design and construction challenge without the aid of power tools. Using handsaws and a boring machine, as well as an age-old wood-joining technique, the team constructs a king post bridge, a type of truss bridge that gets its strength from ultra-rigid triangles that will not easily bend or twist.

Background Essay

Bridges and other engineered structures are subject to forces that push and pull on them. The load, which includes the weight of the structure and anything supported by it, pushes down. Depending on how the structure has been engineered, different parts are said to be in tension (stretched) or in compression (pushed on) as they manage this load.

As illustrated in this video segment, a truss is an assembly of interconnected triangles that is most commonly used to strengthen bridge decks. Horizontal bars, called chords, run along the top and bottom of the length of the deck, usually on both sides. The upper and lower chords are joined to diagonal struts and vertical braces called ties. Together, they create a triangular configuration. A triangle is a particularly rigid shape that will not easily bend, twist, or otherwise deform because a force pulling on one of the three sides is balanced by pushing forces on the other two sides.

Looking closely at how forces move through a truss bridge, the force of a load puts the upper chords in compression, squeezing in from either end of the chords. The lower chords, by contrast, are stretched or lengthened by a pulling force called tension. The diagonal and vertical braces that connect the upper and lower chords carry these forces so that tension or compression acting on one member of the truss is countered by a balancing force acting on an adjoining member. In this way, the load is distributed evenly through the framework design of the truss.

In this video segment, the student design team uses an age-old technique to secure the timber framing. Simple and strong, the mortise-and-tenon joint has been used for millennia to join two pieces of wood. Mortise-and-tenon joints come in several variations, but they all consist of a mortise, which is a recess cut into a piece of wood, and a tenon, or "tongue," at the end of a board that fits into a mortise. The force that runs through the bridge members, then, is transferred at these joints. For the truss bridge, the team, under the guidance of an experienced timber framer, cut a "wedged dovetail" joint. This design allows room for the tenon to be inserted into the mortise with a wedge-shaped key (also made out of wood), preventing its withdrawal and locking it in place.

Discussion Questions

• Is one type of bridge best for all conditions? Why or why not?
• Make a simple drawing of the truss bridge. Use arrows to describe the forces on each element of the bridge.
• The engineering design process describes a series of steps, including testing the designed object and making changes and revisions. Are these steps evident in the video? Which steps are not evident?
• Can you find a bridge in the area where you live? Make a sketch. What parts of the bridge are in tension? In compression?

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