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Estimating of buildings vulnerability is based on a well-organized and detailed database of buildings and their characteristics. Creation of the buildings database of the city Osijek is in progress. This database contains, for each building, information regarding its location, geometric and structural characteristics, materials which were used for structural elements, and other relevant data. This paper presents numerical and statistic values of some characteristics of the buildings from the database. Different methods can be applied for the prediction of damage probability in the field of earthquake risk assessment. With the empirical Macroseismic method and the analytical Capacity Spectrum Method, the vulnerability of the few blocks of buildings, typical for the city Osijek, is estimated. For unreinforced masonry structures with flexible floors, the probability of reaching a certain degree of damage is estimated by the two selected methods, and the obtained results are compared.

The limitation of deformation, the desired level of bearing capacity of the structural elements and the dissipation of the energy produced by an earthquake ensure sufficient bearing capacity, stiffness and ductility of structures. These requirements are met by buildings which are designed and constructed in accordance with the latest technical regulations for buildings exposed to the earthquake. Most of the existing buildings in the city of Osijek do not meet these requirements. It is possible to use a capacity spectrum method for the purpose of considering possible responses of such structures to earthquakes and their estimates of seismic vulnerability. Calculation of seismic vulnerability by this method is based on the expected behavior of the building, obtained by overlapping the demand curve and capacity curve. The performance point is the point at which the capacity curve intersects the reduced response curve, at which capacity and demand are equal. The values of spectral displacement obtained for the performance point of a specific building class are used as the input parameters for the fragility curve for different levels of damage. This method will be applied to the collected database of traditional unreinforced masonry buildings of the city Osijek in Croatia. The database is prepared and the main characteristics of the buildings are processed using geographic information system.

Due to increases in the number of inhabitants and their concentrations in densely populated areas, there is a growing need in modern society to be cautious towards the impact of catastrophic natural events. An earthquake is a particularly major example of this. Knowledge of the seismic vulnerability of buildings in Europe and around the world has deepened and expanded over the last 20 years, as a result of the many devastating earthquakes. In this study, a review of seismic risk assessment methods in Croatia was presented with respect to the hazard, exposure, and vulnerability of buildings in the fourth largest city (Osijek) in Croatia. The proposed algorithm for a detailed risk assessment was applied to a database and is currently in its initial stage.Sustainability 2020, 12, 1796 2 of 24 to estimate the losses. These losses can be assessed in a material form, through the damage of the building stock or non-structural elements of buildings, or in the form of casualties or injuries during the earthquake [10,11]. The last step is to analyze uncertainties, costs, decision-making criteria, etc. The ultimate goal of studying earthquakes and their impact on people and buildings is to create a safer environment, in case an earthquake occurs.The basic elements of the earthquake risk assessment process are shown in Figure 1.Sustainability 2020, 12, 1796 2 of 25 probable intensity in a given geographical area). Then, based on the available data from the exposure model, evaluate the damage using one of the existing vulnerability assessment methods in order to estimate the losses. These losses can be assessed in a material form, through the damage of the building stock or non-structural elements of buildings, or in the form of casualties or injuries during the earthquake [10,11]. The last step is to analyze uncertainties, costs, decision-making criteria, etc. The ultimate goal of studying earthquakes and their impact on people and buildings is to create a safer environment, in case an earthquake occurs. The basic elements of the earthquake risk assessment process are shown in Figure 1.

Assessing earthquake risk and building vulnerability requires an exposure model. These exposure models quantify the building stock in terms of structural characteristics, spatial location, and occupancy. The most significant exposure parameters are the structural characteristics of buildings, which must be uniformly covered by structural typologies. Structural typologies that take into account the regional specificities of design and construction provide more accurate and reliable exposure models. Despite the long history of earthquake engineering in the Republic of Croatia, the assessment of exposure and vulnerability of buildings is a rather new concept, hindered by the fact that no city in the Republic of Croatia has a database on the number, types, and characteristics of existing buildings. The article presents the creation of a building exposure model for the city of Osijek, points out the problems and concerns that the realization process brings, and details the practical solutions and strategies that have been used to achieve the set goals.

Most of the buildings in old city cores of Croatia, built between 1860 and 1920 with wooden floors, are mainly designed to bear vertical loads. In this paper we propose a methodology for seismic vulnerability assessment of unreinforced masonry buildings with flexible floors. The methodology is based on the calculation of Damage Index (DI), a numerical value indicating the level of structural damage. In this methodology, the structure is represented using an SDOF model determined by damping, weight, elastic base shear capacity, elastic stiffness and post-elastic stiffness. Using accelerograms of earthquakes, step by step time-history numerical integrations are provided along with the results: top displacement, yield excursions, cumulative energy and base shear-displacement. These results serve as parameters which are then input in the formula for Damage Index (DI). The results of the paper are presented in the form of diagrams with DI values on the y axis and fundamental period of the structure on the x axis. These spectral functions of DI, along with knowledge of fundamental period and chosen accelerogram, can be used to quickly determine the level of damage for unreinforced masonry buildings with flexible floors.

In this paper, we demonstrate how UHS-based seismic microzonation can be applied in low-to-medium seismicity areas with deep local soil and deep geological deposits under the local soil. The case study area surrounds the city of Osijek, Croatia, which is in the south–central region of the Pannonian Basin. New frequency-dependent scaling equations are derived, and the empirical response spectra are compared to the spectra of real strong motions in the surrounding region. Empirical calculations for deep soil atop deep geological strata show a 37% reduction in short-period spectral amplitudes when compared to rock locations. This demonstrates that local soil amplification is mitigated by energy dissipation in deep soils. For vibration periods longer than 0.3 s, spectral amplitudes are being amplified. This amplification goes up to 2.37 times for vibration periods around 0.5 s. UHS spectra for Osijek are computed using regional seismicity estimates, data on local soil and deeper geological surroundings, and newly created regional empirical equations for scaling various spectral amplitudes. UHS amplitudes for Osijek are also compared to the Eurocode 8 spectra for ground type C. The results show that ratios of the maximum UHS amplitudes to PGA values are up to 46% larger than the corresponding 2.5 factor that is recommended by Eurocode 8 for horizontal spectra. The UHS results might be viewed as preliminary for Osijek and regions with similar seismicity and local soil and deep geology conditions. When the number of regional strong-motion records grows many times beyond what it is currently, it will be feasible to properly calibrate the scaling equations, resulting in more reliable and long-term UHS estimations for the area under consideration.

The severity of vertical seismic ground motions is often factored into design regulations as a component of their horizontal counterparts. Furthermore, most design codes, including Eurocode 8, ignore the impact of local soil on vertical spectra. This paper investigates vertical pseudo-absolute acceleration spectral estimates, as well as the ratios of spectral estimates for strong motion in vertical and horizontal directions, for low to medium seismicity regions with deep local soil and deep geological sediments beneath the local soil. The case study region encompasses the city of Osijek in Croatia. New regional frequency-dependent empirical scaling equations are derived for the vertical spectra. According to these equations, for a 0.3 s spectral amplitude at deep soils atop deep geological sediments compared to the rock sites, the maximum amplification is 1.48 times. The spectra of vertical components of various real strong motions recorded in the surrounding region are compared to the empirical vertical response spectra. The new empirical equations are used to construct a Uniform Hazard Spectra for Osijek. The ratios of vertical to horizontal Uniform Hazard Spectra are generated, examined, and compared to Eurocode 8 recommendations. All the results show that local soil and deep geology conditions have a significant impact on vertical ground motions. The results also show that for deep soils atop deep geological strata, Eurocode 8 can underestimate the vertical to horizontal spectral ratios by a factor of three for Type 2 spectra while overestimating them by a factor of two for Type 1 spectra.