[DuncanWaring]

A licensed engineer probably did sign-off on the design.

A soon-to-be-formerly-licensed engineer.

[Alberta's Child]

The design may have been fine. In fact, if I were to list the probabilities of the three primary causes of failure here I would list them in this order:

1. Flawed construction process (most likely)

2. Poor materials (somewhat likely)

3. Flawed design (not very likely)

I say #3 is the least likely scenario because I've been working in the industry for years and I know that it's the one phase of a project that has the most eyes on it.

Of course, there's also the possibility that a combination of these factors came into play.