Soft omega joint for selective weakening of core walls in large precast commercial buildings

Commercial malls and similar large low-rise multi-storey building typologies are often constructed employing precast frame structures. Wall cores surrounding stairs and elevators often occupy a small area if compared to the frame area of large floors. Generally, these cores need to be decoupled from the frame structure, which is designated as the lateral load resisting system, given that, if rigidly connected to the frame, their high stiffness would draw seismic actions not compatible with their capacity or with economical/feasible foundation systems. The necessary core decoupling from the frame is hence generally made by horizontal structural joints of considerable width to avoid pounding, given the peculiar lateral flexibility of precast frame structures. However, these large joints are in reality highly complex, since they need to be compatible with internal finishes, pedestrian distribution and often fire compartmentation requirements. The present paper proposes an alternative to the use of decoupling frame-core joints consisting in deformable vertical joints made by omega-bent aluminum sheets and insulation strips to be inserted during production in the wall elements. The proposed omega-profiles, while avoiding the use of horizontal joints and keeping the fire compartmentation capacity of the core walls, selectively weaken the wall elements, lowering their stiffness and allowing for lateral deformation capacities compatible with that of frames. The mechanical behaviour of the proposed joint is assessed by lateral cyclic tests on full-scale wall panels with inner vertical voids surrounded by a solid frame connected with mechanical devices to the foundation beams. The comparison with the test results of a benchmark precast wall not provided with the proposed joints allows to quantify the enhanced flexibility and displacement capacity of the wall with omega joints and to interpret the role of the joints in modifying the resistance mechanism of the wall subjected to lateral loads. Moreover, the results of a non-linear numerical model based on an ideal joint behaviour allow to indirectly highlight the soft joint contribution to the wall behaviour under lateral load.