Wind impact study on the structure of your buildings.

High winds: a complex and dangerous structural action

The action of wind on buildings is inherently three-dimensional and strongly context-dependent. High winds generate aerodynamic loads liable to affect not only the load-bearing structure, but also the building envelope, the technical equipment, the fixing systems and the outdoor fittings.

Beyond the global forces, it is the local effects that constitute the main risk factor. The combination of height, geometry (sharp corners, setbacks, complex roofs), the possible porosity of the façades and the immediate surroundings (obstacles, other buildings, terrain) generates advanced aerodynamic phenomena: intense overpressures and suctions, high pressure gradients, local accelerations of the wind, Venturi effects between built volumes, corner vortices and unstable recirculation zones.

These phenomena can lead to extreme loads on very localised zones, often poorly represented by global code-based methods. In dense urban environments, the interactions between buildings amplify these effects, making the wind assessment particularly sensitive to the calculation assumptions.

A poor appraisal of these mechanisms can lead to:

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  local under-sizing of façade or roof elements;

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  tear-off of technical equipment;

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  risks to users in exposed pedestrian areas;

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  premature deterioration of the works and significant remediation costs.

Controlling the effects of wind is therefore a matter of safety, durability and cost control for the project.

An advanced engineering tool to master wind risk

Computational fluid dynamics (CFD) makes it possible to approach the action of wind through a realistic physical approach, going beyond the limits of simplified code-based methods. It provides a three-dimensional, local and continuous representation of the airflow around buildings, incorporating the real geometry of the project and its immediate surroundings.

CFD not only quantifies the forces, but above all helps to understand the physical mechanisms behind the loads. This understanding is decisive in guiding design choices, adjusting architectural shapes, positioning equipment and defining suitable construction solutions.

A local and directional analysis of the loads

Unlike approaches based on the Eurocode's average coefficients, CFD makes it possible:

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  to finely map the pressures and suctions over all the walls;

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  to identify the pressure peaks linked to corner and edge effects;

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  to analyse the local accelerations of the wind and the shear zones;

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  to characterise the aerodynamic interactions between neighbouring buildings.

This local approach is particularly relevant for high-rise buildings, unconventional architectural shapes, dense urban projects and sites subject to marked prevailing winds.

Decision support integrated into the design

Brought in early, CFD becomes a genuine design tool. It makes it possible to compare different variants (layout, massing, orientation, screens, windbreaks), to optimise the sizing of the exposed elements and to technically justify the chosen options to inspection offices and insurers.

Feedback shows that bringing CFD in early significantly reduces the risk of later rework and the associated extra costs.

A mastered regulatory framework: Eurocode 1

The studies carried out by EOLIOS fall strictly within the regulatory framework of Eurocode 1 — Wind actions (NF EN 1991-1-4) and its National Annex, an essential tool for designing wind-resistant buildings. CFD is not used as an alternative to the standards, but as a complementary, consistent and defensible tool.

Terrain characteristics

The Eurocode's National Annex gives the values of z₀ (apparent roughness length) according to the nature of the site. For urban studies, the roughness used in the calculations corresponds to a category IV urban roughness. This roughness directly governs the wind velocity profile, a fundamental input for defining the inlet conditions of the CFD model.

Defining the reference wind speed

The basic wind speed v_b,0 corresponds to an extreme but rare wind event (mean return period of about 50 years). It is defined over a 10-minute period at 10 m above open countryside, dependent on the geographical zone through the Eurocode zoning. It can be corrected by two coefficients to form the reference wind speed v_b:

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  Directional factor c_dir — the strongest winds do not always blow from the most unfavourable direction; it reduces the speed when the critical orientations are unlikely.

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  Seasonal factor c_season — extreme winds do not occur evenly throughout the year. For permanent works it equals 1; for temporary works (scaffolding) it can be lower.

Orography factor c₀(z)

The orography factor accounts for the influence of the terrain relief (hills, ridges, escarpments) on the wind speed. Orographic effects may be neglected if the average upwind slope is below 3°, in which case c₀(z) = 1.

Roughness factor c_r(z)

The roughness factor c_r(z) accounts for the apparent roughness of the terrain (buildings, trees, towns) to compute the mean variation of speed with height. It depends on z₀ (roughness length), z_min and z_max (validity limits) and k_r (terrain factor):

Mean wind speed

The mean wind speed v_m(z) combines the reference speed v_b with the roughness parameters of a category IV terrain. Iterating the equation yields a logarithmic velocity profile:

Gust winds

From the mean-speed profile, the profile of peak gust speeds is computed, integrating the turbulence intensity and temporal variability of the wind (k_l = turbulence factor of the terrain category). This second profile serves as the domain inlet in the simulations:

A robust and controlled CFD approach

Reconstructing the built environment

The study area is modelled over a perimeter large enough to guarantee the full development of the flows. The surrounding buildings are included in order to capture the sheltering effects, channelling and aerodynamic interaction. The geometric simplifications are carried out in a controlled way so as to preserve the physical mechanisms while ensuring the numerical stability of the computations.

Modelling the atmospheric boundary layer

Wind can be described through the concept of the atmospheric boundary layer, which breaks down into three sub-layers:

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  The outer layer, or inertial sub-layer, with a thickness of the order of a kilometre (0.5 to 3 km).

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  The surface boundary layer (10 to 100 m), about 10% of the total thickness; a region of strong velocity and temperature gradients, where the wind direction stays relatively constant.

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  The roughness sub-layer, a few metres deep; here the flows have a three-dimensional, disordered structure, strongly affected by obstacles.

At ground level, the wind is slowed by obstacles and roughness. Higher up, in the undisturbed layers of the geostrophic wind (≈ 5 km), it is no longer influenced by the earth's surface. Between the two, the speed varies with height following a logarithmic profile: this is the vertical wind shear.

Multi-case directional analysis

The simulations are run for the eight main directions of the wind rose, corresponding to an extreme gust wind per Eurocode 1. This multi-case approach identifies the most penalising orientations and characterises the directional site effects. The intermediate directions (NNE, ENE, ESE…) are estimated by linear interpolation of the nearest simulated directions.

Advanced exploitation of the results and validation

Physical analysis and expert interpretation

Beyond the result fields, EOLIOS's expertise lies in the physical interpretation of the flows. The simulations make it possible to pinpoint the origin of the pressure peaks, to understand their mechanisms (separation, vortex/structure interaction, channelling) and to propose optimisation levers. The deliverables include:

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  maps of pressure and pressure coefficients;

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  comparative directional analyses;

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  identification of the critical zones;

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  design recommendations.

Cross-check with the analytical Eurocode approach

To validate the computations, analytical calculations following the Eurocode can be carried out on greatly simplified geometries to provide an order of magnitude. An estimate of the peak velocity pressure allows mean aerodynamic pressure values to be established on the faces, by coupling this pressure with standard pressure coefficients taken from the Eurocode charts:

In addition, for h > 2b (h = tower height, b = base length perpendicular to the wind), we take q_p(z) = q_p(h) for h−b < z < h. It follows, for z > h−b:

The pressure coefficient varies with the size of the surface considered. Its maximum value is c_pe,1 (area A < 1 m²), used for the design of small fixing elements; Eurocode charts provide these coefficients according to the geometry. The maximum pressure averaged over 1 m² is then written:

Expertise serving well-controlled projects

By combining a fine command of the regulatory requirements, advanced CFD expertise and the ability to physically interpret the results, EOLIOS supports its clients in securing and optimising their projects against wind-related risks. CFD becomes a strategic tool for risk reduction, technical justification and cost control, in the service of safe, durable and high-performing buildings.

Why complement the Eurocode with a CFD study?

Eurocode 1 provides an essential regulatory framework for assessing wind actions, based on idealised geometries and global coefficients. It guarantees regulatory compliance, but reaches its limits as soon as the project sits within a complex real-world context. CFD simulation makes it possible to go beyond these limits by incorporating:

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  the real geometry of the building (complex shapes, setbacks, corners, atypical roofs);

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  the urban environment and site effects (neighbouring buildings, sheltering, urban canyons, Venturi effects);

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  a local and directional analysis of the loads (pressure peaks, critical zones);

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  a fine treatment of turbulence and gusts, essential for sensitive elements.

Combined with the Eurocode, CFD becomes a decision-making engineering tool, helping to secure the design choices, technically justify the sizing assumptions and limit the risks of local under-sizing or over-conservatism.

Project: Wind impact on high-rise buildings — Tours Olympiades, Paris