# Innovative Seismic Microzonation Maps of Urban Areas for the Management of Building Heritage: A Catania Case Study

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

_{SSI}, higher than the fixed-base structure one, T

_{fixed}, and the response spectrum of a flexible-base structure lies below the fixed-base structure one due to the lower damping ratio of the flexible-base structure. Generally, the spectral ordinates corresponding to the fixed-base structure, S

_{a}(T

_{fixed}), are higher than the flexible-base case, S

_{a}(T

_{SSI}) (Figure 1a). However, sometimes the trend of response spectra modifies this behavior, leading to an underestimation of seismic actions (Figure 1b). Moreover, while S

_{a}(T

_{fixed}) corresponding to T

_{fixed}can be estimated in a straightforward manner, the spectral acceleration S

_{a}(T

_{SSI}) corresponding to T

_{SSI}requires energy dissipation mechanisms generated in an oscillating soil-structure system through radiation and soil hysteretic damping, with no counterpart in fixed-base structures.

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15. Finally, it was possible to make interesting considerations on the reliability of the Italian building code, NTC2018, prescriptions [10].

## 2. Evaluation of SSI Effects for the Estimation of the Design Accelerations

_{SSI}) may be computed by means of the following equation [12]:

_{fixed}and k

_{str}are the fundamental period and the horizontal stiffness of the fixed-base structure, h

_{eff}is the effective height of the structure equal to 0.7H (except for single-story buildings where h = H), and k

_{h}and k

_{r}are the translational and rocking stiffness of the foundation, respectively [13].

_{fixed}) is estimated according to the easy-to-use equation suggested by the old Italian Technical Code [17]:

_{1}is equal to 0.075 for concrete structures and 0.050 for masonry structures and H is the height of the structure.

_{str}is obtained by reversing the known equation:

_{h}) and rocking (k

_{r}) stiffness of the foundation may be computed by the following equations [12]:

_{θ}is a dimensionless coefficient that depends on the excitation period, the dimension of the foundation, and the properties of the supporting medium [18] assumed equal to 1 without accurate studies.

_{0}and I

_{0}stand for the area and moment of inertia for the foundation, respectively. In particular, A

_{0}is the footprint area of each structure (it refers to a rectangular footprint area of dimension B

_{eq}= √ A

_{0}); in this manner, the moment of inertia for the foundation I

_{0}may be computed by B

_{eq}

^{4}/12.

_{a}and 0.75r

_{m}for the translational and rocking stiffness of the foundation [13]. In particular, the degradation of the shear modulus of soil is considered with the deformation level G(γ). Therefore, the degradation coefficient is estimated according to the procedure suggested by the EC8 [20].

_{SSI}of a soil-structure system is defined as:

_{0}is a foundation damping factor depending on T

_{SSI}/T

_{fixed}. It is defined as:

_{SSI}is generally higher than the damping factor β

_{fixed}(more than 5% for concrete structures and 8% for masonry structures) with the exception of the rare case of the foundation damping itself being very low (smaller than 5%), and the period ratio being large [21]. In fact, the system damping gradually decreases when the period ratio increases. However, it should be noted that the effective damping may not generally be taken less than the structural damping of 5% [22,23]. These damping ratios will be used to plot the response spectra in accordance with the procedure proposed by [24].

_{SSI}and T

_{fixed}), the spectral accelerations will be calculated and compared, according the flow chart shown in Figure 2.

## 3. The Catania Case History

#### 3.1. The Investigated Areas

#### 3.2. The Utilized Inputs

## 4. The 1-D Site Response Analysis

_{max}at the ground surface for the response spectra described in the following section were evaluated. In particular, since the shown procedure deals with rough analyses were carried out on large scale, we decided to standardize the seismic bedrock for all performed analyses, setting it at 30 m, which is a typical value prescribed via technical codes [10]. Investigations were characterized by trends that sometimes led to different bedrock depths.

_{W}and T3

_{W}stratigraphies in the west area, the T1

_{NE}stratigraphy of the northeast area, the T1

_{N}and T2

_{N}stratigraphies of the north Old-Town area, as well as the T3

_{S}and T5

_{S}stratigraphies of the south Old-Town areas amplified the signal more than the others stratigraphies, due to their poor mechanical and dynamic characteristics.

_{NE}stratigraphy presented higher peaks compared to the other ones.

## 5. New Seismic Microzonation Maps for the Investigated Areas

_{fixed}and T

_{SSI}, the response spectra and the related spectral accelerations, S

_{a}(T

_{SSI}) and S

_{a}(T

_{fixed}). The results were mapped in the Google My Maps environment and presented via pie charts. The procedure was adopted for each individual building of the investigated areas, but the developed maps summarized the results, referring to each identified block of the relative area (representing a pie chart for each block).

_{fixed}values for the investigated areas, summarizing the achieved results for each chosen block. In order to provide a systematic presentation of the obtained results, three different ranges were selected for the fundamental periods: T

_{fixed}< 0.40 s (i.e., buildings having H < 10 m), 0.40 < T

_{fixed}< 0.80 s (i.e., buildings having 10 < H < 20 m), and T

_{fixed}< 0.80 s (i.e., buildings having H > 20 m).

_{SSI}/T

_{fixed}ratios for all investigated areas. As seen from the previous figures, the results refer to three different ranges, indicating probable negligible (T

_{SSI}/T

_{fixed}< 1.15), moderate (1.15 < T

_{SSI}/T

_{fixed}< 1.30), and high (T

_{SSI}/T

_{fixed}≥ 1.30) SSI effects on the fundamental period of the structures.

_{SSI}/T

_{fixed}< 1.15 for almost every building. The greater proximity of the T

_{SSI}values to those of T

_{fixed}was due to the presence of more recent buildings in this area, which were therefore built according to a better geometric configuration of the entire structure and its foundations. This was in agreement with the seismic technical standards after the 1970s. As for the northeast area (Figure 9b), negligible DSSI effects (T

_{SSI}/T

_{fixed}< 1.15) were observed mainly for the T2

_{NE}and T3

_{NE}stratigraphies, characterized mainly by rock soils. Therefore, for these cases, the assumption of the fixed-base structure was acceptable. As for the north Old Town area (Figure 9c), it was evident that for the T3

_{N}stratigraphy, the period of the generic structure in the flexible-base configuration did not differ much from the period of the same structure in the fixed-base configuration: given the predominantly rocky nature of the soil, the assumption of the fixed-base structure faithfully captured reality. Different considerations were made for the T1

_{N}and T2

_{N}soils, which were characterized by a lower stiffnesses: for the T1

_{N}stratigraphy, the T

_{SSI}/T

_{fixed}ratios were mainly between 1.15 and 1.30, while for the T2

_{N}stratigraphy, the ratios were mainly greater than 1.30. Therefore, with reference to the entire north Old Town area, it can be stated that most buildings were characterized by T

_{SSI}/T

_{fixed}> 1.15, for which the effects of the dynamic SSI could be relevant. Finally, as for the south Old Town area (Figure 9d), most buildings were characterized by ratios T

_{SSI}/T

_{fixed}< 1.15. This was in accordance with the nature of the foundation soil, which is mainly rock soil. Nevertheless, a good percentage of buildings with a higher period of ratios allowed us to make interesting considerations relating to the effects of soil-structure interaction.

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratios were evaluated and mapped considering three different ranges: beneficial (S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ≤ 0.85), negligible (0.85 < S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ≤ 1.15) and detrimental (S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15) SSI effected the seismic response of the structures. Figure 10, Figure 11, Figure 12 and Figure 13 show the developed maps, considering the adopted 4 inputs.

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ≤ 1.15) or beneficial, with ratios S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ≤ 0.85. For the 2002 input, the B1 block had a small percentage of S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15, probably due to the poor properties of soil foundation.

_{NE}stratigraphy, as it was constituted by poor soil. The higher spectral accelerations ratios (S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15) were obtained for the seismic inputs of 1818 and 2002, especially in the northern part of the investigated area, i.e., for the T1

_{NE}stratigraphy. This was due both to the irregular development of the response spectra and to the nature of the foundation soil. For the rock foundation soil, i.e., for the T2

_{NE}and T3

_{NE}stratigraphies in the southern part of the investigated area, the spectral accelerations ratios were generally beneficial or negligible. The most worrying cases were related to block “A” (34 structures). In particular, 4 structures for 1818 seismic input and 5 structures for 2002 presented higher spectral accelerations ratios (S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15); these were mainly masonry structures.

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) < 1.15 were achieved. Thus, neglecting the DSSI was almost always a safety advantage. However, for some blocks, there were ratios S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15 and neglecting the DSSI meant working by spectral accelerations lower than those expected to impact the structure. This result was due to the poor soil foundation (T1

_{N}and T2

_{N}) on which old buildings were built.

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15 are highlighted; for these structures, the performed microzonation analysis suggests a more accurate study of the SSI effects before carrying out any seismic retrofitting.

_{a}(T

_{fixed}) and S

_{a}(T

_{SSI}) for all buildings in the analyzed areas with reference to the inputs of 1818 (a) and 2018 (b), according to the numbering assigned to the buildings shown in Figure 19.

_{a}(T

_{SSI}) are also compared with those suggested by the Italian Technical Code [10]. The results in terms of the spectral accelerations ratio S

_{a}(T

_{SSI})/S

_{a}(NTC2018) are shown in Figure 20, Figure 21, Figure 22 and Figure 23.

_{a}(NTC2018) values greater than or almost equal to the S

_{a}(T

_{SSI}) values obtained considering the SSI interaction.

_{a}(T

_{SSI}) were higher than those suggested by [10] (S

_{a}(NTC2018)). In fact, the higher spectral accelerations ratios (S

_{a}(T

_{SSI})/S

_{a}(NTC2018) > 1.15) were obtained for the 2018 input, especially in the northern part of the investigated area, i.e., for the T1

_{NE}stratigraphy.

_{a}(T

_{SSI})/S

_{a}(NTC2018) < 1.15, except for the 2018 input, for which the spectral accelerations of the flexible-base structure S

_{a}(T

_{SSI}) were higher than those suggested by [10] (S

_{a}(NTC2018)). Therefore, the design suggested by [10] was not advantageous.

_{SSI}/T

_{fixed}ratios and the S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratios versus the soil V

_{s}. For lack of space, Figure 24 and Figure 25 show that the results achieved the “south Old Town” area. Figure 24 shows that the ratio between the period of the flexible-base building configuration and the fixed-base one tended to unify as the velocity V

_{s}increased. This behavior was more evident for V

_{s}values higher than 300 m/s. Figure 25 shows that the ratio between the spectral accelerations tended to the unit, as the value of V

_{s}increased. In particular, as the velocity of the shear waves increased, the range of S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratios decreased and it approached unity from V

_{s}> 300 m/s.

## 6. Conclusions

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15.

_{a}(T

_{SSI}) were higher than those suggested by [10] (S

_{a}(NTC2018)).

_{a}(T

_{SSI})/S

_{a}(T

_{fixed})).

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**A schematic explanation of the period elongation effect due to the soil-structure interaction (SSI) on the seismic force imposed on a structure depending on the seismic input and soil conditions: (

**a**) beneficial effect of SSI reflected in S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratio below unity; (

**b**) detrimental effect of SSI reflected in S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratio above unity.

**Figure 3.**(

**a**) Geological map of the city of Catania. (

**b**) Map of average shear wave velocity (m/s) in 0–30 m depth interval for the city of Catania (after [29]).

**Figure 4.**The investigated areas: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; and (

**d**) south Old Town.

**Figure 5.**Profile of V

_{s}for all the considered sub-areas of the four investigated areas and corresponding stratigraphies: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 7.**Division of the areas in blocks according to the urban morphology: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 8.**Spatial distribution of T

_{fixed}for the investigated areas: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 9.**Spatial distribution of T

_{SSI}/T

_{fixed}for the investigated areas: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 10.**Spatial distribution of S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratios for the 1818 seismic input: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 11.**Spatial distribution of S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratios for the 1990 seismic input: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 12.**Spatial distribution of S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratios for the 2002 seismic input: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 13.**Spatial distribution of S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratios for the 2018 seismic input: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 14.**Identification of the buildings for which S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) > 1.15: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 15.**Spectral accelerations S

_{a}(T

_{SSI}) and S

_{a}(T

_{fixed}) evaluated for each building of the west area (shown in Figure 19a) for (

**a**) 1818 input; (

**b**) 2018 input.

**Figure 16.**Spectral accelerations S

_{a}(T

_{SSI}) and S

_{a}(T

_{fixed}) evaluated for each building of the northeast area (shown in Figure 19b) for (

**a**) 1818 input; (

**b**) 2018 input.

**Figure 17.**Spectral accelerations S

_{a}(T

_{SSI}) and S

_{a}(T

_{fixed}) evaluated for each building of the north Old Town area (shown in Figure 18c) for (

**a**) 1818 input; (

**b**) 2018 input.

**Figure 18.**Spectral accelerations S

_{a}(T

_{SSI}) and S

_{a}(T

_{fixed}) evaluated for each building of the south Old Town area (shown in Figure 18c) for (

**a**) 1818 input; (

**b**) 2018 input.

**Figure 19.**Location of buildings in the investigated areas: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 20.**Spatial distribution of S

_{a}(T

_{SSI})/S

_{a}(NTC2018) ratios for the 1818 seismic input: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 21.**Spatial distribution of S

_{a}(T

_{SSI})/S

_{a}(NTC2018) ratios for the 1990 seismic input: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 22.**Spatial distribution of S

_{a}(T

_{SSI})/S

_{a}(NTC2018) ratios for the 2002 seismic input: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 23.**Spatial distribution of S

_{a}(T

_{SSI})/S

_{a}(NTC2018) ratios for the 2018 seismic input: (

**a**) west area; (

**b**) northeast area; (

**c**) north Old Town; (

**d**) south Old Town.

**Figure 24.**T

_{SSI}/T

_{fixed}ratios versus the shear waves velocities V

_{s}for the south Old Town area.

**Figure 25.**S

_{a}(T

_{SSI})/S

_{a}(T

_{fixed}) ratios versus shear waves velocities V

_{s}for the south Old Town area.

**Table 1.**Characteristic values of some representative geotechnical parameters [29].

Lithotype Label | Corresponding Lithotype | γ (kN/m^{3}) | V_{s} (m/s) |
---|---|---|---|

R-Df | Topsoil and fill (R); debris and landslides (Dt) | 17.0–19.0 | 130–220 |

X | Scoriaceous lavas and volcanoclastic rocks | 18.0–18.5 | 180–300 |

Alg | Coarse alluvial deposits | 18.0–19.5 | 210–280 |

Asg | Yellowish or brown clays and sandy silts | 19.3–20.0 | 220–400 |

Aa | Silty clays and grey-bluish clays | 19.5–20.0 | 450–600 |

M | Marine deposits | 18.3–18.7 | 210–280 |

P | Pyroclastic rocks | 16.0–17.0 | 250–500 |

Cc | Calcarenites and block-calcarenites | 21.0–23.5 | 500–800 |

E1-E2 | Fractured to slightly fractured lavas | 22.0–24.0 | 350–500 |

Ai | Clayey interlayers in Cc unit | 21.0–23.5 | 300–650 |

Alf | Fine alluvial deposits | 18.5–19.5 | 130–210 |

SG | Yellow or brown quartz sands | 19.8–20.8 | 350–500 |

Data | M (Richter) | f_{1} (Hz) | Epicenter |
---|---|---|---|

20.02.1818 | 6.0 | 0.58 | Aci Sant’Antonio |

13.12.1990 | 5.7 | 1.59 | Augusta |

29.10.2002 | 4.4 | 0.35 | Santa Venerina |

26.12.2018 | 4.8 | 2.55 | Etna |

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**MDPI and ACS Style**

Abate, G.; Bramante, S.; Massimino, M.R. Innovative Seismic Microzonation Maps of Urban Areas for the Management of Building Heritage: A Catania Case Study. *Geosciences* **2020**, *10*, 480.
https://doi.org/10.3390/geosciences10120480

**AMA Style**

Abate G, Bramante S, Massimino MR. Innovative Seismic Microzonation Maps of Urban Areas for the Management of Building Heritage: A Catania Case Study. *Geosciences*. 2020; 10(12):480.
https://doi.org/10.3390/geosciences10120480

**Chicago/Turabian Style**

Abate, Glenda, Simone Bramante, and Maria Rossella Massimino. 2020. "Innovative Seismic Microzonation Maps of Urban Areas for the Management of Building Heritage: A Catania Case Study" *Geosciences* 10, no. 12: 480.
https://doi.org/10.3390/geosciences10120480