# Seismic Vulnerability Assessment of Historical Masonry Buildings in Croatian Coastal Area
## 2 Seismic Vulnerability Assessment of the Test Site
### Seismic Vulnerability Assessment Using the Vulnerability Index Method  
The adopted vulnerability index method was based on the original vulnerability index method for masonry structures developed by the Italian National Research Council and the Italian National Group for the Defense Against Earthquakes (GNDT) from 1984 onwards [18; 19]. The method consists in filling in a survey with data relating to 11 geometrical and structural vulnerability parameters, the calculations of those parameters, and finally, the calculation of the vulnerability index for the building. The main parameters consider the type and organization of the resistant system, the quality of resistant system, the conventional resistance along two main horizontal directions of the building based on the estimation of the maximum resistant shear of the structure, the position of the building and foundations, the typology of floors, the planimetric and elevation configuration, the maximum distance among the walls, the typology and weight of the roof, the presence of non-structural elements, and the state of conservation. For each parameter, the surveyor must judge the condition among four possibilities, from \"A\", corresponding to an optimal condition, to \"D\", meaning a bad condition. For each judgment, the method provides a numerical score. Weight coefficients are then used, relating to each parameter to account for the relative importance of each parameter in the global definition of vulnerability. Finally, a vulnerability index \(\rm I_{v}\) is calculated in the form:  
\[\rm I_{v}=\sum_{i}s_{vi}w_{i}, \tag{1}\]  
where \(\rm s_{vi}\) is the numerical score for each class and \(\rm w_{i}\) is the weight of each parameter. This vulnerability index is then normalized in a 0-100% range. A low index means that the structure is not particularly vulnerable and has a high capacity under seismic action, whereas a high index shows that the structure is vulnerable and has low seismic capacity. The method was improved with the modifications proposed for the region of Tuscany in 2003 [43] to consider the possible substitution of the original light timber floors with heavier reinforced-concrete slabs. Field observations of the damage states of heritage buildings after earthquakes in the past 30 years has indeed shown that the replacement of timber floors with heavier concrete slabs, when performed on low quality masonry buildings, can substantially change the dynamic behavior of the structures, because it adds a considerable mass on the top of the building, thus increasing the overall in-plane stiffness. These changes can cause the collapse of the structure under earthquake excitation. Therefore, researchers on the Tuscan region corrected the vulnerability index method and updated the weights of parameters 1, 5, and 9. In particular: (1) in parameter 1, the weight changes from 1 to 1.5; (2) in parameter 5, the weight changes from a range 0.5-1 to the range 0.5-1.25, in particular, if heavier floors, such as concrete slabs, are supported by not-so-resistant masonry walls, the weight coefficient is 1.25; (3) in parameter 9, the weight changes from the range 0.5-1 to 0.5-1.5, and the calculation of the weight is the same as the original interval (0.5-1.0), where the weight is 1.25 if a heavier roof, such as a concrete slab, is supported by weak masonry walls, and it is 1.5 if there is also a heavy floor just below the roof. The present work uses the aforementioned modified weights proposed for the Tuscan region [43]. The weights of all the other parameters are assumed as in the original vulnerability index method. The vulnerability parameters, their numerical score values, and weight coefficients are shown in Table 1. The maximum value of the vulnerability index \(\rm I_{v}\) is 438.75.