Why we are using circular pillars for the construction of bridges?

Constant Inertia due to circular shape, Design, & Dynamic load transformation are the main reasons  

it gave a great architectural shape after construction , if its direct placed upon piles( mono piles) no need to bent or disturb pile reinforcement, if its in water ( river, sea, ) to reduce Water erosion effect on concrete

Round Columns/Pillars are theoretically more stable and better for transferring loads to foundation, than square or rectangle cross-sectional columns.

Usually the loads that are transferred to columns in bridges case are dynamic and moving due to traffic, the shape that has a constant inertia along all axes is the circle. This leads to a more economical design with a lower cross section to enhance the visibly through various angles around the pillar

Engineering is what not only provide necessary strength but also focus on economic design. thats why for economic projects circular pillar is used

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Follow-up to the owners of experience in this specialty

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Although this is not my specialist, i read about it, the circular pillars of Bridges they make it, because you don't need to flush them to anything, also circular looks aesthetic there.

Is not a requirement to use the circular columns in bridges. ... Possible use of rectangular section columns .or in the form of T and decide to form by loads falling on the surface section of the column..