Guest essay by Eric Worrall
According to a new study, if civic authorities don’t properly clean clogged bridge expansion gaps, warmer temperatures might hasten the failure of major support structures.
Climate change may see one in four US steel bridges collapse by 2040
Bridges in the US and other high-income countries are ageing and deteriorating. Last year, a large portion of an Italian bridge built in the 1960s collapsed, killing more than 40 people.
One of the most common problems involves expansion joints. These allow sections of a bridge to swell and shrink in warmer weather without weakening the structure. But they cause major structural problems if they malfunction.
Hussam Mahmoud at Colorado State University and his colleague decided to model the effects of increasing temperatures on steel bridges around the US.
Mahmoud analysed data on the condition of around 90,000 bridges across the US and modelled how the expansion joints would be affected under temperatures predicted for the next 80 years.
They found that one in four bridges are at risk of a section failing in the next 21 years, rising to 28 per cent by 2060 and 49 per cent by 2080. Almost all are set to fail by 2100.
“These failures are very serious,” says Mahmoud.
The abstract of the study;
Published: October 23, 2019
Bridges in America are aging and deteriorating, causing substantial financial strain on federal resources and tax payers’ money. Of the various deterioration issues in bridges, one of the most common and costly is malfunctioning of expansion joints, connecting two bridge spans, due to accumulation of debris and dirt in the joint. Although expansion joints are small components of bridges’ superstructure, their malfunction can result in major structural problems and when coupled with thermal stresses, the demand on the structural elements could be further amplified. Intuitively, these additional demands are expected to even worsen if one considers potential future temperature rise due to climate change. Indeed, it has been speculated that climate change is likely to have negative effect on bridges worldwide. However, to date there has been no serious attempts to quantify this effect on a larger spatial scale with no studies pertaining to the integrity of the main load carrying girders. In this study, we attempt to quantify the effect of clogged joints and climate change on failure of the superstructure of a class of steel bridges around the U.S. We surprisingly find that potentially most of the main load carrying girders, in the analyzed bridges, could reach their ultimate capacity when subjected to service load and future climate changes. We further discover that out of nine U.S. regions, the most vulnerable bridges, in a descending order, are those located in the Northern Rockies & Plains, Northwest and Upper Midwest. Ultimately, this study proposes an approach to establish a priority order of bridge maintenance and repair to manage limited funding among a vast inventory in an era of climate change.
Naturally the study uses RCP 8.5 for its most dire predictions, though they also consider other scenarios like the relatively benign RCP 2.6.
Interestingly the study authors appear to have used average rather than peak minimum and maximum temperatures applied to a bridge with dysfunctional expansion gaps, to predict when each bridge will exceed its structural design tolerances.
The use of average is an interesting choice of metric. I have seen plenty of software systems fail because the hardware capacity estimates were based on average load rather than peak load.
Although peak temperatures would have made the modelled risk of future disaster seem more likely, I suspect the use of peak temperatures might have flattened the impact of global warming on their risk scenario, by raising the modelled risk of immediate bridge failure relative to future failure.
Leaving aside the fun with numbers, the real takeaway is, make sure politicians make an effort to maintain road infrastructure. Defective, poorly maintained bridge expansion gaps are a serious safety risk regardless of whether global temperatures rise. A bridge as poorly maintained as the study authors propose is a disaster waiting to happen. Global warming, if it occurs, might hasten that disaster, but the risk of failure of a poorly maintained bridge is severe regardless of what happens to global temperature.