Evening star. [volume] (Washington, D.C.) 1854-1972, 1931-03-08

short Of the critical point You may see the
same action in the movement of the women
who carry Swaying water jars, as Rebecca did,
upon their heads.
II the Great Six were shorter columns they
would swing more Quickly-that is with a
shorter period-and they might tune in with
the vibrations of the ground. In that case at
the end of each swing their own tendency to
return would have added to it the impulse of
the earth movement in the same direction and
they would sway more widely with each return
TIL they came crashing to the ground. Their
slow movement has protected them by prevent
ing that tuning in, that timing with the earth
quake vibrations which is called resonance.
The Great Six, of course, constitute a very
rigid pendulum. They cannot bend, they are
not elastic In this they differ from skyscrap-
ers, which have an elastic frame Of steel and
would bend far more than they do If they were
not stinened by walls and partitions Even
so. the Chrysler might ask the Woolworth
Say, you silded ANTIQUE. what's your period
OF SWtng| And though the Woolworth might
truly answer: How should ] know? That
wasn't thought of when was built" the fact
would still remain that each of them has q
definite period Of swinging dependent on its
height, width, distribution Of weight and
elasticity.
The example of the majestic, earthquake
defying Great Six Of Baalbek points to one
condition of safety: Avoid resonance.
pUT how? Earthquake periods-that is, the
time required by the ground to swing to
one side and back again-are found to vary
between half second and two seconds. Some
times one of a number Of successive periods
viii be smaller or greater, but rarely. And in
general If not always, successive periods in any
one shock differ quite widely. The motion is
chaotic. It follows that If building has a
period which lies between half second and
two seconds there is danger of resonance for a
few vibrations, and therein lies the greatest
danger. Safety lies In avoiding 1t. We must
get below half second or above two seconds.
To secure the shorter period-less than half
second-great stiffness is required, and with
1T should be combined nghtness and strength.
It Is quite possible to accomplish this object in
residences and low business buildings at an adj
u1t1onal cost that varies between half per cent
and 3 or per cent Of the total.
On the other hand, to secure the longer Pc
nod In excess of two seconds, hexibility must
De combined with strength. The steel towers
more than 30 or 20 stories high probably meet
these conditions, which their very magnitude
imposes or demands. Measurements are being
made upon them as they sway in the wind and
their periods are known TO be long. regard
this as reason for thinking that they are reIa-
lively safe.
Between stories and 20 stands the great
number Of buildings whose vibration periods lie
within the dangerous interval, where resonance
Is likely to occur. II Of such proportions and
STRUCTURE Inae the period Is probably short
the structure should be designed for rightly
and strength. II the indicated period is longer
but not long enough then o considered effort
may be made to introduce greater fexibility
without sacrifice Of strength and so to lengthen
the period sti1 further.
Upon our shaking table stands q model OF
26-story building. The model is constructed
BY Jacobsen In such q manner that It possesses
very close dynamic simiiitude" with the great
structure that is to be erected-that is to say
it swings with the same period and suffers the
same distortion as would the building itself
We have subjected it to all kinds of EARTH
quakes, even to good imitation of the shock
that destroyed Yokohama and Toxic in 1928
We have motion pictures Of it and can DEM
onstrate the points at which it needs to he
strengthened to meet the inevitable success
fully.
But am getting into the realm of expert
nent, which may be described as that OF
crystalhzed speculation. There are other specu-
lations which are still in fluent state I re-
ferred little way back TO the earth as a
heat engine. It certainly Is q very hot and
heterogeneous globe In. which heat loiters and
wanders, playing tricks like a mischievous
spRtte. Much Of the rock Is crystalline, and
in the crystals are locked up very powerful
forces Each atom or molecule Of @ crystal is a
balanced system that yields energy IL disturbed
That wandering sprite, heat Is, think, the
disturbing imp. He releases the energy that
produces pressure, raises mountain ranges and,
incidentally, causes earthquakes. We would
like TO know exactly bow.