Seismic Upgrading of the Heritage-Protected Reinforced Concrete Warehouse in Rijeka, Croatia

Abstract

Despite Croatia experiencing two strong earthquakes in 2020, Rijeka was not directly affected, underscoring the importance of proactive seismic assessment and strengthening in all seismic regions. This paper presents a comprehensive case study on the seismic strengthening of a 20th-century concrete building located in Rijeka, Croatia, originally designed according to Austro-Hungarian construction norms and practices. As a heritage-protected structure, the building’s architectural features and construction practices were examined and contextualized within its historical background. The assessment and renovation phases of this project are discussed in detail, demonstrating the practical application of modern seismic strengthening techniques while preserving the building’s historical integrity. This case study aims to highlight the need for such measures to protect heritage structures and to show the implementation of rapid and new (ad hoc) norms for earthquake-damaged buildings in Croatia. This study serves as a reference for engineers, architects, and conservationists involved in the preservation of heritage buildings, demonstrating that it is possible to enhance their structural safety without compromising their architectural authenticity.

1. Introduction

Recent seismic events in the Mediterranean have once again underscored the high vulnerability of existing buildings [1,2,3]. Over the past four years, earthquakes have inflicted damage on or led to the demolition of thousands of buildings across the Mediterranean region [4,5,6], including Croatia [6]. In Croatia, the year 2020 was particularly devastating due to two moderate earthquakes that caused extensive socioeconomic and material damage in the capital, Zagreb, and its surrounding areas [7,8,9]. The impact of these earthquakes was severe, resulting in eight fatalities, the displacement of hundreds of families nationwide, and significant damage or destruction of numerous historically and culturally significant buildings. The total damage of the two earthquakes (Zagreb and Petrinja) was estimated at EUR 19.9 billion [10]. According to data from the Croatian Center for Earthquake Engineering, over 82,000 buildings were damaged in both earthquakes [11]. The majority of the buildings damaged were old and built with unreinforced masonry (URM) with timber roof and floor structures. Several Croatian case studies of heritage URM buildings that were investigated and retrofitted after the earthquakes or documented before earthquakes are already published, and more information can be found in the following articles: [12,13,14,15]. Reinforced concrete (RC) structures, while generally more resilient, can still suffer significant damage due to design flaws, construction defects, inadequate maintenance, etc. Additionally, older RC buildings may not meet the modern seismic codes, rendering them vulnerable to seismic forces. This highlights the importance of comprehensive assessments and retrofitting to ensure the safety and integrity of all building types. This vulnerability was demonstrated in the recent earthquakes in Türkiye and Albania, where both unreinforced masonry and RC structures experienced substantial damage [16,17,18,19,20,21,22,23,24]. In Croatia, although RC buildings were not as extensively damaged, there were still instances of structural compromise, reinforcing the need for ongoing improvements in seismic design and retrofitting practices [14,25,26]. In addition, URM structures are commonly recognized as heritage buildings [27,28], whereas reinforced concrete (RC) structures often are not, despite their historical significance. Protecting cultural and historical heritage is crucial, as these structures represent numerous values of societies, connecting us to our past and informing our future. Due to their age, construction techniques, and materials, many historical buildings may lack the resilience needed to withstand seismic forces, making it essential to assess and reinforce them to protect their integrity and historical significance, whether they are built using URM or concrete. Recent advancements in the strengthening methods have focused on approaches that maintain the original geometry and design of these structures. For example, innovative techniques that provide effective strengthening without altering the geometry have been developed [29,30]. In today’s era of concrete, these structures are becoming increasingly significant, and their vulnerability under earthquake effects needs more attention. They should be researched and presented to a wide audience as important masterpieces of modern heritage, deserving of the same level of preservation and care as older, more traditional heritage buildings.

Although there is substantial research on the preservation and strengthening of heritage RC structures, further study and focus are necessary to ensure their protection and longevity [20,31,32,33,34,35]. Known techniques for repairing and retrofitting these structures include the use of fiber-reinforced polymers (FRPs) and fabric-reinforced cementitious matrices (FRCMs) [36,37], steel jacketing [37], and the application of shotcrete or concrete jacketing [38,39,40]. Additionally, base isolation can be employed to enhance the seismic performance and overall structural integrity of heritage RC buildings.

In Croatia, awareness of the need to construct earthquake-resistant buildings has existed for many years. However, the systematic design and construction of earthquake-resistant buildings began to gain importance and implementation with greater attention during the 20th century, especially after various seismic events in the region which highlighted the vulnerability of some buildings to seismic loads [41].

At the beginning of the 20th century, the construction of earthquake-resistant buildings was not regulated by standards. However, engineers learned from their mistakes, observed the behavior of buildings during and after earthquakes (such as cracks, tilting, wall out-of-plane failures, collapses, etc.), improved their profession, and established “rules of the trade”. Despite some initiatives aiming to define the vulnerability of buildings in Croatia, most of the research was conducted from a purely scientific perspective, and policymakers largely ignored the results.

The first attempts to introduce criteria for construction in seismic zones in Croatia were made at the end of the 19th century. Andrija Mohorovičić (1857–1936), a renowned seismologist, meteorologist, astronomer, and globally acclaimed scientist, published the work “The Effect of Earthquakes on Buildings” in 1909, following a strong earthquake that struck Zagreb in 1880. This was a visionary work emphasizing the necessity of adhering to “special regulations for the construction of buildings in earthquake-prone areas”, giving specific principles to be adopted. Mohorovičić was born near Rijeka, the location of the case study building examined in this research. However, Mohorovičić’s work in the scope of building science and practice did not receive significant attention, and the first official regulations on construction in earthquake-prone areas were only introduced in Croatia in 1964 after the Skopje earthquake in 1963. Destructive earthquakes in Banja Luka (1969) and in Montenegro (1979) resulted in significant material damage. After the earthquake in 1979, the “Regulation on technical standards for the construction of buildings in seismic areas” (Official Gazette 31/1981) was enacted. After Croatia gained independence, as part of the process of it joining the European Union and harmonizing its national regulations with European standards, the introduction of Eurocode 8 began. The implementation of Eurocode in Croatia started with the pre-standards of the HRN ENV 1998 series (in 2005), and the full adoption of Eurocode 8 (through the HRN EN 1998 series of standards) came into effect in 2008. The whole process of the regulations is shown in Figure 1.

The graph shows that in the time of the construction of the case study building, there were no regulated requirements for earthquake resistance. The blue line represents the seismic resistance required by the regulations for new buildings, while the blue dashed line represents the seismic resistance for renovated buildings. The levels of reconstruction (Level 1 to 4) are defined by the Croatian regulation Amendment to the Technical Regulation for Building Structures (Official Gazette 75/2020). Level 1 represents 25% of Eurocode 8’s demand, Level 2 50%, and Level 3 75%, while Level 4 represents the current Eurocode 8 demands.

Further analysis indicates that the regulations increasingly demanded earthquake resistance in buildings as awareness of the consequences of the damage caused by strong earthquakes grew. The Law on Reconstruction of Earthquake-Damaged Buildings in the City of Zagreb, Krapina-Zagorje County, and Zagreb County [42] was enacted six months after the Zagreb earthquake but solely in the areas affected by the first earthquake in 2020. After the Petrinja earthquake, consequently, in February 2021, a revised Law on Reconstruction of Earthquake-Damaged Buildings, encompassing the City of Zagreb, Krapina-Zagorje County, Zagreb County, Sisak-Moslavina County, and Karlovac County, was issued [43]. This law, along with the Amendment to the Technical Regulation for Building Structures [44], outlines four distinct levels of reconstruction based on achieved mechanical resistance and stability. In the renovation process, each building must meet the earthquake resistance standards as specified by HRN EN1998 [45]. The reconstruction levels defined by the Technical Regulation for Building Structures are determined by the degree of damage, the building’s importance and purpose, and the financial resources of the investor. It is worth noting that these rapid post-earthquake assessments and subsequent legislative developments and renovation procedures were heavily influenced by Italian experiences.

This paper presents a case study from a region in Croatia that was not affected by recent earthquakes. It highlights the issues arising from the fact that the new laws are not applicable to this case study, underscoring the challenges and limitations faced in the context of seismic strengthening and renovation for areas that have not experienced direct earthquake damage.

In this paper, a brief overview of the construction practices and the historical context of the Rijeka region, Croatia, is provided. A detailed explanation of the case study begins with the historical background and the significance of the building from a historical perspective. This includes a description of its architectural and structural properties, supported by archival documentation and drawings.

Next, the assessment phase is elaborated on, involving preliminary modeling and 3D representations of the building, as well as on-site material testing and investigations. Following this, various strengthening ideas are explored, and the final solution is presented in detail. Each strengthening method is thoroughly explained and illustrated with 3D drawings.

In the final section, conclusions are drawn, and the need for a comprehensive and holistic design approach for existing heritage buildings is emphasized. The importance of integrating historical context, architectural integrity, and modern engineering practices to ensure the longevity and resilience of these structures is highlighted.

This paper is important because it addresses a gap in the understanding and application of seismic strengthening laws in regions that have not recently experienced earthquakes. This study from Rijeka, Croatia, serves as a significant case, highlighting how the current regulations and policies may not be fully applicable or effective in regions without recent seismic activity. This underscores the broader challenges faced in ensuring comprehensive safety measures for all buildings, especially heritage structures. Furthermore, this paper emphasizes the importance of a multidisciplinary approach to the preservation of heritage buildings, combining historical context, architectural integrity, and modern engineering practices. Most studies concentrate on areas recently impacted by seismic activity, where the urgency of renovation and strengthening is apparent. However, this paper brings attention to the need for proactive measures in regions without recent earthquake experience, emphasizing the importance of preparing for potential future events. By presenting a range of strengthening methods with detailed illustrations, this paper not only contributes to the academic discourse but also offers practical insights for engineers, architects, and policymakers. Moreover, this paper introduces the idea that modern concrete structures, often perceived as less vulnerable, are increasingly significant and at risk due to their age and material properties. By advocating for the recognition of these structures as modern heritage worthy of preservation, this paper expands the definition of heritage conservation beyond traditional masonry or timber preservation.

Highlighting this approach can serve as a model for similar regions worldwide that face similar regulatory and preservation challenges.

2. Historical Context on Architecture and Construction in Rijeka, Croatia

At the beginning of the 20th century, in Croatia, traditional materials such as brick, stone, and wood were most commonly used to construct building structures. These materials were widely available and had a long tradition in construction.

Clay brick was one of the most common materials for wall construction. It was readily available and affordable and enabled the construction of steady and durable buildings. Such buildings typically consisted of thick load-bearing walls, lightweight ceiling structures made of wooden beams, and/or stone and brick vaults above the basement and ground floor. The walls were connected by steel ties, contributing to the integrity and stability of the structure. The number of floors did not exceed five, and the load-bearing walls generally extended in only one direction [46].

RC structures in Croatia from the first half of the 20th century were built as RC skeleton structures, with masonry façade walls added to the RC frames within the plane of the façade [47]. Initially, the façade walls were constructed as RC infills, but shortcomings were quickly observed, primarily their poor thermal insulation properties. Consequently, the construction of exterior walls made of RC, especially thin RC panels, was abandoned. The traditional material, brick, returned as the best and safest choice for exterior walls [48].

Timber was used to construct ceiling and roof structures, as well as a secondary material in making interior elements such as floors, doors, and windows. In Rijeka, Croatia, during that time, steel was not used to the extent of traditional materials, but its usage became increasingly significant as technology and industry developed throughout the 20th century.

It is important to emphasize that structures were designed so that the systems for transferring vertical forces were clearly visible. The horizontal forces that were considered were mainly related to wind, and the mass of such “heavy” structures was sufficient to resist these forces. Although circumstances at the beginning of the 20th century in Croatia dictated construction in traditional materials, that period also marked the beginning of the era of constructing complete structures made of steel as well as RC—one such building is the subject of this study.

Figure 2 shows typical cross-sections of RC and masonry multistory buildings from Rijeka, Croatia, at the beginning of 20th century. Figure 3 shows a view of the case study building.

Local Conditions at the Beginning of the 20th Century

The location of the case study building is situated in the port of Rijeka, which has a rich historical heritage. The rapid development and spatial expansion of Rijeka occurred in the second half of the 19th century with the construction of a port-industrial complex, a railway junction, and new city districts. The development of shipping and an increase in maritime traffic imposed the need to build a new, large, and technically equipped port of Rijeka. The former port, which often needed to be cleaned of sludge and stones, became completely inadequate and unsuitable for modern needs. After the great flood of Rječina in 1852, the mouth of Rječina was moved in 1855 to a new riverbed a few hundred meters east. This created the beginning of today’s Delta, which, with the backfilling of the sea, grew larger [49].

The strongest development of Rijeka occurred after the Hungarian–Croatian Settlement, when, as corpus separatum was administered in 1868, it was annexed to Budapest and became the only exit port of the Hungarian part of the Monarchy. The first and main project of the Hungarians was the construction of the railway in two directions; Rijeka–Budapest via Karlovac and Pivka–Rijeka (connection with the rest of Europe). At the same time, Rijeka became the main point for the transit of raw materials.

After the construction of the railroad, along with road connections, the preconditions for the development and construction of the port of Rijeka, which was a priority of the Hungarian government, were set [49].

The construction of the port can be divided into three stages and is linked to three contractors.

• 1872–1879—The first phase of the breakwater; construction began from east to west. There was a section for the railroad and a small port for the naval academy.
• 1880–1888—The completion of the central part of the breakwater, following the pier, the coast, and Petrolejska port.
• 1889–1894—The western part of the port–the port for wood and the completion of the breakwater [50],

Figure 4 shows the coastline of the city of Rijeka before backfilling [51], and the red marker indicates the location of the case study building.

The overlap of the former coastline of the city of Rijeka with the current coastline formed by the backfilling described is shown [51] in Figure 5—the red marker indicates the location of the case study building.

From Figure 4 and Figure 5, it is evident that the building is constructed on the backfilled shore in the city of Rijeka, which greatly influenced the foundation concept—more about this is described in the existing description of the structure.

After completing the phase of backfilling the coastline, the ground was prepared for the construction of internal port facilities, including the port warehouses.

Various types of warehouses and their combinations were constructed. The most common were multistory warehouses, where the ground floor served as a receiving warehouse, while goods that stayed longer were stored on the upper floors. One such warehouse is warehouse XXII, which is part of a large complex of five warehouses in the far western part of the port (Figure 6). Construction of this complex took place from 1909 to 1914 [49]. Figure 7 and Figure 8 show archival photos of the warehouses.

The specificity of this complex is that this building is one of the first examples of using a combination of RC and brick masonry as a structural system in Croatia.

The building is well documented, with the names of contractors, designers, and architects. According to the archival documentation, the construction participants for the Metropolis complex and warehouse XXII were as follows: the construction took place from 1912 to 1913, with Rolbertz Wehler as the author of the design and Otto Prister as the designer of the detail design. The contractors involved were Ignác Berényi and Holstein és Sterner testvérek from Budapest. The Metropolis complex was constructed using very modern materials for that time. The entire load-bearing structure, including the roof, was built as a skeletal structure with reinforced concrete (RC) floor slabs, while the façade walls were made of masonry (Figure 9).

In the intriguing collection of archival documentation, special attention is drawn to the drawings of the RC structure, particularly the reinforcement drawings from 1912 (Figure 10, Figure 11 and Figure 12).

Warehouse XXII, originally built as part of the Metropolis complex, is currently classified as protected cultural heritage.

3. Case Study Building—Warehouse XXII

3.1. Structural Aspects of the Case Study

The structural system of warehouse XXII is divided into three sections. In seismic zones, it is necessary to design plan layouts in square shapes with approximately similar aspect ratios, rather than rectangular ones with unfavorable aspect ratios. If the architectural requirements dictate a rectangular shape for the building, the structural engineer should separate or partition the structure to achieve approximately square shapes. Architecturally, it remains a single building, but structurally, in our case, it consists of three separate structures. Each structural unit is calculated independently and is not influenced by or dependent on the other units. This approach significantly reduces seismic masses and consequently seismic forces, as well as structural displacements. The total dimensions in all three sections are 20 × 120 m. However, structurally, the building is divided into three units along axes 8 and 14. Each unit is calculated separately. The plan dimensions of the units are as follows: Unit 1: 41 × 20 m; Unit 2: 36 × 20 m; and Unit 3: 41 × 20 m.

Warehouse 22 is a three-aisled hall with main beams approximately every 6.0 m in the longitudinal direction, while secondary beams are located approximately every 2.0 m in the transversal direction (Figure 13).

Vertical loads are transmitted through a thin continuous floor slab, with a span of 2.0 m, to the secondary beams and consequently the main beams. The horizontal structure transfers its load to the columns, which have a variable cross-section—the cross-section reduces through each level of the hall. In the basement, the columns have dimensions of 1.0 × 1.0 m, while on the top floor, they are reduced to a cross-section of 27 × 27 cm.

In order to ensure the load-bearing capacity of the storage space, the secondary beams were constructed at 2.0 m intervals with an average height of 50 cm. The secondary beams are indirectly supported by the main beams or columns, depending on the position of the secondary beam. Both the secondary and main beams are thickened at their ends to ensure their load-bearing capacity against transverse forces at the location of the supports. The structural grid spacing is 6.0 × 6.0 m in all sections. Cross-sections of the case study building are shown in Figure 14 and Figure 15.

Along the perimeter of the entire building, a masonry façade wall was constructed, with a thickness of 78 cm in the basement which gradually reduces ascending from floor to floor. The façade wall of the top floor is 48 cm. Additionally, during the operation of the warehouse, masonry walls of elevator shafts and staircase cores were constructed. Subsequently, these added elements were designed and constructed so that their contribution to the horizontal resistance of the structure was negligible.

Due to Rijeka’s exceptionally important geo-positioning, the entire port area was subsequently backfilled, which led to significant foundation issues. To reduce the settlement of the structure as caused by these extremely poor soil conditions, a concrete slab approximately 1.5 m thick was constructed beneath the entire building (Figure 16).

In addition to reducing settlement, the concrete slab also bears the vertical compressive load from the columns. The concrete slab is not reinforced; instead, a reinforcement mesh with dimensions of approximately 1.7 × 1.7 m was placed at the connection plane between the columns and the concrete slab at a depth of 25 cm below the top level of the slab. The vertical reinforcement of the columns is anchored into the concrete at a depth of only 25 cm.

For the purpose of understanding the existing building comprehensively, a BIM model (Figure 17) was created in Autodesk Revit, which also served as a working model for designing the new strengthening solution.

During the operational period, warehouse XXII was regularly adapted to the needs of users, indicating subsequent installation of the following:

• Staircase cores (between the two world wars);
• Masonry lift shafts (1976);
• Enlargement of large entrance openings (in the second half of the 20th century);
• Additional layers of cast asphalt flooring, 3 cm thick (time unknown);
• Replacement of the roof tiles with tile metal sheeting.

The aim was to achieve better utilization of the Metropolis warehouse complex, which served for the long-term storage of goods.

Warehouses XXII and XXI were constructed simultaneously as the second and third warehouses in the complex. Shortly after their completion in 1914, there arose the need to connect them. Consequently, two passages were added on the ground floor which enabled communication between the northern and southern sides of the warehouses.

3.2. Preliminary Analysis of the Existing Structure

The structure of warehouse XXII is divided into three sections, and each section is analyzed separately. Calculation of the overall mechanical resistance and stability of the existing load-bearing structure of each section was carried out using the Dlubal RFEM 5 software package [52].

The global model, or its structural stability, is defined by columns connected to single span beams (Figure 18). The floor slabs are modeled as in-plane rigid diaphragms, with a thickness of 13 cm, on all floors. The left-hand figure represents the members (beams and columns), while the surface elements are represented in the right-hand figure. The differences in colors represent different cross-sections of the members and different thicknesses of the surface elements.

Analysis of the embedded reinforcement according to the archival structural drawings concluded that the arrangement of the reinforcement does not ensure the full transfer of moment from the beam to the column (Figure 19). As a result, the transfer of horizontal seismic loads to the foundation soil is not achievable through the frame action of the beams–columns. The arrangement of the existing reinforcement (left figure) does not fully ensure the moment transfer from the beam to the column as required by the EN1998 standards (right figure). Consequently, the addition of shear walls is necessary.

More precisely, the requirement according to the EN 1998-1 standard to ensure that the column absorbs 30% more moment than the beam for the structure to demonstrate ductility is not met [53].

The façade across the perimeter of the building was constructed as a masonry wall. During the building’s lifetime, the façade wall served as a stiffening element for the longitudinal “frames”. Given the building’s considerable length compared to its width and its lack of shear walls, the structure is considered unstable in the transverse direction. Particularly unfavorable situations occur in the areas around the joints because each section is considered a single structural unit, and there are no stiffening elements (shear walls) along those axes.

The existing structure was analyzed using a linear analytic model to provide illustrative representations of the lack of stiffening elements in all sections in the transverse direction (Figure 20).

In the 3D global model, the surfaces of the masonry walls were modeled in accordance with data from structural investigation works. Connections between the masonry walls and the RC elements were defined as hinged. To avoid instability in the global model and to prevent the occurrence of mechanisms, the connections between the columns and beams were modeled with minimal stiffness.

According to the rules for simple structures, as outlined in the Eurocodes, the percentage of walls is an important factor in ensuring their safety and adequate seismic performance. Likewise, at the connection points with the foundation, masonry walls cannot resist tensile reactions, as the possibility of tensile forces in the masonry wall stiffness matrix is excluded.

Table 1. Percentage of the walls in the sections of the warehouse.

WAREHOUSE XXII	SECTION 1		SECTION 2		SECTION 3
	Walls in X Direction	Walls in Y Direction	Walls in X Direction	Walls in Y Direction	Walls in X Direction	Walls in Y Direction
Existing warehouse	5.72%	1.32%	2.86%	0.00%	5.64%	1.04%
Strengthen warehouse	6.85%	2.60%	3.08%	1.22%	6.58%	2.10%

clearly shows that the percentage of walls in the y direction is significantly low. This lack of walls in the y direction can negatively impact the overall structural stability and performance, emphasizing a critical area of concern for seismic strengthening and retrofitting efforts.

Nonlinear analyses were not conducted due to the demands of the investors. Instead, a modal analysis was performed, and the results of this analysis are presented in Figure 21.

Section 2 is the central section that lacks shear walls or any transverse stiffening elements (Figure 22). Section 3 represents the edge section, which features shear walls but only on one side (Figure 23).

The foundation of the structure of warehouse XXII is ensured by minimal foundation pads with dimensions of 170 × 170 cm and a thickness of 25 cm. It is important to emphasize that such foundation pads were constructed on a concrete base with a thickness of 150 cm beneath the entire structure of warehouse XXII. The vertical reinforcement of the columns is anchored into the concrete at a depth of only 25 cm, which cannot be considered sufficient for anchoring the reinforcement, nor for potentially absorbing tensile forces or moments due to horizontal forces (Figure 24).

The geometry, particularly the thickness of the concrete slab beneath the entire warehouse structure, ensured the transfer of compressive forces from the columns to the soil. Since the horizontal effects of seismic actions were not considered during the design of the structure, there was no need for a larger RC cross-section of the foundation or for deeper anchoring of the reinforcement into the foundation.

The unreinforced concrete slab had a dual purpose (Figure 25). Firstly, it acted as pre-consolidation for the backfilling to speed up settlements before the construction of the structure started. Secondly, it played a role in accepting the vertical force from the columns and transferring it to the lower layers of the soil. The substantial height of the concrete slab reduced the contact stresses beneath the foundation.

3.3. Investigation of Material Properties

Testing of the construction materials for a building located on the coast, exposed to an aggressive environment, particularly chlorides from the sea, has become imperative given its age of approximately 120 years. During this period, the material has undergone various stages of property changes, potentially leading to degradation of its characteristics. Considering the challenges posed by such a location to the building, thorough material testing was conducted. The aim of the testing was to assess the current structural condition, identify potential issues, and determine restoration measures to ensure the building’s longevity and safety in accordance with modern regulations.

All essential structural elements important for mechanical resistance and stability design were covered by the quality investigation works. The investigation works were conducted to assess the general condition of the materials used and to verify the consistency of the data from the archival documentation with the existing state.

To classify the materials in the structure and obtain input data for a high-quality computational model, the testing included the following:

• Compressive strength of the existing concrete;
• Protective layer of the reinforcement;
• Arrangement of the embedded reinforcement in the concrete elements;
• Corrosion of the reinforcement and quality of the reinforcement steel;
• Compressive and shear classification of the masonry;
• Compliance with the archival documentation.

A testing program for the structure was created to conduct a comprehensive and sufficient examination of the construction. The testing was carried out according to Eurocode EN 1998-3 [45], a standard that defines the number of samples per element, floor, and detailing. An extensive survey of the mechanical properties of the materials is comprehensively presented in Figure 26 and

Table 2. Results of determining the degree of reinforcement corrosion in columns.

Floor	Structural Element	Corrosion Grade of Reinforcement Cu/CuSO4 > −200 Low −200 < Cu/CuSO4 < −350 Medium
100—basement	column	low	to	medium
200—ground floor	column	low
300—first floor—connecting building	column	low
400—second floor	column	low
500—third floor—connecting building	column	low
600—fourth floor	column	medium

. Figure 26 provides a detailed illustration of the specific locations where both non-destructive and semi-destructive testing methods were applied. In total, 56 tests were performed.

For the basement level, the compressive strength is consistently high at 37.2 MPa across all dilatations. On the ground floor, the compressive strength is around 19 MPa for all dilatations and 12.8 MPa for the connection building section, aligning with concrete grade C16/20. No samples were taken on the first, third, and fourth floors, so the values from the second floor were applied to these levels. Overall, the results indicate consistent compressive strength values where the samples were taken, with a noted reliance on the second-floor values for the other floors due to the absence of direct sampling. Based on the results of the concrete compressive strength testing according to HRN EN 12504-1 [54], an assessment of the concrete’s compressive strength has also been provided in accordance with the HRN EN 13791 [55] standard. Figure 27 shows an overview of the compressive strength results for the RC elements across different building sections.

In addition to the compressive strength of the concrete, the classification of the reinforcement steel, the diameter and spacing of the reinforcement bars according to the elements, and the protective layer were essential prerequisites for the final verification of the reinforced concrete element.

The main reinforcement bars have the following diameters:

• 20 to 36 mm, with spacing between 37 and 48 mm (basement);
• 16 to 35 mm in diameter with spacing between 19 and 31 mm (ground floor);
• 23 to 28 mm, with 22 to 44 mm spacing (first floor);
• 20 to 32 mm, spaced 22 to 33 mm apart (second floor);
• 20 to 32 mm with 19 to 50 mm spacing (third floor);
• 10 to 16 mm with 18 to 25 mm spacing (fourth floor).

The transverse bars on all floors have diameters ranging from 8 mm to 16 mm, with the spacing varying between 13 mm and 100 mm, which clearly shows potential weaknesses in the building’s load-bearing capacity. Figure 28 shows weak parts of the structure when it is subjected to seismic loads. The minimum protective layer was 24 mm.

In old RC structures exposed to an aggressive environment, the impact of reinforcement corrosion can significantly reduce the cross-sectional area of the reinforcement and consequently the load-bearing capacity of the RC element. The following table shows the degree of column corrosion by floor level. See Supplementary Materials for specific data sources.

All the elements were tested and sampled in the same manner. In addition to the columns, beams, slabs, foundations, and the floor slab were also examined. If we retroactively compare this to the concrete grades in the 20th century, the first classifications were made by the construction administration, which established the regulations. These regulations specified the following concrete grades: M70, M110, M160, M220, and M300. The grade, or number, represented the compressive strength (up to failure) of test cubes measuring 20 × 20 × 20 cm. Besides concrete, cement was also crucial. According to the former JUS B.C1.010 and B.C1.011 norms, factories produced three main types of cement from Portland cement clinker: Portland cement (PC), metallurgical cement (M), and pozzolanic cement (P). Each type was produced in three quality grades (based on strength), labeled as classes 250, 350, and 450. These numbers indicated the compressive strength up to failure of test prisms in kp/cm² (note: 1 kp = 1 kilopond; 1 kp, where 1 N = 0.10197 kp, was one of the basic units in the old technical unit system). Specifically, for the compressive strength of the columns, the testing showed that the compressive strength of the basement columns was f_ck = 37.2 MPa, which can be classified as concrete grade M300 (29.41 MPa). The columns on the higher floors had a significantly lower compressive strength, which can be classified as concrete grade M220. Regarding the masonry, shear tests were carried out (Figure 29), and the compressive strength of the masonry units was tested. The shear test, although it is not completely comprehensive [56], is a common procedure for testing masonry in Croatia [57,58]. In total, 16 bricks were tested. The analysis revealed varying degrees of shear strength across different positions in the building. The basement walls exhibited a high compressive strength, while the upper floors showed a noticeable decline. The shear strength (f_vk) ranged from 0.15 N/mm² to 0.80 N/mm², with estimated loads (σ_v) consistently around 0.11 N/mm². The shear strength without load (f_vk0) ranged from 0.04 N/mm² to 0.68 N/mm². These results indicate that while the basement is robust, the upper floors require reinforcement to meet the structural requirements. Overall, the building’s structural integrity decreases with height, necessitating targeted strengthening measures.

The analysis of the masonry units revealed varying compressive strengths across different positions in the building. The compressive strength ranged from 11.4 N/mm² to 35.1 N/mm² in the basement walls, 18.2 N/mm² to 28.6 N/mm² on the ground floor, 7.7 N/mm² to 25 N/mm² on the first floor, and 9.9 N/mm² to 22.7 N/mm² on the third floor. This significant variation indicates potential weak points in the structure.

4. Design of Seismic Strengthening Methods

4.1. Analyses of Strengthening Methods

Considering the requirements of the client and the historical heritage of the building in question, it was necessary to propose different options for seismic strengthening of the structure.

In the redesign of the building, the warehouse areas were converted into office spaces. This conversion defined the requirements for safe evacuation measures, with a particular focus on the stairwell cores, which also played a role in the seismic stabilization. However, the stairwell cores alone were not sufficient to stabilize the structure of each section individually. Due to the lack of stiffening elements in both directions, additional strengthening was necessary to stabilize the structure.

To find the optimal solution, several strengthening methods were considered. Given that the building primarily consists of RC with masonry façade walls, the first solution involved a strictly RC structure with RC cores, additional reinforced concrete walls, and the reinforcement of the masonry façade walls with shotcrete (Figure 30). The second strengthening method consisted of steel frames with vertical stabilization between the RC columns (Figure 31).

The building’s significant mass, coming from the RC and façade walls, accumulates and generates a considerable force which is amplified by seismic acceleration. Additionally, implementing steel frames between the columns would be nearly impossible due to the high beams and the thickening at the axis lines. Furthermore, frames along the longitudinal line of the façade were also necessary, which would result in the closure of openings across the entire height of the building.

The preliminary estimate of the quantity of steel required for the steel frame strengthening option was 84 kg/m².

Therefore, the conclusion of the analysis of the two methods mentioned was unanimous regarding all the selection parameters:

• Cultural heritage—the first complete RC structure of a warehouse in Croatia.
• Simplicity of execution.
• Functionality—the steel structure closes the openings on the façade.
• Feasibility of implementation and connection of existing and new structures.
• Financial feasibility—due to the cost of steel and fireproofing requirements.

The following subsection describes in detail the seismic strengthening method selected, which preserved the original structure while bringing it up to current regulatory standards.

4.2. Seismic Strengthening

In addition to strengthening the building against horizontal actions according to HRN EN 1998 [55], the structure was also verified for vertical loads in accordance with the latest regulations and standards. This primarily refers to the local verification of floor slabs and secondary and main beams, as well as columns. The detailed drawings from the archival documentation and the precise position plans of the structure (both layout and elevation) were of great assistance.

Global mechanical resistance and stability design of the load-bearing structure, including seismic modal analysis, was carried out with the Dlubal RFEM 5 software package [54]. Local calculations of the horizontal structure under vertical loads were carried out with the FRILO Software GmbH 2023-1 software package [59] according to the European standards for concrete structures.

The following, Figure 32, shows a workflow diagram related to the design procedure for the entire structure.

The calculations for all of the horizontal elements/structures carried out for the new building programming meet the requirements given by the current regulations; therefore, further information given refers exclusively to demonstrating the seismic strengthening.

The seismic reinforcement of warehouse XXII was designed as the implementation of a new RC structure and strengthening of the masonry walls (Figure 33).

The positioning of the new RC structure is primarily defined by the new staircase cores and elevator shafts. The need for new cores and their positioning is defined by architectural requirements regarding space organization and rules for safe evacuation in new building programming. The quantity of new walls was insufficient to withstand horizontal forces in the transverse direction; therefore, the structure was strengthened with additional walls in that direction. The positioning of the shear walls was designed to prevent torsional effects in each section (Figure 34).

In the longitudinal direction, the horizontal force is transferred by longitudinal masonry façade walls, which are reinforced with shotcrete. The aim was to prevent tensile forces in the masonry wall.

4.2.1. Analyses and Design of the New Proposed Structure

The new RC structure and strengthening of the walls of warehouse XXII were designed to withstand seismic actions with a return period of 475 years. According to the technical regulations in Croatia for building structures, it is possible to reduce the effects of seismic action for buildings that are older than 8 October 1964, which is taken into account with an importance factor (IZO) value of 0.75. Below are listed all the parameters related to seismic actions on the structure of warehouse XXII:

• System: Dual system
• Ductility class: DCM
• Nominal ground acceleration: a_gR = 0.202 g (475 years)
• IZO Factor 0.75 (according to TPGK 7/22) a_gR = 0.15 g
• Ground type: C—according to the geomechanical report
• Importance factor: γ_I = 1.0
• Damping: η = 5%

The connection between the existing and new structures allows the new system to absorb horizontal forces and transfer them to the new foundation. This concept of strengthening is illustrated in Figure 35. A minimum shotcrete thickness of 8 cm was specified for all floors. The shotcrete used is concrete of class C25/30, with the reinforcement mesh varying between Q503, Q335, or Q257 depending on the specific position. In the graphical representation in Figure 35, the green walls indicate the new RC structure, while the brown walls represent the masonry walls strengthened with shotcrete. The characteristics of the existing brick material were analyzed, and an analysis of the shotcrete material was performed. Subsequently, the equivalent material properties were calculated based on the thickness of the brick wall and the thickness of the shotcrete, taking into account their elastic moduli and shear moduli. The masonry wall with shotcrete was modeled as a shell element with equivalent elastic moduli and equivalent shear moduli. Figure 36 shows a graphical interpretation of the mechanical properties of the materials used for numerical modeling, which are subsequently listed in

Table 3. Mechanical properties of materials used for numerical modeling.

CONCRETE ELEMENTS
Elements:	Beams and Slabs		Columns		Foundation Slab		New Structure
Compressive strength	C20/25		C20/25		C30/37		C30/37
Elastic modulus E (MPa)	30,000		30,000		33,000		33,000
Shear modulus G (MPa)	12,500		12,500		13,750		13,750
Poisson’s ratio	0.2		0.2		0.2		0.2
Type of modeled element	Isotropic linear elastic surface		Isotropic linear elastic surface		Isotropic linear elastic surface		Isotropic linear elastic surface
MASONRY WALLS
Elements:		Existing masonry wall		Shotcrete part of wall		Strengthened masonry wall
Compressive strength of brick fb (MPa)		12		--		12
Compressive strength mortar fm (MPa)		9		--		9
Compressive strength masonry wall fk (MPa) or shotcrete fck (MPa)		6.05		25		--
Masonry wall weight (kN/m3)		18		25		18.7
Elastic modulus E (MPa)		6054		31,000		13,690
Shear modulus G (MPa)		2421		12,917		5476
Poisson’s ratio		0.25		0.20		0.25
Type of modeled element		Material model: Orthotropic Elastic 2D Surface stiffness: Orthotropic				Material model: Orthotropic Elastic 2D Surface stiffness: Orthotropic

.

In Section 1, new RC elements were designed, including a staircase core, an elevator shaft, and two shear walls. Additionally, the masonry façade walls were strengthened. In Section 2, shear walls were designed in the transverse direction, while in the longitudinal direction, masonry façade walls were strengthened with shotcrete. In Section 3, the construction of an elevator shaft and a ramp for parking on the first floor was required due to the architectural design. Additionally, new shear walls were designed for the seismic stabilization of the building. As the building’s purpose has changed from warehouse areas to office areas, the intensity of the vertical loads acting on the structure was reduced. Furthermore, the extremely robust and dense horizontal structure effectively transfers new live loads to the vertical elements.

A spectral modal analysis was conducted, revealing that the structure of Section 1, in its first translational mode, has a period of 5.471 s, which is extremely high. This is reflected in the displacements of the structure, indicating a significant lack of stiffness and ductility to withstand seismic actions (

Table 4. Section 1: Analytical comparison of the displacements.

		NEW STRUCTURE				EXISTING STRUCTURE
Storey	Storey Height	Relative Displacement	Limitation of Interstorey Drift dr,x/n ≤ 0.005 h			Relative Displacement	Limitation of Interstorey Drift dr,x/n ≤ 0.005 h
	hetaže	dx,e				dx,e
	[mm]	[mm]	[mm]	[mm]	[%]	[mm]	[mm]	[mm]	[%]
FLOOR 600	3150	4.7	2.8	15.8	17.8%	196.0	19.9	15.8	126.3%
FLOOR 500	3150	1.9	0.4	15.8	2.5%	176.1	19.3	15.8	122.5%
FLOOR 400	3150	1.5	0.4	15.8	2.5%	156.8	24.1	15.8	153.0%
FLOOR 300	3150	1.1	0.3	15.8	1.9%	132.7	31.0	15.8	196.8%
FLOOR 200	4200	0.8	0.2	21.0	1.0%	101.7	57.3	21.0	272.9%
FLOOR 100	2800	0.6	0.6	14.0	4.3%	44.4	44.4	14.0	317.1%
FOUNDATION LEVEL	0	0.0	0.0	0.0	0.0	0.0	0.0	0.0	0.0

). After the strengthening of Section 1, the period of the first mode is reduced to 0.86 s, resulting in a rigid and stable load-bearing structure.

The existing structure of Section 2 in the first translational mode has a period of 5.172 s. This is also evident in the displacements of the structure, which evidently lacks the stiffness and ductility to withstand seismic actions. After the strengthening of Section 2, the period of the first mode is 2.40 s in the longitudinal direction, while in the second mode, the transverse direction is activated in 0.79 s due to the new RC structure. A comparison of the results can be seen in Figure 37.

The existing structure of Section 3 in the first translational mode has a period of 3.366 s. After the strengthening of Section 3, the period of the first mode is 0.74 s.

Analysis of the displacements according to HRN EN 1998-1 [53] with a 95-year return period shows the displacements of the new structure in the transverse direction:

• Section 1: d = 3.1 mm.
• Section 2: d = 47.0 mm.
• Section 3: d = 24.4 mm.

4.2.2. Construction of the RC Structure

The primary function of the new RC structure is to transfer horizontal seismic forces to the foundations and to the ground. All the vertical load-bearing elements have to be connected with rigid diaphragms at the level of the floor slabs. Since all new walls are constructed along the axes, beneath the existing beams, the width of the wall is adjusted to the width of the beam. Therefore, the wall is 3 cm wider on each side to ensure the continuity of the longitudinal reinforcement across the floors. First, the concrete cover of the beam is removed, followed by the installation of longitudinal reinforcement, and finally, the entire structure is filled with concrete. Holes are drilled into the 13 cm thick floor slab to insure the placement of the reinforcement for the continuation of the upper wall. The primary function of the new RC structure is to isolate the existing columns from the effects of seismic forces and to transfer all the seismic forces to the foundations. A graphical representation of the new RC structure is shown in Figure 38.

4.2.3. Construction of New Foundations

The new RC structure requires new foundations, whether they are foundation strips or foundation slabs. The transfer of the vertical load to the foundation soil is not problematic because the original building was designed for warehouse use, with each floor slab having a load-bearing capacity of 15 kN/m². In contrast, the new building’s usage for office and parking space requires live loads that are three times lower.

Foundation-related challenges arise from seismic actions that result in tensile forces at the ends of walls. To meet the mechanical resistance and stability requirements, specifically to prevent the uplift of the foundation structure, all the foundations for the elevator shafts, staircase cores, and individual shear walls are connected to the existing 150 cm thick concrete slab which was constructed beneath the entirety of warehouse XXII. The holes for anchoring reinforcement are filled with an epoxy-based adhesive to achieve chemical anchoring into the existing concrete. This connection increases the weight of the foundation structure sufficiently to withstand the tensile forces occurring in the new RC elements (Figure 39).

4.2.4. Strengthening of Masonry Façade Walls

Seismic strengthening with shotcrete was applied exclusively to the inner side of the walls. Applying shotcrete to the outer side of the wall was not an option due to the requirements of preservation of the original form of the façade. To ensure the proper connection and joint action of the existing masonry walls and shotcrete, square holes 25 × 25 cm and 20 cm deep were drilled into the masonry walls, and reinforcement was installed for connection. After installing the reinforcement of the holes, the holes in the masonry wall were filled with concrete, followed by shotcreting. This process connected the old and new elements into a single new cross-section (Figure 40 and Figure 41).

After presenting all the aspects of the case study, including its historical background, architectural and structural properties, and the proposed strengthening methods, the following photos (Figure 42 and Figure 43) illustrate the case study building at the beginning stages of its renovation.

5. Conclusions

Croatia ranks among the most seismically at-risk countries in Europe. Recent earthquakes in Zagreb and Petrinja highlighted numerous vulnerabilities in Croatian construction, including aging buildings, poor materials, various design and construction flaws, and a slow inconsistent legal framework for reconstruction. Many buildings constructed before the implementation of regulations requiring seismic-resistant designs are still in use today. Unlike their ability to bear vertical loads, these structures often fail to adequately withstand horizontal forces resulting from natural phenomena. This study underscores the importance of seismic strengthening for such existing structures, particularly those with heritage protection. The preservation of cultural heritage and the specific requirements of users heavily influence the choice of strengthening methods. Other critical factors include the feasibility and quality of solutions and their economic viability.

To ensure that the chosen method of strengthening is effectively implemented and complies with the current regulations, it is essential to thoroughly evaluate, analyze, and identify the “weak points” in the existing structure. Through investigative work, reliable input parameters must be secured for the calculation of mechanical resistance and stability, enabling a comparison with the current state. In such cases, archival documentation of the buildings being reconstructed is of great importance and can significantly aid in developing solutions that bring the structure up to the standards required by the current regulations.

This paper presents an extensive case study on the seismic strengthening of a 20th-century heritage-protected warehouse in Rijeka, Croatia. Originally built according to Austro-Hungarian norms, the warehouse required modern seismic upgrades while preserving its historical integrity.

The seismic upgrading of warehouse XXII in Rijeka, Croatia, serves as a good case of balancing modern engineering practices with the preservation of historical integrity. By conducting a comprehensive assessment, including material testing and 3D modeling, this study demonstrates the practical application of modern seismic strengthening techniques. This process not only enhances the building’s structural safety but also ensures the preservation of its architectural and historical value.

The detailed analysis and the resulting strengthening measures highlight the need for a holistic approach when dealing with heritage buildings. It is possible to meet the current regulatory standards for earthquake resistance while maintaining the cultural and historical essence of such structures. This case study serves as a reference for engineers, architects, and conservationists, showcasing the importance of integrating historical context, architectural integrity, and modern engineering practices.