1. Mohammed Zaid “ Wind Tunnel Test for structures”
W
affects the way that wind acts on
ind structures. In addition, buildings and
structures induce additional
Tunnel fluctuations in the flow, which mean
that pressures fluctuate constantly
Test over the surfaces of buildings. This is
why pressures on buildings reduce as
the averaging area increases. Thus, a
Introduction simple summation of peak local
T here are two generally‐used pressures would result in vast
methods for determining wind overestimates of the maximum
loads on buildings and overall pressure acting on a building
structures: at a given instant in time. Early in the
history of wind engineering, it was
1‐ code‐based design and recognised that it was necessary to
2‐ wind tunnel testing. take account of these turbulent
fluctuations in order to accurately
While code‐based methods may be
predict loads. This is the reason why
adequate for the majority of buildings
wind tunnel testing of buildings and
and structures, there are a number of
structures must be conducted in what
cases where code approaches are not
are known as boundary layer wind
suitable, or alternatively can result in
tunnels. This type of wind tunnel is
excessively conservative design. The
designed to model the characteristics
following paragraphs will give a basic
of the natural wind as it approaches
guide to the background of each to
the model being tested. Wind loading
assist the designer in understanding
codes and standards also take
when each approach may be
account of these fluctuations in an
appropriate for a given project, or to
analytical manner.
recognise when it may be beneficial
to seek specialist wind engineering The wind characteristics (variation of
advice. wind speed with height, and
turbulence characteristics) at a site
Background to Wind Loading are dependent on the storm type
being considered, the local terrain,
Wind loads on buildings and and the effects of surrounding
structures have two components that buildings. Design wind speeds for a
are of interest to designers: global given site need to take account of a
wind loads that will affect the design wind climate analysis, and then
of the main structural elements and adjusted for the effects of terrain and
local loads that will affect roughness. Typically, wind climate
components such as secondary analyses are based on long‐term
members or the cladding of the meteorological records at local
building envelope. The wind is airports or other meteorological
turbulent in nature, and this gustiness stations.
M_as@hotmail.com
M_as3@yahoo.com

2. Mohammed Zaid “ Wind Tunnel Test for structures”
Overall wind loads for structural with strengths and weaknesses. One
design may also need to take account key commonality that almost all
of wind‐induced dynamic responses contain is that they define wind
for flexible structures such as masts, speeds and loading coefficients, as
large roofs, and tall buildings. There well as a number of other necessary
are a number of different types of parameters. One area where they
responses that need to be considered. differ, though, is that wind speeds are
For example, for tall buildings it is defined differently in different codes,
necessary to cover along‐wind and the coefficients and other
response (responses in the direction parameters may differ in order to
of the wind, caused by turbulent work with the wind speed definitions
buffetting), crosswind response in determining the correct results. For
(responses orthogonal to the wind this reason, it is not advisable to
direction, and caused by vortex‐ combine values of different
shedding), and torsional responses components from different wind
resulting in twisting of the building. loading codes unless the basis of the
The magnitude of these responses is component values is fully understood.
very dependent on the dynamic
structural properties of the building The intent of wind loading codes is to
such as the natural frequencies, mode provide design values that envelope
shapes, and structural damping the majority of building designs within
values. the limitations of the code. Usually,
codes will give guidance on their
Overall, the loads on a dynamic applicability and when further advice
structure comprise three major may be sought. The majority of the
components: mean, background, and science contained in wind loading
resonant. The mean loads are the codes has been developed through
average loads that occur as a result of wind tunnel testing, and wind tunnel
a static response to the mean wind testing has also been the source of
speed. The background response is a almost all of the loading coefficients.
quasi‐static response as the structure The values from a number of wind
responds to the gusts in the wind. The tunnel tests are then enveloped to
resonant response is due to the cover the majority of building shapes.
structure vibrating at its natural However, it needs to be recognised
frequencies due to excitation by the that wind loading codes and
wind flow around it. standards are not intended to be
applicable to all buildings and
structures, rather they are intended
to cover the majority of cases but may
Wind Loading Codes and not give adequate design loads for
unusual shapes or configurations that
Standards fall outside the scope of the code.
There are many wind loading codes
and standards around the world, each
M_as@hotmail.com
M_as3@yahoo.com

3. Mohammed Zaid “ Wind Tunnel Test for structures”
Two key differences between wind Wind tunnel tests must be conducted
loading codes when it comes to the in a boundary layer wind tunnel
design of tall buildings are the where the characteristics of the
considerations of torsional responses natural wind are modelled in the wind
and crosswind responses. Some codes tunnel. This is usually achieved by a
have only very simple or no combination of spires, trip boards,
consideration of torsional responses. and floor roughness over a long fetch
Of the few codes that consider of the tunnel upwind of the test
crosswind response, some have section. Often these combinations of
analytical approaches based on wind boundary layer generating elements
tunnel tests of isolated tall buildings are changed for different wind
of simple shape, while others have a directions depending on the terrain
more empirical approach. Neither upwind of the site. Typically, wind
approach is particularly accurate, loading and cladding pressure tests
although research literature may are conducted for thirty six wind
provide additional relevant directions at equally spaced
information. increments. Normal length scales for
building testing are usually between
Most wind loading codes and 1:200 and 1:500. The output of the
standard explicitly provide for wind wind tunnel tests can be presented as
tunnel testing as an alternative route coefficients relative to a reference
to compliance. wind speed or pressure. The reason
that boundary layer wind tunnel
Wind Tunnel Testing testing works at small scale is that
these coefficients are the same in
Wind tunnel tests can be used to
model and full‐scale, as long as a basic
determine overall wind loads and
minimum wind speed (or more
local cladding pressures. Properly
correctly Reynolds Number) is
conducted wind tunnel tests should
achieved in the wind tunnel.
provide more accurate results than
can be obtained from the use of wind Local pressures and cladding loads are
loading codes. In many cases, this can determined through pressure testing.
result in lower loads and pressures. It This requires a model of the building
should, however, be recognised that or the structure to be tested with
wind tunnel testing can also produce adequate pressure taps (points where
higher loads and pressures than the pressures are being measured) to
would be obtained from code capture the peak pressures in
approaches. When this occurs, it different areas of the building or
ensures that the resulting design will structure. This requires an increased
have an adequate level of design density of pressure taps in areas
reliability, and reduces the risks of where the pressures are expected to
failure. be largest or large pressure gradients
are expected to exist. This typically
means a higher number of pressure
M_as@hotmail.com
M_as3@yahoo.com

4. Mohammed Zaid “ Wind Tunnel Test for structures”
taps close to building corners and the model as an analogue mechanical
other architectural discontinuities. integrator of the wind pressures. In
this case, the key is to use as light and
Overall structural loads and responses stiff a model as possible to ensure
can be determined in a number of that the combined balance/model
ways. The simplest approach for non‐ natural frequency is much higher than
dynamic sensitive structures such as the scaled natural frequency of the
low‐rise buildings is to integrate the prototype building. This is to avoid
pressures over the building. This contamination of the signal by
requires adequate pressure taps to model/balance resonance at the
quantify the pressure fields over the frequencies of interest for
building, and time histories of the determining the response of the
pressures. Time histories of the building. Models for this type of test
overall loads on the building can thus may be constructed from materials
be determined and key load effects such as high density expanded foam
(such as base shears or bending or balsawood. This type of test is
moments) can be identified using recommended for buildings with very
areas of influence associated with complex architecture or with
each discrete pressure tap. floorplates that do not allow sufficient
When dynamic responses are pressure tubes to be extracted
anticipated, then the same pressure simultaneously.
integration approach can be The final type of test for tall buildings,
employed but with the added step of or wind sensitive components, is
calculating the resonant response conducted much less frequently and
component. This is done using is the aeroelastic test. In this type of
dynamic properties (natural test, the model/test rig incorporates
frequencies, mode shapes, mass the scaled dynamic characteristics of
distributions, and damping ratios) the prototype structure. Because of
provided to the wind engineer by the this, the model will vibrate in the
structural engineer. This approach is wind tunnel and loads and responses
the method that allows the most can be measured directly. This
accurate floor‐by‐floor distribution of approach has the advantage of
structural loads, but is best suited to measuring the full effects of
buildings where the architecture is aerodynamic damping, but the
relatively straightforward and a large disadvantage that the results may be
number of pressure tubes can be of more limited use if the structural
extracted from the model dynamic characteristics change
simultaneously. through the design process after the
The other common approach to wind tunnel testing has been
determining wind loads on structures conducted. This type of test is
is to mount them on a high‐frequency normally only used where there is the
balance. This measures loads at, or potential for negative aerodynamic
near, to the base of the building using damping where the building or
M_as@hotmail.com
M_as3@yahoo.com

5. Mohammed Zaid “ Wind Tunnel Test for structures”
component motion starts to drive the American Society of Civil Engineers
excitation mechanism leading to (1999): ASCE Manual on Engineering
potential aerodynamic instabilities. Practice No. 67 ‐ Wind Tunnel Model
Studies of Buildings and Structures
A complete guide to wind tunnel
testing can be found in ASCE Manual Australian Wind Engineering Society
of Practice No. 67, and basic (2001) : AWES‐QAM‐1‐2001, Wind
minimum standards are published in Engineering Studies of Buildings
AWES QAM‐1‐2001. The Council on
Tall Buildings and Urban Habitat will Council on Tall Buildings and Urban
also shortly publish a Guide to Wind Habitat (in preparation): Guide to
Tunnel Testing of Tall Buildings. Wind Tunnel Testing of Tall Buildings
When to use Wind Tunnel
Testing
Wind tunnel testing should be used when:
1. The building or structure is of a
complex, unusual, or irregular
shape not covered by simplified
wind loading codes; or
2. The building or structure has
response characteristics that
make it susceptible to crosswind
loading, vortex shedding,
aerodynamic instabilities such as 3D view of 55 Floor structure during wind tunnel testing
galloping or flutter; or
3. The building or structure is sited
such that it is particularly
sensitive to potential channelling
or buffeting due to upwind
obstructions; or
4. Where the building designers
wish to design with the maximum
economy consistent with
reliability.
References
M_as@hotmail.com
M_as3@yahoo.com