Episode 2 – Bridge Sufficiency Rating and Ethical Obligations

Topics: I-35W bridge collapse; bridge sufficiency rating; ethics and legality of being a professional engineer. 

Talking Traffic Episode 002 – Bridge Sufficiency Rating and Ethical Obligations

Good day, and thanks for listening to Talking Traffic, the podcast about explaining the basics of transportation issues. I’m Bill Ruhsam and I host this podcast and its sister website, talkingtraffic.org. Today is August 20, 2007 and this is episode 2, the Bridge Sufficiency Diaries.

If you listened to episode one, you know that I intend these podcasts to be educational, describing the engineering issues so that you may discuss transportation with people from an informed point of view. Normally, the term “Bridge Sufficiency Rating” would not be a topic that came up in casual conversation, but with the tragedy of the Interstate 35 W bridge in Minneapolis/St. Paul, it seems to be on the lips of every pundit and media critter around. I receive Google Alerts with the keyword “Transportation” and last Saturday, every single hit was about the bridge.

There is another topic I’ll touch on here, which is very important to me, but we will cover that after talking about bridges.

The questions I will try to answer in this episode are: What is a structurally deficient bridge? Is a bridge rated as Structurally Deficient unsafe? Who makes these evaluations, anyway?

The term “structurally deficient” comes from the Federal Highway Administration, or FHWA. In 1967, the Silver Bridge joining West Virginia and Ohio collapsed, killing 46 people. This sparked congress to pass legislation requiring every bridge on the Federal Highway System to be evaluated regularly and their conditions recorded in the National Bridge Inventory. The various states are responsible for the periodic inspections.

As a part of the evaluation, many bridge elements are examined including the beams, piers, foundations, pavement, and other components of the bridge. These elements are scored and a sufficiency rating for the bridge is calculated according to a complicated formula. The sufficiency rating is a number between zero and 100 with zero being the worst and 100 being the best. No extra credit is allowed. A quick note about the inspections, though. They are called “arms length” inspections because the people involved with the evaluations get to an arms length of the critical structure and evaluate it visually.

A bridge is deemed structurally deficient if it has a score of 50 or less. However, a bridge is not necessarily unsafe because it’s rated as deficient. It may merely require more maintenance to keep the bridge in active service or the maximum load may need to be reduced by an appropriate load limit sign. A label of structurally deficient MAY mean that a bridge should be closed immediately, but no high volume bridge should ever get to that point. The biannual inspections should catch any serious problems before they become disastrous ones. Fundamentally, the bridge sufficiency ratings are used by the states to prioritize their bridge maintenance and construction projects. There’s a big catch, though. Federal funding is not usually available for “regular maintenance” on bridges. It only comes into play during major construction. This is perceived as a flaw in the system by many experts.

I just finished saying that a structurally deficient bridge is not necessarily unsafe. The I-35W bridge is the example that disproves this generalization. It was evaluated as structurally deficient, but wasn’t in a condition that warranted immediate repair or replacement. Or so the Minnesota department of transportation thought. Unfortunately, disastrously, they were wrong. Why they were wrong is under investigation. The investigation into the bridge collapse will be thorough and I’m sure they will determine the root cause of failure. It is up to the engineers involved to learn from this tragedy and disseminate the lessons so similar collapses may be averted.

This brings me to a more personal topic. I’ve mentioned that I am a professional engineer, or PE. As a PE, I am charged with maintaining the safety of the public foremost in my endeavors. If my actions cause harm or loss of life, I am morally, ethically, and LEGALLY liable for those actions. If my signature is on the plans, I am responsible, and that thought gives me the shivers when I think about the I-35W bridge collapse. I understand through news reports that several different teams of engineers have evaluated that bridge since 1997. Each rated it as deficient, but capable of maintaining the traffic load. Each team head accepted personal responsibility for the accuracy of that assertion. It’s possible that one or more of those professional engineers will be blamed, or sued, or prosecuted for the 5 fatalities and more than 100 injuries that occurred during the collapse. Now, understand that I have no firsthand knowledge of the situation and that, for all I know, there is an engineer who is, in all respects, responsible for this disaster. It’s happened before. What makes me shiver is that the public, or the government, will go hunting scapegoats within the group of people who may have done everything right, within the bounds of policy, yet go caught in the gears. I hope that we will not need to sacrifice people on the altar of public scrutiny before we can learn how to avoid disasters like this.

To wrap up, in this episode we learned that bridges are evaluated every two years (or more often) and given a Bridge Sufficiency Rating. This evaluation is conducted by individual states, but is mandated by the Federal Government. We also learned that calling a bridge structurally deficient does not mean it is unsafe. I barely scratched the surface of the national bridge inventory, but if you are so inclined, you may see the episode notes and read to your heart’s contentment.

I had several comments on episode 1, concerning the basics of traffic signals. Mike wanted to know if bicycles are picked up by the loop sensors cut into the roadway. Indeed they are, as Mike 2 asserted through his experience in a fast-service establishment with a loop sensor at the drive through order menu. Mike also wondered about the differences in signal operation around the country. His full comment was “One thing I’ve noticed since moving to Michigan is that left turn signals operate differently than most places on the East Coast where I have previously lived. Dedicated left turn arrows illuminate following the green cycle for the road as compared to before the main road gets their green. When the main road has a green, the left turn signal flashes red, indicating left turn is authorized contingent on oncoming traffic. Any thoughts on the pros and cons of the two different modes?” Well, Mike, you’re asking about signal phasing which is a topic suitable for several episodes. I’m sure we’ll see it here in the not too distant future.

This podcast is distributed under a creative commons, attribution, Non-commercial, no derivatives license which means you may share it with others, but don’t sell or alter it. Other information including episode notes can be found at www.talkingtraffic.org and you can reach me by email at Bill@talkingtraffic.org. Comments and suggestions are always appreciated. The music background is Steam Train by John Williams found at magnatunes.com along with many other excellent artists. Have a great day, and always remember to respect your fellow travelers.

Episode Notes:

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