PSI - Issue 78

Valentina Picciano et al. / Procedia Structural Integrity 78 (2026) 1167–1174

1173

from the inner corner to a compressed zone depth x from the upper beam surface). From this, the required precompression force ( F p ) is calculated to achieve a target increase in shear capacity ( ΔV ). The depth x of the compression zone is obtained from finite element analysis results or conservatively assumed as 1/20 of the beam height in practice. For the two modelled specimens, the calculated F p to achieve a 50% increase in capacity was 249.9 kN and 491.6 kN, respectively. The corresponding external tendons’ diameters (assuming only 50% yield strength used) were 24 mm and 30 mm. Steel plates were dimensioned accordingly and modelled with elastic-linear behaviour. Moreover, the post-tensioned bars were modelled considering a bilinear stress-strain constitutive law. A series of nonlinear FE analyses were performed varying the pre-tension level from 5% to 40% of the external tendon yield strength. The analyses were structured in three stages: (i) application of self-weight only; (ii) activation of post-tensioning; (iii) displacement-controlled analysis to failure. The results were analysed with a focus on both service (SLS) and ultimate (ULS) performance improvements. As shown in Fig. 5, at SLS, crack load increased almost linearly with pre-tension level: for the specimen 1-OL1, the increase ranged from 17% to 60%; for 2-OL3, up to 86.4%. At ULS, strength gain was largely independent of pre tension level—up to ~50% increase was achieved even at 5% of pre-tension. This indicates high effectiveness at relatively low stress levels. The tensile stresses in the external bars remained within or slightly above the elastic range (max ~85–100% of f yt ), validating the initial design assumption. Under service conditions (e.g., internal reinforcement yielding), tendon stresses were between 30–60% of f yt , confirming their elastic behaviour. Finally, post-tensioning affects the failure mode by inducing a general embrittlement of the structural behaviour. This embrittlement is primarily associated with cracking due to concrete crushing, particularly at high pre-stress levels. As a result, the crack pattern shifts from diagonally oriented cracks near the junction between the nib and the main beam to more vertically oriented ones located deeper within the dapped-end region.

(a)

(b)

Fig. 5. SLS and ULS improvement as a function of the prestressing applied related to sample a) 1-OL1 and b) 2-OL3.

Additional analyses were also carried out on a Gerber saddle from the Musmeci bridge located in Potenza (southern Italy), thoroughly examined in previous research by Santarsiero and Picciano (2023), to evaluate the effectiveness of external post-tensioning in the presence of reinforcement corrosion. By simulating chloride-induced degradation over 45 and 95 years, and activating the post-tensioning intervention after these two different corrosion periods, the study confirmed the trends observed in the previously discussed models. In particular, the results once again highlighted a linear increase in the cracking load and a nearly constant ultimate load with increasing post-tensioning force. These findings confirm the reliability of the intervention even under advanced degradation, further supporting its role in restoring and improving the structural performance and durability of Gerber saddles.