Bridges are usually built to last between 75 and 100 years—and many stay in use even longer. But heavier traffic, larger vehicles, and changing weather patterns are putting more stress on them. Climate change, for example, can accelerate the stress of expansion and contraction caused by larger temperature fluctuations. A University of Minnesota research project led by Lauren Linderman, associate professor in the Department of Civil, Environmental, and Geo- Engineering and a CTS scholar, is exploring the impact of temperature changes on the strength and service life of Minnesota’s bridges.
As concrete heats up or cools down unevenly from top to bottom, it expands or contracts unevenly, too. This uneven movement, called a thermal gradient, creates internal stresses in the bridge, Linderman explains. A positive thermal gradient means the top of the bridge is warmer than the middle, while a negative thermal gradient means the top is cooler than the middle. When these temperature differences occur, they can push (compress) or pull (tension) parts of the bridge. Over time, that compression or tension can lead to cracking and other damage.
Previous studies of the I-35W St. Anthony Falls Bridge in Minneapolis found that temperature changes caused more stress than traffic loads—and that existing design standards underestimate how strong these temperature effects can be. Because Minnesota already experiences large temperature swings and is expected to have warmer winters in the future, understanding these thermal effects is especially important here.
The researchers looked at future temperature and sunlight predictions for Minneapolis using detailed climate models. They considered two climate scenarios—moderate warming and high warming for two future time periods: years 2040–2059 and 2080–2099. When investigating historic gradients and future environmental conditions, they found that the worst positive gradients (top much warmer than the middle) happen when temperatures rise quickly from one day to the next and when sunshine is abundant. On the other hand, the worst negative gradients (top much cooler than the middle) happen when temperatures drop sharply, there’s little sunshine, and it’s windy. Researchers considered the 10 worst case scenarios fitting these criteria for each of the different climate projections. Using computer models of the I-35W bridge, they simulated how these future temperature changes could affect the bridge’s temperature distribution and the resulting internal stresses.
Climate change has the biggest impact on negative thermal gradients—occurring when the bridge’s top and bottom is cooler than the middle. These future negative gradients could be about three times stronger than what has been measured in the past. The projected positive thermal gradients, while greater than design gradients, are in line with historic measured gradients on the I-35W St. Anthony Falls Bridge. Although positive gradients are still important, the larger negative gradients pose a bigger long-term risk because they cause tensile stress (pulling), which can lead to cracking. Cracks allow water and road salt to seep in, speeding up deterioration.
Minnesota’s bridges may face significantly higher thermal stresses in the future, especially when considering negative gradients, which are less seasonally dependent. Future bridge designs, retrofit strategies, and maintenance operations should plan for this—by updating design standards, improving materials, and adjusting maintenance schedules—to make sure the state’s bridges remain durable for decades to come, Linderman says.
This project was supported by CTS seed funding. Awarded biennially, this funding aims to help CTS scholars develop expertise in emerging areas and foster strategic relationships that position them for future funding opportunities.
—Adapted from content contributed by Lauren Linderman.
