PSI - Issue 78

Leandro Pancottini et al. / Procedia Structural Integrity 78 (2026) 1103–1110

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Shear checks on columns significantly increase the seismic fragility of buildings, even at low values of spectral acceleration (S a ), n both Bare Frame (BF) and Infilled Frame (IF) models. In Pushover (PO) and Multiple Stripe Analysis (MSA), the stairwell columns are most vulnerable to shear failure. Excluding these four columns from the checks substantially alters the fragility, especially in the BFsm model, with a lesser impact in other. 5. Conclusions This study investigates the impact of staircases on the seismic vulnerability of RC buildings designed for only gravity loads, a common practice in Italy during the 1970s (D.M. 1972). The analysis examined a 6-story prototype building, comparing six distinct models based on staircase configuration and the presence of infills. The structural elements were modeled in OpenSees using the "Beam-With-Hinges" element with concentrated non-linearities at the ends and fiber sections. Infill walls were represented using a phenomenological equivalent strut model, including openings. Both Pushover analyses (PO) and Nonlinear Time History analyses (NLTH) were conducted. The Pushover analyses highlighted a significant difference in stiffness and base shear between the two principal directions of the building. The presence of the staircase increased stiffness in the direction parallel to the knee beams, with negligible effects in the other direction. Post-processing checks showed that brittle failures (shear) in the columns always preceded ductile failures (cord rotation). The infills demonstrated significant stiffening of the models, increasing base shear and reducing lateral displacement. Nonlinear dynamic analyses, employed to derive fragility curves with the Interstory Drift Ratio (IDR) as the Engineering Demand Parameter (EDP), showed that the staircase and its position increase fragility for the irreversible damage state D5, particularly in Bare Frame (BF) models. IF models exhibited lower fragility due to the reduction in lateral displacement. The position of the staircase (central or corner) had a greater impact on BF models, given the crucial role of infill walls in absorbing lateral forces. The results of this study underscore the critical importance of including staircases in the vulnerability and fragility assessment of reinforced concrete buildings. The staircase significantly alters the modal parameters, lateral stiffness, and collapse mechanisms of the structure, which tend to become predominantly brittle due to early shear failures in the short columns of the staircase. Therefore, their accurate modeling is indispensable for a faithful representation of the building's actual seismic response. . References Li, B., & Mosalam, K. M., 2013. Seismic performance of reinforced-concrete stairways during the 2008 Wenchuan earthquake. Journal of performance of constructed facilities, 27(6), 721-730. Ricci, P., Manfredi, V., Noto, F., Terrenzi, M., Petrone, C., Celano, F., De Risi, M.T., Camata, G., Franchin, P., Magliulo, G., Masi, A., Mollaioli, F., Spacone, E. & Verderame, G. M., 2018. Modeling and seismic response analysis of Italian code-conforming reinforced concrete buildings. Journal of Earthquake Engineering, 22(sup2), 105-139. De Risi, M. T., Di Domenico, M., Manfredi, V., Terrenzi, M., Camata, G., Mollaioli, F., Noto, F., Ricci, P., Franchin, P., Masi, A., Spacone, E. & Verderame, G. M., 2023. Modeling and seismic response analysis of Italian pre-code and low-code reinforced concrete buildings. Part I: bare frames. Journal of Earthquake Engineering, 27(6), 1482-1513. Di Domenico, M., De Risi, M. T., Manfredi, V., Terrenzi, M., Camata, G., Mollaioli, F., Noto, F., Ricci, P., Franchin, P., Masi, A., Spacone, E. & Verderame, G. M., 2023. Modeling and seismic response analysis of Italian pre-code and low-code reinforced concrete buildings. Part II: infilled frames. Journal of Earthquake Engineering, 27(6), 1534-1564. Belletti, B., Michelini, E., Pappalardo, M., De Risi, M. T., & Verderame, G. M., 2024. Preliminary numerical investigation of the effects of staircases on the seismic behaviour of existing pre-code RC framed buildings. Hoseini, M. H., Jafarnejad, H., 2015. Evaluating the performance of stairs’ elements in reinforced concrete structure during earthquakes, 2th National Conference on Earthquake, Qazvin, Iran. Alirezaei, M., Omidi, H., Fallahi, S., & Dehghani, M., 2014. Evaluation and comparison of linear and nonlinear analysis of modeling staircase on concrete buildings. In 5th National Conference on Earthquake and Structures, Kerman, Iran. Ministerial Decree 30 May 1972. Norme tecniche alle quali devono uniformarsi le costruzioni in conglomerato cementizio, normale e precompresso ed a struttura metallica. Official Journal n. 190, 22 July 1972 (in Italian).