Design Optimization of Composite Road Bridges using Genetic Algorithms
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Examensarbete för masterexamen
Programme
Model builders
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Abstract
Steel bridges today are mostly constructed in traditional carbon steel, with corro sion as a common issue leading to high maintenance costs and a limited service
life. This problem can be minimized by using stainless steel as it is less prone to
corrode. However, stainless steel is more expensive leading to a higher investment
cost. The aim of the thesis is therefore to develop a design optimization program of
concrete-steel composite road bridges. The program, written in Python, is based on
the Eurocode design procedure and optimized with the use of a genetic algorithm
with the option to optimize towards minimum life cycle cost (LCC), environmental
impact (LCA) or steel material usage. The life cycle performance tool used is de veloped in a parallel master’s thesis by Nissan and Woldeyohannes (2022).
A case study is conducted where the program is set to redesign an existing bridge
with flat web girders of carbon steel S355. The optimization is run for two concepts:
girders of carbon steel grade S355 with flat webs and Duplex stainless steel girders
with corrugated webs. The main study is on minimizing the life cycle cost, where
sub-studies are performed to see how the result is affected by limitations of the web
height, the amount of traffic, and the material price. Additionally, the program
is run to minimize the environmental impact (CO2 equivalents) and steel material
mass to better understand the design choices of the program.
The results shows that the optimization program reduced the material use with 20
% for the carbon steel concept and with 30 % for stainless steel concept, compared
to the original design. The first concept design also gave a life cycle cost reduc tion of 6 %. Comparing the two concepts with each other, the results showed that
when allowing a web height of up to two meters, considering today’s steel prices (60
SEK/kg for Duplex steel and 20 SEK/kg for S355), the stainless steel alternative is
9 % more expensive. However, when allowing a web height of up to three meters,
the stainless steel alternative is 3 % cheaper. Further, with decreased material prices
and increased amount of traffic, the stainless steel alternative could be proven even
more competitive.
Lastly, an important conclusion is that the optimizations against mass and mini mize life cycle cost gave similar results for the stainless steel alternative, showing it
enough to optimize against mass. However, for the carbon steel alternative, where
the maintenance aspect is highly important, a life cycle cost optimization is required.
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Keywords
Composite Bridge, Corrugated Web, Stainless Steel, Optimization, Ge netic Algorithms, Life Cycle cost (LCC), Life Cycle Assessment (LCA)