staircase material
The framing material which supports the steps and risers in an object's stairways.
The staircase material for any built object is the framing material used in the structural support of stairways within that object. It may or may not be covered in various non-structural finishing materials (such as carpet, tiles, linoleum, wood veneer, etc.).
The framing elements of a stairway include horizontal, vertical, diagonal, and/or curved members which bear the weight of individual steps in a stairway from the ground or the floor below, and also stabilize it against the adjacent walls or as a freestanding or hanging structure within the overall structure of the built object.
The supporting frame carries and stabilizes the steps. A carpet or wood finish can cover that frame without being its structural material, so a stairway's visible walking surface need not reveal its supporting construction.