At a Glance: Students research, design, and construct a model truss bridge from balsa material and perform a destructive test to determine its load-bearing capacity.
Concepts:
Students discover the considerations of structural design such as: loads, compression, tension, shear, strength characteristics, and materials.
Details: First, students sketch several three-view bridge designs that incorporate crossbeams to distribute the forces of tension and compression and, depending on design criteria, a substructure to increase bridge stability and strength. They select their best design and draw it to scale on graph paper to use as a construction pattern.
When building their model, students may use the Pitsco Timber Tester to identify the strongest pieces of balsa and the Pitsco Timber Cutter to safely cut the balsa strips to the precise size and miter angles to make strong joints. Construction of the bridge is best done on a construction board (preferably 3/16" foam board or equivalent). This allows the timbers to be held in position by straight pins strategically placed beside the members. The bridge pattern is taped to the construction board and then covered with waxed paper. Individual members are cut and positioned over the pattern, and glued together using Pitsco Colored Structure Glue or Pitsco HD Bond adhesive. (CA or super-glue adhesives are not recommended). After a bridge side is completed and the glue is dry, the first side is removed from the foam board and the bridge's second side is constructed.
When both sides are completed and the glue is dry, the bridge sides are positioned vertically on the construction board, parallel to each other at the given roadbed width. Cross members are cut and glued into position between the two bridge sides forming the roadbed, overhead, and substructure portions of the bridge.
Completed bridges are tested on the Pitsco Science Olympiad Structure Tester, a specially designed table with an adjustable bridge span on which a student bridge is positioned. A test block is placed on the bridge’s roadbed and a bucket is suspended from the test block with a chain. Sand is slowly poured into the bucket, progressively increasing the load on the bridge. The design that supports the greatest mass before breaking is the winner.
At a Glance: Students research, design, and construct a model truss bridge from balsa material and perform a destructive test to determine its load-bearing capacity.
