Selected Papers from 1st Croatian Conference on Earthquake Engineering (1CroCEE)

This special issue of Buildings Journal brings together outstanding papers presented at the 1st Croatian Conference on Earthquake Engineering 1CroCEE, held at the Faculty of Civil Engineering, University of Zagreb, Croatia, 22–24 March 2021. The motivation is to keep the momentum created by the ongoing recovery process from the recent earthquakes in Croatia and contribute to an increased awareness to seismic risk. At the same time, it is intended to boost the current state of earthquake engineering research and practice in Croatia.

The organizational leadership for 1CroCEE was provided by the Faculty of Civil Engineering, University of Zagreb. The extensive Organizing Committee included professors and students from all the departments of the Faculty of Civil Engineering and several other faculties with the University of Zagreb. Exceptional assistance, in particular related to seismological issues, was given by the Faculty of Science. A number of earthquake engineering associations supported the organization of the conference, such as the International association for Earthquake Engineering, the European association for Earthquake Engineering and the Croatian association for Earthquake Engineering. Significant and practical help was provided by the Croatian Ministry of Physical Planning, Construction and State Assets, the Ministry of Education and the Ministry of Culture and Media. As well, additional support was made available by the University of Zagreb, Croatian Academy of Engineering, City of Zagreb, Embassy of Italy, Civil Protection Directorate including the Croatian Platform for Disaster Risk Reduction, Croatian Engineering Association, Croatian Chamber of Economy, Croatian Chamber of Civil Engineers, Croatian Association of Civil Engineers and many other organizations and individuals. Numerous sponsors were also brought on-board crucial for the logistics and smooth organization of the conference.

Croatia is located in a relatively active seismic region in Southern Europe. Although earthquakes do occur, disastrous earthquakes are much less common giving population and decision makers a false sense of security. In these circumstances, even when a moderate earthquake takes place, the potential risk increases from what could otherwise be expected from such events. This was literally the case with the recent M5.5 22 March 2020 Zagreb earthquake. The failure to foresee and prepare adequately for this event left the population vulnerable to the impacts of the seismic shaking. The estimated direct and indirect economic losses from the Zagreb earthquake amount to more than 10B euros. The surprisingly heavy social impacts were amplified by a combination of circumstances such as damage concentrated in the dense urban city center, the number of people requiring alternative accommodation and the relatively cold weather. To make the situation worse, the COVID-19 pandemic and lockdown were in progress and aggravated the emergency response in the aftermath of the event.

The 2020 Zagreb earthquake caused significant damage in older brick masonry buildings, not properly designed to withstand lateral loads, and/or to residential buildings which have undergone interventions in the structural system. A number of hospitals, churches and other cultural heritage buildings were among the damaged structures. It is estimated that approximately 17,000 buildings were affected in a certain degree by the earthquake. About 1300 buildings were unusable with serious structural damage or scheduled for demolition, 5000 buildings were temporary unusable with short term repairable damage, and approximately 20,000 of the inspected buildings were tagged usable with only minor or without any reparation. The structural damage consisted mainly of in-plane shear cracking and rarely out of plane collapse of the bearing walls, and damage to stairways and to floor slabs. Most of the direct economic loss, however, was attributed for frequent reparations of non-structural damage, e.g., partial damage or collapse of chimneys and attic gable walls, damage to wooden roof structure, detached and falling decorative elements and architectural finishes, cracks in ceilings and partition walls, cracking and partial collapse of lintel and vault sections, etc.

In December 2020 and January 2021, barely nine months following the Zagreb earthquake, a series of earthquakes struck Petrinja and Sisak, about 50 km south of Zagreb. The strongest quake occurred on 29 December 2020, with a magnitude of 6.2. It was felt throughout Croatia and neighboring countries, causing casualties and structural damage in the epicentral region.

Disastrous earthquakes have occurred in Croatia in the past and will hit again. If not adequately addressed, losses to life and property can once more be significant. The conventional disaster risk management wisdom based on ‘known risks’ is insufficient for building resilient societies. Interdisciplinary research and training in particular in the fields of Earthquake Engineering and Seismic Risk assessment and management is crucial for improving the design of new structures, retrofitting the existing ones and efficiently respond and recover from disastrous earthquakes.

Despite the surge of scientific papers in the field of earthquake engineering, full text papers focused entirely on subjects specific to the construction practices in Croatia and neighboring countries have rarely been brought out at the same place. This was the principal objective the special issue. Seven papers comprise this special issue, spanning a range of various themes related to the assessment of the seismic performance of buildings. The authors list and a brief résumé are provided in the following.

Haladin et al. [1] studies the influence of tram traffic-induced vibration on earthquake damaged Buildings. Most of the damage occurred on historic masonry residential buildings in the center of the city of Zagreb. Although traffic-induced vibrations generally do not affect surrounding buildings, they can be harmful to buildings damaged by a previous earthquake. The impact of tram-induced vibrations on earthquake-damaged buildings in the city of Zagreb is investigated in this paper. The analysis is conducted on a tramway network scale to identify critical locations by performing continuous monitoring on the tramway network and risk analysis based on the distance of buildings from the track, vibration amplitude at source, and building damage.

Medić and Hrasnica [2] analyze in-plane seismic response of unreinforced and jacketed masonry walls. Low-rise residential and public masonry structures constitute a large portion of the building patrimony, yet they were erected during the massive reconstruction of Southeast Europe after World War II before any design rules existed in the engineering praxis. To determine lateral strength, stiffness, and capacity of energy dissipation of the URM walls, in-plane tests were performed. Two full-scale plain walls built with solid clay brick and lime-cement mortar and two walls strengthened with RC jacketing on both sides were subjected to cyclic lateral loading under constant vertical precompression.

Ereiz et al. [3] deals with influence of seismic action on the tie rod system in historic buildings using finite element model updating. Historic buildings have a high architectural value and their maintenance, repair and rehabilitation require a special approach. In this context, this paper presents an approach for the analysis of seismic action on the tie rod system in a historic building. The analysis was performed by combining the on-site experimental testing and the finite element model updating (FEMU) of the local models of tie rods and the global model of the structure. It was shown that the combination of analyzing local and global structural models, experimental on-site testing and FEMU is a viable solution for assessment of historic buildings’ load bearing capacity.

Blagojević et al. [4] use simplified seismic assessment of unreinforced masonry residential buildings in the Balkans. The paper presents a study on the existing low-rise unreinforced masonry (URM) buildings constructed in the period from 1945 to 1980 in Serbia and neighboring countries in the Balkans. Buildings of this typology experienced damage in a few earthquakes in the region. The focus of the study is a seismic design approach for Simple masonry buildings according to Eurocode 8, Part 1, which is based on the minimum requirements for the total wall area relative to the floor plan area, which is referred to as Wall Index in this paper. Although the intention of Eurocode 8 is to use WI for design of new buildings, the authors believe that it could be also used for seismic assessment of existing masonry buildings in pre- and post-earthquake situations.

Folić and Čokić [5] develop fragility and vulnerability curves of RC building. In this paper, the seismic response of a five-story reinforced concrete (RC) frame system building is analyzed through fragility analysis. For the analysis of the response of a structural system to earthquake actions, the methods of nonlinear static (NSA) and nonlinear dynamic analyses (NDA) are applied and, based on the obtained results, fragility curves are constructed using statistical methods. A relationship between the intensity measure (IM) and engineering demand parameters (EDPs) is needed in order to estimate a fragility curve.

Balić et al. [6] deals with seismic analysis of the bell tower of the Church of St. Francis of Assisi on Kaptol in Zagreb by combined finite-discrete element method. The paper presents a failure analysis of the bell tower of the church of St. Francis of Assisi on Kaptol in Zagreb subjected to seismic activity using the finite-discrete element method—FDEM. The bell tower is a masonry building, and throughout history it has undergone multiple damages and reconstructions. It was significantly damaged during the earthquake in Zagreb which occurred on 22 March 2020 with a magnitude of 5.5.

Jamšek and Dolšek [7] develop the reduced-degree-of-freedom model for seismic analysis of predominantly plan-symmetric reinforced concrete wall and frame building. A reduced-degree-of-freedom (RDOF) model for seismic analysis of predominantly plan-symmetric reinforced concrete (RC) wall–frame buildings is introduced. The RDOF model of the wall–frame building consists of elastic beam–column elements with concentrated plasticity used for simulating cantilever walls and predominantly plan-symmetric RC frame buildings that are represented by the improved fish-bone (IFB) model. In this paper, the capability of the RDOF model is demonstrated for two frame buildings and two wall–frame buildings. The RDOF models were defined directly from the building information model. This is an advantage of RDOF models with respect to single-degree-of-freedom (SDOF) models, while the computational robustness of the RDOF models also makes them attractive for the seismic analysis of building stock.

We hope these papers will make significant contributions to the understanding of various aspects of interest for the engineering community in Croatia and elsewhere. The above papers were reviewed by a team of external experts led by Buildings Journal. The editors express their gratitude to the peer reviewers for the rigorous analysis of the manuscripts. Last but not least, the publication of this issue would have not been possible without the dedicated work and valuable contributions by the authors. Their input to this special issue cannot be overstated.