Retrofit Optimization for Resilience Enhancement of Bridges multihazard Scenario

Open Access
Author:
Chandrasekaran, Sandhya
Graduate Program:
Civil Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
April 28, 2014
Committee Members:
  • Swagata Banerjee Basu, Thesis Advisor
Keywords:
  • Optimization
  • seismic retrofit
  • resilience
Abstract:
Bridge performance is often controlled by the strength of its critical sub-structural components (Teng et. al 2000). The seismic response of highway bridges is governed significantly by the axial strength and ductility of its columns. Prior to 1971, bridge failures were often characterized by column failures at the plastic hinge zones due to poor detailing and insufficient confinement (Ramanathan 2012). The Caltrans Seismic Retrofit Program adopted the technique of column jacketing to provide the additional confinement required to restrict the lateral disintegration of column concrete. Steel jackets have been established as a means to increase the deformation capacity of concrete beyond its unconfined compressive strength and consequently improve the column rotational ductility under lateral loading. However, fiber reinforced polymer (FRP) is growing as a preferred alternative owing to its high strength to weight ratio, resistance to corrosion and superior confinement through hoop action coming from the orientation of its constituent fibers (Hajsadeghi et. al 2010). FRPs also provide the advantage of easy and economical installation, although their manufacturing costs are much higher in comparison to steel. The inherent disparity in the mechanical properties and the associated costs of these different materials gives rise to a trade-off between cost and performance when it comes to retrofit operations. This study explores the aforesaid trade-off and aims to optimize bridge retrofit design configurations with respect to cost and resilience. The study is a two-objective optimization problem that aims to minimize column jacket retrofit cost and simultaneously maximize the retrofitted performance measured in terms of bridge resilience. Multi-objective evolutionary algorithm, namely Non-dominated Sorting Genetic Algorithm II is used to carry out the optimization owing to its implicit elitism and simplicity in use. The variables in the parameter space include the choice of material for the retrofit, the choice of column in the bridge to be retrofitted and the thickness of the retrofit material for each bridge column. Three different materials, steel, carbon fiber and glass fiber composites are investigated, each associated with different values of strength and unit cost. Required thickness of jacket and unit cost of jacketing differ for each material for the same target resilience. The algorithm hence, searches the domain to arrive at parameter values which are most favorable in terms of cost as well the resulting resilience of the retrofitted structure. Results from the optimization, called Pareto-optimal set, include solutions that are distinct from each other in terms of the associated cost, contribution to resilience enhancement, and values of design parameters. The user is offered a wide range of superior solutions to choose from, based on more specific preferences. It is of interest to investigate seismic resilience enhancement due to column retrofit under the multi-hazard effect of earthquake and flood-induced scour in continuation to previous study by Prasad and Banerjee (2013). Hence the example bridge is evaluated for its seismic resilience for various retrofit configurations taking into account the bridge columns being exposed to pre-existing scour resulting in reduced structural stiffness.