Sunday, March 29, 2015
A Bridge Too Close
Construction accidents that lead to loss of life are always tragic, but it's unusual that anyone other than construction workers are killed or injured in them. When hazards to the public are involved, most construction organizations take extra precautions to make sure that public safety isn't jeopardized by potentially dangerous operations or conditions. Last week, a freak accident at a construction site came uncomfortably close to turning me into a statistic, but instead picked off another unfortunate member of the public whose safety engineers are supposed to protect.
Most new interstate bridge construction in Texas these days involves those huge prestressed-concrete beams—the ones you will see now and then rolling down a highway after being turned temporarily into a long trailer by hitching the front to a tractor and the back to a set of wheels. Whenever one of these beams is hoisted into place across an active roadway, the Texas Department of Transportation (TxDOT) usually waits until late at night and diverts traffic away from the overpass under construction, so that if something goes wrong, no passing vehicle will be squashed like a bug under a falling beam. But once the beam is in place, especially if the roadway involved is as busy as IH-35 that goes from Laredo on the Mexican border up through Austin and Dallas-Fort Worth, the usual thing is to let traffic pass once again. And that is usually all the precautions that are needed. But it wasn't enough on Thursday, March 26.
Somehow—the investigation is still ongoing—a flatbed tractor-trailer carrying a bucket-truck-type load wound up going north on I-35 near the small tourist town of Salado, about 50 miles north of Austin. As part of a months-long construction project to widen the interstate in that area, TxDOT had recently placed the last of about a dozen of the concrete-beam monsters across both northbound and southbound existing lanes as part of a new overpass. So the picture was that the beams were bridging the roadway, but the structure wasn't yet a bridge—just a set of beams laid between the end buttresses. Once the superstructure of the bridge was completed, TxDOT would "armor-plate" the leading edge of the bridge with a thick steel L-beam so that any overheight loads would simply damage the armor plating without threatening the structural integrity of the bridge. But there was no armor plating in place on Thursday.
Concrete is great for bearing evenly distributed compressive loads. Concrete beams even perform in tension well if prestressed to be in compression, as these beams no doubt were. But anyone who has tried to make a hole in concrete with a chisel knows that if you concentrate a lot of pressure at a small point, concrete will crack. And that's exactly what happened about 11:30 AM on March 26.
Despite height-warning signs of a 13.5-foot clearance (there was actually more room than that), part of the flatbed truck's load was high enough to strike the leading concrete beam with such force that it not only cracked, but slammed forward into the adjacent beam and caused it to fail too. Once that happened, many dozens of tons of concrete beams plunged onto the busy roadway. It was just a matter of chance that one of the people driving under the bridge at that moment wasn't yours truly.
Earlier that day, I had left San Marcos by car for Wichita, Kansas—to attend a funeral, as it happens. The logical route would have been straight under the beams in question, but lately, I have encountered so many traffic delays on I-35 that I've taken to driving a back-road route to the west. It goes through small towns and beautiful hill-country vistas, and although it takes about an hour longer than you can do on I-35 when I-35 doesn't have any slowdowns, I believe the last time I-35 didn't have any slowdowns was sometime around 2003. So about the time the beams were crashing down in Salado, I was leaving I-35 in south Austin for my leisurely westerly route.
An unfortunate pickup driver was crushed beneath one of the beams and died, and three other people were treated for minor injuries. The driver of the flatbed that apparently caused the accident was uninjured and made it through the crash to the other side, but will no doubt face a lot of questions from authorities and investigators. And TxDOT may start thinking about putting armor-plating on concrete beams the minute they are put in place, rather than waiting until the bridge is finished to do so.
In one of the news reports, a TxDOT spokesman objected to the headline "Bridge Collapse" because the structure in question wasn't yet a bridge. This is a distinction without a difference to most people, but to engineers, especially bridge engineers, you can't have a bridge collapse until you have a bridge. Technically, this was a construction accident, not a bridge collapse. But the man in the pickup is just as dead, and it's possible that compartmentalized thinking has contributed to this accident.
Clearly, the beam was vulnerable to damage caused by traffic from the moment it was put in place and traffic was allowed to pass. But somehow, the sequence of construction didn't allow for the armor plating (as I'm calling it) to be attached until later in the process. Because the circumstances are so unusual, construction crews have avoided such an accident up to now. It's rare that any type of overheight load gets onto the interstate and goes very far, much less one that has the concentrated mass and speed to actually break a large concrete beam. But we have seen that it can happen, and I bet that somewhere in Austin engineers are rewriting construction specs to make sure that any beams exposed to traffic have their armor-plating on from the start.
Besides death and injuries, this accident caused annoyance to thousands of beleaguered denizens of I-35, which has the reputation of harboring some of the worst freeway traffic in the U. S. Situations like this are exactly why I no longer drive long distances on that stretch of roadway if I can avoid it. My sympathies are with the family and friend of the man who died, with everyone who got stuck in the 10-mile backup for hours, and for the engineers who are sadder but wiser after experience has taught them a better way to build bridges—a category of engineers that stretches back hundreds of years to the dawn of history.