PSI - Issue 78

Filippo Dringoli et al. / Procedia Structural Integrity 78 (2026) 395–403

400

5. Case Study and Selected Ground Motions for the Analysis This study focuses on the analysis of a lateral frame belonging to the structure of a twenty-story office building, designed in San Francisco to withstand seismic actions in accordance with the 1988 UBC provisions. The frame, located along the perimeter in the most critical direction, is modeled with pinned base supports and consists of beam column elements made of A36 steel, with mechanical properties derived from the literature. The main geometric and mechanical characteristics of the frame are presented in Fig. 7.

Fig. 7. (a) Isometric view, (b) geometric characteristics, (c) Cross-sections of the elements, (d) Axial and flexural strength of the elements (axial load is 106N, bending moment is 109Nmm)

The vertical loads include the self-weight of slabs, partitions, mechanical systems, and finishes, amounting to 4.79 kN/m² for both typical floors and the roof. The weight of the moment-resisting frames is calculated based on the properties of the rolled sections, while the exterior curtain wall adds an average load of 1.67 kN/m². Service loads are assumed to be 3.84 kN/m² for typical floors and 0.96 kN/m² for the roof. For the computation of nodal masses, 20% of the live load is considered. Wind effects are neglected due to their minor influence compared to seismic actions. The dynamic analysis determined a first natural period, including P-Delta effects, of 3.51 seconds. To perform the Nonlinear Time History Analysis, three ground motion records were used, as detailed in Table 1. The Loma Prieta and Amatrice earthquakes were scaled by factors of 3.35 and 1.29, respectively, to match the pseudo acceleration at the fundamental period of the structure. This scaling ensures the comparability of the results presented in the following section. The computer program DRAIN-2DX is used in the analysis.

Table 1. Ground motions used on the analysis.

Earthquakes Northridge Loma Prieta

Date

Station n°

PGA

t 90

17/01/1994 17/10/1989 24/08/2016

24322 57562 AMT

0.865 [g] 0.195 [g] 0.529 [g]

8.3 [s] 10.6 [s] 5.3 [s]

Amatrice

6. Capacity Improvement Against Instability Through Local Modifications This paragraph presents the results in terms of the critical scale factor I c , defined as the amplification coefficient of the seismic action at the point where the non-linear time history analysis exhibits an anomalous displacement indicative of structural collapse due to instability. The analysis is conducted with reference to three different structural configurations: