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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?

EDIT1:

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".

enter image description here

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  • $\begingroup$ sometimes its about aesthetics too. $\endgroup$
    – agentp
    Commented Mar 27, 2017 at 20:59
  • $\begingroup$ @ Florian F: thanks for the edit. $\endgroup$
    – Narasimham
    Commented Jun 14, 2022 at 17:52
  • $\begingroup$ Simple support = simpler calculations? 1876 predates calculators, but slide rulers were in full swing. With increased computation power, quality of materials and understanding of material science, we have been able to refine designs, and cut excess fat from build. Take a look at your bridge code from the 90s and compare it to your current one. In my next of the woods the number of pages has more than doubled. Over build and keep things simple is a hallmark of engineering projects from the past. $\endgroup$
    – Forward Ed
    Commented Jun 14, 2022 at 22:09
  • $\begingroup$ It's hinged at the bottom. There is no bending at the bottom. $\endgroup$
    – david
    Commented Feb 7, 2023 at 22:23
  • $\begingroup$ Hinged at bottom like in sketch added to the question? Would it not impose extra stress at M, the midspan? The Luis Bridge built soon after Eiffel has fixed connections so maximum bending there. $\endgroup$
    – Narasimham
    Commented Feb 8, 2023 at 19:20

2 Answers 2

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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
    Commented Jul 29, 2019 at 7:06
  • $\begingroup$ @Narasimham is 1876 considered contemporary design period? $\endgroup$
    – Forward Ed
    Commented Jun 14, 2022 at 22:02
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The picture below is my opinion:

enter image description here

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  • $\begingroup$ Um, I think the question was about the Eiffel Bridge - not the Eiffel Tower! $\endgroup$
    – Transistor
    Commented Jun 15, 2022 at 9:54

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