Eads Bridge

Length of Spans

There is an apparent discrepancy between the length of the bridge's spans in the schematic design as described in the 1868 Report of the Engineer in Chief and in the final working drawings. The 1868 report gives the spans as 497, 515, and 497 feet. The finial drawings show the “as-built” dimensions of 502, 520, and 502 feet.

In his 1982 article The Designing of the Eads Bridge, John Kouwenhoven suggests that the 5 foot increase to each span must be due to a corresponding reduction in the width of the piers and abutments. He expresses surprise that such a significant change escaped the notice of Woodward and other writers. Careful review of the design drawings reveals why the discrepancy did not invite comment. The difference is an artifact of the dimensioning systems used in the drawings not a physical change to the size or spacing of the piers.[1]

Undated drawing TR430 in the archives of the Washington University Libraries, in St. Louis, sheds light on this puzzle. The drawing depicts an intermediate stage in the bridge's evolution. The railroad deck and the arch ribs are shown in approximately their final configuration but the top and bottom chords still bear on a single skewback block, as in the schematic design. The drawing gives the schematic design's values of 497 and 515 feet for the spans, measured to the intersection of the lower arch chords and the skewbacks. Later drawings, such as TR191 in the WU archive and Plate XX in Woodward's A History of the St Louis Bridge, show the spans as 502 and 520 feet, but now the dimension is to the pin in the first node of upper chord. The apparent 5 foot increase to the spans is due to the combined effect of measuring to the upper not the lower chord and of eliminating the skewback blocks. The distance between piers and the dimensions of the piers are not changed.

In his article, Kowenhoven cites Carl Gayler's anecdote about Eads overruling the design staff and reducing the width of the piers for aesthetic effect. The event described by Gayler must have occurred while the schematic design was still under development and prior to publication of the 1868 Report of the Engineer in Chief. The size and spacing of the piers remained constant after issue of the Report and was not in play during the transition from schematic to final design.

Above: Drawings from WU archive. [2]

On left, detail of undated drawing TR430 shows 37'-6” between dimension-points at lower chord and the wedge-shaped skewback blocks. On right, TR119 (July 26 1870) shows arch tubes bearing directly against the pier and the dimension line moved from bottom to top chord. The 37'-6” dimension is reduced to by 5 feet, to 32'-6” (twice 16'-3”) and the span dimensions are increased accordingly. These changes do not affect the size or location of the masonry pier, which remains 32'-6” wide in both drawings.