High wind speeds are a risk factor to consider in the construction industry and the wider built environment. Weather forecasts typically give the average wind speed one might expect on a particular day. Real wind conditions, however, are influenced by gusts, turbulence, terrain and surrounding buildings, making wind difficult to predict and even harder to manage. Pedestrians, cyclists, vehicles and construction workers at building sites are at risk of high winds.

Traditionally, wind tunnel experiments are conducted to evaluate the impact of wind on a particular site. Wind tunnel testing is expensive and, thus, mostly confined to the larger projects in the construction industry. It also lacks the iterative speed and accuracy needed to adapt to rapidly changing conditions. Computational Fluid Dynamics (CFD) is a type of engineering simulation used to simulate wind. CFD on the cloud enables a fast, accurate and much more responsive method of assessing risk.

The Problem of High Wind Speeds

In construction, 50% of all fatal accidents occur as a result of falls, mostly roof-related. The Advisory Committee on Roof Safety (ACR) has developed specific guidance for working on roofs in the U.K. The issue of climate and wind is mentioned repeatedly, reflecting the impact of high wind speeds and their corresponding risk. A wind speed of 23 mph (10 m/s) is enough to impact a persons’ balance significantly. Gusts can easily be twice the average wind speed in any location, and doubling the wind speed increases the wind-induced pressure by a factor of four.

For example, in “Safe Handling of Solar Collectors and Other Large Items on Roofs,” published by the ACR, the impact of wind on installing solar panels/collectors is mentioned several times. Due to the large surface area and weight of solar products and the relatively higher wind speeds on roofs, it is necessary to ascertain the wind-related risk. This risk has a direct impact on project feasibility, insurance, and safety.

Figure 1 shows a simulation of the wind-induced pressure on solar collectors on a flat roof. Simulations can be run at various wind angles and speeds to give a representative annual risk profile.

The results can inform the appropriateness of a roof/site for installing solar collectors, installation procedures, available time-on-site and extreme risk scenarios, such as in a storm. Multiple wind angles are simulated as is the terrain and surrounding building context. The turbulence caused by built-up areas and variation of wind speeds are also explicitly accounted for, thus providing a realistic and useful output.

The National Federation of Roofing Contractors from the U.K. has a publication called “Roofing and Cladding in Windy Conditions,” highlighting wind-related issues in roofing design and planning. For significant types of roofing and cladding work, the guidance gives upper limits on wind speeds when work must cease immediately. Most of the work undertaken by roofers must stop at 23 mph (10 m/s), and many other activities have a lower limit of 17 mph (7.7 m/s).

The guidance also highlights the importance of wind directions. The wind direction given in a weather forecast is not necessarily the direction of the wind onsite. Surrounding buildings and site characteristics can drastically alter the incident wind direction, which is also exacerbated by wind turbulence. The wind speed must also be measured at working height on a construction site and not from the weather forecast speed given for a standard 10m height. The guidance places critical importance on knowing the wind characteristics of any construction site before work commences.

Figure 2 illustrates the CFD simulated wind pressure and speeds on a building with a flat roof.

The wind is coming head-on to the building, and the higher wind speeds are shown in red. This type of simulation in a web browser can take only minutes to complete and give individual wind direction results as well as annual, averaged results for up to 36 wind directions using local climate data. Manually inputting much higher wind speeds to test gusting will calculate any extreme risk scenarios from storms. Wind loading results on buildings, cranes, towers, and large machinery are also easily simulated.

In built-up areas, various building aerodynamic phenomena can cause safety issues from high winds. Air flowing from a prevailing direction into a built-up area will have a specific upstream profile that is relatively steady (before it reaches the built-up areas). Once it comes closer, the amount of interference in the wind profile increases due to surface roughness, terrain friction, obstacles such as trees/vegetation, and the buildings themselves.

Once the airflow is in contact with buildings, the air will behave under the influence of several drivers:

• channeling effect, where the wind flow accelerates through a narrow flow path, increasing the wind speed;
• downwash, the forcing down of wind onto ground and pedestrian areas by a neighboring taller building;
• corner acceleration, flow separation and acceleration around corners of buildings; and
• turbulence, unsteady and unpredictable wind movement caused by changes in pressure.

The above aerodynamic behavior of wind in built-up areas has caused many safety issues and accidents, giving rise to multiple wind comfort and safety guidelines.

Table 1 summarizes the common wind comfort and safety criteria in use today and compares simulation results using the same model. It represents criteria for three different guidelines: Lawson (Top), Davenport (Middle) and NEN8100 (bottom)

Figure 3 shows the typical classification methodology used by all the primary standards. Although different measures will have their own definitions, they are based on a similar approach of categorizing comfort criteria using color-coded bands corresponding to a wind speed range.

For example, in Lawson LDDC (Figure 3), Band E means that if the average wind speed is above 8 m/s for more than 5% of the time, the location now becomes uncomfortable. Band S is considered unsafe and refers to the average wind speed above 15 m/s for more than 0.022% of the year (annual climate data is used). When designing a new building or site, anything falling under Band E and S, in this case, would require redesign to avoid high wind speeds. This could be in the form of mitigation via trees/vegetation or windscreens, or, more fundamentally, the building’s shape and size might be altered to improve the overall wind characteristics of the site.

Assessing wind related risk on construction sites and elsewhere is an increasingly common activity. Many new standards and guidelines are available that specify how such assessments must be performed.

Key to these methods is the use of computational fluid dynamics (CFD) that allows fast and accurate simulations of a 3D model. CFD outputs are useful to quickly evaluate a site safety profile due to wind and gusts and must be adopted by the construction industry as standard practice.