Engineering Principals

Balsa Wood Bridge Building Exercise

You are to design and construct a balsa wood bridge that will span a minimum of 11 ¾ inches.

The bridge cannot be longer than 14 inches.

There will be usable abutments at both ends 11 ¾ inches apart.

The bridge substructure can extend down to use the abutments no more that one inch below the bottom of the road bed.

The bridge superstructure cannot be taller than 3 inches.

The inside height of the bridge must be a minimum of 1 ¾ inches.

The road bed must be a minimum of 2 ½  inches wide with no obstructions.

The bridge cannot be any wider than 3 ¼ inches.

The center of the bridge must have an opening to accept a ¼ inch bolt for testing.

The test block (3 1/8” X2” X ¾” must pass through the bridge.

Materials:

1 Sheet of graph paper

20 sticks of balsa wood

Wood glue

Masking tape to hold drawing and wax paper

Tools:

Scale

Exacto knife

Pencil

Pitsco Timber Cutter (I only have one that must be shared)

Pins

Construction Procedure:

1.     Sketch the bridge on graph paper and get the design approved by the instructor(Grade 1)

2.     Tape new graph paper to cardboard and draw a side view of the bridge. Get the drawing approved by the instructor (Grade 2)

3.     Cover graph paper with wax paper, tape in place.

4.     Collect the materials for the bridge.

5.     Construct one side of the bridge and let dry.

6.     Construct side two of the bridge and let dry.

7.     Construct cross sections and let dry.

8.     Assemble the cross sections to the bridge sides and let dry

9.     Present the bridge for inspection to the instructor. (Grade 3)

10. Destructive bridge test performed by the instructor (Grade 4)

Vocabulary word:

Load – weight added to a bridge

Compression - stress that pushes together

Tension - stress that pulls apart

Racking – stress that distorts

Stress – the force that produces strain

Superstructure – The truss portion above the roadbed.

Substructure – that portion of the bridge under the roadbed.

Span – the distance between supports.

Truss - a structural load-carrying member with an open web system designed as a simple span with each member designed to carry a tension or compression force.