Spiral staircase 3D model is available on CGTrader for $5. Original author: HEZE3D. Specs: 20364 polys, 23560 verts.
OBJ (OBJ, filesize: 2.25 MB), MAX (Autodesk 3ds Max, filesize: 10.7 MB), TEXTURES (Textures, filesize: 2.33 MB), JPG (JPG, filesize: 2.33 MB), FBX (Autodesk FBX, filesize: 16.1 MB), SKP (Sketchup, filesize: 5.43 MB), 3DS (3D Studio, filesize: 826 KB)