A History of Research and a Review of Recent Developments

Civil buildings 187
exterior of the brick shelter was covered by steel mesh, held in position by an
additional 4.5 in brick skin. A typical reinforced brick shelter used brick side
walls combined with a concrete floor and roof. Reinforcing bars were introduced
into the cavity between bricks before the cavity was filled with concrete. When
attacked by blast weapons surface brick shelters were of course much more
vulnerable than buried shelters.
Brickwork was also protected during the Second World War by internal
strutting, which was placed so that there was enough lateral stability to eliminate
the possibility of collapse from unsymmetrical loading. Strutting was also
used to support the load of debris on the roof of the building if destruction of
the walls occurred. Tests on brick shelters during the Second World War were
described in a series of reports by the Research and Experiment department,
Ministry of Home Security, and examples are given in refs [8.7] to [8.9]. The
lateral velocity imparted to a surface shelter by earthshock from a bomb of
250kg at 15 ft horizontally from the shelter wall was found to be about 10 ft/
sec, as measured and reported by Walley [8.10].
The amount of reinforcement required to keep a brick surface shelter intact was
calculated very simply by Christopherson. He supposed that when an unreinforced
shelter breaks up the maximum relative velocity of roof and walls is 5 ft/sec for a
shelter 30 ft×8 ft in plan area with walls 1 ft thick. If the roof is assumed to be 5 in
thick, with a density of 144 lb/ft 3 , its kinetic energy is (60×25)/64 ft lb/ft 2 . The
kinetic energy of the roof per foot run of wall is (60×25×240)/64×72 ft lb. If there
are A in 2 of reinforcing bars per ft run of wall, having a yield stress of 50 000 lb/
in 2 , and if their extension is d ft, then A.d. 50 000=the kinetic energy per foot
run. If, as suggested earlier, a minimum area of reinforcement of 0.06% is used,
then in a 1 ft thick wall, the value of A=0.06×1.44, and d=(60×25×240)/(64×72×50
000× 0.06×1.44)=0.22 in. Thus the separation of wall and roof cannot be greater
than about in , and could well be less. This indicated that the shelter components
would remain virtually intact.
This wartime research and testing was still used as a basis for design rules
over the next thirty or forty years. Thus, when the US Army Technical Manual
on Protective Design (Non-nuclear) appeared in 1965 [8.11], it contained
wartime information on the thickness of reinforced and plain brickwork to
provide protection against fragments and blast from general purpose bombs
detonated at a distance of 40 feet. The required thicknesses are shown in
Table 8.1.
It was also recommended that brick structures designed with these thicknesses
should be spaced 120 ft apart. Blast resistant protective brick walls were also
required to be buttressed at intervals not greater than 10 ft, with the buttresses
reinforced horizontally.
In Britain, the response of brick walls was often based on information
given by a wartime bulletin issued by the Ministry of Home Security [8.12],
which gave the minimum distance in feet at which only slight damage would
occur from surface blast. The required distances are shown in Table 8.2.