Bridge Unit Study - Lesson 3: Trusses

My son built and tested a truss bridge.

Trusses are formed when two straight pieces come together to form a rigid joint. Most often they are triangular in shape and trusses are used in all types of construction from roof tops, to the Eiffel Tower and of course bridges.  

The book Bridges: Amazing Structures to Design, Build & Test (Kaleidoscope Kids), explained how trusses work and then suggested a project for building a truss bridge. Again, my son jumped in.

 Using craft sticks and glue my sun built a truss bridge full of triangles.

 It took several days to build the bridge as he needed to wait for the glue to dry after each new side was constructed.

 Once it was finished and dry, testing began.

 His plan was to attach a road to the bridge (a ruler) and hang a bucket full of weights from the road.

 The basic idea worked well, but the bridge turned out to be much stronger than he anticipated.

In order to test the bridge to the point of failure, a stronger road surface was required as it was the weakest link in the design.

 Throughout the process, he took notes which included a sketch of his design as well as estimates of how much weight the bridge could hold.

Test set-up

Weighing coins to determine how much weight was applied to the bridge

A stronger ruler (road surface) was used

Free weights were used instead of coins as they were heavier

Books, weights, soup cans......... lots of heavy stuff - the bridge still held strong

Finally he got a hold of heavier weights.

 The bridge reached failure. 

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