I guess maybe the core was built on a steel hoop made from bent round bar, or something like that? Not sure how you hold the back end positioned to
1/8" accuracy, though.
=============================== Chaplets.
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This notorious collapse of a cast iron bridge brought attention to the difficulties of sand casting and prompted improvements.
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Fortunately stronger rolled structural steel was just then becoming available to displace wrought and cast iron.
Many defects were found in the column castings, including shifted cores that left a thin wall on one side. The foundry supervisor couldn't be interrogated as he had emigrated (fled?) to Australia. Railway management had pushed the designer to cheapen, and thus weaken, this bridge compared to his earlier projects.
Two cylinder steam locomotives are inherently unbalanced. The wheel counterweights are sized to divert the vibration from horizontal which the passengers would feel, to vertical which increases the pounding load on the tracks and bridges. The effect increases with speed, and trains reportedly ran over the Tay bridge much faster than they were supposed to.
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