Structures - Part 2

Oct 18, 2017


Corbett Freeman, P.E.

In part one of my two-part blog series on structures, I discussed rehabilitation of structures as a cost-effective solution to prolong the life of infrastructure. By stretching our infrastructure dollars, we can do more to improve our systems and replace structures that are beyond repair at least in an economic sense.


There is one area where, despite news articles each year to the contrary, we’ve done a good job at systematically replacing decaying infrastructure. Since 1978, the Highway Bridge Program has provided federal funding for repair and replacement of bridges. Even though the program has “highway” in its title, funds are also used for replacing non-highway bridges which are called off-system bridges. With local and TxDOT matching funds of 10% each, the number of structurally deficient bridges has decreased from 2,433 to 998 from the year 2001 to September 2012.


A few questions remain to be answered, however:


1. Aren’t 998 structurally deficient bridges still a lot of structures? Yes, and no. Close to a thousand bridges seems like too many. However, we have over 600,000 bridges in the United States and a tenth of those bridges are structurally deficient. Just going by the numbers that means that less than two percent of structurally deficient U.S. bridges are in Texas. When you consider the sheer volume of bridges in Texas, the picture is just as good. There are over 50,000 bridges in Texas. That’s again only 2 percent of Texas bridges that are structurally deficient compared to the national average of 10 percent.


2. What is a structurally deficient or functionally obsolete bridge? A structurally deficient bridge has deterioration severe enough to cause an extreme restriction on load carrying capacity. A functionally obsolete bridge is unable to serve the current traffic demand because of some design deficiency. This could be the clearance under the bridge, how much load it can carry, or simply that the bridge is in a tight roadway curve.

 

3. Is a structurally deficient bridge in danger of falling down when I drive across it? A typical span can carry at least two lanes of HS-20 loading. (Note, new bridges are built to an even higher standard.) This is equivalent to 144,000 pounds. So, even severe damage that reduces the load capacity to 10% would still be enough for typical traffic of cars and pickup trucks. Failures are typically caused by heavy trucks that disregard load posting signs advising of the bridge capacity or by impacts from these same trucks beneath the bridge.

 

4. What is a fracture critical bridge? Is that bad? Fracture critical bridges have elements of the structure that, if compromised, can cause a progressive collapse of the bridge. In other words, one element fails which yields another failure and ultimately the collapse of the structure. We typically avoid this type of design for that very reason. It’s not necessarily a bad design, just a structure that needs more careful examination and consideration on a regular basis.

 

5. What do we do now? Keep it up. Our bridges are going to continue to decay and 70,000 more households move to Texas every year. So, just because we are doing a good job doesn’t mean we can rest. We should continue to replace these bridges as quickly as we can due to the increased demand. Trump’s infrastructure plan should help, but we already have many tools to use for roadway and bridge construction here in Texas.

 

Lastly, the Federal Highway Administration is shifting to just calling decaying bridges by what they really are, “poor” in condition. This will clarify why we are replacing a particular bridge and avoid the use of the term “structurally deficient” as a headline or scare tactic. Here’s one gem that I recently read in an article about bridges, “Nobody wants to be the poor soul in the last car that finally collapses one of them.” As discussed, this is not a likely concern, just an argument to paint a gloomy picture. In fact, Departments of Transportation around the country utilize performance measures to determine bridges to replace instead of the sometimes confusing or alarming terms that engineers use when talking about these bridges among themselves.


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