In this exercise, you are going to design a spiral staircase with some particular requirements.

A spiral staircase is one whose steps are placed around a vertical axis. Several types of spiral staircase can be designed, but in this case we are going to create one in which all the steps are connected to the vertical axis, not leaving any space in the inner part of the staircase, as in the example of the right.

As indicated in the image below, the inner diameter must be 2,4 meters while the inner hole must be 0,4 meters. Therefore, the steps will be exactly one meter in width. Apart from that, the staircase will have 18 steps, each of them rotated 20º with respect to the previous one:

Finally, each step will be 15 centimeters in height (0,15 meters).

To finish the project, a vertical column of 0,4 meters in diameter will be added as indicated in the image on the right.

Once finished, the model could be customized by adding little details, colours or textures.