Seismic retrofitting of concentrically braced frames by rocking walls and viscous dampers

In this paper, a design procedure for seismic retrofitting of concentrically braced frames is proposed and validated. The reasons leading to the need of an enhancement of the seismic performance of these frames may be multiple. As an example, the expected response of concentrically braced buildings may not match the desired performance objectives because (1) the initial design of the structures was deficient or (2) the design performance objectives have changed in time due to an update of the seismic hazard map or to more demanding requirements of the owners of the building. In this paper, rocking walls are proposed to ensure an almost uniform distribution of the storey drifts along the height of the building. To achieve a direct and efficient control of the ultimate peak ground acceleration, the design procedure is founded on the displacement‐based approach. The equivalent viscous damping ratio capacity of the concentrically braced structure with rocking walls is calculated based on semi‐empirical relationships specifically calibrated for concentrically braced frames. Viscous dampers are added and arranged between the existing structure and the rocking walls. The design internal forces of the rocking walls are evaluated considering also the contributions of the higher modes of vibration. The proposed design procedure is applied to an eight‐storey chevron braced frame and its effectiveness verified by incremental dynamic analysis.