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Windloading Part 2

Following on from last month’s article, Simon Havard delves further into the issue to consider what criteria is used to calculate windload and how signs can be designed to counteract this problem

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Windloading—what does it mean?

There are six basic criteria that are used to calculate windload; windspeed, altitude and geography, distance from coat, country or town, height and area. Once it has been calculated, the resultant ‘load’ or ‘force’ can be used in the practical design of a sign.

The Eurocode Standard’s tables
and mathematical formula help to
provide a ‘worst case scenario’ for
wind pressure or force across the
UK

The Eurocode Standard provides a map of the UK with different windspeed zones, each zone using an average speed in metres per second based on readings taken at 10m height over a number of decades. By plotting your sign location on this map you can determine the basic windspeed. Distance from the coast could also be estimated,—windspeed reduces the further inland you go—but I would advise the use of Google maps, or similar, for this as you can also determine geographical features and whether the sign location is in the country or a built up area using Google satellite images as well.

Geographical features are only significant in calculating windload if the signs are, for example, situated near cliffs or steep hills. Determining whether a sign is in a built up area is open to interpretation but can make quite a difference—generally windspeed is lower in built up areas; however, closely sited tall buildings can cause a ‘funnelling’ effect which can increase windloading.

Open areas within towns and less-densely built up areas on the edge of towns should be treated cautiously as these will not reduce windspeed as higher density areas will. If in doubt, assume open country. As windspeeds tend to increase with height, the taller the sign, or the higher up a building it is fixed, the greater the windload. The overall area of the sign will impact directly on the overall windload on the sign as this is the ‘resistance’ offered to the wind.

All this information is then used in conjunction with tables and mathematical formula from the Eurocode Standard to provide a worst case wind pressure or force. A pressure is usually expressed in kiloNewtons per metre square (kN/m2). For instance ,1kN/m2 is the equivalent of 100kg on every square metre; a force or load is expressed in kN where 1kN is equivalent to 100kg.

Technical terms

So what does all this technical information mean in practical terms? For example, specifying posts for a totem sign; if the totem sign is on the edge of Glasgow, the windload is likely to be much higher than one in the centre of Oxford. Therefore the one in Glasgow will require larger posts, baseplates, rootcage and concrete foundation, which will probably translate into greater cost. Or putting it another way, the totem in Oxford will cost less to produce than the one in Glasgow. Why? Because Glasgow is in a higher windspeed zone, it is nearer the coast, the sign is on the edge of the city so will be taken as in open country—all of which add up to greater windload.

The Eurocode Standard prescribes different windspeed
 zones across the UK, with each zone corresponding to
a different set of structural demands for signage

I can hear groans all round—too technical and too complex. Actually it does not have to be technical at all, an engineer will deal with the figures. However, it is important to appreciate that rarely does a ‘one size fits all’ actually provide a reasonable solution. A sign design for Oxford may well not be safe for Glasgow, and a design for Glasgow may well be over specified for Oxford and therefore more costly than necessary.

Obviously this has to be tempered with regards to savings made with larger volumes. But having an appreciation of what criteria will affect windloading will provide for more informed decision making when it comes to the chose of materials and design. This in turn can only help make signs safer.


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