Secondary frame action in concentrically braced frames designed for moderate seismicity: a full scale experimental study

European seismic design codes do not take into account the strength and stiffness of the secondary frame action provided by bracing gusset plates of concentrically braced frames (CBFs). This is an attractive assumption for practicing engineers, as it provides simplifications during the analysis and design phases. However, when efficiency and economy are concerned, especially in low-to-moderate seismic regions, this normally neglected frame resource may be interesting to consider in design. Gusset plates can provide a certain degree of stiffness and strength following the bracing failure, and may even prevent global collapse. In particular, when the shear deformation demand of the braced cell remains limited, as in the case of low-to-moderate seismic actions, it may become reasonable to take this extra stiffness and strength into account. Ongoing research project RFSR-CT-2013-00022 MEAKADO investigated this phenomenon by means of experimental and numerical studies with the perspective of setting new inputs for the design rules of the future generation of Eurocodes. This paper presents the results of full scale tests performed inside this research project, which characterized resistance, stiffness, and ductility resources of CBF systems designed for moderate seismicity. The paper also quantifies the effective contribution of the frame action, provided by gusset plate connections, to the global performance of CBF frames.