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You are right. we can't tell from the picture what they had in place. It takes two girders side by side to hav diaphragms between them. WE used different methods to get temporary bracing or support while erecting them depending on the situation. One way is to assemble girders in pairs on the ground if not too heavy to lift. Another way is to use two cranes and hold one girder until the adjacent girder is braced to it. Another way is to rig temporary guys or shores if the wind is not strong. But you must calculate what you are going to do before you start. The slenderness of the bare girder is beyond the range of specifications for girders in finished atructures, and we
(the bridge company i worked for) developed our own formula to calculate this. We never had a failure.

Question for you: It's been many years since I worked for the highway department (in Colorado, no less) but looking at the picture where the left side of the girder is still up on the smaller pier, it seems to me that this was probably the first of the set to be placed (probably last thing on Friday before knocking off for the weekend). Typically, the girder would have been set on its pier bolts and the nuts tightened down. Some sort of lateral bracing would definately have been needed (as you pointed out), but do you think that perhaps there was a failure of the anchor bolts, or concrete or such, that allowed it to tip? Or would the lack of lateral bracing (combined with maybe some strong winds) be enough to start some movement back and forth and thereby work loose or damage the bolts?

I just can't imagine somebody not securing that beam properly considering how careful they usually are in all stages of handling them. My understanding is that those are custom made for each placement and any excess twisting or such while placing them can render them useless by causing the holes to not line up with the preset anchor bolts. What do you think?

I think the primary question is this:

Was the beam in the process of being lowered into place when it failed? If so, then why was traffic permitted to operate underneath it before it was fully secured?