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"There's no kind of existing bridge defect that could be aggravated by a spell of 90F temperatures?"

Absolutely not! Engineering problems arise at temps lower than ~10 in steel and cycling around the freezing point of water. this is far from the boiling point of water, and the central concern is just that the different expansion rates are accomodated for in the design. Problems arise when people fail to analyze the design and situation. In this case it's the effect of the cracks, what caused them, and what the new stressesin the bridge look like. Other than the effect of water, the steel and concrete ect... follow the Kelvin temp scale. They had failures with stress redistributions that they failed to recognize.