Seismic upgrade of a historic warehouse in Rijeka

How can historic buildings be protected without jeopardizing their architectural identity? When the Warehouse XXII was built, seismic safety was hardly an issue compared to today, in times of increasing seismic activity. In our case study “Seismic Upgrading of the Heritage-Protected Reinforced Concrete Warehouse in Rijeka, Croatia”, we took an in-depth look at this topic based on this renovation task - together with Mislav Stepinac from the University of Zagreb.

The challenge: lack of standards that can be applied across the board
Numerous earthquakes have caused severe damage in the Mediterranean region in recent years. The year 2020 was particularly devastating for Croatia, with two moderate earthquakes in the capital Zagreb and the surrounding areas. Although Rijeka was not affected, the regulations for seismic reinforcement must also be consistently applied there. This poses a particular challenge for historic buildings, as there are often no generally applicable standards. Our case study highlights this problem and shows possible solutions.

Example building: Reinforced concrete warehouse in Rijeka
Warehouse XXII is a reinforced concrete industrial building from the early 20th century. It was built according to Austro-Hungarian construction standards - long before there were binding standards for earthquake-proof construction. It was one of the first buildings in Croatia to combine reinforced concrete with brickwork as a load-bearing system.Our task was to adapt this historic warehouse to today's safety requirements without changing its characteristic design.

Existing structure
Warehouse XXII is a three-bay hall with longitudinal main girders at 6 m intervals and transverse secondary girders at 2 m intervals.The vertical loads are transferred to the beams via a thin, continuous floor slab with a span of 2 m. The horizontal loads are transferred via the columns.The horizontal loads are transferred via the columns with a variable cross-section, which decreases as the height of the hall increases.Both the secondary beams and the main beams are reinforced at their ends to ensure their load-bearing capacity against shear loads at the support points. There is a masonry façade along the entire perimeter of the building. Its thickness is 78 cm in the basement and gradually decreases as the height of the building increases. The building also contains masonry walls for elevator shafts and stairwell cores. Due to the exceptional location at the port of Rijeka, the foundations were a special feature: The entire harbor area was subsequently filled in. As a result, the building had to be erected on a 1.5 m thick unreinforced concrete slab. This was originally intended to reduce settlement, but was not designed for horizontal loads or seismic forces.

Methodology and key findings
The first step was a detailed analysis of the existing building using various methods:

• Historical research: archive material and building plans helped us to understand the original construction and choice of materials.

• 3D modeling and analysis: We created a detailed BIM model to analyze the load-bearing behavior of the structure.Seismic resilience simulation models revealed structural weaknesses, particularly in the horizontal bracing and foundations.

• Material investigations: On-site investigations of the concrete and masonry quality revealed severe material ageing and corrosion damage to the reinforcement.

Seismic reinforcement methods
Taking into account the client's requirements and the historical heritage of the building, we developed various options for the seismic strengthening of the structure. As part of the architectural redesign of the building, the storage areas were to be converted into office space. This change of use defined new requirements for the evacuation measures, particularly in the stairwell cores, which also contributed to seismic stabilization.
However, the staircase cores alone were not sufficient to stabilize the structure of each section individually. Due to fewer stiffening elements in both directions, additional reinforcement was required to ensure the stability of the structure.

We considered several reinforcement methods to determine the optimal solution. Since the building is predominantly reinforced concrete with masonry facades, we proposed the following two primary solutions:

• Variant 1: Reinforcement with reinforced concrete walls

• Variant 2: Reinforcement with steel frame

After a detailed analysis, the decision was made against the steel frame solution as it proved to be unsuitable for several reasons: The considerable mass of the building, due to the reinforced concrete and façade walls, leads to a considerable accumulation of forces, which is amplified by seismic acceleration. In addition, it would be almost impossible to install steel frames between the columns, as the main beams are very high and have thickenings at the axis lines.
Frames would also be required along the longitudinal axis of the façade, which would result in the openings being closed over the entire height of the building.

The choice therefore fell in favor of the reinforced concrete solution:

• New construction of load-bearing reinforced concrete walls: To stabilize the structure, we integrated new reinforced concrete cores for stairwells and elevators. In addition, we strategically placed shear walls to improve the seismic resistance of the structure.

• Reinforcement of the existing brick walls: The façade received an additional thin layer of shotcrete with reinforcement on the inside and was connected to masonry walls. This technique enables a significant improvement in load-bearing capacity without changing the external appearance of the building.

• Optimization of the foundations: The new reinforced concrete structure requires the construction of new foundations. To anchor the reinforcement, holes were drilled into the existing concrete and filled with an epoxy-based adhesive to ensure chemical anchoring. This connection increases the weight of the foundation structure sufficiently to absorb the tensile forces in the new reinforced concrete elements.

Conclusion: Engineering in monument protection
The seismic retrofitting of Warehouse XXII shows how engineering technology and monument protection can be successfully combined.Through comprehensive as-built analysis, material testing and 3D modeling, the study illustrates the practical application of modern seismic strengthening techniques. This process not only contributes to increasing the structural safety of the building, but also ensures the preservation of its architectural and historical value.The project illustrates the need for a holistic approach when dealing with listed buildings in earthquake-prone regions.

Case Study
In their case study “Seismic Upgrading of the Heritage-Protected Reinforced Concrete Warehouse in Rijeka, Croatia”, the authors Berislav Bošnjak and Nikola Pekas, structural engineers at ATP architekten ingenieure, Zagreb, and Mislav Stepinac from the University of Zagreb, Department of Civil Engineering, demonstrate the need for proactive measures in seismically active regions.The scientific article was published in the renowned journal Buildings in the special issue “Sustainable Preservation of Buildings and Infrastructure”.

To the study: https://www.mdpi.com/2075-5309/14/9/2912