Wind and vibration studies for the longest cable-stayed bridge across the Mississippi River
The U.S. 82 Bridge, which crosses the Mississippi River at Greenville, has a cable-stayed main span of 1,380 feet and a total length of 13,560 feet. The current U.S. 82 Bridge was completed in 2006, replacing an upstream bridge built in 1940, the same year as the Tacoma Narrows bridge, whose collapse just a few months after it was opened to traffic was one of the most notorious engineering failures of the 20th century.
To design safe, resilient long-span bridges, designers must quantify and mitigate the effects of wind on the deck, pillars and cables, as well as the effects of vibration – both from weather conditions and from automotive traffic. As they developed their design for the new U.S. 82 bridge, the engineering team at Howard Needles Tammen and Bergendoff (NHTB) of Kansas City approached us for help both modeling wind conditions at the bridge site and testing the efficacy of their planned mitigation strategies.
Our extensive experience with long-span bridges gave our team a strong understanding of the challenges the bridge designers faced. Our diverse in-house team – including specialists in climate and meteorology and vibration, as well as several of the world’s top wind engineers – enabled us to deliver a comprehensive set of studies to inform NHTB’s plans.
We began by developing a precise picture of wind conditions at the bridge site. Using hourly wind records from the Greenville Municipal Airport as well as nearby recording stations in Mississippi, Arkansas and Tennessee, we constructed a rich historical wind record for the area and used it to create a detailed statistical picture of expected wind loads. This gave us a foundation for anticipating the bridge’s aerodynamic stability both during construction and following its completion.
Having helped the bridge engineers gain a complete understanding of the wind environment at the site, we embarked on the next phase of our work: assessing computer models of the engineers’ preliminary designs with a specific focus on preventing aerodynamic instability. Our team’s past experience with similar structures – and familiarity with the published data on the performance of other long-span bridges under various wind conditions – allowed us to deliver this analysis with efficiency and confidence.
Having supported the NHTB during early design stages, we helped complete the evaluation process through a series of wind tunnel tests. We provided:
- wind tunnel testing of both sectional and full-bridge aeroelastic models. This made it possible to accurately assess the aerodynamic stability of specific sections of the bridge and to determine wind loads on the entire structure.
- wind studies of both the completed bridge and the bridge as it would stand at several critical construction phases. This intelligence enabled the designers to optimize the safety of the structure throughout the building process as well as after the opening of the bridge to the public.
- assessment of wind-induced vibrations of the cables for implementation of cross-ties.
We used criteria for acceptable aerodynamic stability to assess the bridge’s behaviour during the wind tunnel simulations and, where necessary, our team recommended modifications to the structure that would bring the design in line with these parameters.
Detailed wind tunnel tests can be valuable because although published design codes offer some guidance on standard force coefficients, wind tunnel testing allows engineering teams to understand the forces at work on a specific structure at a specific location. Testing in the tunnel can also capture dynamic effects that published design codes can’t anticipate, such as variations in force on different structural elements due to buffeting, or random vibration due to unsteady turbulent winds. We produced wind reports that accounted for the full range of conditions the U.S. 82 bridge – a unique structure in a unique location – could experience. This allowed the bridge’s engineers to proceed with confidence that their design had been fully evaluated.
The U.S. 82 Bridge has been operating successfully since its opening in 2010. When it opened, it was the third-longest cable-stayed bridge in North America.