Full scale experimental assessment of concentrically braced frames designed for moderate seismicity

Abstract

Concentrically braced frames (CBF) represent a very effective structural form against horizontal loading, providing high lateral resistance at the same time limiting lateral displacements. Global cyclic behavior of these structures is determined by several phenomena acting together such as yielding of tension bracings, buckling and post-buckling of compression bracings, slip of bolts, and frame action provided by semi-rigid joints. A large amount of research has been conducted for CBF systems in order to maximize their performance in regions exhibiting a very significant seismicity level. Indeed, current code previsions are quite advanced for high-ductility design of CBF structures. However, although these systems are very popular also in low-to-moderate seismic regions, there is not a clear distinction for their detailing requirements between high-seismic and moderate-seismic design. Since the strict application of the high-seismic design principles in regions of low to moderate seismicity leads to solutions with a significant increase of the building costs, designers usually choose “non-dissipative” design, neglecting any detailing provisions that enhance the seismic performance. This approach may lead to unsafe solutions with significant life-safety and economic consequences. This situation gives rise to the need of an optimized design (both in terms of safety and economy), with specific design rules more compatible with the buildings located in areas susceptible to low-to-moderate earthquakes. Ongoing research project MEAKADO (RFSR-CT-2013-00022) aims to develop specific design methodologies for steel and steel-concrete composite structures in regions characterized by a low to moderate seismic activity, with an appropriate reliability level. The present paper presents the preliminary results of the full scale testing program realized in this research project. Tests focus on the contribution of the compression diagonal, assessment of the parts of the horizontal loads that are effectively resisted by the bracing system and the part that is actually taken by the frame, and investigate the inherent ductility provided by “nondissipative” bracing connections. Thanks to the observations obtained from these tests, possibilities to improve seismic performance of concentrically braced frames built in moderate seismic have been discussed.