If a building is too flexible, commonly occurring strong winds may cause the building to shake excessively. Although this is not a safety issue, it is a nuisance and a serviceability issue. For high-end buildings, such strong wind motions are unacceptable and can lead to claims or complaints.
Problem + Cause
When wind blows against the building, it imparts mean + fluctuating forces, causing the building to shake. The shaking is usually measured using the peak or root-mean-square (RMS) accelerations, in units of cm/s^2 (gal) or milli-g (%g x 1000).
In general, if the peak accelerations are above 5 milli-g, they will be perceptible to about 50% of building occupants (but this does not mean that such accelerations will become a nuisance).
Analysis Methods
The most accurate and reliable way to estimated wind induced accelerations is by undertaking a wind tunnel test, combining the data with the building's structural properties (mass data, frequencies, mode shapes etc), and calculating the peak acceleration.
※ What are the Building's Structural Properties?
There are also code-based analytical procedures available. For example, the Architectural Institute of Japan provide an excel spreadsheet to estimate the wind loads and accelerations considering the along-wind, across-wind and torsional response of the building (only available in Japanese).
Wind Speed
Wind loads for strength design are calculated based on extreme wind speeds with high return periods (e.g. 500-year return period). However, wind-induced accelerations are calculated for much more common winds, such as the 1-year or 10-year return period wind speeds.
In order to calculate the wind induced accelerations, the basic wind speed (usually defined at the 50-year return period or higher) first needs to be scaled to these smaller return periods.
※ How to Scale Wind Speeds to Different Return Periods?
Damping
For strength design, the damping ratios for wind are considered to be a constant value between 1.5- to 2.0%. However, for the lower wind speeds (and therefore smaller amplitudes of motion) considered for serviceability, typically a damping ratio between 1.0- to 1.5% is considered.
Criteria
※ What are the Criterta for Wind-Induced Accelerations? Explained in 1-Minute
Mitigation Measures
※ How to Reduce Wind-Induced Accelerations? Explained in 1-Minute