Role of compression diagonals in concentrically braced frames in moderate seismicity: A full scale experimental study

According to European building code provisions, compression diagonals should be neglected during the analysis stage of concentrically braced frames (CBFs) with X and N type bracings, and the inelastic capacity of the tension bracings only should be considered in the design. This provides simplifications at analysis and design stages for practising engineers. Such an assumption can be rational in the high-seismicity context, where the compression bracings undergo buckling at the early stages of the seismic event, and the shear demand is very high. On the other hand, in moderate seismicity areas (that is estimated as the 90% of the seismic regions of the world), where the shear deformation demand for braced frames and the number of high-amplitude cycles are very limited, it may be reasonable to consider both tension and compression diagonals in the analysis. Accounting for compression diagonals at the analysis stage, and exploiting their post-buckling resistance and dissipative contribution in design, may allow using a higher behaviour factor, and increase the economic efficiency of CBF structures in moderate seismic regions. To understand the real seismic performance of braced frames in moderate seismicity areas, a sound characterization is needed, focusing on behaviour of compression diagonals. In the literature, many tests have been performed to analyse the behaviour of bracing elements, but they were mostly designed to meet high seismicity criteria with significant connection over-strengths. The European research project RFSR-CT-2013-00022 MEAKADO investigated the influence of compression diagonals on the global performance of CBF structures, by means of experimental and numerical studies. This paper presents the results of full scale tests performed within this research project, focusing on the stiffness and post-buckling performances of double-angle bracings with bolted connections, which are the most common bracing configurations in the European construction market characterized by low-to-moderate seismicity. Experiments have shown that extra stiffness and strength provided by the contribution of the compression diagonals to the structural response are significant for moderate seismicity drift and shear demands, and may be worth considering in the analysis and design phases.