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[er_spsvasan]

Hello Aravan,

The IS 875 seems to be using the term Pressure
Cofficient Cp (= Cpe-Cpi) when dealing with normal
buildings. The term Force Coefficient Cf seems to be
used for structures like chimneys, watertanks, lattice
towers etc, for which the the term internal pressure
coefficient does not have any meaning.

We have to use either Cf or (Cpe-Cpi) depending upon
the type structure.

While the force coefficient and the pressure
coefficients give the value of pressure normal to a
surface, the drag coefficient gives the value of
pressure parallel to the surface (due to friction
effect between wind and the surface). The use of drag
coefficient depends upon the circumstance. For
example, if we are designing a factory building, when
we design the trusses, the drag force is not
considered since the drag force is perpendicular to
the truss and the truss is not expected to resist the
same. However when we design the bracing system the
drag force is to be considered while calculating the
forces on the bracing system. Similarly when we design
the frame supporting the trusses, the drag forces are
to be considered. The drag forces are transmitted to
the supporting frame through the bracing system.
Ofcourse, if we are doing a 3 dimensional analysis,
the drag forces are always to be considered.

Regards
S.P.Srinivasan

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