what year did builders start using structural steel to support first floors blockwork instead of timber?

Pre-first world war the common layout for houses was to have 2 storey a loadbearing wall running across the house at roughly midway. This supported the chamber joists and ceiling. Sometimes there were 2 walls enclosing the staircase. Between the wars there was some ingenious use of timber to support loads and quite often partitions on the first floor were supported by the floor joists, quite often these were masonry and sometimes loadbearing. This practice gradually became less common and in the late sixties there were very few loadbearing walls in houses with joists spanning side to side, roof trusses spanning the complete length of the building and all the partition walls of timber studwork. Timber beams are quite often used to carry loads, i.e. purlins and stair trimmers but steel will do the job with a smaller beam size.

sorry he is not an architect he is a chancer