RELIABILITY ANALYSIS OF REINFORCED CONCRETE BRIDGE DECKS SUBJECT TO FATIGUE

In this thesis the reliability analysis of reinforced concrete bridge decks subjected to high cycle fatigue was undertaken. A simply supported (single span) reinforced concrete bridge deck was specifically used for the investigation. The statistical models of capacity loss were derived. The uncertainties in structural resistance and the applied loadings were fully accommodated using probabilistic method. The limit state function(Failure Modes) for the flexural capacity of the deck slab, shear capacity of the deck beam and flexural capacity of the deck beam girder was developed and evaluated using the First Order Reliability Method(FORM). The entire process was implemented through a developed MATLAB program “dc_fatigue.m”. Analysis was carried out on all the failure modes considered by varying the geometrical and material properties of the system and their respective safety indices were determined. Failure due to shear in the deck beam gave the least safety index range of 6.06 to -2.11 at 3cycles/min for 10years and 40years respectively indicating the most critical section, when compared to the flexural failure with a safety index range value of 7.59 to 0.25 at 3cycles/min for 10years and 40years respectively. It was generally observed that the reliability index increased as the depth of section and concrete grade increased. Also, the coefficient of variation due to the concrete strength and depth of section decreased with increase in reliability index, from a safety index range of 7.61 to – 0.47 at 5% covariance and from 3.47 to -0.23 at 20% covariance under stress load of 3cycles/min. This trend is expected as quality control plays a very important role in achieving this level of safety in the system. Thus, for a single span bridge of 15m and less under high level of traffic load, a deck beam depth of 1200mm, deck slab of 250mm and concrete strength class of 30-35 are adequate for design and construction.

Researches on fatigue behaviour of concrete materials started at the end of the 19th century, due to failure in many concrete structures caused by fatigue rupture of the concrete (Rteil et.al, 2011). Results of experimental and theoretical studies of the fatigue properties of plain concrete, reinforcing bars, prestressing tendons, and also of structural concrete members, have been accumulating steadily over the past thirty years.

The assessment of existing structures will become a more frequent task for engineers in the near future due to the increasing age of existing infrastructure. These may be due to reasons such as(Jansen, 1996); Change in intended use of the structure, new regulations with higher load requirements for the structure, indications of ongoing deterioration in the structure, unusual incidents during use (e.g. vehicle impact, fire, earthquakes), inadequate serviceability, discovery of design or construction errors.

The purpose of reliability analysis in this research is to verify the overall stability and establishment of action effects, i.e. the distribution of internal forces and moments. In turn, this will enable the calculation of stresses, strains, curvature, rotation and displacements. In certain complex structures, the type of analysis used (e.g. finite-element analysis) will yield internal stresses and strains and displacements directly.