Theoretical and experimental study on the cyclic behaviour of X braced steel frames

The paper presents a theoretical and experimental study on the slenderness of the diagonal members of concentrically X braced steel frames. According to Eurocode 8 and Italian Standard, the non-dimensional slenderness of the diagonal members should be lower than an upper limit, in order to provide sufficient hysteretic energy dissipation, and greater than a lower limit, to avoid overloading the columns in the prebuckling stage. This seismic design provision requires the realistic evaluation of the diagonal member slenderness. A reduced scale test bench was designed to study X braced steel frames with different geometrical characteristics of diagonal members and with restraints as similar as possible to the theoretical ones. The results confirm the theoretical prediction of the effective length factor of the diagonal members which exhibited good hysteretic behaviour with large inelastic deformation capacity under cyclic loading. The low values of the effective length factor suggest the use of plates to realise the diagonal member instead of the classical angles as shown in the design example.