Influence of the corrosion damage scenarios on the residual life of bridge grillages

The reinforcement corrosion is nowadays considered one of the primary causes for the decrease of the load carrying capacity of r.c. structures. This phenomenon is particularly felt for bridge structures very often subjected to aggressive environment and/or to thaw salts, as witnessed by the need for continuous maintenance and important repairs or strengthening due to structural deterioration. The rebar diameter reduction, induced by the corrosion, and particularly the pitting effects, seems to influence both the steel yield stress and the ultimate steel strain. In fact, as the load increases, the plastic deformation localizes in small zones and the consequent reduction of yielding spread can lead to a brittle failure. Aim of the paper is the analysis of the behavior of a reference r.c. bridge grillage, representative of a wide class of this structural typology, subjected to corrosion. Different damage scenarios are considered and the most critical situation is highlighted. The analyses are performed with numerical procedures; the corrosion effects on the structural elements are calibrated on the basis of the results of experimental tests carried out by some of the authors. The study has been conducted on a r.c. bridge grillage representative of a wide class of Italian viaducts, selected through a critical analysis of different geometrical parameters. The chosen case presents a static scheme of simple supported beams, and this very common peculiarity allows a clear interpretation and understanding of the phenomena related to the rebar corrosion. The system is composed of 4 main beams with a 6 meter spans and 4 transverse beams characterized by 3 m spans, completed with a r.c. slab with constant thickness of 22 cm, with load amount and distribution in agreement with the Italian code. The simulation of the grillage response is carried out with the well-known program SAP9-NL, by performing a static non-linear analysis, through an accurate modelling of the moment-rotation capacity of the plastic hinges. In this preliminary case, in order to have a clear interpretation of the results, the corrosion effects are simulated only with a reduction of the yielding and post-yielding strengths. In order to investigate different damage scenarios, localized corrosion phenomena are considered (pitting corrosion). This kind of attack is particularly dangerous for the structure, due to its local nature, that causes, even if in smaller zones, highest strength reductions, if compared with an "equivalent" uniform corrosion. On the basis of the results obtained by the experimental tests, related to a uniform corrosion, highlighting a reduction of 30% of the ultimate bending moment, for a pre-fixed corrosion level (assumed as 100%), it appears reasonable to consider a 50% of strength reduction in pitting attack. The critical zones to be subjected to corrosion are detected on the basis of a linear elastic analysis of the grillage, allowing the definition of the most stressed sections and with the aid of indications provided by the Italian Highway Society on the most recurring damage zones. As a result 5 damage scenarios have been assumed, with corrosion localized in defined sections of the edge beam or near the transverse joints. For each corrosion distribution a series of non-linear static analyses are performed, by increasing the applied loads, while the corrosion amount in the critical sections is kept fixed. The obtained results are interpreted and discussed mainly through capacity curves. The evolution of the plastic hinges up to the collapse is also analyzed and the behavior of the structures is judged in the framework of performance criteria based on different performance levels of the plastic hinges. The safety level respect to the failure is evaluated as the ratio between the maximum collapse load and the load provided by the current Italian code. The obtained results allow to point out the safety of the damaged structure respect to the sound one, as a function of the corrosion distribution, and then the most dangerous damage scenario for the structural scheme. In particular the following aspects are highlighted: 1. the strength supply of the sound grillage; 2. the strength supply of the damaged grillage as a function of the corrosion scenario; 3. the stress redistribution, after yielding, in the sound and corroded grillage; 4. the most dangerous damage scenario. The obtained results have highlighted that the global behavior is strongly affected by concentrated damage on the longitudinal beams, while the corrosion of the transverse beams leads to almost negligible effects. For the analyzed geometry and load distribution the most critical situation is related to the corrosion in the edge beam midspan. In this case the loss of safety level is equal to the loss of local strength (about 50%) that mean that no redistribution occurs in the structure. © 2006 Taylor & Francis Group.