As an engineer it sure looks to me like a stupid design guaranteed to fail. I'm going to speculate that someone, either a young inexperienced engineer, or an old one whose mind is losing it's acuity, input some wrong numbers into a design program and didn't have enough common sense to see that the output was bad. Nor did anyone else in the firm who reviewed the design. I read that some cables were loose and were being tightened when it collapsed. I would not be the least bit surprised if the 'loose' cables had already broken inside the concrete due to being seriously undersized and over stressed. There's a reason why bridge beams have a 'T' shape or a box shape. From the photos it looks like this one did not.
I fear we may have reached the point where experience derived by grunt mathematical work is being replaced by total reliance on, "But that's what the computer said!" I hope this is the wake up call STEM educators need to realize that students have to understand what all those formulas in the computer programs are really doing. This incident is truly sad, but the cost of the wake up call this way is a whole lot lower than if an airliner were to crash due to a computer programming screw up. The formulas that are used to calculate stresses and forces in a bridge have been in computer code for decades now. I wonder how many of those formulas have been carried forward into the programs used today with no one having any idea how they were derived in the first place, nor what the long hand form of the equations are that are the basis of the calculations.
This incident scares me.