With the purpose to investigate the behaviour of the lattice girder composite slab (LGCS) at construction stage, a push-out experimental investigation has been carried out on 9 lattice girder specimens having not only different diameters of web bars, top and bottom chords, but also different heights. The results obtained from tests are here presented and analysed. The paper focuses on the behaviour of the flare groove welds among web bar and bottom and top chords. For each tested specimen a F.E.M. model has been developed, schematizing the stiffness of the flare groove welds by local springs, whose stiffness has been calibrated by means of the load-displacement curves obtained from tests. A chart that allows to estimate the aforesaid stiffness of a lattice girder beam in function of the triad of top, bottom chords and web bar diameters, and in function of the inclination between the web bar and the beam horizontal transversal axis is proposed. This paper highlights that the chart proposed here provides stiffness values with which numerical models can more accurately predict the behaviour of LGCS during construction. Furthermore, this paper highlights that the weld stiffness of the flare groove influences the buckling length of the web bar.
