Internal pressures - Wind Loading of Structures - page 160

Equation (6.16) can be applied with A w taken as the combined open area for several
openings on a windward wall and C p w taken as an average mean pressure coefficient with
similar treatment for the leeward/side walls. It has been applied to give specified values
of internal pressures in design codes and standards (see Chapter 15), in which case the
coefficients are used with mean pressure coefficients to predict peak internal pressures,
making use of the quasi-steady assumption (see Section 4.6.2).
Measurements of mean internal pressure coefficients for a building model with various
ratios of windward/leeward opening area are shown in Figure 6.4. The solid line in this
figure is Equation (6.16) with C p w taken as +0.7 and C p L taken as −0.2. These values were
the values of mean external pressure coefficients on the walls at or near the windward
and leeward openings, respectively. It may be seen that the agreement between the
measurements and Equation (6.16) is good.
6.3.2 Fluctuating internal pressures
The analysis of fluctuating internal pressures when there are openings on more than one
wall of a building is more difficult than for a single opening. In general, numerical
solutions are required (Saathoff and Liu, 1983). However, some useful results can be
obtained if the inertial terms are neglected and the damping term is linearized (Vickery,
1986, 1991; Harris, 1990). The neglect of the inertial term in comparison to the damping
term is justified when there is background porosity in the walls of a building, but may not
be so when there are one or more large openings.
It can be shown (Harris, 1990) that when there is a combined open area on a windward
wall of A w and external pressure coefficient C pw , and on a leeward wall with total open
area A L and external pressure coefficient C P L , then there is a characteristic response time
given by:
(6.17)
There is some similarity between Equations (6.16) and (6.5) for a single opening, but
they are not exactly equivalent. External pressure fluctuations which have periods much
greater than τ are transmitted as internal pressures in a quasi-steady manner—i.e. they
will follow Equation (6.15). Fluctuations with periods of the same order as τ will be
significantly attenuated; those with periods less than τ will have negligible effect on the
fluctuating internal pressures.