NEWSWEEK: How long after construction does a bridge stay sturdy before it requires reinforcements or renovations? Edwin Rossow: There’s no set period of time. It depends upon the environment of the bridge and the maintenance of the bridge, and that can vary greatly. If you have a bridge that is near salt water, the detrimental effect of the salt spray or the salt in the air from the water can cause deterioration faster than on a bridge that is over fresh water or railroads. Really, the manner in which you have to assess the safety of a bridge is through inspection, and that requires looking at the elements of the bridge to make sure that there are no circumstances where they have deteriorated or there has been a change in the manner in which the bridge is being used that has accelerated deterioration. Maybe a foundation has moved—that’s not a widespread problem, but there are circumstances that may lead to foundations moving. There may be unusual temperature variations, which causes problems with supports on the bridge for example. But civil engineers are educated to create safe designs that have a certain degree of latitude for error, for construction or for later deterioration. When things are well done and when the maintenance is well carried out, the structure will function for—in quotes—forever.
What are the structural problems most frequently seen in U.S. bridges? What causes them? The most general conditions are just general deterioration due to age, and it’s very much like the walls of your house. Over time, cracks appear. Wallpaper comes loose, paint peels. There’s a constant maintenance program that needs to be used to maintain the whole integrity of structures, and even though bridges are designed with a consideration of loss of capability because of deterioration, you can’t design it for significant loss. So they have to be maintained. Steel bridges need to be painted, reinforced concrete bridges need loose pieces removed and replaced with fresh concrete.
The American Society of Civil Engineers reports that 27 percent of American bridges are “deficient” or “obsolete.” Is that as bad as it sounds? With those terms, we’re really just talking about the fact that the bridge—or the structure, or the highway—is so old that it no longer meets the requirements of modern design specifications. And this means, for example, lane widths are not sufficiently large, the barriers that are used to help keep traffic on the highway in case they wander off to the side don’t meet current specifications. Also, the loads that can be carried by trucks have slowly increased over the years, so that means that the bridge, or even a highway, is not designed for or capable of carrying these heavier loads. So that’s functional obsolescence. Structural deficiency comes up because the old design is not equipped to carry the load or endure the loss of structural capability from corrosion or from fatigue.
The Minneapolis bridge was among those deficient 27 percent. When will we know what kind of “structural deficiency” it suffered? This comes through the inspection process, which is generally two to three years for major structures such as the Minneapolis bridge. The inspection process is used to assess the structural capably of the bridge compared to its capability when it was designed and first constructed.
Should we take this bridge collapse as a warning sign, or is it just a fluke? Well, it’s not a fluke. I guess what you can say is that this unfortunate disaster in Minneapolis is a reminder of the fact that our infrastructure—highways, our bridges, as well as other things—need to be maintained much in the same way that the roof over your house needs to be maintained in order for everything to function the way it is intended to function. So when we say 27 percent don’t meet what we want them to meet, what we’re saying here is it’s a warning that we need to address the upgrading of the infrastructure, the maintenance of the structure, the attention to the most severely corroded or deficient structures and repair them or take them out of service.
For large structures such as this, this is extraordinarily unusual. I would have no problem crossing most any bridge in the United States, because this occurrence is so unusual that it does not say we’re in deep trouble with all major bridges, but it does remind us that in order to have functional and safe structures we need to maintain them.
With 700,000-plus bridges being monitored by inspectors across the nation, what does it mean that this one, which passed inspections in 2005 and 2006, crumbled? The odds that all of those are being maintained regularly is pretty small. But the flip side is that they are being monitored, maybe not as much as they should, but they’re being monitored, and the worst are being repaired, replaced or taken out of service. And, well, it eventually comes down to the same old thing, it’s just a matter of money. Some states are better off for transportation funding than others.
What precautions are being taken to prevent disasters like this one? People should be concerned about whether the inspections are done in a timely manner. The inspection cycle is every two or three years. And it should be done by competently trained people, and the results of the inspections need to be appropriately evaluated. Each state makes its own judgment calls about bridges based on these inspections.
Aside from the inspections, what kinds of precautions are being taken? Inspections turn up problems, and fixing them is all part of taking precautions. They reveal that there’s a need for painting or removing loose concrete and replacing it, or protecting reinforcing bars from environmental problems, so often times an inspection will lead to these maintenance requirements.
There were construction workers on the bridge when it collapsed. Could they have caused it? My understanding is that the bridge was being resurfaced, so they would be at least grinding down the surface on that side and then putting down a new surface on top of it. But to the best of my knowledge from what they’re saying they weren’t working on the structural elements. But yes, it could cause [the collapse] by changing the weight distribution on the bridge, because traffic is being funneled to one side of the bridge, or changing the weight of the decking. There are small changes, they should not be significant but there are small changes that occur [from superficial construction work].
The blogosphere is abuzz with talk of expansion joints malfunctioning due to debris. What does that mean? Expansion joints are meant to allow the bridge to change linear dimension with temperature. I don’t know if you’ve ever seen them or not, but they’re sort of two sets of fingers that intersect, and when the bridge contracts in cold weather, they kind of move apart, but they still provide the necessary support for traffic to run. And in hot weather, they close, but they don’t completely close, which would cause stresses across the top of the bridge. There’s a drawback to this of course. If debris gets into the joints, then they don’t function as they’re designed to function and can cause problems, although that’s a little bit unusual. The other thing is that when the joints are slightly open, you then get debris falling through, and in particular the concern is that if you use salt to get the snow or the ice off of the road surface then that drips down through and salt is very corrosive.
Could de-icing, which Minneapolis does with potassium acetate, have contributed to unexpected corrosion? I’m not that familiar with the particular chemical, but I do know that Minnesota is very concerned about the effects of salt, so this is probably better than salt would be. Salts are ineffective when roads get below 20 degrees. And of course Minnesota gets below that level fairly frequently during the winter, so they don’t use it very extensively.
Methods for building truss bridges have changed in recent years. Is that a sign that something was wrong with the old method? No. The changes relate to the manner in which the structure is designed. In other words it may be designed to carry the loads in a particular way, and the economics of the construction process as new methods of erecting bridges come about they’re put into use. But if a structure is successfully birthed, then it’s OK.
What does it take for a bridge to collapse? Is it a matter of one car too many or 10 tons too much weight? These are really very complicated questions, because the fact that the structure is up and is functional is very, very high indication that it’s all right, but it’s not a guarantee of 100 percent. There may be defects in the construction process that were not detected, but generally speaking, for properly constructed bridges, the problem of collapse as we’ve seen here comes about because of slow deterioration or perhaps a totally unexpected circumstance that has occurred, or perhaps inspection was improper—in other words, something was overlooked and so that’s a human error. But as I said, you know, no structure is forever. And even the pyramids show signs of wear. So you have to understand that if you want to maintain the full 100 percent functional capacity of the structure, you have to pay full attention to the maintenance of it.
But is a 40-year-old bridge old enough to have suffered so much wear and tear? A 40-year-old bridge is not an old bridge.
