Last Thursday, a bridge under construction collapsed west of Miami killing 6 and severely engineering others. The why is a big question with no answer so far. I have been studying the original proposal, photographs and videos and have come up with the following conclusions so far: First diagonal at north was in compression. It can be seen failing looking up:" fiu bridge video slow motion". You can see it start to buckle downward in video. When this member stopped carrying compression load, the next member to the south which was a tension member became a compression member, dropping the vertical gravity load that would have been carried in the failed compression member into the bottom cord of the truss (the walk way) as a downward load. The system at this time was no longer a full truss, but a beam in the area of failure. This same conclusion can be reached by looking at failure modes of the system and comparing them with the photos after the collapse. The above assumptions predict the same mode of failure that the videos show. Why did the first compression member fail? That would take plans to determine the stresses, etc as well as knowing about the state of the tendons and how the bridge was built, but it may have had something to do with the reported loosening of tendons which were said to have been in the process of being re-tightened. The compression load of the diagonal under gravity from the unshored span may have loosened the tendons and required the re-stressing of the tendons. Any structural engineers here who would care to discuss it?