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EARTHQUAKES 
257 
which yield slightly, so that the stress is spread widely and 
not concentrated at one level. Closely knit iron girders 
give the same firm but flexible connection between roof and 
walls. The roof can also be supported on posts that rise from 
a platform of stonework or cement, which forms the sole- 
plate of the building; the posts are connected by cross- 
beams and rafters, and as the roof moves with the ground, 
there is less tendency to horizontal fracture. The roof should 
have a gentle slope, or the tiles may be shaken off. The 
leverage of the roof is less on low than on tall buildings; 
hence in some earthquake areas houses are restricted to two 
or three storeys. Doubts have been expressed as to the 
safety of American sky-scrapers with their 35 or 50 stories ; 
but these buildings have the advantages of strong construc- 
tion and a firm foundation, 
LeveL oF GREATEST DamagE—A building when disturbed 
by an earthquake, away from the epicentre, sways like an 
inverted pendulum with a period dependent on its height. 
The level most liable to damage depends on the rate of vibra- 
tion of the earthquake and the oscillation period of the 
building. If the latter be the longer the upper part tends to 
continue its sway forward while the lower part has begun 
to swing back, and a rupture may occur between the parts 
moving in opposite directions. Thus in the San Francisco 
Earthquake of 1006 a tall building 20 storeys high, had an 
oscillation period of 26 times a minute. The oscillation 
of the ground was 50 a minute, and the greatest stress on 
the building was two-thirds of its height from the ground. 
Lower buildings with an oscillation period of 50 a minute 
moved as a whole; but still lower buildings with an oscil- 
lation quicker than that of the earthquake were broken 
close to the ground, for adjacent parts of a wall often moved 
in opposite directions. 
SurrasiLity oF Materiars—Dewell (Building against 
Earthquake Shock, Commonwealth, San Francisco, 1st 
September, 1925) classifies constructions in order of resistance 
to earthquakes as follows: (1) the best is a structural steel 
frame with walls of re-enforced concrete; (2) low ferro- 
concrete buildings; (3) steel frame and brick walls; (4) 
re-enforced concrete frame and brick walls; (5) the weakest, 
brick buildings without structural frame, 
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