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Recent earthquakes, which caused widespread damage to bridges around the world, prompted significant advances in earthquake-resistant bridge design and retrofitting. Because the ductility requirements were not included in the seismic codes until later, the bridges constructed in the early 1980s were not intended for sufficient seismic resistance. Due to their lack of ductility and seismic resilience, bridges built before to this year may be seriously damaged by even mild earthquakes.
The effects of the soil-structure interaction are sometimes ignored by conventional structural design techniques. The structural response of a bridge is greatly influenced by soil-structure interaction (SSI), particularly when there are heavier structures and soft soil conditions. Again, considering the scouring effect in bridge design is essential to ensure the safety of bridge infrastructure.
The present study is focused on retrofitting of RCC Bridge using two different retrofitting schemes (concrete jacketing and steel jacketing) considering SSI and scouring effect. A reference model of RCC Girder Bridge was selected for this study. It is a 100 meter long 5 span bridge and length of each span is 20 meters. The bridge was modeled using a commercial Finite Element Software. SSI was considered during the analysis using two types of soil springs such as: linear springs and multi-linear springs. Three basic models were created for the RCC girder bridge: one considering soil structure interaction with linear pile spring, one considering soil structure interaction with multi-linear pile spring and another one without considering soil structure interaction with fixed base at the point of fixity. Comparing these three models, it was observed that base shear demand is almost 35% less in the model that considers soil structure interaction with multi-linear pile springs and almost 30% less in the model that considers soil structure interaction with linear pile springs from that of the model that considers the base of the bridge as fixed at point of fixity. As base shear, for all load combinations, axial force, shear force and bending moment of the piles were found almost 30% less in the model that considers soil structure interaction with multi-linear pile springs and almost 25% less in the model that considers soil structure interaction with linear pile springs from those of the model that considers the base of the bridge as fixed at point of fixity. Displacement and fundamental period of the structure were found maximum in the model which considers soil structure interaction with multi-linear pile springs and minimum in the model that considers the base of the bridge as fixed. The base shear and moment obtained from static analysis was
slightly higher compared to response spectrum analysis. The shear and moment obtained from response spectrum analysis was slightly higher compared to time history analysis, this was due to variation in amplitude and frequency content of the ground motion.
To observe the effect of scour on the bridge, each model was analyzed for full scour, half scour and no scour conditions. When half scour was considered displacement, natural period, base shear, axial force, shear force and bending moment increased (10%-15%) than the no scour condition. Again, when full scour was considered displacement, natural period, base shear, axial force, shear force and bending moment increased (8%-10%) than the half scour condition.
In the pushover analysis, it was observed that the base shear capacity was almost 30% less in the model that considers soil structure interaction with multi-linear pile springs and almost 25% less in the model that considers soil structure interaction with linear pile springs from that of the model that considers the base of the bridge as fixed at point of fixity. When full scour depth was considered, the piers of all the three basic models did not satisfy the required reinforcement for the Extreme Events, Response Spectrum and Time History load cases. So, the bridge piers were designed for retrofitting by concrete jacketing and steel jacketing.
With the retrofitting design, displacement, natural period, base shear, axial force, shear force and bending moment slightly increased (2%-3%) due to the increase in self-weight of the bridge. In pushover analysis it was observed that the performance of the plastic hinges improved after retrofitting. The base shear capacity increased around 20% after concrete jacketing and around 30% after steel jacketing. The displacement ductility increased around 10% after concrete jacketing and around 15% after steel jacketing. Intersection of demand curve and capacity curve found near the elastic range after concrete jacketing and steel jacketing which represent that, the structure has good resistance and capacity against earthquake loading after retrofitting. |
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