PSI - Issue 84

L. Fieno et al. / Procedia Structural Integrity 84 (2026) 1127–1135

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The removal of the Gerber hinge support resulted in a modification of the static scheme, whereby Pier 4, previously monolithically connected to the approach viaducts, is now supported by double sliding bearings. As a consequence, its slenderness increased, only partially compensated by the enlargement of the pier cross-section along the shaft. Moreover, the increase in mass led to a worsening of the seismic vulnerability. Similarly, the modification of the structural scheme at the arch crown, not accompanied by an analogous intervention on the adjacent lower piers, resulted in a significant reduction of seismic safety. In some cases, even the strengthening interventions themselves were conceived incorrectly. For instance, the bending strengthening of the deck girders was designed without adequate consideration of the force transfer mechanism between the existing section and the new cast elements. Furthermore, the longitudinal reinforcing bars were not effectively anchored, thus losing a substantial portion of their effectiveness along the beam length.

Figure 2: Bending strengthening interventions on the deck girders;

The viaduct under consideration does not exhibit significant deterioration; however, following a Level 4 Assessment (VAL4), the structure has been classified as Transitabile according to the LLGG20 guidelines, due to the intrinsic structural deficiencies of the system. The main critical aspects identified during the study are as follows: 1. The slenderness of the piers, which lack transverse bracing systems; 2. The shallow foundations of the footings supporting the approach viaducts, whose dimensions are insufficient to withstand seismic horizontal loads; 3. Insufficient bending capacity of the deck girders, particularly under seismic actions; 4. An inadequate bearing configuration, unable to ensure an effective seismic-resistant mechanism; 5. Insufficient seismic capacity of the arches under combined compression–tension–bending and shear, further aggravated by deterioration affecting the springings. The interventions were designed to retrofit the structure with respect to seismic actions (Vr = 50 years) and to allow the structure to be classified as Operativa according to the LLGG20 guidelines, thereby removing the current traffic restrictions. The proposed interventions include: A. Enlargement of the shallow foundations of Piers 1, 2, 3 and their symmetric counterparts; B. Structural strengthening of all piers; C. Strengthening of the deck girders under combined compression–tension–bending through the application of steel plate bonding;

D. Rehabilitation and structural strengthening of the arches; E. Introduction of a structural discontinuity at Piers 7 and 7’; F. Modification of the bearing configuration through the installation of seismic isolators;

G.Various secondary, non-structural interventions, such as concrete surface repairs, replacement of expansion joints, upgrading of the deck drainage system, and environmental restoration works at the end of construction activities.