JR North wrote:

I don't think I buy it. Follow the money and find where the payola is
squashing the real cause. How old was that bridge? 45-50 something?.
More likely scenario: the construction gang on the span at the time of
collapse-"Hey Boss-I done pulled thet pin yoo toll me two..."
"NOT *THAT* PIN!!!!AAAAGGGHHHHH............."
JR
Dweller in the cellar

Certainly a possibility.

More likely the bridge was engineered correctly at first, and somewhere
in the drafting to the final plans the gusset size was misapplied as
"half this thickness per side", instead of "this thickness per side" and
then the construction crew didn't mention to the engineer that the
thickness was half of what they were used to from other bridges.