Design Procedure and Behaviour of Advanced Flag-Shape (AFS) MDOF Systems
The concept of Advanced Flag-Shaped (AFS) systems, in which alternative forms of energy dissipations (yielding, friction or viscous/visco-elastic damping) are combined in series and/or in parallel together with re-centering elements (un-bonded posttensioning tendons or Smart memory alloy(SMA) elements), has been previously introduced by the authors. Based on numerical analyses on SDOF-systems, the unique combination of friction or hysteretic dampers in series with viscous dampers, further combined in parallel with re-centering and hysteretic dissipation elements, has been shown to be very effective in controlling both force and displacement responses for either far-field and near-fault ground motions. Experimental validation of the effectiveness of the systems based on shake-table testing on wall systems is presented in a companion paper. In this contribution, the concept of AFS systems is extended to MDOF systems. Preliminary suggestions for a simplified design procedure for AFS connection systems are given within the framework of a Direct Displacement-Based Design (DDBD) approach. Using case-study prototypes of five-storey moment-resisting frame, incorporating four different connection systems, a comparative MDOF study is carried out by the means of non-linear time-history analyses using suites of far-field and near-fault earthquake excitations. The non-linear time history analysis results for both far-field and near-fault earthquakes provided satisfactory validation of the design procedure, though being, as expected, on the conservative side when dealing with velocity-dependent dissipating systems. As per the results of SDOF systems, AFS systems appear to be capable of providing beneficial attribute to the response of a MDOF system, particularly when dealing with velocity-pulse earthquake record, typical of a near-field event. In addition to providing reduction of peak displacement/drift response and a negligible residual deformation, floor accelerations and column shears due to the higher mode effects are also lessened. In the global performance matrix, AFS systems would achieve a much higher performance level in comparison to the conventional systems. There is however, less than expected contribution from the excitation velocity on dampers’ energy dissipation up the building heights. Based on these results, an approximation for the velocity-dependent devices’ velocities at a given storey is proposed. In conclusion, a brief discussion on limits and potentials for the practical implementation of AFS systems is given, along with anticipation of ongoing and further investigations.