ON THE EARXUQUAKE AND VOLCANIC PHENOMENA OF JAPAN. 307 



3. ArcJnoork. 



An ordinary arcli ia undoubtedly stable for vertically applied forces, 

 but for horizontal stresses it is cue of the most unstable structures that 

 could be erected. Archwork has so often been the cause of ruin when 

 shaken by an earthquake, that in Italy and Manila special rules have 

 been drawn up respecting such structures. Thus, in Manila intersecting 

 vaults are not allowed, and ordinary vaults are only permissible when 

 strengthened in a particular manner by iron. In Liguria vaults can only 

 be used in cellars, and even there the rise must be at least ^ of the span. 

 The law of Norcia also only permits the use of arches in cellars, and their 

 thickness and the method of construction are defined. In Ischia arch- 

 work with a rise of 7^- of the span, and with a thickness of '20 m. at the 

 crown, may be iised, but only in cellars. 



Speaking generally, the use of archwork above ground has been 

 prohibited, and if it has existed after an earthquake, all governments who 

 have paid attention to building have ordered its removal. Underground 

 its use is permitted providing that the arches are not too flat. This, 

 however, only tells us that the motion beneath the surface is too small to 

 destroy even a bad form of structure, and therefore such a form of 

 structure, providing it is underground, is allowable. 



In the writer's first paper on this subject, already referred to, he has 

 given instances where archwoi-k in Tokio has been ci-acked by exceed- 

 ingly small earthquakes. In case of a severe earthquake it is not 

 unlikely that much of the archwork in buildings, like the shops in the 

 Ginza in Tokio, and in public structures, will suffer very severelj'. If for 

 architectural reasons it is a necessity that arches exist, these should not 

 be too flat ; they should have a specified thickness, be protected by an 

 iron or wooden beam, and curve into the abutments. The Ligurian 

 regulations provide that above windows there should be two iron bars. 



4. Doors and Windows. 



In the building regulations for Norcia and Ischia it is stated that 

 openings should be placed vertically above each other. It ap^Dcars to me 

 that if we have a series of openings like doors and windows in a wall 

 placed vertically above each other, it is very much the same as if we 

 had here and there built our wall with the joints of a line of bricks or 

 stone continuously above each other — that is to say, we have destroyed the 

 uniformity of the wall by lines of weakness which will readily give way 

 to horizontally applied stresses. 



The subject is not one of great importance, but the writer inclines to 

 the opinion that the doors and windows in successive tiers of openings 

 ought not to be above each other, but so arranged that any line of open- 

 ings, when regarded vertically, is as much broken as possible. 



To arrange doors and windows so that they may form ready means 

 of escape is certainly a matter worthy of attention. 



An important point mentioned in the Ischian law is the position of 

 doors and windows relatively to the freely vibrating end of a building, 

 the limiting distance being 1'50 metre. Similar regulations are found 

 in the regulations for Norcia and Liguria. This distance should, if 

 possible, be made to depend upon the materials of which a wall may be 

 constrncted, its dimensions, and the size of the openings. 



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