Stabilità in fase I del traliccio metallico delle travi reticolati miste

The results of a numerical and analytical in-vestigation on hybrid steel truss RC beams are described. Hybrid steel truss RC beams are beams where, during the construction phase and the concrete casting (Phase I), the steel truss sys-tem only carries the vertical loads. In this con-struction phase, the verification against failure due to the instability of the upper steel bars or diagonal elements is the most important design require-ment.In the present study, four different beams are considered, with different steel trusses in Phase I condition. Finite Element modelizations of the structure are made and failure loads are obtained, taking into account second-order effects due to large displacements. It is shown that the stability of the steel truss strongly depends on the lateral stiffness of the diagonal elements of the truss. Analytical models for the prediction of the fail-ure load are proposed, based on the instability of a beam on elastic foundation and the lateral buck-ling due to bending moment. Starting from the proposed analytical model, some design criteria are illustrated and discussed.