Just to clarify, the I-35W failure was the result of a design error in the sizing of a connection gusset plate - part of a bolted connection between multiple structural sections. The error resulted in the connection being grossly under designed and incapable of supporting the code loads at the time of the bridge design (1965). It was not a case of old standards not meeting current requirements. It was an engineering and detailing mistake, plain and simple. The error was missed by the QA/QC program of the original design firm, and by the design reviews conducted by the state and federal DOT. It was an accident waiting to happen, and no one knew the design defect was there. If there was a failure in maintenance, it was that some subtle clues that the plates were undersized were missed. Minor warping and distortions had been seen during routine inspections, but those findings were considered incidental, non-critical and never caused anyone to go back to the original design. No one thought those huge plates were simply too thin.
Actual bridge applied loads caused by typical traffic are normally much lower than the loads the codes require for design, and so-called "safety factors" are applied to provide a margin between code loads and failure loads. That is why the bridge did not collapse sooner. Ironically, it was bridge maintenance that finally overloaded the plate and caused the failure. They were in the process of repairing the bridge deck and had stored some of the equipment and materials on the bridge. The added dead load of those stored items in addition to the normal traffic load, though well below the assumed capability of the bridge, overstressed and failed one of the gusset plates, and caused the bridge collapse.
The design flaw was first discovered in the course of the
NTSB post collapse investigation. For structural engineers, the I-35W failure brought back some horrid memories. A failure due to bad connection detailing? See
Hyatt Regency Kansas City. That was a failure that forever changed the way structural connections were designed, detailed, and reviewed. Unfortunately, the I-35W design occurred 16 years before the Hyatt failure. The lessons learned and engineering practice changes resulting from the Hyatt failure were not in place in 1965.
The I-5 failure was caused by an oversize vehicle striking a cross member in a through truss. The often-cited inadequacy of the bridge design is that it is "non-load-path redundant," which in English means that the failure of one structural segment will result in the failure of the bridge. Obviously, if a bridge that is non-load-path redundant fails, had that bridge been load-path-redundant, it might not have failed. However, there are untold numbers of bridges and other structures that lack redundancy. Being non-load-path redundant does not mean that bridge, or others like it, is unsafe and should be replaced. Given infrastructure needs, replacing such structures that are otherwise perfectly adequate would seem to me to be a monumental waste of money. For bridges like this, it would seem to be more cost effective to find ways to prevent oversize trucks from hitting truss members.
As for infrastructure funding needs, I am well aware of that. The American Society of Civil Engineers (ASCE) produces the annual
Infrastructure Report Card - the report commonly referenced when the need for infrastructure spending is promoted. I know that because I'm a structural engineer, a member of ASCE, and despite being "retired," still actively participate on an ASCE technical committee (on my own dime just because I think it's fun). In a way, I contribute to that report.
I absolutely agree that there is a critical need for infrastructure spending. Are the I-35W or I-5 collapses good examples of that need? In my opinion, no.
