PSI - Issue 78

Simona Coccia et al. / Procedia Structural Integrity 78 (2026) 1318–1325

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1. Introduction Recent seismic events affecting the Campi Flegrei area have revealed a significant vertical ground motion component in addition to the traditional horizontal component. This finding is particularly important for structural vulnerability assessments and damage potential analyses. Historical evidence has demonstrated that locally intense vertical motions can substantially influence damage patterns and even lead to structural collapse (Papazoglou and Elnashai, 1996). The vertical component is considered significant when the ratio between vertical and horizontal peak ground acceleration (PGA) exceeds 0.6 – 0.7 (Bozorgnia and Campbell, 2004). Such seismic events typically originate from sources characterized by impulsive energy release and vertically propagating fault rupture, leading to a predominance of P-waves that amplify vertical ground motion (Goto and Morikawa, 2012). The main seismic events exhibiting high vertical-to-horizontal (V/H) ratios are summarized in Table 1. Among these events, several have highlighted the detrimental effects of vertical ground motion on masonry structures, particularly in historical centers built with masonry containing coarse aggregates, where local failure mechanisms are influenced by poorly connected walls, flexible floor diaphragms, and vulnerable roof systems. For instance, f ollowing the L’Aquila earthquake, out of-plane collapse of perimeter walls was observed due to the combined effect of horizontal shaking and vertical uplift forces, accompanied by roof detachments facilitated by tensile vertical pulses (Indirli et al., 2013).

Table 1. Seismic Events with High V/H Ratio.

Source Mechanism Strike-slip (Nojima fault)

Horizontal PGA [g]

Vertical PGA [g]

V/H (max)

Event

Date

Mw Depth [km]

Kobe (Japan)

17/01/1995

6.9

16

0.83

0.63

0.77

Central Italy

2016

6.0 – 6.5 7.5-10

1.42

0.87

1.26 Normal fault

L'Aquila (Italy)

06/04/2009

6.3

9

0.66

0.57

1.30 Normal fault

Crustal reverse fault Oblique reverse fault

Wenchuan (China)

12/05/2008

8.0

19

0.96

0.95

1.45

Kahramanmaraş (Turkey)

06/02/2023

7.8

5.5-24

~2.00

~1.38

1.45

Subduction megathrust

Tohoku (Japan)

11/03/2011

9.1

29

2.73

1.92

2

Shallow reverse fault Volcanic unrest (bradyseism)

Christchurch (NZ) Campi Flegrei (Italy) †

22/02/2011

6.3

~5

1.41

2.21

4.68

2022 – 2025 2.3-3.9

0 – 4

1.14

0.58

5.51

† For Campi Flegrei, V/H ratios vary significantly from event to event (some > 2), based on INGV data.

Provided that the masonry quality is such to ensure the wall monolithic behaviour (good quality masonry with sufficient transversal connections among the leaves), under an earthquake the walls perpendicular to the seismic action experience an out-of-plane motion and eventually collapse by lack of balance. In this condition, the rigid-block assumption can properly describe the actual behaviour of masonry walls, as confirmed by the dynamic experimental investigations on masonry elements subjected to free-vibration without detachment or falling of the single stones (Degli Abbati and Lagomarsino, 2017). The feasibility of models based on rigid-block assemblies to estimate the seismic vulnerability of existing masonry structures characterized by the macro- elements’ formation is well established (Roca et al., 2010; Lourenço et al., 2011). Consequently, current standards adopt methodologies based on these approaches in both the dynamic and kinematic fields (CS.LL.PP., 2019). Concerning the dynamic approach, the out-of-plane motion starts once the kinematic acceleration is exceeded by the earthquake and the failure condition can be set in terms of maximum angle of rotation associated with the element rocking before the overturning. The dynamic behavior of a rigid block rocking on a rigid base under free and forced vibrations was firstly theorized by Housner (Housner, 1963), in the aim to understand the seismic performance of inverted pendulum structures like elevated water tanks, petroleum-cracking towers, stone columns, electric power