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 On January 15, the National Transportation Safety Board
released a safety recommendation letter report to the FHWA related to
the I-35W bridge collapse in Minneapolis
Minnesota that claimed the lives
of 13 people and injured 145. The safety
recommendations are based on the findings of an interim report from the FHWA Turner-Fairbank Highway Research Center that some gusset plates, components of the
steel trusses, were undersized (not thick enough). This deficiency was
confirmed to be a flaw in the design and not construction-related based on
review of the original drawings from the 1960s and inspection of the wreckage.
Whether this was a calculation error or a drafting error will perhaps never be
known as only portions of the original design calculations were located. But
the point is that it was never caught by any reviewers.
When this event first
happened back on August 1, I remember being very shaken up by it. After my
initial sadness for the victims of the accident, my first thought as a
geotechnical engineer was: “were the foundations at fault.” As more information
came out, it quickly became evident that the failure did not have anything to
do with the foundations but that it was related to the superstructure of the
bridge. But this still was something that profoundly affected me. (Continues…)
Gusset Plate Free Body Diagram from FHWA Report
 Just because this event did not have anything to do with
geotechnical engineering, it doesn’t mean we should not take it as a reminder
of what could happen. This type of failure is my absolute worst fear as a
practicing engineer. It was not only a catastrophic failure of the structure,
but a tragic loss of life. I take my profession very seriously, especially my
obligation to serve the interests of the public. This is why I decided to join
the Order of
the Engineer and why I wear the ring on my pinky as a symbol of that
obligation.
If you are an engineer and are not familiar with the Order
of the Engineer I encourage you to look into it and to see if there is a link
(chapter) near you. At the ring ceremony there was a story I was told about the
origin of the ring (which incidentally Wikipedia says is
not true). The story was that the original rings were made of iron from the
Quebec Bridge that collapsed in 1907 as a
result of poor engineering, killing 75 construction workers. The parallels between
that story, true or not, and the I-35W bridge collapse are rather symbolic for
me. Despite the story being false, the
ring is still a symbol of the ethical responsibility I have to practice in the
best interests of the public.
 Clearly I am not a structural engineer, but I have a very
hard time understanding why this design deficiency was not caught during a
review process. First of all, the original designers (Sverdrup & Parcel,
since acquired by Jacobs) should have had some form of internal review. Then
there had to have been a reviewer for the State. So somehow this error made it
through and the bridge was constructed.
I guess I can imagine something sneaking through like that. (Photo of Gusset Plate at Southwest pier in 1996)
However, as you may have heard, the bridge had gone through
two major renovations, one in 1977 and one in 1998. As a part of these
renovations, the deck thickness was increased from 6.5 to 8.5 inches and the
center median barrier and outside barrier walls were increased in size. During
these modifications the bridge structure would have been evaluated for the
increased loads, why did they not look at the gusset plates? The NTSB letter
indicates that they could not find all of the original design calculations. If
you were checking the bridge for these additional loads and you were missing
calculations from the original designer, wouldn’t you want to go and check
everything yourself? Especially since this bridge was termed “fracture
critical” or “non-load-path-redundant”, meaning that “a failure of any one of a
number of structural elements in the bridge would cause a complete collapse of
the entire bridge.” The NTSB letter provides this (in my opinion) rather weak
explanation for failing to catch the deficiency during the 1977 or 1998
reviews:
In addition, gusset plate design calculations are not
usually reviewed during major modifications on bridges. Generally, the weakest
point of a bridge is evaluated to determine if the additional loads or stresses
can be accommodated, with the assumption that the remaining portions of the
bridge can withstand the change. For example, as previously mentioned, the
accident bridge underwent two major renovations, which added significantly to
the overall weight of the structure. Information obtained from Mn/DOT indicates
that Mn/DOT engineers followed generally accepted practice and recalculated the
anticipated stress levels in what they believed at the time were the weakest
members of the bridge. Normally, there would be no reason for them to question
the strength of the gusset plates relative to these weaker structural members.
So what do I personally take away from this tragedy? First
and foremost, I have a renewed sense of the responsibility I have in my day to
day work as an engineer. As I write this, I am on my lunch break and when I
come back I will be working on reviewing a set of drawings and calculations
submitted by a MSE retaining wall vendor for a freeway widening project
currently under construction. I can’t tell you how tedious this can be, wading
through a stack of their spreadsheet runs and looking at sheet after sheet of
drawings and performing my own parallel calculations. But every time I find
myself getting tired of it or wanting to take a shortcut, I just think about
failures like the I-35W bridge. Even when I am out standing behind a drill rig
classifying soil samples or counting blows of the hammer, I think about how
that information will be used to design a foundation for a bridge or retaining
wall or other structure that someone’s life might depend on and it helps to
keep me focused.
And what about the failure of the review process to prevent
the failure of the bridge? The company I work for does quite a bit of review of
other engineer’s designs, primarily on behalf of state or local transportation
departments. I am very fortunate that the corporate culture here is one of
meticulous attention to detail and a firm belief that nothing should ever leave
our office without being thoroughly checked by someone else. In fact one of our
principals is a retired Civil Engineering professor with a long and
distinguished consulting career as well. He is known for his incredibly
thorough reviews of our reports and calculations. We even joke that he signs
the company Christmas cards in red ink!
Demand / Capacity for Various Gusset Plates (From FHWA Report)
I know for a fact having observed it first hand, that not
every engineer and firm has the same dedication. I have heard people say things
like “it’s not my stamp” in reference to drawings or calculations. While they
may be technically correct in the sense that the bulk of the liability for any
failure would lie with the designer, the reviewer would not be entirely without
liability in the legal sense, and they would DEFINITELY not be free of ethical
and moral obligations. Granted, not every project is a “fracture-critical”
bridge 108-ft over a river, but that is no excuse for not performing proper
reviews.
The original I-35W bridge was designed in the early 1960s.
Even a very young engineer who might have had some part in the design of the
bridge would be at least in their mid-70s now. And likely the more senior
engineers responsible for the overall design or the review would be even older
if they are still alive. But imagine if it was you and that you were still
alive. You would have been retired for a number of years now, probably in the
twilight of your life. Before the collapse you probably looked back with pride
on the many unique and challenging projects you had worked on throughout your
long career. Now imagine the guilt you would feel at having been partially
responsible for the death of 13 people, and an altered life for many of the
injured people to say nothing of the profound economic and psychological
impacts the collapse caused. Your entire professional career would be
tarnished, no matter how prestigious it was. I don’t claim to be
perfect, far from it. In engineering as in life, we are only human and so were
the engineers involved in this tragedy.
But I pray that this kind of failure never happens in my career. If it
does, I hope that I can look back at the work that I did and say that I did
everything I could have done and met all of my moral and ethical obligations to
the profession, to the public, to myself and to my God.
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