2000 Structural Engineering Traveling Fellowship
Shaping the Wind and the Face of Tomorrow's Skyscrapers

We marvel at the skill of the ancient builders, at the sheer scale in which they cast their masterpieces, setting a precedent that we to this day still follow: using our built environment as a testament to our civilization, a tribute to our technological prowess and financial strength. Throughout time, civilizations have relied upon structures of ever-increasing height to celebrate their progress and prosperity. Today, that same spirit is embodied in the race towards the heavens undertaken by modern skyscrapers, which have come to define the thriving metropolis worldwide.

The Price of Height

The rapid growth of cities during America’s infancy necessitated the efficient use of real estate, forcing buildings upward, but the lack of sufficient materials to carry the added loads of upper floors placed limits on these designs. In fact, the skyscraper was not a practical undertaking until the advent of electricity, fireproofing, and most importantly, E.G. Otis’s elevator. These components were first synthesized in 1885 in the first metal-framed structure by William Le Baron Jenney to yield the Home Insurance Building of Chicago. From this first skyscraper, America’s skyline has grown with materials and construction technologies, yielding structures towering at amazing heights. Unfortunately, as a consequence of their lightweight design, these structures often suffer from increased flexibility and a lack of sufficient damping. These deficiencies have not left them well suited for the new challenges that await them at higher altitudes in the complex environment created by wind, where the interaction between the wakes of neighboring structures can play havoc with today’s skyscrapers.

Lofty Challenges

With every advance in height comes a new challenge, such as ensuring that the structure remains functional under the action of wind. Even though the structure may satisfactorily carry all the lateral loads, it still must satisfy serviceability requirements in the form of drift limits and perception criteria, as occupants often feel discomfort in the form of dizziness, headaches, and nausea resulting from the accelerations of the building due to the dynamic nature of wind. Traditionally, the use of more efficient structural systems permitted designs to accommodate the challenges imposed by their increased height; however, even with the use of core and outrigger systems and bundled and braced tubes to increase stiffness and limit lateral and torsional motions, the final design may still be forced to consult additional avenues to satisfy occupant perception criteria

Shaping the Wind

Just as the aerospace industry has tailored its designs for optimal performance in wind, the design of tall buildings can use these same considerations to eliminate the problem of wind-induced vibrations at its source. While there may be reluctances that particular aerodynamic modifications will detract from the aesthetics of the structure, the following examples illustrate that these considerations can be integrated into the design of tall buildings without sacrificing their appearance and often creating a signature for that structure.

Modifications to Corner Geometry and Building Shape

The inclusion of chamfered corners, horizontal slots, and slotted corners have been found to considerably reduce the response of buildings, in comparison to the performance of a basic square plan, with these improvements becoming more marked as the corners are progressively rounded. [1] The first series of hand sketches illustrates the spectrum of corner modifications, confirmed by wind tunnel testing to markedly reduce rms displacements. In particular, the chamfering of corners has proven especially beneficial in the design of the Mitsubishi Heavy Industries (MHI) Yokohama Building in Japan, following realization that the wakes of peripheral tall buildings would induce excessive response. While the aerodynamic superiority of this configuration is evident from wind tunnel studies, the modification also improved the appearance of the structure, adding additional depth and contrast to the facade, with the vertical shadows created by this chamfering effect eliminating the redundancy of the horizontal elements along its face.

Setbacks and Tapering

Some of the most graceful and notable structures incorporate a series of setbacks and tapering in their design, accentuating the height of the structure, but also serving practical aerodynamic purposes. Improvements in response have been observed in buildings that vary their cross-sectional shape with height or which reduce their upper-level plans through tapering effects, cutting corners, or progressively dropping off corners with height. [2] The sketches compare the response of a building that reduces its plan with height to one without such modifications, illustrating a marked reduction in response. In fact, the more sculptured a building’s top is, the better it can minimize wind responses, as evidenced by the 450-meter Petronas Towers. The repeated use of multilevel setbacks to taper the towers and reduce their plan with height provided a sleek and elegant appearance while still allowing the integration of Islamic geometric patterns in the cross-sectional shape to merge cultural imagery with modern architecture.

Further evidence of the aesthetic benefits of aerodynamic modifications is provided by the 421-meter Jin Mao building, which uses setbacks to gently taper its facade. While the setbacks draw the eye’s attention up the structure toward its crown, they more importantly gradually reduce and redefine the shape of the structure at the upper levels, where the effects of wind are most critical. The structure is literally transformed by subtle setbacks from a square plan to a complex cruciform, as if slowly twisted from above by an unseen hand. The performance under wind is further enhanced by shifting the ornate crown of the tower about its central axis. The end result is a breathtaking structure that not only pays tribute to the Chinese culture, with its ornate tiers reminiscent of the ancient pagodas, but can also withstand typhoon winds, in part due to the judicious choice of lateral system and the use of aerodynamic tapering.

Through-Building Openings

The inclusion of openings completely through the building, particularly near the top, provides yet another means for improving aerodynamic response, significantly reducing vortex-shedding forces. [3] This design strategy was integrated into the proposed world’s tallest building, the Shanghai World Financial Center, which features a diagonal face that is shaved back with a 51-meter aperture to relieve pressure at the top of the building. The utilization of both the opening and shifting and decreasing the cross section with height essentially tapers the 460-meter tower and reduces the wind loads on the structure, as evidenced by the dramatic decrease in the spectral peak as a result of the addition of through-building openings. The shaving of the face of the structure provides a unique shape to the building’s upper plan, yielding a smooth transition from the entrance toward the captivating opening above. Unfortunately, this design has resulted in a loss of rental space in these upper floors, though in the case of the World Financial Center, this feature now serves as a priceless trademark for the building. However, the presence of openings does not have to be as drastic as in this example, as research has also established the benefits of configurations of vents, slits, or smaller openings that do not compromise as much rental space.

Tomorrow’s Vision

With each step toward the heavens man becomes more aware of his limitations, yet with much tenacity, finds innovations to overcome them. As we embark upon a new millennium in tall building design, the significance of dynamic wind effects becomes increasingly important, presenting a new challenge to conquer in the quest for new heights. While the advancements in auxiliary damping devices, which have sparked great interest in Japan, provide one viable solution, it is my intent to reiterate the significance of the merger between traditional structural engineering and aerospace fundamentals as equally viable mitigation strategies. The next generation of skyscrapers must build upon the foundations laid by the innovative structures discussed in this work and consciously incorporate aerodynamic modifications into their designs, using the wind tunnel as a tool to optimize the building’s form, from the perspectives of serviceability and occupant comfort. As these prototype structures have illustrated, the simple modifications of corners and the more sophisticated tapering and progressive alterations of plan can achieve stunning visual presentations, while achieving equally stunning performance objectives, when coupled with a judicious choice of structural system. In this respect, tomorrow’s structures can truly integrate the advancements in structural and aerospace engineering without sacrificing architectural vision, as we inch every bit closer to the gods.

Notes

[1] K.C.S. Kwok, “Aerodynamics of Tall Buildings,” (Proceedings of Ninth International Conference on Wind Engineering, New Delhi, 1995).
[2] K. Shimada and K. Hibi, “Estimation of Wind Loads for a Super-Tall Building (SSH),” The Structural Design of Tall Buildings (1995): 47–60.
[3] R. Dutton and N. Isyumov, “Reduction of Tall Building Motion by Aerodynamic Treatments,” Journal of Wind Engineering and Industrial Aerodynamics 36, no. 2 (1991): 739–47.