We uncovered details of the original construction of the balcony guard during demolition. The cast iron balustrading was secured by screws to a triangular surface, which we discovered is a piece of moulding turned upside down. This was nailed down through the floor deck. Since most of the floor joists were cut short, every third joist was extended to support the balustrade with a scabbed-on bit of wood. Needless to say, this doesn't provide a rigid connection, or one that's adequate for a safety-critical application.
The first photograph shows a piece of the original balustrade and wood support left in place; the beginnings of the steel reinforcement can be seen along the joists underneath. The second photograph shows an end view of the new steel reinforcing channel replacing the triangular wood moulding, bolted to steel angles along the joists. An overview of some completed reinforcing is visible in the last image.