RWDI-Anemos are wind engineering experts who can help with the proper application and interpretation of BS6399-2. There has been negative public perception of higher loads in BS6399-2 compared to the old CP3 code, but calibration studies show loads are actually lower on average. The complexity of BS6399-2 means expert guidance is needed for accurate application to complex designs. RWDI-Anemos offers services like audits, software, and training to help designers appropriately apply the code.

1. BS6 3 9 9-2 Wind loading
Information Sheet 2 July 2003
BS6399 Loading for Buildings The second calibration showed that with
Part 2 Code of practice for wind loads misapplied terrain categories in CP3, BS6399
How can we help? would appear to give design pressures up to
Friend or foe? 25% greater.
• Expert advice
There has been a lot of negative public UK location facts
• Desk studies comment about BS6399-2 since it was first
introduced in 1995 - mostly ill-informed and To show you how important the altitude
• Wind tunnel testing caused by misapplication of corrections and topography are, look at
• Failure investigation
CP3:ChapterV:Part 2 over many years. This Table 1. This shows the percentage of
is not all the fault of users, CP3 was badly postcode sectors on the GB mainland that
• Software design worded in places. Fortunately, the old code have high altitude and/or significant slopes
is now withdrawn and should not be used. within +/-1km of their centroid. It also shows
• Training / Seminars
the distribution of terrain roughness, using a
There is a lot more useful information in the parameter called z0. For open country, z0 is
new code, and it is described in a less about 0.03m, in town z0 is 0.3m or more.
ambiguous way. This helps to avoid too many
arguments about interpretation. Equally, There are many locations where slope and
there is less scope for misunderstanding and altitude are significant, and yet it is surprising
misapplication. Much more background how many designers have not routinely used
information on wind speeds and pressure the topography rules that were introduced
We’re on the web coefficients was available to be included. All into CP3 in more than 17 years ago.
www.rwdi-anemos.com
of this is good.
Nevertheless, like all codes, BS6399-2 can
only cover basic shapes and when it moves
into more intricate arrangements the clauses
do get more complex, or perhaps don’t exist
- which is where we can help you.
“...negative How does it compare with CP3?
public comment It may be several years since BS6399-2 was
introduced but this is still a popular question.
about BS6399-2 Reported perceptions are that design loads
are significantly higher with the new code
has been mostly
but extensive calibrations show this to be
ill informed…” untrue. One calibration based on a 10km
grid covering the whole of Great Britain
(Figure 1), showed that, on average, the
design wind pressure was 8% lower for
inland sites and 15% for coastal sites. A
separate study, using 9167 postcode sector
centroids showed -8% to +4%, depending on
the assumptions.
RWDI-Anemos Ltd
Naturally, there is a range around these
Charleston House
13 High Street average figures but the perception of higher
Hemel Hempstead loads is wrong. A major reason for this is the
Herts HP1 3AA misapplication of CP3 in two main areas -
Figure 1 - BS6399-2
Tel: 01442 212292 • applying terrain categories incorrectly Basic mean wind speed contours
Fax: 01442 256155 with 2% risk of exceedence per annum
• not using the topography rules of CP3
enquiry@rwdi-anemos.com

2. Page 2 BS6399-2 Wind loading
Conservatism bespoke applications with BREVe2
embedded, such as a roof tile fixing specifier
Often the enemy of the client, if not the and design package for conservatories.
designer, conservatism is a necessary part
of any codified approach to design. Audits and advice
BS6399-2 gives two methods for calculating You can harness our expertise in several
wind loads - standard and directional. The different ways, very cost-effectively. Of
simpler method contains conservatism that course, we are happy to do the calculations
depends on site location and terrain. It for you and present a detailed report of our
arises when the worst case for the various assumptions and findings. But perhaps you
parameters contributing to the wind speed just need reassurance and guidance.
(distance to coast, distance in town, terrain
roughness, altitude, slope, etc) do not Ask for advice before you start and we
occur for the same wind direction. will spend a short time with your design
Screen shots from
team to advise them how best to
BREVe2 There is a smaller amount of conservatism approach the wind load calculations.
in the pressure coefficients. Both of these Then carry out the calculations in-house.
sources of conservatism were also inherent
in CP3. Both sources can be minimised with Increase confidence in your design by
asking us to audit wind loading
intelligent use of the code.
calculations. We will advise on errors,
Expert interpretation how to correct them, remaining
conservatism and whether the
To get the best of the code in complex
calculations lie within the acceptable
situations, you need a clear understanding
“...RWDI- of the detail and the underlying behaviour range of interpretation for the code.
of wind around structures. The audit option can be especially useful
Anemos have when the calculations must be justified to a
RWDI-Anemos are probably the most licensing or regulatory authority, such as a
developed a experienced consultants in this field and can nuclear safety review or public enquiry.
series of design advise on where the conservatism lies.
Nicholas Cook was principal author of the Free design aids
aids to help code and knows it inside out. He was also RWDI-Anemos have developed a series of
responsible for writing the only effective free design aids to help reduce your
reduce your piece of software used to automate the workload. Interpolation for the terrain and
estimation of design wind speeds - including building factor, Sb, is a good example. Often
workload…” the effects of terrain and topography. needing a 4-way interpolation, and strictly a
BREVe2 has established an excellent logarithmic interpolation, a quick solution
reputation, along with its sister BRECp that should be very welcome. On the RWDI-
provides validated pressure coefficients. Anemos web site, there are a set of printed
BREVe2 is now built into a number of other design charts that reduce the task to
well-known structural design packages. reading from a graph. If you prefer a high-
tech solution, try out our simple Windows
Paul Freathy has presented more than 40 utility to calculate Sb automatically. It uses
training courses on the use of the Code, the approach that minimises conservatism,
many in combination with Nick Cook. He but gives you the Standard Method value for
has also been responsible for developing comparison as well.
Sector terrain roughness
Sector altitude Sector max slope over 1km
(z0)
Altitude (m) % of sites Slope (%) % of sites z0 (m) % of sites
<25 28.1 <2 76.8 <0.025 0.7
25-75 36.7 2-4 16.9 0.025-0.1 18.7
75-125 20.4 4-6 4.2 0.1-0.3 30.8
125-200 11.8 6-8 1.3 0.3-0.6 44.0
Free design aids from >200 3.0 >8 0.8 >0.6 5.8
www.rwdi-
anemos.com
Table 1 - Distribution of altitude, slope and terrain type over the GB mainland