This article investigates how across-wind loads impact rectangular tall buildings, what the Eurocode and other global standards say about them, and where these provisions fail to capture the full picture.
Wind loading is one of the most complex and misunderstood actions in structural design. Unlike gravity loads, wind actions are highly dynamic, varying with height, location, terrain, and building geometry. Among these complexities, the across-wind response—oscillations perpendicular to the prevailing wind direction—remains one of the least intuitively grasped behaviors, yet one of the most structurally significant.
Design codes often offer simplified formulations to deal with these complex wind effects. However, the across-wind response of tall rectangular buildings frequently falls outside the reach of these simplifications. The result is a risk of underestimation, sometimes leading to discomfort, serviceability issues, or even structural inefficiencies. Engineers must look beyond the formulas and toward the physical response mechanisms and limitations imposed by the very codes they are required to use.
This article investigates how across-wind loads impact rectangular tall buildings, what the Eurocode and other global standards say about them, and where these provisions fail to capture the full picture. More importantly, we explore how practicing engineers can detect, assess, and mitigate the structural consequences of these dynamic effects.
Understanding Across-Wind Response
Wind hitting a building produces two types of structural response: along-wind and across-wind. The along-wind response aligns with the wind’s direction and is relatively straightforward. Across-wind response occurs when vortex shedding behind the building causes alternating low-pressure zones on each side, inducing lateral motion perpendicular to the wind.
Unlike along-wind response, which is largely governed by average wind pressure, the across-wind response is driven by turbulent phenomena, resonance, and building geometry. This makes it more difficult to predict using deterministic calculations. Yet in many cases, it governs the design of lateral load-resisting systems, particularly for buildings with slenderness ratios exceeding 6:1.
In low- to mid-rise buildings, across-wind effects are often negligible. However, as height increases, the excitation frequency of the structure may approach the vortex shedding frequency. At this point, resonance can occur, leading to large amplitude oscillations. Such behavior not only affects the structural integrity but can also lead to occupant discomfort, facade damage, or even local failure in secondary systems.
Code-Based Approach and Its Shortcomings
Eurocode 1 (EN 1991-1-4) addresses wind actions in general terms. It provides a framework for calculating wind pressures on various types of buildings, with the assumption that the along-wind load is the dominant design action. The across-wind response is acknowledged, but no explicit design procedure is offered. The code merely advises that specialist dynamic analysis be performed if resonance is suspected.
This creates a critical gap. Engineers working with Eurocode must often rely on judgment or external references when the across-wind response is non-negligible. While Annex E of EN 1991-1-4 provides basic dynamic amplification factors and resonance checks, they are not tailored for complex geometries or aerodynamic instability.
The 2017 paper “Across-wind load on rectangular tall buildings” by Tanaka and Davenport underscores this deficiency. It reveals that code provisions often underestimate the peak response by 20% to 40%, particularly for buildings with aspect ratios greater than 7 and located in open terrains. Such underestimations can lead to significant structural vulnerability or necessitate costly retrofitting after construction.
The American ASCE 7 standard and Canadian NBCC include more detailed guidance on across-wind effects using empirical data from wind tunnel tests. Yet even these stop short of replacing the need for physical or CFD-based modeling in complex structures. The codes offer starting points, not reliable conclusions.
Governing Parameters and Structural Dynamics
The across-wind behavior of rectangular tall buildings is governed by three main parameters: building aspect ratio, wind turbulence characteristics, and structural damping. These interact with each other to amplify or suppress dynamic motion.
The aspect ratio of a building, particularly the height-to-width ratio in the crosswind direction, determines susceptibility to vortex shedding. When this ratio exceeds 6 or 7, the building becomes aerodynamically unstable. Even small wind fluctuations can lead to significant sway.
Turbulence intensity plays a major role as well. High turbulence can disrupt vortex shedding, thereby reducing resonance. Conversely, moderate turbulence levels can sustain vortex lock-in conditions over longer durations, increasing response amplitudes. The terrain category—urban, suburban, or open—greatly affects this turbulence.
Damping is the only structural mechanism that counters resonance. Unfortunately, damping ratios in tall buildings are typically low, often around 1% to 2%. Supplemental damping systems such as tuned mass dampers (TMDs) or viscous dampers can help, but their use remains limited due to cost or architectural constraints.
These parameters interact in nonlinear ways, making closed-form predictions unreliable. For this reason, code simplifications become increasingly inadequate as buildings become taller, more slender, and more exposed.
Longer Zone: Vortex Shedding and Lock-In Mechanism
Vortex shedding occurs when wind flows around a bluff body such as a rectangular building and separates into alternating vortices downstream. These vortices create low-pressure zones that alternate between sides, pushing the building back and forth in the crosswind direction.
If the frequency of vortex shedding coincides with the building’s natural frequency, a phenomenon known as “lock-in” occurs. During lock-in, vortices reinforce the building’s motion instead of canceling it out. This leads to a sharp rise in lateral displacement and acceleration.
The danger lies in the self-sustaining nature of lock-in. Once it begins, it can persist across a wide range of wind speeds, not just at resonance. Structural damping and turbulence can mitigate it, but not always sufficiently. Buildings without sufficient dynamic separation—where the shedding frequency stays far from the natural frequency—are especially vulnerable.
Code provisions often ignore this lock-in mechanism. They may check resonance at a single wind speed but fail to recognize that lock-in can span 10 to 30% of the speed range. This oversight is critical, especially in cities with high wind variability or seasonal gusts.
Case Studies and Physical Evidence
In cities like Tokyo, Toronto, and Chicago, full-scale measurements have consistently shown that across-wind loads dominate in tall slender buildings. Wind tunnel tests conducted by RWDI and Tokyo Polytechnic University have validated that real-world pressures are often higher than code estimates.
For instance, the benchmark CAARC building model, tested across multiple facilities, has demonstrated peak across-wind accelerations that exceed code-predicted values by nearly 50%. These findings are not anecdotal—they are repeatable, validated, and measurable.
Furthermore, studies show that serviceability rather than strength often governs design in tall buildings. Occupants begin to feel motion-induced discomfort at lateral accelerations as low as 10 to 15 milli-g, far below failure thresholds. Such accelerations are typical under lock-in across-wind effects, making the case for better prediction methods.
Alternative Analytical and Experimental Approaches
Engineers facing these shortcomings often turn to wind tunnel testing. Scaled physical models allow precise measurement of pressure distributions, vortex shedding behavior, and dynamic amplification. These tests capture effects that cannot be derived from code formulas, such as interference from neighboring structures or non-uniform turbulence.
Another approach gaining popularity is computational fluid dynamics (CFD). While still less reliable than wind tunnel data, modern CFD tools can model vortex patterns and pressure fields in detail. Hybrid approaches, combining CFD with aeroelastic models, are showing promise in predicting both local pressures and global responses.
Some design teams also use empirical databases like the Tokyo Polytechnic University database or the AIJ Recommendations for Loads on Buildings. These offer data-backed approximations tailored to building shape and height, improving accuracy over standard codes.
Nonetheless, these methods require time, cost, and expertise. Not all projects can afford them, which is why the absence of comprehensive code procedures becomes a design liability.
Design Implications and Engineer Responsibility
The limitations of code-based across-wind analysis do not absolve engineers from responsibility. Eurocode allows—and in fact mandates—deviation from simplified provisions if they are known to be inappropriate. Clause 1.1(4) of EN 1990 places the onus on the engineer to ensure safety, even if it means going beyond code minimums.
Designing for across-wind response must consider serviceability, not just strength. This includes checks for acceleration, sway perception, and facade fatigue. Serviceability criteria vary by location and occupancy, but they must be rigorously verified using dynamic analysis.
Engineers must also work closely with architects, since aerodynamic modifications such as chamfered corners or tapering can significantly reduce across-wind excitation. Collaboration enables performance without overdesign.
In essence, engineering judgment cannot be delegated to the code. Structural safety and occupant comfort demand active interpretation of response behavior, not passive compliance.
Conclusion
Across-wind loads on rectangular tall buildings represent a structural behavior that eludes easy quantification. Eurocode and similar design standards provide a necessary framework, but they stop short of offering a full design solution. The burden rests on engineers to understand the physics, recognize the signs of resonance or lock-in, and seek appropriate tools and tests.
Physical testing, CFD, and empirical databases all provide pathways to safer, more accurate design. But none of them replaces the need for foundational insight into dynamic response. When across-wind motion governs, structural safety becomes more about perception than collapse, and more about precision than margin.
Also See: Wind – Structure Interaction in Tall Buildings
Sources & Citations
- Kareem, A., & Tamura, Y. (2013). Advanced Structural Wind Engineering. Springer.
- Dyrbye, C., & Hansen, S. O. (1997). Wind Loads on Structures. John Wiley & Sons.
- Solari, G. (2016). Wind Engineering: Fundamentals and Applications. Springer.
- Holmes, J. D. (2015). Wind Loading of Structures, 3rd Edition. CRC Press.
- The Structural Engineer (2017). Across-wind load on rectangular tall buildings. https://www.istructe.org/journal/volumes/volume-95-(2017)/issue-3/across-wind-load-on-rectangular-tall-buildings/
