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In bridge design normally base supports are strong,fixed/rigid. Why did he choose thicker arch sections at middle above mid-river and narrow sections at abutment supports? If he chose a simply supported condition, then for what structural advantage?

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Could it be some combination of arch and beam design?

Between arch abutments it spans 5 pillar span lengths.Treating main structure as a long span Beam bridge simply supported at ends bending moment at mid-span is maximum so midsection should be designed deeply trussed as done here.

However does it not compromise benefits of pure membrane arch action ( like St. Louis Gateway arch, Missouri design as a catenary) creating stress concentration by lower section property utilization?

Eiffel Rail-Road Bridge Portugal

"In 1876 they designed the 1,158-foot steel railroad Ponte Maria Pia (Maria Pia Bridge) over the Douro River in Oporto, Portugal".

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  • $\begingroup$ sometimes its about aesthetics too. $\endgroup$ – agentp Mar 27 '17 at 20:59
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"Why did he choose thicker arch sections at middle above mid-river and narrow sections at abutment supports?"

The greatest force is at the centre of the bridge, not the abutments. Since material is the same strength the thicker sections take higher stresses and therefore greater vertical forces which are transmitted down to the abutments and foundations.

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  • $\begingroup$ (When simply supported maximum bending moment occurs at mid span alright) but in contemporary bridge design typified by the Sydney harbor bridge, sections around supports/piers are built so as to be capable of resisting higher bending moment. $\endgroup$ – Narasimham Jul 29 '19 at 7:06

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