Civil Engineering Reference
In-Depth Information
10.6 Wind loads
Wind loads are one of the most significant lateral loads con-
sidered in the design of structures, since they are generally
considered in the design of building structures, compared to
say seismic loads, impact of blast, etc. which are considered
in some cases depending upon location and 'risk'. The mag-
nitude of the loads is dependent upon the geographical loca-
tion, the statistical frequency of storms (climatic analysis), the
roughness of surrounding ground, the geometrical features of
the building and its parts and the dynamic properties of the
structure. Additionally, the magnitude of loading considered to
larger building volumes is less than that to smaller areas, such
as individual cladding panels, as the gusts are larger, with less
speed and correlation of wind over larger volumes is less prob-
able than over smaller areas.
Wind loads for typical shapes of buildings and other
structures are generally determined simply through use of
the various code approaches - BS6399, EC3, IBC (ANSI
ASTM), etc. In recent years, there has been a trend towards
these codes being more consistent and a consensus of
approach between European and North American practice
has developed.
Some rules of thumb which can be used for initial hand cal-
culations are as follows:
adopted, due mainly to speed and cost considerations as well
as questions regarding the 'accuracy' of computational meth-
ods. When using wind tunnel testing it should be recognised
that in the vast majority of cases only the static component
is directly 'measured' and the dynamic component is added
numerically, which in particular for torsional modes effects is
not 'precise' - i.e. even apparently 'precise' results do contain
considerable approximation and need to be understood in such
terms.
For tall buildings typically the guidelines for use of wind
tunnel testing are as follows:
i.
has a shape which is not a typical geometrical form (i.e. it
is not a box);
ii.
natural period higher than 1 second;
iii.
subject to buffeting by wake of upwind structures;
iv.
subject to funnelling effects due to ground conditions or
other structures.
For structures supporting large roof areas it is often possible
to generate 'savings' in terms of wind loadings by using wind
tunnel modelling which looks at the correlation of wind effects
over the whole surface compared to code approaches.
It should be borne in mind that typically something like 25%
of structures which are wind tunnel tested give wind loads that
are higher than in the code approach.
lateral load of 1.5% of dead load;
■
1 kN/m
2
pressure for roof structures, etc.;
10.7 Seismic loads
Seismic 'loads' are those forces generated by the accelerations
related to earthquakes. Whilst the design of structures for seis-
mic loads has not been commonly undertaken in the UK (with
the notable exception of the nuclear industry) it is an important
part of structural engineering design globally, in particular in
those countries where earthquakes of significant magnitude to
cause damage to property and injuries and/or death of people
occur regularly.
The most commonly adopted international codes are those
in the UBC 'family' (UBC97, IBC 2006, etc.), which are US
codes. The Eurocode approach (EC8) is similar to the US codes
in theoretical approach and application. The Japanese seismic
codes are significantly different in approach, being based on a
'strength' rather than ductility approach. In recent years there
has been a trend towards performance-based design methods
and more sophisticated dynamic methods of analysis, reflecting
the general changes in engineering computational methods.
When considering the magnitude of seismic loads it is im-
portant to start with an understanding of what is being designed
for. Typically the approach is:
■
2 kN/m
2
pressure for tall buildings;
values for cladding pressures double those for buildings (i.e. 2 or 4
■
■
kN/m
2
for design of a cladding element or supporting member).
Whilst wind loading may govern some aspects of many, if
not all building types, types of structures for which wind load-
ing is likely to be particularly significant are:
i.
tall, slender structures;
ii.
long span roofs;
iii.
local areas in buildings impacted by funnelling effects (i.e.
two buildings close together, etc.).
Keep in mind that wind will hit tall structures at high level and
be forced down and around, so the local pressures can be sig-
nificantly higher than at that height for a different structure
Wind loads can be thought of as having two components:
the static part which is due to the shape of the building and
the dynamic part which is due to the building's response to the
wind loads and which is dependent upon the mass and stiffness
distribution of the structure and its intrinsic (or augmented)
damping. For common structures, the dynamic part of the
loading can be derived by applying simple factors to the static
part or by using other codified methods.
To deal with more sophisticated cases the currently avail-
able techniques are either computational modelling or wind
tunnel testing. Wind tunnel testing seems to be universally
Structures to survive minor, regularly occurring earthquakes
■
without permanent damage and associated non-structural elem-
ents to have no, or minor, damage.
Structures to behave as predicted for larger magnitude, infrequent
■
earthquakes - extensive damage to the non-structural elements is
acceptable.
