Dr during severe winter storms and of structural damage to temporary structures and older buildings . It is thankfully rare for modern buildings to fail structurally due to wind loading alone , although this is no reason to be complacent . RIDBA is aware of several instances where buildings have swayed excessively during high winds , resulting in complaints from concerned building owners . Excessive deflections can lead to damage to the building envelope and ancillary components attached to the frame , requiring costly remedial action that could have been prevented by proper consideration of the likely wind loading at the design stage . Wind forces on buildings

When the wind blows over or around a building , it is forced to change direction and either speed

up or slow down depending on the shape and orientation of the obstruction . This causes either an increase or decrease in the external air pressure . When combined with changes to the internal air pressure the result is either a net positive pressure ( on windward facing walls and the windward slopes of steep roofs ) or a net suction ( on leeward facing walls , walls parallel to the direction of the wind and on roofs generally ). Importantly , the magnitude of the pressure is proportional to the square of the wind speed , so doubling the wind speed will produce four times the wind loading on the building .

From a building design point of view , the most important point to understand is that wind speed varies enormously with location and building geometry , meaning that wind loading is site and building specific , so should be calculated for each and every building project . Since the magnitude of the wind loading has a direct bearing on the design of

the frame ( e . g . column and rafter sizes ), it follows that the design of every building is unique and should be calculated or at least regularly checked . It should come as no surprise that a 15m barn designed for a sheltered location in Oxfordshire may not be adequate if placed on a hilltop overlooking the coast of Cornwall . Factors affecting the wind speed : Location

Some parts of the country tend to experience higher wind speeds than others and this needs to be taken into account when calculating the wind loading on a building . To enable engineers without specialist meteorological expertise to judge the likely wind speed at a particular location , the available meteorological data has been analysed to produce a contoured “ wind map ” of the UK , which is published as part of the UK National Annex to BS EN 1991- 1-4 and is reproduced over the page . The values shown on the map are magnitudes of the “ basic wind speed ” to which correction

Martin Heywood , RIDBA

Technical Consultant

factors may be applied to take account of wind direction , altitude and exposure conditions . Altitude

Wind speed naturally increases with altitude and this is accounted for by a correction factor that is applied to the “ basic wind speed ”. This is especially important for agricultural buildings since many are constructed at altitudes greater than 200m above sea level , where wind speeds are significantly higher than those in low-lying locations . Distance to sea

The shorter the distance to the sea , the greater the wind speed , since the wind loses energy and speed as it blows across land . The greatest reduction in wind speed