Theory

Stairs:

The first scetion of the aboe ground stair used in the model:

Entirely Steel

The second flight of the above stair:

The below ground staircase: 

Entirely concrete (to achieve minimalist/organic look)

The individual stairs of the above:

Concept for alternate staircase (above ground)

A intial few steps to arrive onto the above:

Second alternate staircase:

Continuation of above (continues onto platform seen in the left of the original):