Eads Bridge

Webpage developed by David Aynardi

Contents

Introduction Background Design & Construction Modifications Spandrel Bracing Length of Spans Storm Damage Collisions Pneumatic Piling West Abutment Floating Cofferdam Aesthetics Theory Numerical Results References Contact Form
Appendix
1  Woodward Ch XXVI 2  Piers & Abutments
Other Projects
Introduction Background Design & Construction Modifications Spandrel Bracing Length of Spans Storm Damage Collisions Pneumatic Piling West Abutment Floating Cofferdam Aesthetics Theory Numerical Results References Contact Form
Appendix 1  Woodward Ch XXVI 2  Piers & Abutments
Other Projects

Spandrel Bracing

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Spandrel Bracing
The curved arch-like brackets between the uprights are bracing added during the 1902 renovation of the bridge.
Detail of illustration from TRRA Report of Chief Engineer[2]
In the first Report of the Engineer in Chief, Eads described the design of his arches. They are self-supporting and do not rely on bracing by other parts of the structure. The posts which extend above the arches serve only to transfer the weight of the decks down to the arches. This distinction between “support” and “supported” is dramatic in old photographs of the bridge, in which the uprights appear as an airy tracery between the decks and the opaque mass of the arches.

The importance that Eads attached to this structural scheme can be seen in his terse response when a consultant suggested adding bracing between the uprights. Eads disagreed:

In this fourth suggestion Mr. Laurie shows his unfamiliarity with the theory of the ribbed arch. The insertion of stays between the adjacent vertical struts would have constituted a sort of spandrel bracing entirely inadmissible”.

From the text of his report, it can be seen that Mr Laurie was concerned about the unbraced length of the uprights not the stability of the arches. Eads interpreted his comment as proposing bracing that would effect the arches themselves.[1]

The bracing was not added on James Eads’ watch but, after the bridge was inspected in 1902, struts were installed between the uprights to stop what the 1902 inspectors saw as “a considerable vibration in the bridge ...due to insufficient bracing given to the posts carrying the upper deck”. Instead of introducing diagonals that would interfere with the normal deflection of the arches, knee-braces were installed to reinforce the connections between the posts and beams at each deck level. The braces stiffen the posts while preserving the overall flexibility of the spandrels.

The designers of the retrofit bracing tried to respect Eads' vision for the bridge. The bracing was given the form of curved brackets, recalling the arcades of the approaches. Because they are located near the decks, the brackets blend into the background clutter of girders, making them as inconspicuous as possible.

At the time of the 1902 retrofit, bracing was added at both decks. In the 1940s, the wood-plank upper deck was replaced with a steel and concrete highway. During this work, the upper tier of curved bracing was removed. The lower tier of curved brackets is retained in the Metrolink upgrades to the railway deck.


Copyright © 2019-, David Aynardi
Footnotes
  1. Woodward, p114 ^
  2. TRRA Report of the Chief Engineer on Improvements made in 1902-3-4, p.318 ^

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