So your convincing evidence for the claim that "plenty of poorly designed steel bridges from the 19th century have collapsed" is a single example of a bridge that collapsed because it was designed "less extensive and robust than [..] previous similar designs"?
Or is your claim a mere circular argument, since we can deduce that a steel bridge was poorly designed if it has since collapsed?
Disagree. In late 19th century steel girder bridges were well understood and many of them survive to this day.
They were usually replaced due to changed demands (wider roads, heavier loads), not because of failure.