# Vulnerability and Seismic Exposure of Residential Building Stock in the Historic Center of Alcamo
## 5 Assessment of Exposure and Vulnerability through Building-Scale Survey  
As can be observed from the procedures and data presented above that the exposure estimates conducted with the described procedure were strongly influenced by the estimated percentage of a given type within the compartment. Moreover, the variation in the vulnerability index related to detailing characteristics (influence of modifiers) was significant for masonry types, while this variation was much lower in the case of reinforced concrete structures. Apparently, a more accurate estimate of exposure and vulnerabilities could be achieved by surveying individual structural units. As is shown, this approach lead to total reliable estimate, only if the survey was performed with great thoroughness, through an inspection of the individual unit from the inside too, and an interview with the owner who could provide details about the construction that could not be obtained by a cursory survey. This procedure was so time-consuming and costly that it was possible to adopt it only for small areas. More frequently and over larger territories, it was possible to conduct a cursory building-scale survey, limited to a photographic survey and an inspection from the outside of the building. In this case, structural features were often not clear, and the survey and attribution of these features were strongly affected by the surveyor's knowledge and skill in correctly identifying elements representative of these features. Based on these premises, the results of an investigation performed through cursory surveys conducted from the outside of the building are presented. The evaluation was conducted through a complete photographic survey of the units, the support of images from the web, and a subsequent in-depth investigation through additional on-site surveys. The photos and results were reported on a geo-referenced database, which was used as support for the calculation of the vulnerability index of the individual structural unit. The parameters of structural interest that were surveyed were the following: number of floors, number of raised floors, state of preservation, regularities in plan and elevation, type of masonry or type of characteristic framed structure, material of the elevation, type of floors, type of roofing, presence of openings near the eaves, percentage of openings on the ground floor, position in aggregate and interaction with adjacent buildings, presence of earthquake-resistant devices, and presence of seismic strengthening or improvement interventions. As an example, thematic maps showing the number of floors (Figure 4a) and the state of preservation (Figure 4b) of residential buildings are presented in Figure 4. The former shows the prevalence of 2- and 3-story buildings in Compartment C01, while the percentage of 4-story buildings, which are more vulnerable to seismic action, is significant in Compartment C02. The second map shows a prevalence of good or at least sufficient states of preservation. Figure 5a shows the EMS98 types assigned to individual units, represented chromatically according to the vulnerability index. The units with a plastered elevation, for which the masonry type could not be assigned with a high degree of accuracy, are presented in light blue (Figure 5a). In fact, the architectural characteristics allowed one to identify, with a certain reliability, among the plastered structures, those that were reinforced concrete, while the attribution of masonry typology was not as simple, except in the presence of strongly characterizing typological indicators [38,42]. The high percentage of buildings with plastered walls (31.6%) affected the accuracy of the cursory building-scale survey for the seismic vulnerability assessment. Aiming at overcoming this problem, based on the previous studies, it was found that the age of construction of the building and the number of floors were among the parameters most correlated with masonry type.