PSI - Issue 64

Galić Josip et al. / Procedia Structural Integrity 64 (2024) 908 –917 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Upon checking and calculating the resistance of masonry walls and reinforced concrete frames, the results indicated that the structure lacks sufficient resistance to seismic action. Walls in the X direction have a calculated resistance for peak ground acceleration a g /g = 0.05, which is approximately 19% of the peak ground acceleration for a return period of T = 475 years (a g /g = 0.249). Walls in the Y direction have a calculated resistance for peak ground acceleration a g /g = 0.063, which is approximately 25% of the peak ground acceleration for a return period of T = 475 years (a g /g = 0.25). Therefore, the existing building complies with 19% a g /g of today's applicable regulations for designing earthquake-resistant structures. Existing masonry walls do not have sufficient shear resistance due to their very low shear strength caused by the degradation of the bonding material. In reinforced concrete frames, concerning horizontal action under seismic loading, the reinforcement of sections is insufficient. The required longitudinal reinforcement of columns according to the calculation is A req = 220 cm 2 , while the column only has 10 Φ 22, which amounts to 38 cm 2 . There is also insufficient shear reinforcement as the required shear reinforcement A sw,req = 49 cm 2 , but only Φ 6 / 30 cm (m = 2) is installed. The beams of the frames are also inadequately reinforced. Based on the analysis of the existing structural condition, it can be concluded that strengthening of the structure is necessary to meet current seismic regulations. 2.4. Damage evaluation In addition to the in-situ tests, a visual inspection of the building was conducted. Following the earthquake in Zagreb on March 22, 2020, a detailed visual inspection of the entire building was carried out. The subsequent earthquake on December 29, 2020, in the Sisak and Petrinja area further exacerbated the existing damage, leading to a re inspection. The most significant damage occurred at lintels and connections between the frames and infill masonry walls, where cracks appeared, as well as on the staircases and non-load-bearing parts of the structure. During the inspection, shear damage to the masonry infill, damage to the junction of the reinforced concrete frame structure and masonry construction, and minor cracks at the column-beam junctions were observed.

Fig. 7. Pictures of damage on infill masonry walls and connection with reinforced concrete frame

On Fig. 7, damages to the masonry infill are visible in the form of shear cracks. In addition to the damages on the masonry elements themselves, there are also damages observed at the junction between the masonry wall and the reinforced concrete frame. These damages confirm what was previously determined in the analysis, indicating that the building lacks sufficient stiffness and resistance of individual structural elements. The damages also confirm that the partition walls in the transverse direction, to some extent, played role due to stiffness of building and resistance. 3. Strengthening of the existing structure 3.1. Strengthening The request for strengthening the existing structure was that it must meet the requirement of seismic resistance for today's standards. The peak ground acceleration for which the building was calculated, and which satisfies the load-