A History of Research and a Review of Recent Developments

Uniformly distributed pressures 73
the box and reaches the rear edge. It then spills down the back wall. On the flat
roof and sides the instantaneous pressure rise is to p 0 (not p r), but because of the
pressure difference at the junctions of the top and sides with the front face,
vortices will be formed there. See, for example, the book by Norris et al. [4.1].
In determining the loads on the structure it is useful to consider the instant
at which the shock front strikes the front face as t=0, and the time for reflection
effects to clear the front face as t=t c, where
t c =3s/ū.
(4.2)
In this equation s is the clearing height, taken as either the half width of the
front face (B/2) or the full height (H), whichever is the lesser, and ū is the
velocity of the shock front. Once the time t c is reached, the pressure on the
front face (taken as uniform) is a combination of the gradually decaying incident
pressure, p, and the dynamic pressure, q. The variation of dynamic pressure
with time is usually given by the equation
(4.3)
which is similar to the expression for overpressure decay but with a different
decay rate. From Eq. (1.28) we note once more that the dynamic pressure in
psi at time t 0 is given by
(4.4)
To obtain a true measure of the dynamic loading due to the blast winds on the
structure, it is necessary to multiply q by the drag coefficient, C d, for the shape under
consideration. So the total front wall pressure (p s) from time t c onwards is given by
p s=p+C dq.
(4.5)
For the front face of a rectangular box structure C d is often taken as 1.0, although
wind tunnel tests show that the average pressure due to blast winds is rather less
than 1.0 q. Some analysts therefore suggest C d=0.85 for the front face.
When the shock wave reaches the rear wall of the structure distant L from
the front wall, vorticity again occurs at the edges, causing a reduction in incident
pressure due to suction. According to some authorities the overpressure on
the rear wall reaches a maximum in a time given approximately by
t (4.6)
L =(L+4S)/ū.
The sides and top of our simplified box are not fully loaded until the shock
front has travelled the full length of the structure. The average pressure is
therefore considered to be the shock overpressure plus the drag loading at a