Assessment of alternative design approaches for seismic upgrading of RC frame structures with steel exoskeletons

The use of external steel bracing systems (named exoskeletons) has recently emerged as a convenient technique for seismic upgrading of existing RC structures, as it possibly limits disturbance or interruption of building occupancy. However, although some conceptual formulations of the design problem can be found in the scientific literature, there are no universally accepted design criteria. The present paper highlights how alternative design choices can reverberate their effects on (i) the seismic performance of the upgraded structural system, (ii) its economic and ecological impact. For the sake of simplicity and consistency with the most common approach of practicing engineers, the present study adopted a force-based seismic design approach. The critical discussion proposed herein was obtained by varying the intensity of the seismic base shear force adopted for the exoskeleton design. Results of the design assessment show that the adoption of larger values of exoskeleton seismic design forces may not be the most suitable choice, especially if economic and ecological impacts are taken into account.