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