Design- Defining the Dilemma

As with any design process, the first step is always google the problem.  Searching for "how to build a spiral staircase around a tree" didn't yield any useful results, so I had to create a design myself from scratch.  Even if I had found a suitable design, it would have to be modified to fit my needs.  And what specifically were my needs?  At this point I set about creating a clear definition of...

The Problem
 I want to build a direct route to the 4th floor<insert hyperlink> of the treefort.  It has to bypass and be completely independent from levels 1, 2, and 3.  My first decisions were to determine where and how this would be possible (if at all), so I took a notebook and tape measure to the 4th floor and started sketching.  My early ideas varied from simple ladders to complex systems of platforms with floating staircases, but I was enamored with the idea of a spiral.  The fourth floor is supported by 4 trees that all lie within the interior of the floor, and each of these trees has at least one knee brace supporting the floor.  The braces made it impossible to use any of the 4th floor supporting trees as a spiral staircase location, so I looked around to see if any close by trees could be used.  I took some measurements and decided a nearby double leader tulip poplar was the best candidate.

Looking down from the 4th floor, spiral will go up tree at top of picture

With my location determined, the next step was to find a way to connect it to the 4th floor.  There's a gap of approximately 8' between the proposed spiral location and the nearest corner of the 4th floor.  I balanced a 2x4 in the gap to help visualize a solution.

2x4 balanced from the 4th floor corner to future spiral location

Eventually, I came up with the idea of replacing the top stair of the spiral with a rectangular bridge straight to the 4th floor.  It could then be supported by the spiral structure on one side and the 4th floor on the other.  Thinking back to my structures engineering courses, I planned to fix the bridge to the staircase end and use a roller type connection to the 4th floor.  I wanted to make sure the bridge and 4th floor were structurally independent to allow for tree movement.  Unfortunately I had neither the know how nor means to build a 'roller'.  Resting the bridge on the 4th floor would allow movement but could possibly damage both.  Some sort of low friction material could be used in between, but I didn't have anything like that.  Plus, it would change the whole look of the 4th floor.  This problem took quite a while to find a solution, but eventually I came up with the idea of hanging the bridge below the 4th floor using steel cables.  Going from the bridge to 4th floor would require a step up, no big deal.  This detail was very important going forward in the design because it set the elevation of the top step and effectively determined the height of the spiral.

4th floor on the left (purple), bridge (blue), and approximate spiral location (light blue) with double tree on right

As shown by the orange arrow, I had inadvertently decided which was the spiral would rotate.  Next up was to figure out how exactly the staircase would work.