Chapter 8 Conclusions and Recommendation for Future Research
8.2 Summary
The major motivation of the present research work is prompted from the fact that use of HyFRC in place of conventional concrete can lead to increase in ductility. The best possible combinations of selected fibres for improving ductility of concrete specimens is first evaluated. It is also observed from literature that the HyFRC has superior shear resistance as compared to conventional concrete. Kumar et al. (2011) studied HyFRC specimens with 50% lesser transverse reinforcement as compared to conventional RC bridge pier and reported comparable seismic performance. Shear capacity of standard prisms are evaluated with 50% reduction in shear reinforcement.
An existing bridge is considered for the study. It is known that damage in a bridge pier is mostly localized at the column-foundation interface region. Post-earthquake rehabilitation works of bridge piers become easier if the crack location is shifted away from the pier-foundation interface zone. This is likely to be achieved by incorporation of some additional features at column-foundation interface. Three different detailing strategies are considered at pier-foundation interface region. While normal reinforcement details are followed for specimen type 1, additional reinforcing bars (dowels) are used in the interface region for specimen type 2 and corrugated sheet duct is used below interface region in specimen type 3. It is necessary to perform an advanced level of experimental investigation in order to evaluate the seismic performance of the bridge with pier made of high performance material. In the present research, performance of all the types of pier specimens with different detailing are evaluated using hybrid simulation.
Hybrid simulation technique can integrate efficiently the seismic behaviour of entire bridge including structural elements with behavioural uncertainties. For this reason, hybrid simulation is considered to observe behaviour of pier-foundation interface zone in the present study. Four input excitations corresponding to different intensity levels are used in the hybrid simulation, which is followed by the quasi-static cyclic test to take the specimens to the ultimate failure level. Hybrid simulation is carried out by physically testing critical elements of a structure (such as bridge piers), while the remaining elements are concurrently simulated numerically. The numerical model is developed in OpenSees platform and the elements of the numerical model are suitably chosen for simulating appropriate behaviour of different components of the sample bridge. The hybrid simulation is performedusing a step-by-step numerical solution of the governing equations of motion for a model that is formulated considering both numerical and physical portions of the structure. The OpenFresco framework, which is used to define the experimental elements, provides a useful
8.2 Summary
and effective set of modules for performing hybrid simulations. It further helps to connect the numerical module developed in OpenSees platform to the experimental elements in the laboratory. The effect of HyFRC for enhancing seismic induced damage resistance is investigated by hybrid simulation of the entire bridge structure.
The results obtained from hybrid simulation of six different scaled pier specimens and subsequent testing under cyclic loading are examined to explore the effectiveness of use of fibres as well as additional detailing features at pier-foundation interface region.
Comparison of measured and observed responses of specimens made of two types of materials are made to highlight changes in damage pattern, displacement ductility and energy dissipation capacities of specimens. Extent of damage at the end of test is significantly lesser in all the three HyFRC specimens than the corresponding three conventional concrete specimens. HyFRC specimen with dowel bar shows least damage amongst all the specimens, while all HyFRC piers shows consistently higher energy dissipation capacity in comparison to corresponding conventional concrete piers.
The use of HyFRC leads to delay in crack formation and crack growth, which in turn reduce strain level in reinforcing bars. Strain gauges are used to measure strain in concrete as well as in reinforcement to explore the relative merits and demerits of both HyFRC and conventional concrete. Strains are monitored at the critical pier locations to correlate the growth of strain with damage in specimens. The strains in reinforcement of HyFRC piers are found to be considerably reduced at all stages of loadings and displacement ductility of HyFRC specimens got improved compared to those of the conventional concrete piers. The lower growth rate of strain in HyFRC bridge piers in turn can be related to the improvement in basic seismic design parameters.
In order to accurately predict the response of the initial numerical model in line with experimental results, modification are made in the model which includes addition of translational spring at the pier-foundation interface and adjustments in damping ratio to capture hysteretic damping effects. The calibrated model is able to capture the stiffness and strength degradation effects at each intensity levels of loading.
Fragility function for the entire bridge system is developed considering the results of current experimental studies. Incremental dynamic analysis (IDA) is used in the procedure for development of fragility curves. In this study, the provisions of FEMA P695 (FEMA, 2009) are followed to select the ground motion. Finally, the seismic demand and failure probabilities of HyFRC bridge piers and conventional concrete bridge piers are compared using these fragility curves.