Report of the British Mission to Japan

BLAST EFFECTS

SCALE AND KIND OF EFFECT

21. As mentioned in the previous chapter it is difficult to estimate the equivalent amount of T.N.T. from the damage caused in Hiroshima and Nagasaki, because the criteria are very insensitive. It may suffice, however, to point out that an explosion of 20,000 tons of T.N.T. would be expected to cause at a distance of ½ mile from the centre of damage an instantaneous pressure rise of about 10 lb. per square inch, falling back to atmospheric pressure in about ½ second : and since, during part of this time, there would be a wind of the order of 500 miles an hour, the pressure initially imposed on parts of a building might be as high as 30 lb. per square inch. Such figures could be multiplied and become meaningless : the reader who finds them so may prefer a summary analogy. This is that the scale of destruction expected would be that which would befall a model town built to the scale of Gulliver's Lilliput, 1 inch to the foot, if there were exploded above it a bomb more than twice as large as the largest British " blockbuster ", which with its case weighed about six tons.

22. This model and what has been said will also help him to understand the major differences between conventional blast effects and those seen in Hiroshima and Nagasaki. These were three in number.

• Mass Distortion.—It is usual for a bomb to damage only part of a large building, which may then collapse further under the action of gravity. The blast wave from the atomic bomb, however, was so large that it engulfed whole buildings, pushing them askew in the manner shown in photographs 5, 6, 11 and 12. The effect, which occurred with all types of buildings, resembles damage done by wind, and operates somewhat in the same way. Thus the pressure which damaged the long wall in photograph 5, which shows the side remote from the explosion, was transmitted in part from the front wall through the roof and floors, and in lesser part was the wind suction which is normally felt on the leeward side of an obstacle.
• Infrequency of Blast Suction.—After the blast pressure has fallen from its peak back to zero, there always follows a period of suction (unrelated to the wind suction just mentioned). Although this suction is weaker than the original pressure, it lasts several times as long, and therefore normally does much damage to objects which had no time to fail under the usually brief initial pressure. Pressures from the atomic bomb, however, lasted long enough to give windows, doors, walls, and even chimneys and telegraph poles time to fail. As a result, effects which could be ascribed to blast suction were unusually scarce, although a few were observed in Hiroshima.
• Downward Thrust.—Because the explosion was high in the air, much of the damage was due to downward pressure. Most characteristic was the " dishing " of the flat roof slabs of reinforced concrete buildings, some of which assumed a saucer shape. For the same reason, telegraph and other poles remained upright immediately below the explosion, but were overturned or tilted at greater distances from the centre of damage. Trees below the explosion remained upright, but had their branches torn downward.

23. Screening from blast by large features, and similar effects, were not unusual ; for example, almost the whole of the smaller valley in Nagasaki was screened by the intervening mountain ridge. The reflection of blast, which adds considerably to its force, was unusually marked but important only in special cases, among them some bridges. Finally there may be remarked the absence of the carriage of any heavy debris over large distances. Small debris such as tiles and battens appeared to have been carried considerable distances, and was found on tail buildings. But larger pieces of debris were always found close to their point of origin, and where massive slabs such as bridge decking had been shifted, the movement, although on occasion critical, was small.

DAMAGE TO COMMERCIAL AND INDUSTRIAL BUILDINGS AND MACHINES

24. Photographs in this report and elsewhere show great areas of destruction in which, rising here and there like islands, there remain reinforced concrete buildings showing few signs of external damage. There were in fact many reinforced concrete buildings in Hiroshima and a number in Nagasaki. They varied from exceptionally strong office blocks (see photograph 7) designed to be proof against earthquake, to lightly constructed industrial buildings (see photograph 10), Between these extremes were some schools and office buildings of more or less normal design such as is usual in Great Britain. In comparison with other forms of construction these buildings resisted the blast well. Owing to the height of the explosion, the behaviour of buildings near the centre of damage was greatly influenced by that of their roofs.

25. For example, the building of normal construction shown in photograph 8 collapsed at 200 yards from the centre of damage because the flat roof was forced in. The Nagasaki school shown in photograph 5, which suffered mass distortion at 500 yards from the centre of damage, while it had special weaknesses, may also be regarded as of roughly normal design. At 700 yards from the centre of damage in Nagasaki, another school of normal design suffered some structural damage, but did not collapse and one wing continued in use. A school and an office block, each at ¾ mile from the centre of damage, remained structurally sound, although some internal walls were damaged. In summary, reinforced concrete buildings of normal construction were usually safe from partial collapse beyond 600 yards from the centre of damage, and from structural damage beyond ½ mile.

26. Reinforced concrete buildings of very heavy construction in Hiroshima, even when within 200 yards of the centre of damage, remained structurally undamaged. Flat roof slabs 6 ins. or 7 ins. thick were often dished, but a roof thickness of 10 ins. appeared to be sufficient to resist permanent deflection.

27. It is appropriate here to draw attention to the building the interior of which is shown in photograph 9. This was less massive than the strongest Hiroshima buildings, but its design incorporated features which made it unusually resistant to side pressure. As a result it remained structurally undamaged, although only 500 yards from the centre of damage.

28. These observations make it plain that reinforced concrete framed buildings can resist a bomb of the same power detonated at these heights, without employing fantastic thicknesses of concrete. The main requirements are a frame designed to withstand heavy side forces from any direction, and a flat reinforced concrete roof perhaps 50 per cent, thicker than would be normal practice. The reinforcement of external concrete walls should also be tied into the supporting frame. It is believed that similar requirements would suffice for steel framed multi-storey buildings, of which Nagasaki contained the only example. These requirements ignore fire and casualty risks, which are dealt with later.

29. Light single-storey concrete buildings, such as are employed for factories (see photograph 10) and warehouses, failed at about a mile from the centre of damage in both cities.

30. Of the industrial buildings seen, those most characteristic of western practice were the steel framed single-storey factory sheds. Useful examples were confined to Nagasaki, where they were plentiful throughout the Urakami valley both north and south of the centre of damage. They were usually of the type found in large engineering works, with travelling cranes and a covering of thin corrugated iron or asbestos cement sheeting.

31. The bulk of the damage to these buildings was by blast. In Nagasaki fire had contributed to the damage in only about 10 per cent, of cases of damage. In Hiroshima the few small steel framed sheds foimd had been damaged further by fire, but this had probably originated in the wooden houses by which they were there surrounded.

32. The most striking feature of damage was the mass distortion, in the direction away from the explosion, of the entire framework of these buildings. This distortion, which is shown in photographs 6, 11 and 12, occurred at distances up to more than f mile from the centre of damage. Its amount naturally decreased with the distance from the explosion. Distortion appeared to be less severe in sheds which had been covered with a material which had itself shattered under the blast, such as asbestos cement, than in sheds which had been covered with a pliable material such as corrugated iron, which had transmitted the pressure. (Roof and wall coverings of both types were destroyed to distances of 2 miles and more from the centre of damage.) Beyond the range of mass distortion, steel framed sheds suffered damage to the structural framework at distances up to roughly 1½ miles from the centre of damage. Like other buildings, sheds suffered more severely when they had a long wall facing the blast, or if they lacked stiffness or bracing.

33. Of the machines housed in these sheds, only 5 per cent, had suffered serious damage from the atomic bomb. This low figure is to be ascribed to the absence of fire, so that damage was caused only by the movement of parts of the structure which crushed or overturned adjacent machinery. The Japanese had, however, allowed the undamaged machines to weather in the damaged buildings without protection, with the result that the majority had become unserviceable.

34. Nearly two-thirds of all machines in the Urakami valley had been housed in smaller workshops and sheds of timber. These shops were burnt down almost without exception to a distance in excess of 1½ miles from the centre of damage. As a result 50 per cent, of the machines housed in these shops were destroyed or irreparably damaged. Their appearance today is shown on photograph 13.

35. Little information could be obtained on machines housed in reinforced concrete sheds with thin concrete roofs, a type of factory construction more common in Europe than in Japan. Such evidence as was found suggests that approximately 75 per cent, of the machines in such sheds (the vulnerability of which has been remarked in paragraph 29) would have suffered damage from the heavy debris formed by the collapsing roof.

36. Reference has been made to timber framed single-storey buildings, which were common as workshops and warehouses both in Hiroshima and Nagasaki. These buildings behaved badly, being excessively vulnerable both to fire and to blast. For example, the absence of internal stiffening, and the great weight of the roof trusses and tiles, made them subject to collapse from mass distortion at distances of 2 miles and more from the centre of damage.

DAMAGE TO HOUSES AND SHELTERS

37. The bulk of the damage in both cities, naturally, was to Japanese houses. These houses are constructed on a frame of 4 ins. or 6 ins. square timbers. The roofs are not trussed in the orthodox manner and are a source of weakness, particularly since their covering of pantiles bedded in mud on ½ in. boarding is disproportionately heavy. The walls are of bamboo covered with 3 ins. of mud, which is sometimes protected by ½ in. boarding ; but as photograph 19 shows, much of the wall space is occupied by paper-covered screens. Complete collapse of these buildings from blast extended to 1½ miles from the centre of damage in Hiroshima, and to an average of 1½ miles in Nagasaki. Fire completed the destruction almost to the same distance, except in one congested area of Nagasaki, where it exceeded it. Beyond the region of complete collapse damage decreased rapidly, the further zone in which houses had been damaged beyond repair being little more than 1 mile wide. Minor damage extended to large distances, 3 miles or more from the centre of damage.

38. Naturally, the subject of major interest outside Japan is the behaviour of unframed brick buildings with load-bearing walls, which make up the bulk of European housing. These buildings are rare in Japan, and those which were found differed in important respects from British housing. The Mission had therefore to draw its conclusions from such isolated examples as that shown in photograph 15. (This building, although somewhat stronger than British houses, had collapsed at 700 yards from the centre of damage.) Interpreting such examples in the fight of its European experience, the Mission estimated that a bomb of the same power, exploding at the approximate height of those in Hiroshima and Nagasaki, would cause the collapse of normal British houses to a distance of 1,000 yards from the centre of damage ;

• would damage them beyond repair to a distance of 1 mile ;
• would render them uninhabitable without extensive repair, particularly to the roof timbers, to a distance of 1½ miles ;
• and would render them uninhabitable until first-aid repairs had been carried out, to a distance of 2 to 2½ miles from the centre of damage.

39. Unframed masonry construction with load-bearing walls of greater thickness is also widely employed throughout Europe for public buildings and blocks of flats. Such buildings are subject to damage of equal severity, at smaller but considerable distances. Photograph 16 shows the damage to the monumental Roman Catholic Cathedral of Nagasaki at 600 yards from the centre of damage. Here the damage was completed by fire and other causes, but effectively the building had already been destroyed by blast.

40. The provision of air raid shelters throughout Japan was much below European standards. Those along the verges of the wider streets in Hiroshima were comparatively well constructed : they were semi-sunk, about 20 ft. long, had wooden frames, and 1 ft. 6 ins. to 2 ft. of earth cover. One is shown in photograph 17. Exploding so high above them, the bomb damaged none of these shelters.

41. In Nagasaki there were no communal shelters except small caves dug in the hillsides. Here most householders had made their own backyard shelters, usually slit trenches or bolt holes covered with a foot or so of earth carried on rough poles and bamboos. These crude shelters, one of which is shown in photograph 18, nevertheless had considerable mass and flexibility, qualities which are valuable in giving protection from blast. Most of these shelters had their roofs forced in immediately below the explosion ; but the proportion so damaged had fallen to 50 per cent, at 300 yards from the centre of damage, and to zero at about ½ mile.

42. These observations show that the standard British shelters would have performed well against a bomb of the same power exploded at such a height. Anderson shelters, properly erected and covered, would have given protection. Brick or concrete surface shelters with adequate reinforcement would have remained safe from collapse. The Morrison shelter is designed only to protect its occupants from the debris load of a house, and this it would have done. Deep shelters such as the refuge provided by the London Underground would have given complete protection.

DAMAGE TO PUBLIC SERVICES

43. It remains to discuss the behaviour of the major public services. Many of these are subject to the consideration which has been implied in the discussion of shelters : that bombs' exploded at such heights have no effect below ground. For example, gas and water pipes were in general undamaged except where they were carried over rivers on bridges which were displaced. Sewers were undamaged in Hiroshima ; they did not exist in Nagasaki. However, in both cities the gas supply was destroyed by severe damage to the gas holders (see photograph 14) up to 1¼ miles from the centre of damage. The producing plant was not seriously damaged at this distance. In both cities, the water pumping station was beyond the range of damage, but that at Hiroshima was out of action for some weeks for lack of electric power.

44. Overhead electricity, tramway, telephone, and telegraph cables and their supports were severely damaged to distances of ½ mile to 1 mile. In addition, the electricity supply was affected by damage to sub-stations, resulting in debris damage to the switchboards and switchgear ; such damage was serious, for example, in the main transformer station in Nagasaki, 1 mile from the centre of damage. The great dispersal of the Japanese electricity system, however, made it possible to supply current to most undamaged areas in Nagasaki within a fortnight.

45. Damage to public transport was not considerable. Railway and tramway tracks were only indirectly affected, by debris, adjacent fire, overturned rolling stock and displaced bridges. The Prefect of Nagasaki reports that slow-running trains reached the city along the main railway line, which runs within 100 yards of the centre of damage, three days after the bombing. Trams, buses, and motor cars were probably destroyed to distances of ½ mile to 1 mile from the centre of damage, and some tram motors were reported to have been burnt out beyond 2 miles. Fuel storage tanks were damaged beyond repair more than 1 mile from the centre of damage.

46. There were 49 bridges within 2 miles of the centre of damage in Hiroshima. Most of these were multiple girder bridges in steel or reinforced concrete, and many had one or more suspended spans ; their overall length was usually between 200 ft. and 500 ft. Only one wooden bridge, and one steel bridge in bad repair, were destroyed by the blast ; and nine wooden bridges were burnt down in the subsequent fires. The remaining bridges were usable, although some of them had suffered small displacement in the direction away from the explosion, having perhaps been lifted by blast reflected from the river bed. Seven of these bridges were destroyed by two floods before the Mission reached Hiroshima.

47. The 35 bridges within 2 miles of the centre of damage in Nagasaki were all small and relatively light. As a result, all bridges within ½ mile of the centre of damage suffered some damage or displacement, most severe in the least massive bridges, but in only four cases did the damage require repair (in two cases, extensive repair) before the bridge could be used. This excellent behaviour in both cities is associated with the fact that bridges, almost alone among the structures which have been discussed, are designed for vertical loads such as resulted from the high burst of the bombs.

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