1. Background
The 23 November 1980 Irpinia–Basilicata (Southern Italy) earthquake (Ms = 6.9) caused deep changes in the urban socio-economic layout, and primary and secondary effects that brought about changes to the natural environment, such as landslides (e.g., Senerchia, Buoninventre, Caposele, Calitri, San Giorgio La Molara, and Grassano) [
1,
2,
3,
4]. It consisted of several rupture episodes, which occurred at 0.18 and 40 s from the foreshock, and it was assigned a surface-wave magnitude of Ms = 6.9 [
5,
6]. A wide area (about 3500 km
2) recorded serious damage, many casualties, and 15 localities were almost destroyed, including Sant’Angelo dei Lombardi, Laviano, Lione, Santomenna, Senerchia, Pescopagano, and Balvano. It was estimated that of a total of approximately 1.85 million buildings involved in the event, 75,000 were destroyed, 275,000 seriously damaged, and 480,000 slightly damaged [
6].
With respect to this event, the documentary sources are based on two main typologies of technical data preserved in local archives: The “Scheda A” and “Scheda B”, which report the damages to the buildings, consisting mostly of reinforced concrete (RC) structures characterized by infill masonry walls (IMWs), which are representative of the Italian residential buildings. Eight damage levels were defined by considering the action to be undertaken, such as repairing works, evacuation, or demolition [
6]. Other important documents are the recovery plans (named “Piani di Recupero”) of the historical centers, the other sources used to analyze the outcomes of the earthquake at the urban scale. An important study regarding the effects of spectral accelerations was proposed by [
7], who analyzed the effects of the soil on the accelerations in several locations, with particular attention to the Naples area. In addition, [
8] simulated the recorded strong-motion data by computing spectral accelerations and peak amplitude residual distributions in order to investigate the influence of site effects and compute synthetic ground motions around the fault. They simulated the expected ground motions varying the hypocenters, the rupture velocities, and the slip distributions and compared the median ground motions and related standard deviations from all scenario events with empirical ground-motion prediction equations (GMPEs). Recent earthquakes, such as the Athens (Greece, 1999) [
9], the Kocaeli (Turkey, 1999) [
10], the Haiti (2010) [
11], and the Gorkha (Nepal, 2015) [
12,
13,
14] earthquakes, showed the importance of taking into account soil amplifications. In the literature, several approaches have been applied to perform ground motion analyses including site effects: hybrid analyses that consist of a combination of probabilistic and deterministic methods (e.g., [
15,
16]), convolution approaches that provide modifications of the rocking hazard (e.g., [
17,
18]), and 1D seismic site response analyses (e.g., [
19,
20]).
Even if the 1980 Irpinia–Basilicata (southern Italy) earthquake is well documented with several contributions (e.g., [
21,
22,
23,
24]) and models proposed [
25,
26], the assessment of the role of the soil on the structural damage is still a relatively unexplored issue and this paper aimed to fill this gap. In particular, the principal aim was to propose numerical simulations of different soil conditions and assess the effects of the soil–structure interaction (SSI), which can significantly affect the seismic vulnerability of structures [
27,
28,
29]. In this regard, when the superficial deposits overlie the bedrock, amplifications of the surface seismic accelerations may not be conservatively predicted by the codes. The so-called site effects consist of a combination of soil and topographical effects, which can modify (amplify and attenuate) the characteristics (amplitude, frequency content, and duration) of the incoming wave field and are primarily based on the geotechnical properties of the subsurface materials [
30]. In particular, the response of the superficial layers is strongly influenced by the uncertainty associated to the definition of the soil properties and model parameters that are fundamental to assess the well-known mechanism of seismic amplifications of ground motion [
31]. Therefore, accounting for the amplification effects of superficial layers has become critically important in seismic design [
32] and widely adopted in many codes’ prescriptions, such as Eurocode 8 [
33], ASCE (American Association of Civil Engineering) standards 7-05 [
34], and 4-98 [
35]. These codes provide soil parameters, generally determined through geological investigations [
36,
37,
38,
39,
40], that can largely vary even within the same area [
41,
42]. The methodology followed in this paper consists of a first step, where free-field (FF) analyses were computed on several layers of soil, and secondly, an SSI (Soil-Structure Interaction) analysis was performed on a selected structural configuration that is representative of the buildings that were damaged during the Irpinia-Basilicata earthquake.
2. Case Study
SSI analyses require the definition of geomechanical parameters that are fundamental to describe the dynamic soil behavior, such as the modulus reduction and damping curves (see [
43,
44]). According to the current state of knowledge on the Irpinia-Basilicata earthquake, strength parameters for superficial layers are not available. Therefore, it was necessary to select representative values based on available information, such as [
8], for a preliminary study. These values are herein determined with free-field analyses since the actual values at each building site will slightly differ when the building characteristics are considered. In particular, the present paper aimed to model a low-rise building based on a relatively shallow foundation assuming that the ground motion amplitude, which decreases at the foundation level with respect to the free field, may be negligible [
42].
The study here proposed was divided into two steps. First of all, several FF models with different soil conditions were considered (
Figure 1), in order to study the effects of soil deformability on the amplification of the motion. In particular, four incoherent soils were performed on the basis of the contributions that were found in the literature. Then, a complete 3D numerical model with the soil-foundation-structure system was performed (
Figure 2). The FF soil models consist of a one-layer 20-m-deep homogenous incoherent material with a 3D mesh (
Figure 1). The penalty method was adopted for the boundary conditions (tolerance of 10
−4), chosen as a compromise for the soil domain definition, which was modelled large enough to ensure strong constraint conditions but not too large in order to avoid problems associated with the equations system conditions. Base boundaries (depth of 20 m) were considered as rigid. Base and lateral boundaries vertical direction (described by the third degree of freedom (DOF)) were constrained, while longitudinal and transversal directions were left unconstrained on the lateral boundaries, in order to allow shear deformations of the soil. The definition of the mesh elements dimension follows the approach already adopted [
45,
46,
47,
48] and, in order to verify proper simulation of FF conditions, accelerations at the top of the mesh were compared with the FF ones, which were found to be identical, confirming the effective performance of the mesh. The benchmark structure was calibrated in order to be representative of the buildings that were present in Irpinia in 1980. In this regard, a 3-storey concrete building with masonry walls was considered.
Dynamic analyses were performed with OpenSeesPL. The selected input motion was chosen from the Italian Accelerometric Archive [
49] and it represents the acceleration registered in Sturno (STN) station (lat: 41.0183°, long: 15.1117°) in Avellino, Campania (
Figure 3), and located less than 5 km from the fault and 33 km from the epicenter (41.76°N, 15.31°E). For more details, see [
8]. The input was defined on soil B, as classified by Eurocode 8, and applied at the base of the model along the longitudinal direction.
2.1. Step 1: FF Analyses
The soil models were built up on a two-phase material following the
u-p formulation [
50], where
u is the displacement of the soil skeleton and
p is the pore pressure. The soil material was based on the following assumptions: (1) Small deformations and rotations, as well as solid and fluid densities remain constant in both time and space; (2) porosity is locally homogeneous and constant with time; (3) soil grains are incompressible; and (4) solid and fluid phases are accelerated equally [
51]. The 20-m-deep soil layer was defined by the
PressureDependMultiYield02 model [
52,
53], based on the multi-yield-surface plasticity framework developed by [
54], in order to reproduce the mechanism of cycle-by-cycle permanent shear strain accumulation in clean sands (
Figure 4).
Table 1 shows the adopted parameters, such as the low-strain shear modulus and friction angle, as well as the shear wave velocities and permeability. Soil fundamental periods were estimated considering an equivalent uniform linear layer, following [
55]. The number of yield surfaces was equal to 20 for all soil models.
Figure 5 shows the backbone curves for all the selected soil models.
The 3D soil models consist of a 100 m × 100 m × 20 m mesh, built up with 8000 20-node
BrickUP elements and 9163 nodes to simulate the dynamic response of solid-fluid fully coupled material [
52,
53]. For each
BrickUP element, 20 nodes describe the solid translational degrees of freedom, while the eight nodes on the corners represent the fluid pressure 4 degrees of freedom. For each node, Degree of Freedom (DOF)s 1, 2, and 3 represent solid displacement (
u) and DOF 4 describes fluid pressure (
p), which were recorded using OpenSees Node Recorder [
52,
53] at the corresponding integration points. The element dimension increases from the structure (center of the model) to the lateral boundaries, which were modelled to behave in pure shear and located far away from the center of the mesh.
2.2. Step 2: SSI Analyses
The study considered an RC structure with infill masonry walls as a benchmark, in order to represent the Italian residential buildings that were mostly damaged during the 1980 Irpinia–Basilicata earthquake. The benchmark structure was built with a 4 × 2 column scheme (4 columns in the transversal direction (8 m spaced) and 2 in the longitudinal direction (10 m spaced)) and modelled to have periods in the range of those of residential buildings, considering 3 floors (a 3.4 m storey height, with a total structure height of 10.2 m). The structure was modelled as a superposition of two schemes (
Figure 6). Both vertical and horizontal elements were composed by RC concrete columns and beams, respectively, and characterized by fiber section models.
Concrete02 material [
56,
57] was chosen to model the core and the cover portions (
Figure 7a,b, respectively) of the section (0.40 m × 0.40 m) and with the parameters defined in
Table 2. The ratio between the unloading slope (related to the maximum strength) and the initial slope was taken as equal to 0.1. A total of 30 bars were used and represented by
Steel02 material [
58], with the properties shown in
Table 3 and the ratio between the post-yield tangent and initial elastic tangent equal to 0.01 (
Figure 8). The parameters that control the transition from elastic to plastic branches were assumed R0 = 15, CR1 = 0.925 and CR2 = 0.15, as suggested by [
53]. The masonry walls were modelled as equivalent diagonal
elasticBeamColumn elements [
52,
53], in both the longitudinal and transversal directions. The masonry walls’ properties were selected based on the Italian code provisions, with low-to-medium mechanical characteristics (Table C8A.2.1 [
59]), as shown in
Table 4.
Table 5 shows the vibration periods of the structure with and without the infill masonry walls. It is worth noting that the masonry walls affect the structural natural period (from 0.3012 s to 0.2085 s), since they increase the lateral stiffness of the whole structure (as shown in [
60]). In particular, the infill masonry walls introduce different mechanisms that may significantly modify the seismic behavior of the structure. The foundation was modelled as a 0.50-m-deep rectangular concrete raft foundation (28.4 m × 34.4 m) in order to represent the recurring shallow foundation typologies for residential buildings. These types of foundation can be particularly vulnerable due to their bearing capacity, which depends only on the contact pressure and not on the frictional mechanisms (as in the case of deep foundations). The considered foundation was assumed to be rigid, by tying all the columns base nodes together with those of the soil domain surface, using
equalDOF [
52,
53]. Horizontal rigid beam-column links were set normal to the column longitudinal axis to simulate the interface between the column and the foundation. The foundation was designed by calculating the eccentricity (the ratio between the overturning bending moment at the foundation level and the vertical forces) in the most detrimental condition of the minimum vertical loads (gravity and seismic loads) and maximum bending moments. The foundation was modelled with an equivalent concrete material, by applying the
Pressure Independent Multi-Yield model [
52,
53] (
Table 6). This model consists of a non-linear hysteretic material with a Von Mises multi-surface kinematic plasticity model, which can simulate a monotonic or cyclic response of materials whose shear behavior is insensitive to the confinement change. The nonlinear shear stress-strain backbone curve is represented by the hyperbolic relation, defined by the two material constants (low-strain shear modulus and ultimate shear strength) [
52,
53].
The 3D soil models consist of a 118.4 m × 124.4 m (20.5 m thick) mesh, built up with 31,860 nodes and 35,868 20-node
BrickUP elements to simulate the dynamic response of solid-fluid fully coupled material [
52,
53] and with the same assumptions considered for the free-field models (
Section 2.1). As explained in
Section 3.1, S2 was implemented amongst the soil materials that were considered in step 1. The first 0.5-m-deep soil layer around the foundation was modelled with a backfill defined by the
PressureDependMultiYield [
52,
53] model, based on the multi-yield-surface plasticity framework developed by [
54].
Table 7 shows the adopted parameters, such as the low-strain shear modulus, the friction angle, and the permeability. The number of yield surfaces was equal to 20.
Figure 9 shows the backbone curves.