Failure Portrait 4

by Don Friedman on December 14, 2015

It’s been a while since I posted one of these, but structure hasn’t stopped failing.

Before steel pieces large enough and heavy enough to serve as columns were made – in short, before wide-flange sections were created – columns in steel-frame buildings were typically made by riveting together plates, angles, and channels to form larger shapes. Here is an example on the San Francisco-Oakland Bay Bridge.

The use of box columns composed of two channels with lacing (small plates arranged like truss diagonals) or cover plates was common in New York from the 1890s into the 1920s. They perform well as structural members but like all steel they are subject to rusting when they get wet. The old practice of enclosing columns in masonry piers for fireproofing has the effect of hiding the condition of the columns from view. Rust-jacking eventually damages the masonry enough for the deterioration to be noticed, but the smaller pieces of a built-up column, particularly a laced column, cause less obvious cracks than larger columns.

Remove some brick and you find the channels are rusted. The visible part of the column that’s been exposed here is the left channel of the pair; the diagonal pieces on the right of the steel are the lacing that connects the two channels with the rivets heads visible. The channel flange, particular the flange further away, are ragged from material loss.

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Then you get close up and look at the channel web.

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Oops.

 

[Picture credits to Mona.]

 

 

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