Lawson 2001 (UK)

Back to Pedestrian Wind Comfort and Safety

Lawson 2001 (UK) is ArchiWind’s default scheme. It provides comfort classes for sitting, standing and walking, together with a separate safety classification.

Pedestrian Wind Comfort

Strolling means leisurely walking; Business Walking means walking purposefully to a destination.

Exceedance probability is the percentage of time the wind speed exceeds the threshold wind speed, after combining all wind directions. Read the table from the top: each location takes the first category whose condition it meets. Comfort is classified on the mean wind speed; the gust-equivalent mean (GEM) is used for safety only.

Pedestrian Wind Safety

Lawson 2001 checks mean wind speed and gust-equivalent mean (GEM) against 15 m/s and 20 m/s, assessing each separately and using the worse safety class. GEM converts short gusts into an equivalent average wind speed.

Exceedance probability is the percentage of time the wind speed exceeds the threshold wind speed, after combining all wind directions. Read the table from the top: each location takes the first class whose condition it meets. Unsafe means winds exceed 15 m/s too often to be Safe, while the time above 20 m/s remains within the limit. Dangerous means winds exceed 20 m/s more often than the limit allows.

For an annual result, 0.023% is approximately two hours per year. Calculation details and sources are provided below.

Safety Wind Speed: Mean and Gust-Equivalent Mean

Safety is assessed using both the mean wind speed and the gust-equivalent mean (GEM); comfort uses the mean wind speed only. GEM converts a short gust into a comparable mean speed:

U_GEM = U_gust / 1.85

The gust speed is estimated from the mean wind and its fluctuations:

U_gust = U_mean + 3.5 × σ_u

Here, U_mean is the local mean wind speed, U_gust is the estimated gust speed, and σ_u is the standard deviation of wind-speed fluctuations, all in m/s. Greater fluctuations produce a larger gust allowance.

For safety, calculate the mean-based and GEM-based classifications separately, including their wind-climate probabilities, and take the worse class. Do not average those classes.

The calculation follows the statistical peak-factor method: mean wind plus a multiple of its standard deviation, using a peak factor of 3.5. The 1.85 GEM conversion is specified in the City of London wind guidelines. The peak factor is part of the gust estimator, rather than a universal requirement of the Lawson criteria.

For ArchiWind’s steady-RANS calculation, the simulated turbulent kinetic energy k supplies the fluctuation estimate:

σ_u ≈ sqrt(2k/3)

This follows from the definition k = (σ_u² + σ_v² + σ_w²) / 2, assuming equal fluctuation strength in the three directions (isotropic turbulence). Here, k is in m²/s². The result is a statistical gust estimate, rather than a resolved gust time history.

Both U_mean and σ_u scale with the reference wind speed of each wind direction, so GEM is combined with the wind statistics in the same way as the mean wind speed.

Sources

• T. V. Lawson, Building Aerodynamics (2001).
• City of London guidelines, GEM conversion and Annex A’s separate mean/GEM assessments.