simple_stairway_steps
From Valve Developer Community
A prefab located in the Prefabs HL2 folder, containing a stairway, covered in dev textures.
The stairway is actually a single func_detail, to prevent unneccessary vis-clipping.
Tip:If inserted with the Entity tool, its standard dimensions will match the dev_measurestairs01a texture on its side prefectly, suggesting that this measure is a good standard when in doubt.