**4. Conclusions**

This paper shows the design and the laboratory validation tests of a new low-cost energy dissipation system, for application in precast concrete structures composed of precast footings, precast structural walls, and precast concrete slabs. This energy dissipation system basically consists of a specific connection between the precast footing and the precast structural wall, formed by a set of threaded steel bars that connect both elements. During an earthquake, the steel bars undergo plastic deformation, absorbing most of the energy generated by the earthquake and preventing damage to the rest of the building. The additional advantage of this solution is that steel bars can be easily replaced after the seismic event. Additionally, a flexible connection between walls and slab has been used.

A testing campaign was carried out, including three phases. First, pushover tests were carried out on isolated structural walls formed by one precast structural wall and a precast footing. Second, pushover tests were carried out on structural frames, composed of two precast structural walls placed over two precast footings and connected to a precast slab. Thirdly, seismic tests using a shake table were carried out on a real-scale three-storey precast concrete building, consisting of two precast structural walls placed over two precast footings, two intermediate precast slabs, and a flexible steel roof.

The aim of this structural solution is to fulfill the requirements of the American standard ACI374.2R-13 [34] and more specifically, fulfill the structural performance level of "immediate occupancy", which means that the building can be used without collapse risk once the seismic event has occured.

The pushover tests on isolated structural walls revealed that this solution exhibits a linear-elastic behavior until and beyond the critical drift (which is 0.5%) and no cracks were observed in the structures. The maximum load was reached at a drift of 3%, which was six times greater than the critical drift. Beyond this drift, the structure began to show a plastic behavior. However, no structural damage was observed in the concrete elements, which means that the majority of the energy dissipated by the structural element was through the low-cost energy dissipation device. Additionally, a grea<sup>t</sup> ductility of the solution was observed.

The pushover tests on structural frames revealed that the flexible connection between the walls and the slab exhibited an excellent structural behavior. Again, this solution exhibited a linear-elastic behavior until and beyond the critical drift (which is 0.5%) and no cracks were observed in the structures. The maximum load was reached at a drift of 2%, which was four times greater than the critical drift.

Beyond this drift, the structure began to show a plastic behavior. At this moment, the flexible connections worked as an asymmetrical plastic hinge, able to transmit relevant negative bending moments but almost negligible positive bending moments. This reduction in the overall horizontal sti ffness of the frame resulted in an increased plastic behavior of the structure and, in consequence, an increased capacity to dissipate seismic energy. In this case, small horizontal cracks in the walls (especially in the external face of the Wall 2 where the tension strain is larger) were observed.

The seismic tests revealed an excellent behavior of the real-scale three-storey precast concrete building. The structure was subjected to a main earthquake and six foreshocks. The earthquake used was "El Centro" (the eathquake that occurred in the city of El Centro, California, USA in 1979). Additionally, two one-cycle impulsive tests were performed, one before the seismic events and the other after the seismic events, in order to measure the dynamic parameters of the building (natural frequency and damping ratio) before and after the seismic events.

The visual inspections carried out after the seismic tests revealed that no structural damage was observed in the building (i.e., no cracks in the walls or slabs appeared and, of course, no concrete crushing occurred). This means that the seismic energy was completely dissipated by the connections, i.e., by the low-cost energy dissipation systems placed on the connections between the walls and the footings and the flexible connections between the walls and the slabs.

At the end of the seismic event, the residual longitudinal displacements of the both concrete slabs and the lightweight roofs were almost zero, i.e., the building recovered its original position.

The impulsive tests revealed that the seismic events caused a decrease in the natural frequency andanincreaseinthedampingratio,whichillustratesthedamagegivenbytheseismictests.

Finally, it is concluded that the building reached the performance level of "immediate occupancy", according to ACI374.2R-13 [34].

**Author Contributions:** Design of the structural solution, J.F., D.M., and A.C.J.; design of the testing campaign, J.F., D.M., A.C.J., J.M., and M. Á.V.; development and supervision of the tests, Á.M., J.F., D.M., J.M., and D.C.G.; data interpretation, Á.M., J.F., J.M., and M. Á.V.; funding acquisition and project administration, D.M. and A.C.J.; writing, reviewing and editing the paper, Á.M. and M. Á.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** This project has received funding from the Eurostars-2 joint programme with co-funding from the European Union Horizon 2020 research and innovation programme. The project name is "Novel Seismic-Resistant construction system composed of precast structures" (SEISMPRECAST) and the main partners are ICONKRETE 2012, S.L. and EXERGY, LTD.

**Conflicts of Interest:** The authors declare no conflict of interest.
