# Base Isolation Compared to Capacity Design for Long Corner Periods and Pulse-Type Seismic Records
## 2 Materials and Methods  
### Hospital Building  
The analysed RC frame structure is part of a public hospital in Bucharest. The building has plan layout dimensions of approximately 12 \(\times\) 28 m and an aboveground height equal to 16.55 m. The building has one underground and five above ground storeys. The storey heights range between 3.20 m and 3.50 m. A 30% area setback is present at the first two levels (see red area in Figure 1a), generating in-plane and elevation irregularity [18]. Plan irregularity is generated also by the positioning of the vertical circulation wall assembly. Displacements along the structural perimeter for an earthquake in X-direction are amplified by 37% compared to the mean floor displacement. Details regarding the influence of irregularity on the structural seismic response have been explained in [30]. The hospital building was originally designed according to the capacity spectrum method based on the provisions of P100-1/2006, [19]. The project was completed in 2012. Limited research on the seismic behaviour of base-isolation buildings located in long predominant period seismic regions for pulse-type accelerograms has been performed so far [1]. To increase the confidence of structural engineers in the base isolation system as an alternative to the classical structural solutions, under special seismic conditions, the current study has been planned. Bucharest was chosen for this study as it is situated in the southern part of Romania, in a seismic area with corner period Tc = 1.6 s. Pulse-type accelerogram recordings were obtained in Bucharest. In this attempt, the base-isolated building was designed considering the original building layout. The dimensions of the main structural elements are shown in Table 1 for both design approaches. A 60 cm thick foundation mat was considered for both models. The isolation level was placed beneath the underground level. Isolators are supported by the mat foundation and connected to a foundation beam girder, which supports the entire structure. This solution turned out to be more economic than a double raft foundation, although it preserves less usable area. The structural analysis for the design of both structural solutions was performed using the ETABS software 2013 [29]. The capacity spectrum method was used for structural analysis. Once designed, the seismic performance of the structures was investigated in the \begin{table}
\begin{tabular}{c c c} \hline
**Element** & **Classic Design** & **Design with Base Isolation** \\ \hline beams (bw \(\times\) hw) & \(40\times 50,40\times 60\) & \(25\times 50,25\times 60\) \\ columns (bc \(\times\) hc) & \(60\times 60\) & \(50\times 50\) \\ walls’ thickness (bw) & \(20\) & \(20\) \\ slabs’ thickness (hsl) & \(15\) & \(15\) \\ \hline \end{tabular}
\end{table} 
Table 1: Current structural element dimensions (cm). Figure 1: ETABS structural model [29; 30]: (a) ground floor plan layout; (b) 3D-view. non-linear range of behaviour by using the PERFORM 3D software 2006 [31]. Non-linear static analysis and non-linear time history analysis were performed. A 3D view of the PERFORM 3D software structural model is shown in Figure 2. While EATBS gives the opportunity of a more refined structural model, PERFORM 3D brings the benefits of fast non-linear dynamic computation. The computed base shear, lateral displacement and horizontal accelerations were compared for both design approaches. Double curved sliding isolators produced a structural design optimization by isolator dimensions reduction. Isolators were designed for a chosen isolation period (4.4 s) in order to obtain a minimum response spectrum ordinate. According to [16] the design earthquake for the Ultimate Limit State has a 225 year return period (20% probability of exceedance in 50 years). The spectrum compatible records considered in this study comply to the design spectrum. A 475 years return period (10% probability of exceedance in 50 years) was considered as maximum expected earthquake [32]. Isolators were designed to reach 64.2 cm displacement capacity.