F. K. Kong MA, MSc, PhD, CEng, FICE, FIStructE, R. H. Evans CBE, DSc, D ès Sc, DTech, PhD, CEng, FICE, FIMechE, FIStructE (auth.)-Reinforced and Prestressed Concrete-Springer US (1987)

420 Practical design and detailing
and convert to dynamic pressure q (N/m 2 ) using
q = 0.613V; N/m 2
Values of q, therefore, vary between about 61 N/m 2 for a Vs value of
10 m/s and 3000 N/m 2 when V 5 = 70 m/s.
(4) Using given external (Cpe) and internal (C i) pressure coefficients,
and taking their signs (i.e. negative if suction) into account, the force
F can be taken as
F = ( Cpe -
Cpi)q · A
where A is the surface area of the element or structure being
considered.
Cpe values vary considerably depending upon the building's aspect ratios
(height: width; length: width), the wind direction and the surface being
considered. When the wind is normal to the windward face the value of
Cpe for that face is generally + 0.7, whereas the leeward face then
has a value varying between -0.2 and -0.4 depending upon the plan
dimensions.
Cpi is also very variable, but it has a value of -0.3 if all four faces are
equally permeable, i.e. for the windward face mentioned the value of
( Cpe - Cpi) is equal to 1.0.
The second method, which is only applicable to a limited range of
rectangular (in plan) building shapes, gives the force directly as
F = Cr X q X Ae
where Cr = the force coefficient obtained from tabulated values, which
range from 0. 7 to 1.6 depending upon aspect ratios;
q = obtained as above;
Ae = the effective frontal area of the structure.
Figure 11.4-3 gives the equivalent forces due to the wind (1.2wk) for
the example here, applied at roof and floor levels, as is normally assumed.
A further assumption is now made with regard to the distribution of
these forces across the frame. A choice has to be made between the two
methods usually used:
(1) The cantilever method; or
(2) The portal method.
(1) The assumption made in the so-called cantilever method is that the
axial force in a column, due to the wind loading, is proportional to its
distance from the centre of gravity of all the columns in that frame.
When the system is symmetrical in both column size and position
the calculation is straightforward, e.g. in Fig. 11.4-10 where the
centre of gravity is at the centre:
l~il= 1~ 2 1= 1~ 3 1= 1~:1
i.e.
IN11-= IN41 = I2.6N21 = I2.6N31