Abstract: A steel joist formwork and a composite steel and concrete floor structure provided with a top chord, a bottom chord and a web joining the top and bottom chords with the top chord being serpentine shaped like an S or Z and including top and bottom, generally horizontal portions and an intermediate portion integrally connecting opposite edges of the top and bottom portions with the top and intermediate portions substantially being adapted to be embedded in the concrete floor to cause the floor and steel joists to act structurally as a composite beam. Either the web or a leg depends vertically from the free end of the bottom portion of the top chord, and the web joining the top and bottom chords is substantially solid or is, an open web formed of a zigzag bar member. There is also disclosed a building system and erecting method utilizing steel joists spaced apart by spanner bars which also support rigid panels which act as formwork for the pouring of concrete.