Georgians are freaking out over this photograph that went viral over the holidays. I guess there is some reason to be cautious given that an entire elevated section of Interstate 85 in Atlanta collapsed last year. I remember that well because my family was in Atlanta for a volleyball tournament, and it became a nightmarish commute. That incident turned out to be caused by a fire underneath the bridge. In the social era that we live in, it did not take long for the picture at this link of an apparent "gap" in the road to go viral. It was originally posted on Reddit. When I saw the picture, I immediately knew what it was. You might be surprised to find out that there is a meteorology connection.
Adam Dean is a colleague and senior weather product architect with The Weather Group. He made the following point in a message on my social media page:
I took a look on Google Street view today just for fun and there are gaps like this all over Spaghetti Junction for at least 11 years...
The picture provided by Adam Dean (above) is from 2007 and is very similar to the viral picture (below) that the Georgia Department of Transportation had to debunk (more on tis later). Gaps like the viral picture have always been there. Someone with a social media account and a phone camera just happened to notice this one. The power of social media gave it a life of its own.
As noted, I can understand why this looks scary to many people. If you are not familiar with the Atlanta freeway system, this picture is taken at an interchange at Interstate 285 and Interstate 85 called the Tom Moreland Interchange. To many people in Atlanta, it is called "Spaghetti Junction" because of the intricate weaving of bridges and ramps. It is so scary that many people will not drive across it (see this link for a look at why so). I routinely find myself on Spaghetti Junction. My point here is that the interchange is already scary and then you throw a viral picture out of context into the mix.
What we are actually seeing in the picture is called an expansion joint. Georgia Department of Transportation Twitter site tweeted the picture above and noted,
We’re sure you’ve seen this photo that caused concern. This is an expansion joint designed to expand and contract as needed & is a common design element among bridge design & general construction worldwide. The bridge is fine & is designed as it appears. Thanks for your concern!
There is actually some meteorology behind why expansion joints are needed. This explains why a meteorologist and atmospheric sciences professor at the University of Georgia (me) is writing about it. It is a teachable moment. The EMSEAL (Expansion Joints and Precompressed Sealants) website glossary clearly states:
an expansion joint is a mid-structure separation designed to relieve stress on building materials caused...thermal expansion and contraction caused by temperature changes, sway caused by wind, seismic events, static load deflection, live load deflection...Expansion joint systems are used to bridge the gap and restore building assembly functions while accommodating expected movements.
Expansion joints are found in buildings, bridges, rails, and other engineered structures. Ok, Dr. Shepherd, so where does the meteorology come into play? During a given day, the diurnal temperature range (the difference between the maximum and minimum temperature) can be large, especially during transition seasons like Spring and Fall. In Atlanta, the average high temperature is 70.3 deg F, and the average low temperature is 52.5 deg F in a given year. However, extreme highs can exceed 90 deg F, and extreme lows can go well below 30 deg F. Basic physics reminds us that when structures heat up, the building material (steel for example) expands. As materials cool down, they contract. The expansion joints in bridges are needed to account for annual, weekly, monthly or even daily variations in temperature. Which means that the gap in the joint opens and closes to provide "give" as temperature changes. If this gap was not there, there would be dangerous stresses on the bridge and structural fractures. That's when we really would need to "viraling pictures" and worrying about driving across that bridge.
In Georgia over the past few weeks, temperatures have varied. According to data from the Accuweather website, the diurnal temperature range for Atlanta has been around 20 deg F in recent weeks. This means that high temperatures have been in the mid-to-upper 50s and the low temperatures have been in the mid-to-upper 30s. This ominous looking gap is an example of science and engineering in action.
