Bridging the Gap

The news in Minneapolis this week was unreal. A highway bridge collapsed over the Mississippi during rush hour with vehicles going over the bridge. This tragedy is a rare event that is making many people think about bridges and wonder about how you know if they are safe.

I started doing this in college. My sophomore curriculum in Mechanical Engineering included a Statics course. This is where you learn about the forces in structures, how things want to bend, and how weights and levers affect bodies at rest. The Statics textbook authors loved to use little bridges to illustrate the principles it was teaching. I loved the little bridge designs. The ME's would go on to take this information and use it to design tools and machines. The Civil (Structural) Engineers would actually design bridges. Every freshman engineer is taught about a phenomenon called resonance with the example of the Tacoma Narrows Bridge, a structure that tore itself to pieces because of wind induced harmonics. You want engineers in training to be very worried about the consequences of failure.

After school I entered the Army, in their Corps of Engineers. Some of the things we were supposed to know was how to inspect and rate existing bridges, how to design and construct bridges, and how to destroy them. Can you guess why we needed to know this? Army Engineers are there to make sure that the rest of the forces can get where they need to get and that the bad guys can't. A river is called a "water obstacle" and the thinking is to either erect that line as an obstacle for the enemies movement and at the same time, to breach that line to facilitate your own movement. So we learn about the breach points, the bridges.

Demolitions of bridges was a fascinating study. You learn that it is surprisingly hard to knock most bridges down. The trick is to find the weak key point and blast away at it, hoping it would take down the rest of the bridge. There are examples of people that drop a center span of a bridge, but not the approaches, so you could easily rebuild the gap and restore the bridge. There are lots of examples of artillery cutting the wires out to the charges and the demolition failing, or charges badly placed that did nothing. Most bridges have a lot of redundancy built into them. Another note about bridge failure, again involving resonance. We were taught that when large formations of soldiers cross bridges, you go to "route step", which is a marching term meaning you break step, or stop having everyone slam their foot down at the exact same time. Supposedly, the combined force of all those footfalls happening simultaneously is enough to sometimes collapse a bridge.

We also had to inspect bridges and rate their load carrying capability and their integrity. This was surprisingly hard to do, because most of the bridges I looked at were made of wood, and most of the guide was about steel bridges. We had to make sure that bridges could handle heavy construction equipment, and most of the guides rated bridges in terms of main battle tanks. I guess a bulldozer is pretty close. This process of inspecting bridges always bothered me. We learned in materials that steel fails by a mechanism called cyclic fatigue. This is like when you bend a metal hanger enough times and eventually it breaks. You can't see cyclic fatigue, usually. The other mechanism that occurs is stress fracture propagation. Many people believe that tiny cracks on the surface of the metal eventually propagate into the center of the structure and widen, weakening the metal. The problem is that you don't know to what degree metal has weakened, and it's not often evident from a visual inspection. You could see these fractures and indications of fatigue with x-rays, but you can't x-ray a whole bridge. So often, we'd inspect and rate a bridge and I would wonder if we missed something or if our estimate was even in the ball park. One time, I was driving with 2 surveyors in the back country in Panama and we came to a tiny sagging suspension bridge in the country. I didn't think we would ever be able to put a pickup truck across it without breaking the bridge. I barely trusted it for foot traffic. The driver insisted, and as I got out and watched from the safety of the road, he slowly drove over the bridge. Stopping in the middle, he looked up and made eye contact with me, smiling, and started bouncing up and down in his seat. It was very funny. He made it across, but I still wonder how that nasty little bridge held him. Another bridge I remember was an ancient suspension bridge 100' over a river in Ecuador. The wooden floor of the bridge was very old and crumbling and several boards were missing. The first time I went over it, my knees were shaking so bad, I could barely walk. After watching a man drive a herd of cattle over the bridge one day, I stopped worrying about it as much. Later, I always stopped in the middle and got the bridge swaying, because by then it was like a fun amusement park ride.

By far, the most fun was building bridges. At school, they always get students to make a span with popsicle sticks or toothpicks, then add weight until it failed. Calvin and Hobbes had a great cartoon where the father explains to Calvin that the way they made highway bridges in a similar way, overloading and breaking it first and then rebuilding it. The Army uses a bridge called the Bailey bridge, which is the tinker toy method of constructing bridges. You use these truss panel sections, and just stack them up differently for different lengths and weight classes. It was in a manual like a book of recipes for bridges. I got to work on building two of them and also found a few already in place that had been in service for years.

When I got to Panama, I got to see one of the coolest bridges in the world, the Bridge of the Americas. This structure, shown in the picture above, spans the mouth of the Panama canal. It is beautiful, but has a strange feature. There are 4 lanes on the bridge. You go up to and over the bridge on two lanes, then it necks down to one lane. So there's only 3 lanes on the approaches (the ones that are still on the bridge, just not the central span). It's harrowing during rush hour traffic, because you rush across the bridge, speeding for placement at the other end, and then you merge dangerously before the lanes come together, honking your horn the whole time. That was the way you drove in Panama. I crossed the bridge every day to go to work.

Sharon, one of my coworkers, read that people's fear of crossing bridges is second only to public speaking of the biggest fears there are. This week's collapse in Minneapolis is probably not going to help that.