## 3 Experimental Results and Discussion
### Seismic Tests on Real-Scale Three-Storey Precast Concrete Building
#### 3.3.2 Longitudinal Accelerations of the Shake Table and the Building  
Figure 20 shows the diagrams of the longitudinal acceleration versus time during the \"El Centro\" earthquake at the following locations: shake table, Concrete Slab 1, Concrete Slab 2, and lightweight roof.  
Figure 20: Diagrams of longitudinal acceleration vs. time at different locations, from shake table to lightweight roof. (**a**) Acceleration versus time in shake table; (**b**) acceleration versus time in concrete slab 1; (**c**) acceleration versus time in concrete slab 2; (**d**) acceleration versus time in lightweight roof.  
Figure 20 reveals that this structure is especially resistant to the earthquake \"El Centro\", since the maximum measured longitudinal accelerations of the concrete slabs is smaller than the one on the shake table. The maximum measured longitudinal acceleration at the lightweight roof is a bit larger than the one on the shake table. Specifically, the maximum measured longitudinal acceleration is 2.8 m/s\({}^{2}\) on the shake table, 1.7 m/s\({}^{2}\) on the first concrete slab, 2.1 m/s\({}^{2}\) on the second concrete slab, and 4.0 m/s\({}^{2}\) on the top of the building.  
The dominant excitation frequency of the earthquake \"El Centro\" is around 1.6 Hz (Figure 21), and the natural frequency of the building is around 3.4 Hz before the seismic tests (Figure 22) and 2.6 Hz after them (Figure 23). This large difference between the excitation frequency and the natural frequency implies that the longitudinal accelerations that the earthquake causes in the building are small. Consequently, the horizontal inertial forces are also small, as well as the internal forces caused by the earthquake.  
The seismic event results in a reduction of the natural frequency of the building by 0.8 Hz, that is, 23%. Since no concrete cracks were observed, it is concluded that the damage is completely focused on the connections, both the low-cost energy dissipation system and the flexible connections between slab and walls.  
Additionally, the measured damping factor of the building is 4.7% before the seismic tests and 6.2% after them. This increase in the damping factor is also a good indicator of the damage caused by the seismic tests.  
When the seismic response of the building is compared to Eurocode 8 [38], it is observed that the measured elastic response spectrum S(T), defined as the ratio between the maximum acceleration of the building and the ground acceleration (i.e., the maximum acceleration of the shake table) is 1.43, which is smaller than the theoretical S(T) provided by this European standard. This means that the standard is conservative, as expected.  
Additionally, it is highlighted that the natural frequency of the structure is below 4 Hz before the seismic event (Figure 22b), which is the lowest threshold of the dangerous region of the \"El Centro\" earthquake (see Figure 10c) and no relevant accelerations are developed during the seismic events. Moreover, the loss of stiffness caused by the seismic events reduces the natural frequency (Figure 23b) of the structure and, consequently, reduces the horizontal accelerations caused by the earthquake, which prevents the structure from aftershock earthquakes.