Spiral Structure I- Connecting to the Tree

It had now come time to design what I had always thought would be the hardest part of the staircase- the step supports.  They had to be substantial enough to support the structure, but small enough and placed correctly to allow comfortable passage (not bonk your head).  Mock ups and some trial and error led me to believe a 9 inch rise between steps would be suitable.  At 12 steps per revolution and 9 inches between each step, each revolution would ascend 9 ft.  I previously determined the last step/walkway needed to be about 19.5 ft <check notes> off the ground to connect to the 4th floor.  Two full revolutions took me to 18 ft, so I placed the first step 1.5' above the ground.  See pictures below for a better illustration:

The left picture shows the side view of the steps with the starting height, step height, and revolution height labeled.  On the right, the same model is shown from a downward perspective.  Now that I had the stairs positioned it was time to figure out how to support them.  I set about designing...

The Brace 

Many of the designs I saw online featured an individual brace for each stair, essentially forming right triangles with the tree vertical, step horizontal, and brace making up the hypotenuse.  The steps were anchored perpendicularly to the tree at their inside and at the bottom of their brace.  I knew very early on in my design that I wanted the bracing for the staircase to connect at a tangent to the tree as opposed to perpendicular connections.  This was due to the limitations of my materials and my inexperience with perpendicular connections directly into a tree.  Also, I wanted to minimize the number of holes I'd need to make in the tree.  I already knew what materials I had available for the build --2x6 planks for the braces connected to the tree with lag bolts-- so my design from the very beginning incorporated these pieces.  Looking over what I had thus far, I saw the stringer pieces needed a support at each end (corners of the square).  From each of their corner intersections, I drew support beams back tangent to the tree:

Beams (white) connect the square corners to the tree.  Bolt locations shown in gray.
Stringers will connect to the end of each beam

Each revolution would have 4 support beams running horizontally outwards from the tree to the square support corners.  I considered supporting these 'main beams' from below with a normal knee brace and also from above like a roof frame.  Ultimately I decided to use knee braces due to the amount of space and required board length for roof frame style supports.  Since the spiral will go around the tree twice, each main beam will line up vertically with another.  This lead to the possibility of using one large brace for both main beams by putting the lower beam within the big brace supporting the upper.  After exploring this idea for a while, I eventually decided against it due to concerns over constructability.  To support the main beams, I settled on using a 2x6 knee brace at a 45° angle going 2 ft below the beam.  This left me with a minimum of 7 ft of headroom for stair climbers:

Knee brace positioned on tree, bolts shown with gray circles

My next step was to figure out how the stringers would connect to the main beam.  The stringers were to run along the square sides as shown in the earlier diagrams.  This meant the main beams ends needed to support 2 stringers: the top of the lower stringer and the bottom of the next.  I'd already had the idea to put railing posts at the beam ends.  Since the stringers intersected at right angles, a 4x4 post would fit perfectly.  The post would extend above the main beam for a railing support and go below to support the stringers.  A notch in the 4x4 at the right spot would allow it to slide perfectly onto the end of the 2x6 main beam:

Knee brace with 4x4 (green).  Orange area is where stringer will connect

This frame allows me to connect to the tree at a tangent, support the stringers, and gave me railing posts.  It's strong enough to support the stairs and compact enough to not block the stairs.  Next up is deciding how to support the steps.