The bridge was to be a “cable stayed” design. Look at the collapse pictures and you can see the bolts where the support cables were to attach coming down from a tower not yet erected. They were pulling “post tension” cables that run through the concrete at the time of the collapse. The cable pulling apparatus can be seen in the photos. These companies (the designers and builders) F-ed up big time and it isn’t the first or even the second collapse they have had during construction of a bridge such as this.
With VT in your screenname, I think you probably have a handle on this that’s not evident in other responses. Few people understand the post-tension concept. I first worked a building with post-tension concrete in 1987. Since then, it has made its way even into home construction as I have PT cables in the slab of my house that was constructed in 2008.
It seems clear the erection schedule provided for the structure to be in place prior to installation of the mid-span column and support cables - without support at the bottom in order to facilitate auto traffic. Ergo, the design team had calculated the structure could span the gap with its dead load. One would suppose the structure could have been substantially post-tensioned while yet on the ground once the fabrication process was completed.
OTOH, what are the odds this was a Hyatt walkway type of failure?