Bridge Technology II: Build, Destroy, Scream Real Loud!

In a previous post, I wrote about the work that I have my kids do prior to building and testing model bridges.  In this post, I address the fun part – the building & breaking.

One of my favorite projects in sixth grade is getting to build a bridge and then test it to failure. I was inspired by a civil engineering course I took as an undergrad at WPI, where we had to construct a raw spaghetti bridge to certain specifications and testing them to the point of failure.  In fact, during my first go-around at this project, I used raw spaghetti and hot glue in an homage to my experience (really, I never thought to use other materials).  Only once, never again, and for a couple reasons:

  1. I wiped out the local grocery store’s inventory of store-brand spaghetti in one fell swoop.  They weren’t happy.  I was told to never do that again (note:  if you do try this, give the store some notice and they’ll make a special order to help you out).
  2. I was finding shards of spaghetti all over my room for the next 18 months.

In lieu of raw spaghetti, a material that creates perhaps the most awesome visuals when it fails, I’ve also tried coffee stirrers and popsicle sticks.  Coffee stirrers are extremely flimsy and my students that year found them difficult to work with.  Popsicle sticks have been my go-to member material for the last 5 years or so; they are inexpensive, readily available in bulk, and make minimal mess when broken.

As for adhesives, hot glue has been old reliable.  Unfortunately, I work in a building erected in the 1950s.  My room has 3 usable outlets in inopportune locations, which are then rigged with extension wires and power strips so all the hot glue guns can be plugged in.  Safe?  Well… probably not.  This year, I switched to white liquid glue.  It doesn’t dry as quickly as hot glue, but holds well when it does. It won’t  accidentally trigger an electrical fire at the school, and doesn’t send as many students to the nurse as did the hot glue and the glue guns – two additional perks!

Students get a copy of the project rules and requirements before breaking into groups to plan their attach.  For years, I used to have students work in pairs – I figured they’d get more hands-on time during the build.  Alas, this gave me 12 projects in each class.  While this doesn’t sound like much, keep in mind that each bridge takes several minutes to set up, test, and ultimately destroy.  Doing it this way, I was able to go test an entire class worth of bridges in about a week.  This year, in an effort to speed things up, I allowed groups of 4.  I thought with twice the people working on the same project, the build time would be faster; it wasn’t. I thought I could coach students into pairing up and working on different pieces at the same time and then bring everything together for a final assembly.  This worked for the first day, but never again.  The kids wanted to watch and help on every part, which showed to be as efficient as having only two kids working on everything.  However, there were less bridges to test and I was able to bring my testing days down to only two!

Prior to the building, students created sketches and “blueprints” for the tops, sides, and bottom of their bridge.  These “blueprints” were full-scale drawings, allowing students to use them as templates.  Many groups strayed somewhat from their drawings, which was permitted and I applauded (well, when it was to solve a problem they were facing; when it was due to sloppy construction I became a little irked).  When building begins, each group receives their “budget” of 150-200 sticks and several mini-sticks of hot glue (or, one bottle of white glue).  I don’t allow students to borrow or use another group’s extra material; they are mandated to budget their resources and stick to it akin to a real civil engineering project.  After about a week of building, each group was set to test.  Groups needed a team name, a bridge name, a group photo (showing off their design), and record the weight of their bridge.  I rigged a testing station by stacking tables on top of one another and separating them to the desired 50cm gap.  Bridges rested on top of the tables.  A hanging contraption (a stiff piece of wood with hooks) is placed in the center of the bridge and a 5 gallon bucket hangs down.  Sand is slowly poured in while the team waits with baited breath until the bridge (or some part of the bridge) fails and the bucket crashes to the ground.  The bucket is weighed with a fish scale and recorded for a mini in-class contest.  I didn’t keep records for the spaghetti or coffee stirrer bridges, but have for the popsicle stick ones.  For popsicle sticks and hot glue, the class record is 106.7 lbs.  For popsicle sticks and white glue, the class record is 54.1 lbs.

Perhaps the most difficult thing for me is keeping the non-testing students engaged while we test the bridges.  I usually run the show – the pouring of the sand, the weighing, etc.  This year, I turned it over to the kids and hung out in the background doing crowd control, but that wasn’t ideal either.  I’ve tried having kids record predictions, film, or put the finishing touches on their creations, but it never fails to degrade into a loud non-paying-attention crowd with ill-spirited comments spewing forth from time to time.  I thought to run each test during lunch and recess time, showing a sped-up video version of the testing during class time.  Perhaps I’ll try that next time around.

For project descriptions and rubrics, check out these links:

Bridge Rubric 2011
bridgeProjectRubric (2008)
2014 Bridge Design Activity
2008 Spaghetti Bridge Contest

And here are videos of the testing phase (link goes to playlist, embedded video is of the playlist)


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