Looking at Google's streetview and of pictures of the start of the fire today, the only thing that appeared to be stored under that bridge segment was rolls of flexible PVC pipe or conduit. And not a whole lot of it, IMO. I'm shocked that it would burn fiercely enough to take down the bridge segment.
But, if that's all it was (and assuming an accidental fire), whoever chose to store that stuff under the bridge is looking pretty stupid right now...
For those of you not from Atlanta, the fire took out the Northbound (out of Atlanta) lanes of I-85 and Hwy 400, which are two of the three main commute paths to Atlanta's Northern suburbs. Just thinking through the timelines, it is gonna be months to fix this. A lot of people and employers will be impacted...
Beyond that, I-85 is a obviously a major commercial route from Atlanta and points South up to Charlotte and NE from there.