The Minneapolis journal., 1902-08-26
BRIDGE VIBRATION
Moving Burdens Caueds
Motion Not Unlike an Earthquake,
SOME JAPANESE INVESTIGATIONS
De1ectiOns and Vibrations Measured
by SeismOgraphs, Just As"
Earthquakes Are.
Engineerine Record.


At the first sight it is difficult to see
much resemblance between the motions
produced by an earthquake and those
generated in an iron or steel brldge
structure traversed by a rapidly moving
load, and yet both classes of motions
have SO much in common that they may
at least be measured by the same kind
Of apparatus. Precisely that class OF
observations has been completed in
Japan by running railway trains over
different types of bridges, including plate
girders and trusses, with spans ranging
from 20 feet to 200 feet. The apparatus
used to measure both vertical defect ions
and vibrations (vertical and horizontaD
consisted Of seismographs placed on the
lower chord or other suitable portion OF
the structure. The advantage of the
seismograph for measuring vertical de-.
fecttons lies In the fact that it Is self.
contained and requires no measurement
to be made with reference to liked
datum, like the earth's surface under the
bridge. In some cases in these Japanese
Investigations, however, the deflections
were measured, as checks, by observing
the actual depression of the lower chords
Of the structure while loaded with ref
erence to the beds of the streams un-
derneath them, and the comparisons were
reasonably satisfactory.
The ordinarily recognized defections
and vibrations of a bridge structure sub
jected to moving loads are vertical and
lateral. A. little consideration will show,
however, that this is but a partial or in
complete view Of the matter. When a
rapidly moving railway train enters upon
bridge structure it actually strikes the
latter a heavy longitudinal blow which
continues more or less as long as the
locomotive and train are on the bridge,
and this holds whatever may be the con-
- t1oA 01bryi0, 11y hn ron Oro-
er the track the heavier and more irregu-
lar will be the longitudinal blow, but they
are bound to be delivered, to some extent
at least, as heavy thrust even with
perfect condition of track. The lack of
balance Of the driving wheels of the loco
motive in a vertical direction leads to un-
symmetrical effects on the two rails,
which, combined with the unavoidable
surging Of the train, produces both verti-
cal and lateral vibrations of the struc-
ture, aggravated by the imperfections of
track, whether they be more or less. The
resultant vibrations of bridge under
moving load, therefore, are necessarily
complicated and may be resolved into the
three CO. -ordinate elements, vertical, lon
gitudinal and transverse, the latter two
being horizontal. Earthquake vibrations
are equally general in character and may
also be resolved into their three CO.
ordinate elements at right angles to each
other, so that 1t was quite rational, after
all, that an extended series of observa-
tions on the dehections and vibrations OF
railway bridges should be made under the
direction and supervision of a member,
| DR.. F. Omor1, of the Japanese Imperial
earthquake investigation committee. The
| bridges used for this purpose were con-
structed in accordance with the speclf1ca-
tions of the imperial Japanese govern
ment railways, and the results are pub
lished in No. of the qualifications" OF
the committee. Two types of instru-
ments were employed, both self-record-
Ing, one called the defectometer and the
| other the vibration measure, both being
seismographs adapted to this particular
class of work. l..r
It is clear from what has preceded
| that these pieces of apparatus
simply measured the vertical and horizon
| tai motions of the structure, the hort-
zontai motions being either Longitudinal
or transverse to the bridge, and inasmuct
| as each seismograph included record
| ing device, the record so taken would dis.
close not only the amplitude Of each vi
| bration but also the time occupied in de-
| scribing 1T. XII
No stress diagrams are given to show
! the stresses in the various bridge members
produced by the actual loading when the
vibrations and defections were observed
SO that relative effects only are exhibited
! Although the actual loads of trains are
| given, nothing is stated as to their rela-
| ton ito the greatest loads prescribed 1r
| the specifications. A double track Warrer
| girder one hundred feet long gave a yer.
! tical detection Of about fve- -eighths Of al
i inch as a. maximum when both tracks
! were loaded, with maximum range O