A Balance Beam Play Structure Inspired by the Logarithmic Spiral

I designed this play structure with Excel and balsa wood two posts ago and my opportunity to build it came after Canada day.  Before going to the store, I revisited my design sheet for all the dimensions so I knew how much wood to buy.

At Home Depot with my calculations in hand my jaw dropped when I saw the price of cedar lumber.  Pine didn’t come in 4x4s and I didn’t want to use pressure treated wood so I bit the bullet.  The hardware wasn’t cheap either but luckily I had my dad’s mitre saw and the remaining tools required.  So, my bill of materials was:

  • 4x Cedar 4x4s, 8 feet long
  • 5x Cedar 2x4s, 8 feet long
  • Box of 100 2.5″ deck screws
  • Box of 50 6″ heavy duty screws with hex heads

Came to about $225 with tax but when I got home and started cutting, the fresh smell of cedar eased my mind and I was happy working with that beautiful wood.

Interestingly enough, during my spreadsheet work, I determined that the angle for each joint remained constant at 120 degrees for this design which means 30 degree mitre cuts.  The logs (inner dimension) grow geometrically so that every segment is 1.23x longer than the previous one. So, you don’t need to be a math genius to build this structure.  30 degree cuts and make each segment 25% longer than the previous one and you are set.  Man, all that spreadsheet work and it boils down to that!

Slide showing math properties of log spiral design

Interesting mathematics properties of this play structure’s design

That made the cutting easy and in pretty short work I had my 4x4s ready.

photo of cut lumber for playstructure

4 x 4’s cut to length with 30 degree mitre cuts

Next was the fun part – assembly!  For each step, I had to:

  • sand the steps to round off the exposed corners as much as possible
  • cut a support to the right size
  • counter sink the holes
  • drive two 6″ screws into each support and then two into the previous segment
Photo of assembly process

Assembly in process. Supports start with 4×4 blocks and then become equilateral triangles.

After a few steps, I had to build triangular supports for improved stability, put together with the 2.5″ deck screws.  In the end, the structure got to about 8′ wide and 3.5′ tall which was my target size.  My neighbour helped me move it into place before I bolted on the last two segments.  Immediately, my son and neighbours kid were on it, traversing the structure and jumping off the end.  My 2 year old daughter also enjoyed it, balancing and hanging from the beams.  I am looking forward to practising my parkour vaults and spot jumps from beam to beam!

Photo of kids playing on play structure

Kids practice balance and enjoy getting to the end to jump off. Beam to beam jumps for more advanced kids (and parents).

Well, I don’t know how much math kids would learn from playing on this structure but it sure was a great math refresher for me!  However, they can intuitively predict the next beam lengths and locations seeing the patterns in play.  They also get a feel for structures, supports and stability.  I’m not a designer by any means but with inspiration from math and nature, even I can make something beautiful!

My mission through Daddy Engineer is to encourage people of all ages to enjoy engineering play.  Between work, kids and actually doing some of these projects, there’s little time left for blogging but I hope you enjoy my efforts.  Let me know what you think, your feedback and encouragement is much appreciated.

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