Assessing the Impact of Seismic Dampers in Bridge Design. Case Study: The Ntungu bridge in Isingiro, Uganda

Abstract

Uganda has several historic bridges from the mid-twentieth century, but there is a significant demand for new bridges in many other places to meet future traffic demands. The report case study is located on a highway in Western Uganda, a region that has experienced several earthquakes. The installation of devices such as dampers in bridges, as designed in our case study, is one option for mitigating seismic effects; however, construction has been halted. The major goal of this research was to assess the influence of seismic dampers. We used Midas Civil, a finite element software to model, analyse, and compare the influence of dampers in bridge design. The two-span steel girder composite bridge was subjected to a seismic action of (PGA = 0.1g), on ground type B, with the respective horizontal elastic response spectrum acceleration curve according to EC8. Several indices including fundamental period, base shear, and nodal displacements, were used to assess the bridge's seismic performance. The required number of modes was 8 modes in modal analysis to attain a total participation mass of 90%. The base shear and nodal displacement decreased with the addition of a damper. A maximum reduction of 11.86% and 39.68% was seen for base shear and nodal displacements respectively. Based on extensive analytical modelling, Finite Element (FE) analysis, the following conclusions could be made: i) Seismic performance of a bridge improves by installing energy-dissipating devices (dampers) as they absorb and dissipate energy during an earthquake, base shear reduces effectively and the accelerations decrease, implying reduced inertial forces. ii) Increase in the thickness of the pier section helps to reduce the displacement response of the bridge. However, increments in the section beyond a certain point cause an increase in the response in the longitudinal direction and in addition of the bridge pier becomes very uneconomical iii) The reduction in the displacement response due to the increase in the pier section (6.9%) is much less than the reduction in displacement response due to the addition of dampers (39.6%). However, the latter proves to be a more cost-effective alternative as compared to the use of Viscous Dampers in the Seismic design of the bridge.