PSI - Issue 78

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

1104

This study aims to analyze the impact of staircase modeling on the seismic performance of an existing RC building, originally designed for gravity loads only. The objective is to quantify the influence of staircase modeling and their plan position on the building's seismic response, employing nonlinear static and dynamic analyses to derive fragility curves. 2. The case study: Design and modeling The building under study is a 6-story reinforced concrete frame, designed for gravity loads only, using allowable stress design principles according to Italian building code DM 1972. The structure, with a rectangular plan of 200 m² and a story height of 3 m, exhibits greater lateral stiffness and strength in the X-direction due to the alignment of beams and columns in that direction. The concrete used has a characteristic cubic compressive strength (R ck ) of 25 MPa, while the Feb38K steel has a characteristic yield strength (f yk ) of 375 MPa. Three configurations were defined: the first without considering the staircases, while the other two configurations include staircases positioned in two different locations in plan. 2.1. Non-Linear structural models This study developed six numerical models, varying the presence and position of staircases and the presence of infill walls (IF models). The six different configurations are summarized in Table 1 and shown in Figure 1.

Table 1: Building models’ labels Model Type

Without Staircase With Staircase (centered)

With Staircase (corner)

Bare Frame

BFns IFns

BFsm IFsm

BFsc

Infilled Frame

IFsc

Figure 1: Building models: (a) without staircase; (b) with staircase (center); (c) with staircase (corner). The staircase is highlighted in red.

For the seismic performance assessment of buildings, numerical models were developed using OpenSees (McKenna et al., 2012), with the STKO pre/post-processor (Petracca et al., 2017). Beams and columns (Terrenzi et al., 2020; Barbagallo et al., 2023) were modeled using the Beam-With-Hinges element, which represents distributed plasticity over a finite length (Scott and Fenves, 2006). Nonlinearities are concentrated at the ends over a length equal to that of the plastic hinge, which was modeled with a fiber section (Spacone et al., 1996). Considering the uncertainty in the plastic hinge length for knee beams, the "Force-Beam-Column" element with 5 integration points was employed. Sections were discretized by adopting the Concrete01 (Kent and Park, 1971) constitutive model for concrete (confined and unconfined) and Steel01 (Menegotto and Pinto, 1973) for steel. Floor diaphragms were modeled as rigid using the 'rigidDiaphragm' constraint, with the addition of an 'axial buffer element' to prevent axial forces in the beams (Barbagallo et al., 2020). Brittle failures of beams, columns, and beam-column joints were not included in the model but were verified in post-processing.