I also had concerns about the speed of the collapse ( I assumed it was harder than that to destroy a permanent piece of concrete infrastructure) . And I'm not a conspiracy theorist. But, looking at motivation, of this were a terrorist attack, then as this was successful, someone would have claimed responsibility. On the other hand, this would be a pointless false flag; just something that will cost money to.fix, and decrease profits while it's broken. Seems legit, for lack of a motivated actor.
Economics might be a part of it. I think I heard the judge say that a bond for the patsy who started things by 'setting a chair on fire' would be $200,000,000 which is the estimated damages. Ya, someone is going to make a dime off this one.
I suspect that a much bigger part is psychological. Now any concrete structure can be dropped by a few bums with a barrel fire and 95% of the sheep will 'know' the official story is possible because 'it happened in Atlanta'.
Certain infrastructure in Europe (at least) is designed to be blown for transportation denial reasons. One of the things combat engineers stationed in Germany would practice was going to each, opening up the doors specially designed for the charges, and inserting dummy explosives. I was not stationed there myself, but friends who were would tell me about it. I don't believe that the general theme is classified information or anything. Just not widely know perhaps. At least not in the U.S..
Point is, mobility denial is an important consideration in military strategy. Someone analyzing the I-85 failure pointed out that the lips were missing from the span bearing, and there was a dearth of re-bar which one would expect to see in such a design feature. I cannot get a close enough look to verify this. In the footage I found and linked above I did see some hanging bar but noted that it was not as much as I would have expected for a lip structure.
Another thing of note is that various people reported booms, and on the footage some could be heard and the camera guy also noted it. I also notice that the 'cop' had a beard and a distinctly 'roid physique. Looked more like a security contractor than a cop to me.
Looks to me like we are setting up for a situation where if the shit hits the fan economically a lot of urban and suburban dwellers are going to be stuck in place. If one is of the opinion that there are to many people, the outcome of such an event should be to one's liking.