By Russell Larsen, PE, SE; Farshad Berahman, Ph.D; Kevin Aswegan, PE, SE

Click here to view the article with images in the June 2024 digital edition of STRUCTURE magazine.

Trust, but verify!”

Russian proverb or political saying aside, how often do engineers look back and confirm whether their designs have achieved their objectives? For the structural engineers at Magnusson Klemencic Associates (MKA), asking and answering this question was top of mind given their design of four notable office and residential towers between 2014 and 2020—Seattle’s Rainier Square and Chicago’s 971-foot-tall One Chicago, 1,198-foot-tall St. Regis, and 752-foot-tall 150 North Riverside.

These towers are similar in height and wind sensitivity, and engineers needed to supplement wind performance beyond base code requirements. Like Chicago, New York, and many other cities, Seattle experiences wind events strong enough to buffet and flex high-rise buildings—a challenge for engineers striving for new achievements in building height, efficiency, beauty, and sustainability while offering safety and structural sturdiness during extreme winds. After completion of MKA’s wind-related design for downtown Seattle’s 58-story, 850-foot-tall Rainier Square tower, the firm revisited its projections to verify design objectives were met, ensuring performance and bolstering confidence in their ability to design the next generation of high-rise towers safely in windy regions.

Rainier Square Case Study: Sloshing Dampers in Practice

Rainier Square is the world’s first high-rise to incorporate the exceedingly strong and stiff American Institute of Steel Construction’s SpeedCore lateral system into its structural design. Rainier Square’s design accounted for the site’s constrained land-use setbacks, development-area limitations, an aggressive construction timeline, and a slender and prismatic upper series of floors.

Early design wind studies carried out by structural wind tunnel testing consultant Rowan Williams Davies Irwin (RWDI) revealed the challenges of delivering satisfactory wind performance at Rainier Square. Built in a seismically active region of the United States, the Performance-Based Seismic Design tower’s square-shaped upper floors also experience notable vortex wind forces at one- to 10-year strength windstorms, the time range at which wind motion for occupants is most critical. Therefore, the seismic design needed to reflect member sizes within the lateral force-resisting system from results provided by wind tunnel-based testing and loads.

With the wind tunnel test findings confirming the need to manage wind motions, the team of the designer, owner, and builder began assessing options:

Structural Stiffening—This option proved too expensive and/or was unhelpful to seismic design.
Reshaping/Adding Exterior Wind Disruption Features—This option did not serve the architectural desires, the usable floor-plate areas, or the setback realities of the constricted sites.

Supplemental Damping—MKA adopted this option, with tuned liquid sloshing mass dampers managing Rainier Square’s motion on windy days. These dampers proved more cost-effective than bespoke mechanical mass pendulum dampers, distributed viscous dampers, and other similar options. Additionally, the owners and contractors were confident that existing jobsite crews could construct and waterproof the damper tanks.

As the project progressed, MKA’s engineers were eager to see if earlier building analyses and wind tunnel findings would materialize in the real-world monitoring data. The first step was to monitor the development of the sway periods during the tower’s construction. Historically, accelerometers detecting ambient accelerations were bulky, expensive, or delicate. Today, economic data collection is possible in real time thanks to Epson’s off-the-shelf microcomputer parts and accelerometers. (See sidebar.)
The accelerometers were deployed to the tower about when the tower reached half its total height to monitor the ambient lateral accelerations caused by day-to-day winds at Rainier Square. Frequency analysis of this data revealed the sway periods. Perhaps without surprise, due to the robust nature of SpeedCore, building sway period predictions closely matched collected data.

Performance Verified

In the months and years after completion, tower monitoring at Rainier Square has continued. Several summer and winter seasons have been observed, including large and small windstorms in Seattle. Acceleration readings have been and continue to be collected. Daily, MKA engineers receive a summary of building performance versus Seattle wind-speed data published by the U.S. National Oceanic and Atmospheric Administration (NOAA).

MKA compares these peak accelerations collected against wind gust data of the same date and time at Rainier Square. Specifically, acceleration and wind speed data are grouped into successive three-hour blocks. From each block, the peak acceleration and wind speed data are collected from the NOAA network; plotting these acceleration points (vertical axis) with wind gust points (horizontal axis) results in the information shown in Figure 1. Data highlights collected at Rainier Square reveal the following:

Acceleration Trends—The blue points in Figure 1 represent the observed storms in 2021 through mid-2023. The solid black line represents the performance expectation established by wind tunnel testing. The results indicate Rainier Square performs as expected. Specifically, accelerations trend at or below the design predictions, as evidenced by the points below the predicted response line.

Large Windstorms—The fewer count of points on the righthand side of Figure 1 illustrates the rare nature of large windstorms. In two-plus years of monitoring, only two relatively large storms occurred: one with slightly less than a one-year recurrence interval and one slightly above the one-year recurrence interval. Neither storm exceeded the performance criteria established during wind tunnel testing.

Wind Direction and Building Response

Figure 1 offers a view of how Rainier Square responds to the wind but omits several interesting facets of the collected data—namely, the building acceleration collected in both axes and building movement (e.g., North-South movement versus East-West movement). MKA considered how data for building directional accelerations versus the direction of wind approaching the building could be visualized and explained simultaneously. In response, Figure 2 was generated, confirming what long-time Seattle residents know—strong and common winds often come from the southwest, or occasionally the north. Furthermore, NOAA data also includes both wind speeds and the directionality of those gusts.

So, what does Figure 2 really tell engineers? Firstly, the diagram includes all the same data from Figure 1 but expands upon it by including approach direction and gust wind speed—with the diagram set up as a compass showing North, South, East, and West directions overlaying the aerial view of Rainier Square tower. Within this compass orientation, the data points from Figure 1 are now positioned as gust wind points around the center with concentric circles indicating gust speed (the further the distance from plot center, the greater the wind gust speed). The overall magnitude of acceleration is then represented by the color of the dot, which is measured in increments of milli-g in the legend—a milli-g being 1/1000th of a “g” or the force of gravity. The relative magnitude in the building X versus Y axis is represented by the length of the whisker lines at select acceleration events.

To further explain, a single circled data point has been identified in Figure 2. This data point communicates the following information:

• Wind gust speed = 45 miles per hour.
• Position within compass shows wind coming from the West Southwest direction.
• The color denotes the peak acceleration within this storm—gray, or 3 to 4 milli-g, in this example.
• Whisker lines show a longer Y axis than X axis, meaning the building is moving more in the along-wind direction.
• This point is also identified in Figure 1 as the “individual storm.”

With all of this collective data plotted and considered overall, several interesting trends emerge from Figure 2:

• Most notably, wind gusts do, in fact, originate from the Southwest (as predicted by the wind tunnel climate models).
• Winds tend to cause across-wind accelerations on par with along-wind accelerations (as predicted by the wind tunnel data and shown in the whisker lines).
• Winds from the North can generate greater “across-wind acceleration” (building X direction), which is consistent with the prediction of the wind tunnel.
• Southwest-approaching winds hitting the flat face of the tower are the source of the most common building response motions—motions which can be along-wind or across-wind.

Added Damping—How’d They Do That?

Given the real-world experience and verified results of Rainier Tower, the data collected furthers the clear path to better and safer high-rise design. As previously mentioned, from the onset of the design of Rainier Square, several wind motion solutions (as well as damping technologies) were considered once it became clear that the wind response would challenge the design. Damping is not the only method to manage wind response. Alternatives to mitigate motion, as described above, include adding mass, where a more massive building accelerates less when pushed by the wind, or adding stiffness, where a stiffer building moves less when pushed by the wind. But each of these options comes with additional costs, as more material equates to more money and, indirectly, can hamper seismic design when larger lateral force-resisting system structural members are required for wind resistance, which in turn elevate the member and connection forces observed during seismic design.

Once the stiffness and added mass “brute force” solutions were off-the-table for Rainier Square, three damping technologies were considered: a sloshing damper, viscoelastic dampers, and a tuned mass damper. In the end, a sloshing damper was chosen. Sloshing dampers operate similarly to a traditional tuned mass damper, but instead of a steel or concrete mass swaying from a cable or mechanical assembly, the natural sloshing of water within a storage volume provides both the damper mass in the form of the water’s weight, and the damper tuning by controlling the sloshing rate of the water. Specifically, in tuned liquid sloshing mass dampers, the water’s depth versus the tank’s length establishes the frequency, or rate, at which water will naturally move from one end of the tank to the other. When properly tuned, a sloshing damper uses water to counter the sway of the building. As the building moves left, the water sloshes up the tank’s right wall. The difference in water pressure between the right wall (higher pressure) and the left wall (lower pressure) creates a net force to the right, thus pulling back the building.

While sloshing dampers have been applied to buildings for at least 25 years, designs before the mid-2000s involved building a scale model of the tank and subjecting it to the expected motions of the wind. Much of this work was carried out at the University of Western Ontario. One of the researchers, Dr. Michael Tait, went on to develop and publish methods that allowed designers to calculate the necessary tank system’s length, width, water depth, and internal baffle parameters. MKA’s engineers applied Dr. Tait’s approach in the initial sizing and final designs for Rainier Square.

In the end, implementation of sloshing dampers carried the day at Rainier Square due to the available rooftop space, the relative advantage of the “simple construction” of a waterproofed concrete box, lack of long-term maintenance, and the reduced lead time compared to the bespoke fabrication of other damper technologies.

Trust and Verify

Engineers deeply trust researchers, code writers, and consultants that they partner with, support, and look to. Seeing the mathematics, methods, and engineering, confirmed strongly in real-world completed projects, is refreshing. As evidenced by Rainier Square and four other similarly tall and damped towers currently monitored by MKA, wind tunnel testing and advancements in supplemental damping allow engineers to build towers that achieve new standards for height, efficiency, beauty, and sustainability while offering safety and structural sturdiness during extreme wind events. ■