Horizontal ground motion generated by vibration tests of the nine-story Millikan Library Building on the Caltech campus was recorded on the surface of the ground in the Pasadena area at distances up to 3 miles from the building. Later it was learned that the vertical component of the motion also was recorded by the seismograph on Mt. Wilson, 6.7 miles from the Library and 4,800 ft higher in elevation. The magnitude of the acceleration varied from 2.04 × 10-^(2)g at the excitation level on the ninth floor of the building to 3.2 × 10^(-7)g at Mt. Wilson. Simple calculations show that multistory buildings are particularly well-suited for inducing large dynamic forces in the ground with relatively small equipment

Jennings, Paul C.

Caltech Authors - Main

Bulletin of the Seismological Society of America. Vol. 60, No. 6, pp. 2037-2043. December, 1970 
DISTANT ~OTIONS FROM A BUILDING VIBRATION TEST 
BY PAUL C. JENNINGS 
ABSTRACT 
Horizontal ground motion generated by vibration tests of the nine-story Millikan 
Library Building on the Caltech campus was recorded on 1he surface of the ground 
in the Pasadena area at distances up to 3 miles from the building. Later it was 
learned that the vertical component of the motion also was recorded by the seismo- 
graph on Mt. Wilson, 6.7 miles from the Library and 4,800 ft higher in elevation. 
The magnitude of the acceleration varied from 2.04 X 10-2g at the excitation level 
on the ninth floor of the building to 3.2 X 10-~g at Mt. Wilson. Simple calculations 
show that multistory buildings are particularly well-suited for inducing large dynamic 
forces in the ground with relatively small equipment. 
INTRODUCTION 
During the vibration tests of the nine-story Millikan Library Building at the Cali- 
fornia Institute of Technology (Jennings and Kuroiwa, 1968, Kuroiwa, 1967), building- 
induced motion was clearly recorded off campus at the Caltech Seismological Labora- 
tory. The building is founded on approximately 900 ft of alluvium overlying bedrock. 
After an additional test confirmed that the building vibration was indeed the source of 
the motion, an experiment was conducted to measure the surface motion in the Pasa- 
dena area. It was found that the motion could be measured easily by portable seismo- 
graphs at distances up to 3 miles from the building site. After the test was completed, 
it was learned that the vertical component of the motion was recorded by the seismo- 
graph atop Mt. Wilson, 6.7 miles distant from the Library and 4,800 ft higher in eleva- 
tion. 
INSTRUMENTATION AND TEST PROCEDURES 
The building was excited into resonance in the N-S direction by two of the vibration 
generators developed at Caltech for the State Division of Architecture under the spon- 
sorship of the Earthquake Engineering Research Institute. Details of the vibration 
generators, now the property of the University of California, and descriptions of their 
use in the Millikan Library tests are available in the literature (Hudson, 1962; Jen- 
nings and Kuroiwa, 1968; Kuroiwa, 1967). The accelerometer-recorder system used to 
measure the building motion and the motion on the nearby soil also has been described 
elsewhere (Kuroiwa, 1967). 
The surface motion in the area not immediately adjacent to the Library was meas- 
ured by six of the portable seismographic stations owned and developed by the Seismo- 
logical Laboratory of the California Institute of Technology. These instruments use 
Ranger-type seismometers a the sensing element and record on film. Their characteris- 
tics have been described in a paper by F. E. Lehner and F. Press (1966). 
Beginning at approximately 7 p.m., December 15 1966, the building was excited 
into resonance, using full weights in the eccentric mass vibrators. The fundamental 
mode in the N-S direction (1.92 cycles/sec) was chosen as this enabled the greatest 
amount of energy to be fed into the building by the two 1½-hp vibrators. As a result 
of this choice, all reported measurements with the exception of that from Mr. Wilson, 
are for N-S motion. The purpose of this first part of the test was to calibrate the seismo- 
2037 
2038 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 
graphic instruments a  accelerometers at the chosen frequency by recording motion at 
the same point by both the seismometers and the aeeelerometers. The aeeelerometer- 
recorder system then was calibrated in a routine manner on a tilt table (Knroiwa, 1967) 
to complete this portion of the experiment. The calibration point, shown in Figure 1, 
was chosen as far from the building as it was possible to obtain usable records with 
the aecelerometers. 
Later examination of the film records from the seismometers showed that the calibra- 
tion was not completely successful in that the motion appeared to be too strong for the 
seismometers, although they were attenuated fully. The motion at the site was very 
nearly sinusoidal according to the aeeelerometers, but this was not the case on the 
r 
i 
\ 
CALIBRATION 
POINT [ 
t 
L 
GROUND SURFACE ACCELERATIONS 
(x10-~} 
NORTH-SOUTH FUNDAMENTAL MODE 
1 
30' 
1.89 
t - - - -  --"3 
I t 
I F 
[ i 
= 
_ ..a_. 
I 
~'~0' 
,41 ; 
15' 
.32 t _  
i 
I 
I 
I 
~----1 
t 
I 
J 
FIG. 1. Ground surface motions measured uring vibrat ions in the N-S fundamental  mode under 
the same loading conditions as the present est. The cal ibration point is on the left. 
seismographic records. Also, the ratio of horizontal to vertical motion differed for the 
two sets of instruments. At sites more distant from the building than the calibration 
point, the seismographic records showed the expected sinusoidal motion. Thus, the 
building motions and the relative values of the ground surface motions away from the 
building site are judged to be reliable, but the absolute values of all ground surface 
motions are not known precisely. The calibration difficulties are thought not to affect 
the absolute values more than a factor of two, however, and the results are presented 
on the basis of a calibration performed with the measured horizontal motions. 
After the calibration, five of the seismographic trailers were located permanently at
selected points in the Pasadena rea, whereas the sixth trailer was attached to a truck 
DISTANT MOTIONS FROM A BUILDING VIBRATION TEST 2039 
and readied to gather data at several sites. At  Ii p.m., when the seismic noise from 
traffic, etc., had subsided substantially, the building was again put in resonance and 
maintained in that state for approximately 4 hr while data were taken. Dur ing this 
FAIR OAKS 
AVENUE 
ti AR:O;O 
/ j -AVenuE ~ 
LAKE 
AVENUE~ 
WASHTNGTON 
BLVD. 
~ ALTADENA 
r~ 
P~ 
>HILL AVENUE 
_ _ ~  SAN PASQUAL ST. 
LL~M IL L, K A N'~L~BR A~ j  
]E BUILDING [~_~ 
HUNTINGTON 
LIBRARY 
.I 
I MILE 
~.AALTAD ENA 
DRIVE 
- s ,~RA MADRE B,~D. 
] l _ _  
- ~  BLVD. CALIFORNIA 
NORTH-SOUTH RELATIVE SURFACE MOTION 
MILLIKAN LIBRARY TESTS 
FI~. 2. Relative values of North-South surface motions recorded in the Pasadena rea. 
time, the vibration amplitude was constant within 5 per cent and the frequency within 
I per cent, according to accelerometers on the ninth floor and first floor of the building. 
TEST RESULTS 
The ground surface motion in the immediate vicinity of the building foundation is
shown in Figure 1. These values are from a previous test (Kuroiwa, 1967) under the 
same loading conditions and are within 5 per cent of those measured in the present 
test as judged by the level of motion at the ninth and first floors of the building. The 
20~0 BULLETIN OF THE SEISMOLOGICAL ~OCIETY OF AMEI%ICA 
amplitude of the N-S acceleration atthe level of excitation (ninth floor) in the present 
test was 2.0 × 10-2g, whereas at the calibration point (Figure 1), the N-S acceleration 
had diminished to 1.2 X 10-3g. 
The relative values of the N-S surface motions recorded in the Pasadena rea are 
LAKE S~ 
/ 
MILLIKAN LIBRARY 
/ •  MOUNT WILSON 
6.7 miles 
one mile 
FIG. 3. Location of Mt. Wilson with respect o Millikan Library and the area shown in Figure 2. 
FIG. 4. A portion of the seismogram (vertical motion) from the Mr. Wilson Station for 
December 15/16 1966. 
DISTANT MOTIONS FROM A BUILDING VIBRATION TEST 204 i  
shown in Figure 2. The northernmost measurement is at the b~se of the hills which 
rise to form the San Gabriel Mountains. The numbers in Figure 2 are such that 100~ 
corresponds to the motion recorded by the seismometers at the calibration point. 
Mount Wilson, elevation 5710 ft, lies 6.7 miles from the Millikan Library as shown 
in Figure 3, and its elevation is approximately 4,800 ft higher than the Library site. 
A portion of the seismographic record of vertical motion for December 15/16 1966 
is given in Figure 4. The time signals occur at minute intervals. The drum revolves 
four times per hour so that adjacent rows in Figure 4 are 15 rain apart. The sinusoidal 
motion in the upper part of the figure was generated uring the calibration test and 
the lower portion of Figure 4 is from the test proper. An enlarged view of the central 
FIG. 5. An enlarged view from the center of Figure 4. 
TABLE 1 
APPROXIMATE AMPLITUDES OF P~ESPONSE 
Relative 
Location Amplitude 
Millikan Library, ninth floor (2.04 X 10 -2 g) 1600 
Millikan Library, first floor 150 
CMibration point 100 
3 blocks 10 
3 miles 1 
Mt. Wilson, 6.7 miles (vertical) 0.02 
portion of Figure 4 is shown in Figure 5. Although the signal-to-noise ratio is only 
about two, the nature of the motion makes it clearly discernible. The frequency of the 
motion is the same as that recorded at the Library site. From instrumental constants 
(James Brune, personal communication) the vertical acceleration at Mt. Wilson was 
found to be 3.2 X 10 -7 g. Motion was not recorded at ally of the more distant stations 
in the local network operated by the Seismological Laboratory. 
The relative amplitudes of the motion are indicated in Table 1 in which the num- 
bers are consistent with Figure 2. The recorded motions diminish at about the same 
rate, both to the north and to the west. Although the data are insufficient to establish 
a precise trend of amplitude with radius, R, the decay seems to lie in the range be- 
tween R -1 and R -3/2. 
~0~2 BULLETIN OF THE SEISMOLOGICAL SOCIETY OF AMERICA 
CONCLUSIONS 
It is not difficult to show that a multistory building is particularly well-suited for 
exciting motions such as reported herein with relatively small equipment. The maxi- 
mum force exerted by the two vibrators in the test was about 5.6 X 103 lb. From the 
mass of the building and its measured N-S fundamental mode shape (Jennings and 
Kuroiwa, 1968), and from the response in this test, the sinusoidal force and moment 
imposed by the building on the soil were found to be approximately 2.0 X 105 lb and 
2.8 X l0 s in-lb, respectively. To apply such forces directly to the soil would require 
relatively large equipment. 
Similarly, if a resonating system were designed to produce a translational force of 
the required magnitude, such as the system shown in Figure 6, it is found again that 
the installation seems prohibitively large. For example, using the two vibration gene- 
rators which can produce an oscillating force with a maximum of 104 lb, and limiting 
the displacement of the mass in Figure 6 to 1 in (for stability of the vibration gene- 
rators), it is found that for 2.5 per cent damping, the mass, M, must weigh 5 X 105 
]b to produce a translational force of 2.0 X 105 lb. This mass is about one-fifth that 
of the Millikan Library Building. 
s in~t  
_& 
M 
c/2 
O O 
~/I'~'~\II/\\vI[A\VIA',\ 
FIG. 6. Schematic drawing of a resonating system designed to produce large forces in the ground. 
Forces of the magnitude of those above will be generated by the University of 
California, Berkeley, shaking table facility at the Richmond Field Station. Efforts 
are being made to measure the distant motions in the San Francisco Bay area that 
appear likely to be produced by the operation of this facility (P. W. Rodgers, per- 
sonal communication). 
The principM advantage of the technique used in the experiment reported herein 
is that it allows relatively large forces to be generated with small equipment. The test 
procedure is limited, of course, by the necessity of having a building that can be 
vibrated, and, for a particular building, by being restricted to a small number of reso- 
nant frequencies. 
Prom the results of this exploratory test, it would seem that this technique could be 
useful for testing the applicability of solutions of problems in the dynamics of elastic 
media to the phenomenon of soil-structure interaction. Also, it may be a useful way 
to measure the effects of local geology on dynamic motions and to ascertain average 
soil properties over distances of the order of miles. 
ACKNOWLEDG~ENrrS 
The writer is indebted to Clarence R. Allen, Ronald F. Scott and Francis E. Lehner for op- 
erating the seismographic instruments and to Julio Kuroiwa, Fred McDonald, and I~afael l%on- 
deros for assisting with the vibration equipment and the accelerometer-recorder system. 
DISTANT MOTIONS FROM A BUILDING VIBRATION TEST 2043 
The experiment was funded, in part, by the National Science Foundation under Grant GK- 
1197X, 
]~EFERENCES 
Hudson, D. E. (1962). Synchronized Vibration Generators for Dynamic Tests of Full Scale Struc- 
tures, Earthquake Engineering Research Laboratory, California Institute of Technology, 
Pasadena. 
Jennings, P. C. and Kuroiwa, J. H. (1968). Vibration and soil-structure interaction tests of a 
nine-story reinforced concrete building, Bull. Seism. Soc. Am. 58,891-916. 
Kuroiwa, J. H. (1967). Vibration Test of a Multistory Building, Earthquake Engr. Research Lab., 
Calif. Inst. of Tech., Pasadena. 
Lehner, F. E. and F. Press (1966). A mobile seismograph array, Bull. Seism. Soc. Am. 56,889- 
897. 
DIVISION OF ENGINEERING AND APPLIED SCIENCE 
CALIFORNIA INSTITUTE OF TECHNOLOGY 
PASADENA, CALIFORNIA 91109 
Manuscript received June 9 1970 


Distant motions from a building vibration test

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