HOW
TO DRAW A SPIRAL STAIRCASE IN PARALLEL PERSPECTIVE

Draw
the distance, support and horizon lines and the vanishing, view
and depth points inside the visual field, as previously explained.
Draw a square corresponding to the diameter of the staircase and
divide it into angles of 45º. Find the bisectrix of these
45º angles as follows: Multiply the length of the angle by
the expression 0.586. Figure nº 25 illustrates a square of
10x10cm (2 x 2 m in a scale of 1:20). In this case, the length
of the angle is 5cm 

multiply it by 0.586
0.586
x 5 = 2.930
round up to 3 cm


Mark
3 cm on the square base (width measurement line) starting at each
vertex. See figure nº 25. From this point to the vanishing
and depth points, find the lateral 3 cm and so forth. After finding
the points on the square edges join these points with the lines
that will pass by the perspective center of the square. 
Now
you have all the angles of the staircase steps already in perspective.
Figure
nº 26 shows a circle drawn on the angles of the steps.
Figure
nº 27 shows the raising of the steps within a box.
Figure
nº 28 shows the spiral staircase already constructed.
