Friday, 01 Jun 2012 11:01 GMT

Windloading

Windloading is something that effects all sign-makers and the products they produce. Simon Harvard takes a look at this issue and discovers how you can avoid this problem when installing signage

Addressing the issue of windloading

All external signs are subject to some sort windloading. Even signs for large internal spaces have to be designed for some air movement—the NEC includes this in their guidelines. However, the principal cause of windloading is the wind—I know this sounds obvious, but it is so obvious it can be overlooked.

Wind of both the speed and strength that is likely to cause damage or failure is infrequent. Yearly winter storms will seek out the seriously understrength, but it is the occasional freak winds—only seen every 10 to 20 years—which can wreak havoc. Unfortunately, no one can predict whether a sign will experience these winds during its lifetime.

Yearly winter storms will seek out the seriously understrength, but it is the occasional freak winds—only seen every 10 to 20 years—which can wreak havoc”

Wind can produce unexpectedly large loads. For instance, if you turn a totem style sign horizontally, severe storms produce a load on the face of the sign that is equivalent to two large people standing on every square metre. It is these loads that British Standards are designed to cater for.

Yes, 99 percent of the time the structure is not needed, but it is there for everyone’s safety during that other 1  percent. We all buy cars or fly in planes with expensive safety features which are there just in case —why should signs be any different when peoples safety is at stake?

All UK windloads are calculated using the current Eurocode 1991-1-4 Actions on Structures: Wind Loading or the recently superseded BS 6399, Part 2:1997 ‘Loading for Buildings, the code of practice for wind loads. The Eurocode is open to a greater degree of interpretation than BS6399.

Traffic signs use a separate standard BS EN12899-1; 2007, which is particular to the design but which draws on the Eurocode for windloading. As a result, windloads can be either lower or higher, than those calculated using BS6399. This has already generated differences of opinion as too the most effective interpretation, only time will tell which is correct. I tend to balance the options and use experience to judge where the higher loadings are most suitable.

We all buy cars or fly in planes with expensive safety features which are there just in case —why should signs be any different when peoples safety is at stake?”

However, both Standards attempt to provide a method of calculating worst-case conditions using the most appropriate criteria. This should mean that a sign designed to a Standard would withstand a very severe storm, only likely to happen once every 10 to 20 years. In other words, it reduces risk of failure to a much smaller percentage than would otherwise be the case.

Calculating windload

The actual criteria that is used to calculate windload is as follows:

Windspeed: The UK is divided into a number of basic wind speed areas. These fan out from the lowest in central southern England, 21.5m/sec, to the highest in northern Scotland of 30m/sec. Unfortunately, the divisions are not even nor particularly smooth.

Altitude and geography: How high from sea level the site is or how high the sign is when attached to a building, and whether its sited on or near a significant geographical feature, such as a cliff or mountain, all effect how windload is calculated.
 
Distance from the coast: How close the sign location is from the coast will make a significant difference to the windloading. This difference falls unevenly, slowing the further inland you travel.

Location: Whether the sign is sited in country—higher loadings, or within a built up area—lower loadings, this will also make a significant difference to design specifications. ‘Built up’ is a difficult term to define as even if within a ‘built up’ area the funnelling of wind between buildings, as well as open areas such as car parks or parks, will increase the loadings once again. Therefore, unless we can be sure that a sign is located within a true ‘built up’ area, without the influence of funnelling or open areas, we use the worst-case ‘site in country’ option.

Height: The overall height of a sign can make a very significant difference to the windloading.

Area: The overall area of a sign can also make a difference to the windloading, particularly if the ‘area’ is horizontally or vertically orientated.

In my next article I aim to explain how the criteria is used to provide a ‘windload’ and what this means in practical terms when designing a sign.

For more on these issues visit www.russellandhavard.co.uk