# DNA/Helix Staircase efficient step modeling

Hi,

I have successfully modeled a DNA/Helical staircase, but I simply don’t think that I did it in the most efficient manner. Basically I created two helix around a vertical line pertaining to their respective diameters (7’ and 4"). I ran into problems, however, creating the stairs efficiently though. My method was to use Sweep 2 to get a surface between the two helix and then divide the surface. Next in order to create the stairs, I split the surface with the divided curves and then projected each segment to the c plane to create a planar iterations. Finally I set the planararized surfaces back onto the double helix. I tried a polar array, but that doesn’t recognize the center helix. The tricky part is to match the negative space from the ends of the stair segments to the helix. Helix Staircase.3dm (790.7 KB)
Any ideas? I wish to teach my students this, but currently my method takes far too long.

Erik

Hi Erik,

In fact that link seems broken…

But some simpler details here: http://www.grasshopper3d.com/forum/topics/spiral-stair

Using the ArrayPolar command with the ZOffset option will help place your treads.

First, decide on number or treads, angle of treads, increment angle between treads (if different than angle of treads), and distance between top treads or total height of unit. Also calculate the “pitch”, distance along the axis of the corresponding helix when it makes one complete rotation of 360 degrees. Example: 13 treads, 23.5 degree tread angle, 22.5 degree increment angle, 9 inches tread height, 108 inches total height ((13-2) * 9), 144 inch pitch (360/22.5 * 9).

Create first tread. Revolve can be used to sweep a line around the center point. Then Extrude for thickness.

ArrayPolar with ZOffset option to create remainder of stairs.

Helix to create a reference line for the stringer. Build stringer using this curve.

Example: Spiral Stairs Example.3dm (1010.3 KB)

Ahh this makes very much sense. I overlooked the ZOffset option!
I am now working on a GH definition, hopefully will have that one up and running soon. I know this is not the right forum to post the definition, but since we’re on the topic I suppose I shall.

Thanks for all the help!! Such a gorgeous staircase.

Best,
ErikHelical Stair Progress.gh (20.4 KB)