I wrote about the FIU bridge collapse this past summer when the OSHA report came out. The more comprehensive analysis from the National Transportation Safety Board has been released, and it brought up a lot of the same issues for me. In short, regardless of the mistakes that led to the specific accident that occurred, the design of the bridge is, to me, bordering on incomprehensible.
The report is thorough and lays out a good case as to where the design went wrong. I strongly urge every engineer to read it. But the report does not discuss in detail the concept of the bridge, because that is not the NTSB’s job. The basic structure of the bridge was a two-span warren truss in reinforced concrete – a single truss down the centerline of the bridge supporting a deck that cantilevered to both sides and a canopy that cantilevered to both sides – with a pylon at the juncture of the two spans from which ran steel pipes in a fan pattern similar to that of a cable-stayed bridge, but not intended as major load carrying elements.
Personally, I find the design, even if it had been executed properly, to be structurally scrambled. As I discussed in June, making trusses out of reinforced concrete means putting a lot of effort in to deal with tension. Because truss members and connections in real life have actual dimensions, not just the lines and nodes we use in design diagrams, the concrete has to be thickened at the connections in unsightly bulges with the appealing name of “blisters.” Because of the load reversals caused by the method of construction (off-site fabrication followed by moving the entire bridge into position), many of the members and connections were heavier than they would otherwise have had to be. Because members in tension are stiffer than members in bending, the design had to actively make sure that the pipes wouldn’t act as stays.
In other words, the specifics of the failure would not have existed if the truss were steel (as is more logical) or if the stays were load-bearing (as is more logical), and might not have existed if the bridge had two trusses (as is more common). There’s nothing wrong with new and unique designs, but a design should still respect the strengths and weaknesses of the materials used and the basic structural action of the form used.
As for the picture above: Bollman trusses are weird, but structurally comprehensible.
