PSI - Issue 78

Francesco Nigro et al. / Procedia Structural Integrity 78 (2026) 1537–1544

1541

design methods are possible, e.g. those followed by Calvi et al. (2003), to consider more rationally any sharing of seismic force between the two structures. 3.2. The designed steel exoskeletons Fig. 3 highlights the layout of the retrofitted structure, where the steel braces are located at the four corners of the existing structure and a single diagonal brace is adopted in each braced bay.

Fig. 3. Layout of the upgraded structure: 3D (a), plan (b)

The design of the bracing system was performed with reference to an individual braced bay, assuming a height wise distribution of storey shear forces and evaluating (by equilibrium) the axial loads on tensile braces only, while neglecting the contribution of braces in compression, as allowed by the current Italian seismic code (D.M. 2018). Moreover, the choice of shapes for the brace cross sections has been carried out considering also the Eurocode 8 (2005) slenderness and overstrength limitations. Finally, non-dissipative members (beams and columns) were designed, according to capacity design principles, to withstand the forces transmitted to them by the dissipative zones (braces) appropriately magnified in accordance with Eurocode 8 provisions (2005). Recently, a second generation of the Eurocodes has been developed and it is close to be completely released to the public. Especially, for the design of steel structures the new code, prEN 1998-1-2 (2024), introduces new methods and criteria compared with the oldest version. In particular, the approach of neglecting the compression diagonals has been abandoned. However, such design method, as well as the detailing rules, were adopted for the scope of this study to investigate the potential performance of steel bracing systems which might have been designed and constructed so far in Italy. Table 2 reports the chosen cross sections for the members composing the exoskeletons, highlighting that the selected corner columns are characterised by an “ Austrian cross-section ” in order to provide the same lateral stiffness along the two plan directions.

Table 2. Members adopted for “Scenario 1” and “Scenario 2” structures

Storey Scenario 1

Scenario 2 CHS Braces (D/t) 88.9 / 3.2 139.7 / 4 139.7 / 6.3

CHS Braces (D/t)

Corner columns

Internal columns (HE)

Beams (IPE)

Corner columns

Internal columns (HE)

Beams (IPE)

6 5 4 3 2 1

114.3 / 5

2 x IPE 550 340 B

400 400 400 400 450 450

2 x IPE 360 260 B 2 x IPE 360 260 B 2 x IPE 360 260 B 2 x IPE 360 260 B

300 300 300 300

139.7 / 10 2 x IPE 550 340 B 139.7 / 12.5 2 x IPE 550 340 B 168.3 / 12.5 2 x IPE 550 340 B 193.7 / 12.5 2 x IPN 550 340 M 193.7 / 16 2 x IPN 550 340 M

139.7 / 8 139.7 / 10 139.7 / 10

2 x IPN 360 260 M 360 2 x IPN 360 260 M 360