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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 soleplate
 of the building; the posts are connected by crossbeams
 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 construction
 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 vibration
 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 oscillation
 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 ferroconcrete
 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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