The Slender Approach

Here’s the best contrast I could find to the heavy bridge at Pearl, Illinois, I talked about recently. That’s the 1889 Smith Avenue High Bridge across the Mississippi River between St. Paul and West St. Paul, Minnesota. That 1905 picture does not give a good sense of the bridge overall – it had 28 spans and was nearly 2800 feet long overall – but it caught my eye because it emphasizes the spidery nature of the construction.

The bridge was a subdivided warren deck truss, built entirely of wrought iron and using eyebars for the lower chord. Its load was considerably lighter than the Pearl bridge, as it carried a road rather than trains, but there’s a difference in design philosophy. The use of a high-level deck bridge (enabled by the bluffs on either side of the river) meant that the trusses could be deep relative to their spans and allowed for the use of a deck truss, where the lateral bracing is simplified by being entirely below the deck. Here’s an 1898 view of the central spans:

Not only are the trusses quite light in appearance – with the lower-chord eyebars looking especially slender – but the towers are similarly light, built-up latticed members braced by slender bars. And now that I’ve repeated this point several times, the logical conclusion: a portion of the bridge was destroyed by a windstorm (reportedly with 180 mile per hour winds) in 1904. The missing spans, including one of the main river spans, were rebuilt per the original design except that steel was used rather than iron. The HABS report (see below) states that as of 1984, the steel replacements were the most severely deteriorated portions of the bridge.

It was determined in the 1910s that the bridge structure was too light to carry street cars, and would require reinforcing for that purpose. Scraping rust for repairing was apparently stopped in the 1930s because the surfaces most in need of cleaning were inaccessible, the faces of steel in contact within connections. By 1977, the damage was considered to be irreparable, and the bridge was demolished in 1985 and replaced with a modern deck arch.

The bridge was designed and fabricated by the Keystone Bridge Company from a general design by A. W. Munster, an engineer working for the city of St. Paul. Keystone was a Carnegie company and was one of the major companies absorbed into the founding of the American Bridge Company. The bridge was important enough and represents its era well enough to be given a thorough write-up by HAER before it was demolished. In other words, it’s reasonable to believe that the design and construction were competent and to the standards of the day. The bridge was caught in rapidly changing times: the design and technology that were standard in 1889 were out of date by the 1910s. This fact led to the eventual removal and replacement of many of the historic bridges I’ve talked about here. On that note, I’ll end with one of the detailed HAER photos, showing the rusting:

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  1. Pingback: The Slender Approach — Old Structures Engineering – The Bridgehunter's Chronicles

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