Prediction and control of pedestrian-induced structural dynamics ensures viewing pleasure
The Hualapai Nation’s Grand Canyon Skywalk offers a unique vantage point overlooking one of North America’s largest and most well-known natural wonders. The horseshoe-shaped cantilever bridge consists of a 10-foot-wide, 2-1/2-inch-thick glass walkway bounded by glass walls 4-1/2 feet tall, all supported by two parallel U-shaped steel box beams. Situated on the west rim of the canyon, the Skywalk extends 70 feet out from Eagle Point and provides visitors with spectacular views of the surrounding canyon as well as the Colorado River, some 4,000 feet below.
From the outset of the project it was apparent that the structural engineering design of the Grand Canyon Skywalk would not be straightforward. One of the challenges that Lochsa Engineering faced was ensuring the structure’s dynamic characteristics would not be prone to excessive, and even potentially catastrophic, vibration caused by wind and pedestrians. Our task was to was to investigate these risks and, in collaboration with Lochsa, develop solutions
Preliminary Design Guidance
Based on a preliminary desktop analysis of the initial design we identified that galloping (a form of wind-induced vibration which can be catastrophic) was a potential concern and that pedestrian-induced vibrations could also cause discomfort for visitors and impose significant loading on the structure. Lochsa Engineering collaborated with us to optimize its design to minimize risk of galloping. However, our analysis indicated that pedestrian-induced vibrations were still a risk that warranted in-depth study.
Comprehensive Dynamic Analysis
We then conducted a comprehensive dynamic analysis to predict the Skywalk’s structural response to patron activity. Our goal was to ensure that various pedestrian activities on the Skywalk – jumping, running, walking, and so forth – would not result in excessive motion due to resonance with the natural frequencies of the structure. We needed to characterize the forces imposed on the structure by such activity and also wanted to make sure the structure’s movement would not make patrons uncomfortable.
We turned to various North American and European codes and design guidelines to determine acceptable limits of structural motion. Because patrons using the Skywalk would be able to see through the glass floor of the walkway into the canyon below, we also had to take into account that this would heighten their sensitivity to motion. We analyzed several scenarios with different loading densities over the Skywalk’s 2,114-square-foot area to account for the different dynamic loads associated with walking, running and other activities, also considering the space available, since fewer people are likely to be running or jumping as congestion increases.
Our analysis identified that the bridge would be safe if designed for the loads associated with pedestrian usage, including a scenario associated with a group of people intentionally attempting to excite the Skywalk by running, jumping or stomping. Also under normal use, with only walking pedestrian traffic, the Skywalk’s motion would not be uncomfortable. However, we also identified that there was a risk that small groups of two or three people running and jumping might push the movement into the uncomfortable range.
Tuned Mass Dampers
We identified that adding supplementary damping to the structure would mitigate the risk of discomfort due to pedestrian-induced motions. It also would eliminate the need for wind tunnel testing to verify whether galloping would be a concern. Understanding this, the Skywalk owner and the design team elected to take a conservative approach by including a tuned mass damping system.
We designed three tuned mass damper units for the Skywalk to mitigate the potential for uncomfortable motion. Custom designed to be contained within the structure’s box beams, the damper system allowed the use of a glass floor configuration to provide optimum viewing of the canyon below, which is integral to the Skywalk’s stunning design.
Since the opening of the Grand Canyon Skywalk it has quickly become a must-see destination. The West Rim of the Grand Canyon previously had fewer than 75,000 visitors annually. However, since the grand opening of the Skywalk, that number has risen to more than 1 million visitors in 2015.
We are proud of our engineering collaboration with Lochsa Engineering on this unique project, which CNN identified it as one of mankind’s greatest engineering achievements.