North Carolina Department of Transport assistant bridge inspection engineer Don Idol said that eyewitnesses had described a classic under-reinforced deck design.
'It certainly seems to have failed in the way it would have been designed to, ' he said.
'Witnesses described several loud 'rifle cracks' before the deck slowly sagged and broke at midspan with the steel yielding rather than a sudden catastrophic concrete failure.'
The injured were sent sliding as the deck buckled. However, once the strands had failed the structure broke in half sending pedestrians crashing on to the highway carriageway.
Localised failure occurred as the four broken beam ends hit the road below. But connections over the supports prevented the beams from falling and crushing the injured.