PSI - Issue 64

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

916

9

In Fig. 11 (a), the ribbed slab and reinforced parts of the column-beam joint are visible, while in Fig. 11 (b) and (c), the steel grid in the construction phase can be seen. Connection of the existing reinforced concrete frame structure and the steel truss is achieved through steel dowels inserted into pre-drilled holes and then filled with adequate repair mortar. A critical detail that was modeled and calculated is the connection of steel diagonals to the vertical column of the steel truss shown on fig. 12 (d).

Fig. 12. (a) Photo on connection of steel truss to beam, (b) Photo of connection steel column to RC column, (c) photo of steel diagonal from inside, (d) numerical 3D model of critical connection 4. Conclusion The urgent need for retrofitting numerous damaged buildings in Zagreb and the limited timeframe given for its execution, in the case of the building shown here, called for inventive and practical solutions that could be easily implemented. By installing an external steel structure and minimal reinforced concrete shear walls, the structure, which possessed only 19% of the needed seismic resistance, was brought up full compliance of today's requirements. Polymer reinforcements of the joints enabled the activation of the frame behavior of the existing structure, which further contributed to avoiding additional structural interventions that would otherwise be necessary. This strengthening method was chosen because it did not affect the internal layout of the existing floor plan, with major interventions focused on the exterior of the building thus allowing for maximum flexibility of the space. Seismic analysis conducted on a numerical model with included strengthening elements ensured that reinforcement meeting all current standards and regulations could be provided in a relatively short time frame without demolishing the existing structure. Presented method is not common for strengthening the structures in Zagreb area after earthquake but has proven to be efficient and simple to implement. Frame structures like this one can be strengthened in this way to ensure sufficient stability and capacity increase while reducing excessive displacements. The only additional intervention to consider when reinforcing existing reinforced concrete frames is to verify if they are adequately reinforced at the beam-column joint. Since it is often the case that the installed shear reinforcement is insufficient, reinforcement by wrapping with CFRP fabric should be planned. In addition to the presented reinforcement of the existing structure, it is advisable to inspect the foundation structure and reinforce it in case of increased stresses due to seismic action or the addition of new elements. Conducting investigative works is highly recommended to facilitate subsequent analyses and comparisons. The execution the presented works proved to be relatively straightforward, and the completion is planned by the autumn 2024.