3.3.1 Sequence of events
It was important to establish the precise sequence of events leading up to and during the collapse. From which part had the collapse started? Why had so much of the structure been destroyed? Was there any prior warning of the failure? What part had the weather conditions at the time played?
Eyewitnesses were plentiful, and each had a different perspective of the bridge as it fell. There were some common parts to their statements. Most of the witnesses, especially survivors from vehicles on the bridge at the time, testified that they heard cracking sounds very early in the collapse. This suggested that brittle fracture was an important failure mode, if not initiating the fall then certainly playing a crucial role in the disaster.
Testimony showed that the bridge vibrated during high lateral winds, and also when traffic crossed, which meant that movement in the eye-bar joints would have been occurring quite regularly. There was generally significant traffic across the bridge since it carried an Interstate highway.
Witness statements attested to unusual vibrations in the roadway just before the collapse. They also noticed that a bolt or cap-like object was seen on the roadway prior to or during the initial period of the collapse.