Wind Loading

Each building is situated in a unique wind environment, with many factors influencing the force which the wind exerts on each part of the structure.
In addition to general location, local geography and topography and orientation relative to surrounding buildings and the prevailing wind, the wind pressure is influenced by, for example, building shape, height and roof pitch.

On the windward side of a building, wall and roof panels experience a positive pressure, while on the leeward side there is a negative pressure or suction load.

As the wind blows over and around the building, at the corners of the walls and at the ridge, eaves and verges eddy currents are formed, which exert varying negative pressures in those areas.

For each building it is necessary to estimate the forces likely to be experienced by Marlon ST Longlife sheets when fitted in areas of the building which have different wind load conditions.

The British Standard Code of Practice BSCP3: (CP3) Chapter V: Part 2 outlines a method for estimating
wind loads on buildings: the following is an example of the procedure which should be repeated for
each area of the building.

1. From the wind speed map in the Code of Practice determine the basic wind speed for the building location.

2. The topography factor S1 is usually taken as 1.However, hills, cliffs or escarpments in the
vicinity of the building can influence this factor: reference should be made to Appendix D of (CP3): Chapter V: Part 2.

3. The ground roughness factor S2 is obtained from Table 3 of (CP3). It is first necessary to
define:

(i) The height above ground of the top of the building, normally the ridge.
(ii)The Ground Roughness Category;
1. Long stretches of open fairly level land with no shelter, eg. coastal fringes, airfields, moorland.
2. Flat or rolling countryside broken by walls, hedges, banks of trees and occasional buildings, ie. typical farm land.
3. Land covered with frequent large obstructions the general height of which does not exceed 10m, eg. forested areas, towns and the outskirts of large cities.
4. Land covered by large obstructions, height generally 25m or more, eg. large town and city centres.

4. The statistical factor S3 is normally taken as 1, but can be greater for buildings in exposed
areas or where a high degree of security is required: consult Appendix C of (CP3):
Chapter V: Part 2.

5. The design wind speed is calculated using the formula:      Vs =V x S1 x S2 x S3

6. Calculate the dynamic wind pressure q using the formula: q = 0.613 x Vs2

7. Refer to (CP3): Chapter V: Part 2 to determine the external pressure coefficient, Cpe, for the
area of the building being considered. This requires a knowledge of the form of the building and the length, width and eaves height dimensions together with the roof pitch.

8. The internal pressure coefficient Cpi is normally taken as +0.2: a higher figure must be selected where the presence of dominant openings in a building admit the wind creating high internal pressures.

9. The total wind pressure F on an area of a roof is calculated using the equation:
F = (Cpe-Cpi) x q (KPa or KN/m2)
 

Selection Of Sheet
In selecting the correct sheet thickness, weight or width, in addition to the loading it is necessary to take into account the type of application. In most applications the material may be fixed in four basic ways:
o Four sides of the sheet are fixed, eg. when the sheet is glazed into a window frame.
o Two sides of the sheet are fixed, eg. conservatory roof.
o Four sides of the sheet are simply supported,
eg. in suspended ceilings.
o Cold curved glazing when the sheets are retained by preformed curved supports.

National Wind Loading Standards