Mary Horst, of Horst Buchanan Architects, Inc., approached me in November of 2010 about reworking a stair in an early ’80s condominium in Jamaica Plain, MA.  The original stair was open, giving the new homeowners visions of late-night tumbles, and the design had not aged well.   Budget and logistics required that the framework of the stair remain, but Mary wanted to add risers and half walls, and eliminate the oak balustrade.  The biggest challenge of the job would be fabricating a new wall-mounted handrail. Mary’s new design required a continuous, smoothly curving handrail with no visible joints.  Because the stair transitioned from straight rise to curving rise and back again, the handrail would have to change pitch in addition to following the curve of…