London Bridges Falling Down

“Hundreds of bridges across Cumbria County in England were under review following the Nov. 20 flooding resulting from the highest rainfall in a 24-hour period on record in the U.K. The flooding caused the collapse of six bridges and the death of one police officer. One of the larger bridges to fail was the 50-meter-long, two-span, brick-built road bridge over the Derwent River in Workington. Official says the bridge was inspected and declared sound in July 2008. The town’s second bridge, the Calva, was closed because of serious damage.”

I am surprised to hear that not just one or two bridges, but a total of 6 bridges collapsed due to flooding and rainfall. However, I am more surprised by how little coverage and news these collapses are getting. We all remember how big the bridge collapse in Minnesota was (I even had one of my college classes do a case study on it) and that was just one bridge, this is 6!  The circumstances are different though, since these collapses were due to a natural event, not a calculation or construction error.  These collapses also might be downplayed because they happened in another country.

Since there is little information on these collapses, I was left with unanswered questions on the incident.  I have yet to find out how old these bridges were and of which materials each bridge was made.  I wonder who (if anyone) will be blamed for the collapses.  Is this just an act of nature that couldn’t have been avoided? Or is this a slight oversight by engineers and now the worst case scenario actually happened, causing a structural collapse? Most likely the first, but it does raise the question, “What should an engineer design for when it comes to acts of nature?”  I am still new to the industry, but I have seen that designing for a 100 year flood seems to be a rule of thumb.  Obviously this flooding event was the most extreme that England has ever seen, so there would have been no way an engineer could design for such a flood.  Luckily, only one death was caused by these collapses, but what if these bridges weren’t in one of the least populated counties in England, but in a highly populated city like London? The death toll would be much larger and the amount of money in damages would be as well.  It again raises the question, “What should an engineer design for when it comes to acts of nature?”  On one hand you can design for what the codes call for and meet the minimum requirements which reduces costs and makes a smaller footprint on the Earth.  On the other hand, designing for the minimum amount increases your chances for a failure.  Even though those chances are small, they do happen.  Cumbria County and its flood is just one example, there was also New Orleans and Hurricane Katrina, as well as San Francisco and its earthquakes.  Since these events have occurred, some engineers have been taking nature’s effects into stronger consideration.  Many buildings in high seismic categories, like California, are being designed to withstand higher earthquake effects.

There is no clear answer to this question and different people will have different opinions. It really boils down to if you want to save money, or potentially save lives.  I vote for saving lives, plus no engineer wants to see one of their projects be in headlines because of a failure, whether it was nature’s fault or not.

-Doug Cantrell

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~ by castle2268 on December 9, 2009.

One Response to “London Bridges Falling Down”

  1. Doug makes an interesting point. Bridges are typically designed for 100 year floods, and this was a flood greater than that, so the engineers could not have or at least did not design for that flood. All of our standards – AASHTO, AISC, ACI, IBC give us design guidelines. These guidelines are based on statistics. They are based on designing to the most probable, not the ultimate condition. We design footing depths to 3.5 feet in this area, but that is statistically the frost depth 99 percent of the time or there about. It is not 100% guaranteed that frost will not go deeper than 3.5 feet in this area. It is just very unlikely. All design codes are based on statistics as well. We use what are called safety factors. Safety factors perform two purposes. One, they cover the unknowns of the real world compared to the perfect calculated world we use to design on paper. Safety factors also take into account the probability a certain event will occur. For instance, the new AASHTO LRFD code has a safety factor based on the importance of the item or element being designed. The more important the element, the higher the safety factor. That right there should tell you that our design codes are not a 100% guarantee.

    If we as designers had to design to 100% guarantee, nobody could afford the construction. I don’t know if many people realize what a structural engineer’s job really is. Pretty much anybody can build something that won’t fall. But can you afford it? A structural engineer designs something that can be built with today’s resources and be “safe” for today’s public and use. Even in engineering there are no guarantees in life. But if we make it seem that way, then I guess we have done our jobs.

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