Actually, it was major design flaws. It had been engineered using designs from other bridges which were shorter spans, and 2 elements in particular were different than those known to have been used successfully before. The bridge had bad aerodynamic characteristics, and didn't have the strength and rigidity to either bear updrafts, or prevent/relieve air pressure below the deck. It actually waved throughout it's construction, and rather than gaining the strength and rigidity the designer thought it would as construction progressed, it actually got worse.
Bad design and high winds.
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