28-09-2012, 11:45 AM
Effect of wind on structure
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Wind produces three different types of effects on structuretatic,dynamic and
aerodynamic.The response of load depends on type of structure.When the
structure deflects in response to wind load then the dynamic and aerodynamic
effects should be analysed in addition to static effect.Sound knowledge of fluid
and structural mechanics helps in understanding of details of interaction between
wind flow and civil engineering structures or buildings
Flexible slender structures and structural elements are subjected to wind induced
along and across the direction of wind. When considering the response of a tall
building to wind gusts, both along-wind and across-wind responses must be
considered. These arise from different the former being primarily due to buffeting
effects caused by turbulence; the latter being primarily due to alternate-side
vortex shedding. The cross-wind response may be of particular importance
because it is likely to exceed along-wind accelerations if the building is slender
about both axes.
At low wind speeds, since the shedding occurs at the same instant on either side
of the building, there is no tendency for the building to vibrate in the transverse
direction. It is therefore subject to along-wind oscillations parallel to the wind
direction. At higher speeds, the vortices are shed alternately, first from one and
then from the other side. When this occurs, there is an force in the along-wind
direction as before, but in addition, there is an force in the transverse direction..
This type of shedding, which gives rise to structural vibrations in the flow
direction as well as in the transverse direction, is called vortex shedding.The
frequency of shedding depends mainly on shape and size of the
structure,velocity of flow and to a lesser degree on surface roughness,turbulence
of flow.