Hi, I have an attic with real 2×4 joists; they hold a lath and plaster ceiling, and are exposed from above.  

The span is 14 feet from outer wall to center wall, which is a structural and load bearing wall from basement up to attic.  So the house is 28 deep.  There are 2×6 strongbacks at about 7’, midway in sections over areas where the 14 span is open, versus areas where partition walls break up the span: hallways, baths, closets, stair hall, etc. within/under the 14 foot span.

If if I want to make the attic walkable and usable for storage, what would be the best solution?

I’m thinking of sistering something to the 2x4s, but do not want to increase height/thickness.  Thinking of A36 C 4x 6.25 channel bolted to 2×4, flange side to wood, then using 1-1/8” plywood screwed to the 2x4s for a floor system.  I’d remove strongbacks when doing so.  

What I’d like to know is how many sisters do
I need?

The 2x4s are 16 oc, 14’ max span as indicated. The attic is 52 x 28 feet, with the center structural wall at midway on 28 foot dimension, as mentioned. The 16oc is along the 52 foot dimension. Would sistering every third or fourth 2×4 joist work?  Or more or less needed when factored with the 1 1/8 thick plywood floor on top.