ML16340A909
| ML16340A909 | |
| Person / Time | |
|---|---|
| Site: | Diablo Canyon  |
| Issue date: | 02/28/1980 |
| From: | Kojahn C, Ragsdale J AFFILIATION NOT ASSIGNED |
| To: | |
| Shared Package | |
| ML16340A907 | List: |
| References | |
| NUDOCS 8004210395 | |
| Download: ML16340A909 (36) | |
Text
A Preliminary Report on S:rong-Motion Records from the Imperial County Services Building
'y Chri stopher Rojahn U.S. Geological Survey
'Menlo Park, California and J. D. Ragsdale
.California Division of Hines and Geology Sacramento, California Introduction.. The Imperial County'Services Building,. a 6-story reinforced-concrete frame and shear-wall building-, is located approximately 4.7 miles (7.6 km) southwest of the Imperial Fault and 18 miles (29 km) northwest of 'the epicenter of the shallow-focus, magnitude 6.7, October 15, 1979 Imperial Yalley earthquake {CIT, Dec. 17, 1979). The building sustained significant structural damage during the earthquake and was instrumented
. with a 13-channel accelerograph system installed and maintained by the
,California Division of Mines and Geology (CDMG). Strong-motion instrumentation at'he site also included a triaxial accelerograph located at ground level approximately 340 feet {103 meters) east of the building. All instruments C
triggered and functioned properly. during the earthquake. The overall significance of the data is highlighted by the fact that this is .the first occasion in which an extensively instrumented structure has sustained significant earthquake-induced structural failure.
Buildin Descri tion. The Imperial County Services Building (figure 1) serves as an office building for the county of Imperial. It was designed in 1968 (using the 1967 edition of the Uniform Building Code) and was
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completed in 1971 at a construction cost of '$1.87 million (oral commun.,
Randy Rister, October 29, 1979). The building is 136 feet 10 inches by 85 feet 4 inches in plan and is founded on a Raymond step-taper concrete pile foundation. The piles are interconnected with reinforced-concrete link beams; they extend 45 feet to 60 feet into the alluvium foundation material composed primarily of'and with interbeds of clay to 60 feet (based on logs from 4 soil borings at the site).
Vertical loads are carried by reinforced-concrete floor slabs (5 inches thick at the second floor and 3 i~ches thick at the upper floors) supported by reinforced-concrete 5 1/2 inch-wide by 14 inch-deep pan joists spanning in the north-south (transverse)'direction; the joists are supported by four longitudinal reinforced-concrete frames at 25 feet on center. The frame columns are typically 2 feet square, and the beams vary in size. Beams in the two interior frames are 2 feet wide by 2 feet 6 inches deep at all levels; those in the two exterior frames are 2 feet by 2 feet 5 inches at the second floor level, 10 inches by 4 feet 5 inches at the third floor through sixth floor levels, and 10 inches by 4 feet 2 inches at the roof level..
Lateral loads are resisted by the four reinforced-concrete frames-in the east-west direction and reinforced concrete shear walls in the north-south direction. The shear walls are discontinuous at the second floor level. Below the second floor there are three interior and one exterior one-foot-thick shear walls, and above the second floor, shear walls exist only at the east and west ends (figure 2). Between the second and third floors, the walls are 7 1/2 inches thick and above the third floor,
they are 7 inches thick. According to the design calculations, the design "K" factor was 1.33 for the n(;-th-south shear walls, 0.67 for the east-west interior frames, and 1,0 for the east-west exterior frames.
Earth uake Damaqe.. The most; significant damaqe was the failure just above ground level of four- reinforced-concrete columns located along the east end of the building (figures 3 and 4). Concrete at the base of each of these columns was badly shattered, vertical reinforcing bars were severely buckled, and horizontal tie bars were widely splayed. Based on measure-ments of the Imperial County Department of Buildings and Grounds, the columns were shortened bv approximately 9 inches during the main shock and by approximately 3 additional inches during the strongest aftershock (oral commun, Randy Rister, October 22, 1979). Less significant damage elsewhere in the building included minor cracking in all columns beneath the second floor level (j'ust beneath the beams); minor crackinq and/or spalling at the base of most columns (just above ground level); and a north-south line of severe cracking in all upper-story floor slabs just to the east of the first'nterior row of columns (from the east face). The pattern of column damage suggests frame yielding in the longitudinal (east-west) direction and north-south overturning at the east end; the floor slab cracks were due to the settlement of the structure at the east end.
Stron -Motion Instrumentation. The building was originally selected for instrumentation under the California Strong-Motion Instrumentation Program because of its structural characteristics, size, and location in a known highly active seismic area (Rojahn and Ragsdale, in press). It was initially instrumented with a 9-channel Kinemetrics CRA-1 acceleroqraph svstem that
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was. located'n accordance with the recommendations of the following three groups: Instrumentation Subcommittee of the Structural Engineers Association of Southern California (SEAOSC);. the California Seismic Safety Comnission's Subcommittee on Instrumentation for Buildings;.and a site visitation committee composed'f personnel representing the various organizations interested in the project (building owner,. design engineer, CDMG, SEAOSC, and U.S.
Geological Survey). The original 9-channel system was triggered by the magnitude 4.9 (hiL)* Imperial Valley earthquake of November 4,.1976;,its epicenter was'ocated approximately 20 miles (32 km) north of the Imperial 1
County. Services Building (Porcella and Nielson, 1976). After a review of the November 4 strong-motion record.and on the basis of the first author' recommendations, the system'as upgraded to its present 13-channel configura-tion,. and a triaxial Kinemetr'ics SMA-1 accelerograph was added at qround level (intended to be a free-field site) approximately 340 feet (103 meters) east. of the building. The revised system was installed in t1ay 1978 under the supervision of the second author and is based on the building strong-motion instrumentation guidelines developed at the U.S. Geological Survey (Rojahn and Matthiesen, 1977). The instrumentation is maintained by the CDMG's Office of Strong-Motion Studies.
The SMA-1 accelerograph at the, ground level site east of the building is located in a standard fiberglass instrument shelter (figure 5) that is
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founded on a small reinforced-concrete pad. The accelerograph is battery powered, is triggered by vertical motion that equals or exceeds .01 g, and records analog signals on 35-ran light-sensitive. film. The system is designed to record acceleration with frequency components in the 0 to 25 Hz range
I and with maximum amplitudes of 1 g (980 cm/sec ). Real time is provided on each record by a WWVB time-code receiver and time-tick qenerator system.
The site for the. SMA-1 accelerograph was selected on the basis of distance. (from the instrumented structure) criteria suggested by R. B.
Hatthiesen (oral commun., 1976)'s well as its proximity to other buildings.
This criteria specifies that sites intended to be free-field should be 1'ocated at a distance from the instrumented structure equal to 1 to 1 1/2 I ~
times the estimated wave length of a shear-wave (at the surface) having a period equal to the fundamental period of the instrumented structure.
The 13'-channel CRA-1 system in the building consists of 9 FBA-1 single-axis force-balance acclerometers located at- various locations throughout the structure, an east-west oriented HS-0 horizontal starter
'at roof level, and one FBA-3 triaxial force-balance accelerometer.
package, one 13-cha'nnel central recording unit, and a VS-1 vertical starter at ground,1evel. The FBA accelerometers have a natural frequency of. approximately 50 Hz and are connected by low-voltage data cable to the central recording unit. The recordinq unit is battery powered, is triggered bv horizontal or vertical motion that equals or exceeds .Ol g, and records on 7-inch (178-mm) light-sensitive film.
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The system is designed to record acceleration with frequency components in the 0 to
'50 Hz range and with maximum amplitudes of 1 g (980 cm/sec ). Real time is provided by a WWVB receiver and time-tick generator system; the recorder is not interconnected with the SI1A-1 accelerograph located east of the building.
The FBA accelerometer locations (figure 2) were selected in order to
overall building response as well as ground inPut provide information on.
motion. The primary purpose of the three north-south orien.ed accelerometers at the roof'-and second floor levels (accelerometers 1, 2, 3, 7, 8, and 9) is to obtain and isolate north-south translational,, torsional,, and in-plane floor bending response.. In conjunction with the north-south oriented accelerometers at ground level (accelerometers 10 and 11), these accelero-meters provide translational and torsional response and mode shape information.
Similarly,. the accelerometers at the ground floor, second floor, fourth floor, and roof levels in the more- flexible east-. west (frame) direction (accelero-meters'4, 5, 6,, and 13) provide east-west translational response and mode C
shape information.. The two north-south oriented accelerometers at ground level (accelerometers'0 and 11) are intended to collectively identify the extent to which differential horizontal ground motion has occurred, and the vertical accelerometer at ground level (accelerometer 12) provides information on vertical motion at this location.. =
There are no vertically oriented accelerometers above ground level..
t Ambient and Forced-Vibration Tests. Ambient and forced-vibration'tests were perfo'rmed on the building in 1979 prior to the October 15 earthquake.
The tests were conducted by G. C. Pardoen of the University of California at Irvine in cooperation with G. C. Hart of the University of California at Los Angeles (oral comnun., G. C. Hart, January 7, 1979). Reports on these tests are expected to be available in early 1980.
October 15, 1979 Accelero rams. Both the 13-channel CRA-1 accelerograph system in the building and 'the triaxial SMA-1 accelerograph to the east at ground level provided'omplete, high quality strong-motion records (figures
6 and 7) of the October 15 earthquake. Peak,.acce1erat'ions at" the base=of'-
the h'ui Idion; near "the "center.; were=0:.29".g~H 9-g=,-.and:0%2;-g for the reference north-south, vertical, and east-west components (figure 6, channels ll,, 12,, and 13),, respectively. Peak north-south acceleration at the west end of the building base (figure 6, channel 10) was 0.35 g, slightly higher than near the center of the building (channel 11), although both components were nearly identical in signature. At..the. ground site .to. the east (hereafter refereed,to as the freefield-.site)-,....,peak ground accelerations were-0;24. g, 0.27. g,/ and .0.24 g for the. 2 "(same as ..reference. north, above) -vertical,.
and.'92 'omponents (figure=2).,'espectively. The maximum durations of motion between the first and last peak equal to or qreater than 0.1 g were approximately 6 and 8 seconds for the vertical and horizontal components, respectively.
ln terms'f frequency content, perhaps the most notable feature of these records is the long-period acceleration pulse occurring in the east-west component at the base of the building (figure 6, channel 13) between second 6 and second 8. At that point, the acceleration remained positive and relatively high in amplitude 'for approximately one second (the 0.34 q peak acdeleration occurred during this time) and no doubt generated large velocity and displacement pulses.
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There are significant differences in the signatures of corresponding components of motion recorded at the free-field site and. at the base of the building. The vertical motion at the free-field site, for example, is generally higher in amplitude than that at the. base of the building.
The horizontal. components, on the other hand, have similar amplitudes but have major differences in frequency content, that is, the horizontal
I components recorded at the base of the building contain low-frequency, relatively high-amplitude components of motion that do not exist in,.the free-field motion.. The difference in horizontal motions appears to be rel'ated to the fact that the building is founded on piles. In other words, the motion recorded at the base of the building incorporates to some significant extent building/foundation system response.
Among the most notable features in the acceleration traces in the upper stories of the building (fiqure 6, channels 1 through,. 9) are, the following:, (1) the abrupt occurrence of long-period motion in the east-west components at the roof,. fourth floor, and second floor levels at second 6.8 (figure 6, channels 4, 5,, and 6); (2) clusters of low-amplitude, high-frequency (approximately 50 Hz) components of motion at various times in all upper-story records .at and after second 6.8; and (3) a 0.5-second-long cluster of high-amplitude, high-frequency (approximately 50 Hz) components of motion near second 11 in the north-south direction on the second floor (figure 6, channel 9) directly above the columns that failed; . (These clusters are not easily discernible in the reproduction shown in figure 6.) The abrupt change in frequency in the upper-story east-west components at second 6.8 is believed to denote the time at which the building's structural frame in the east-west direction began to yield. The clusters of low-amplitude, high-frequency motion are believed to denote the continuation of yielding, whereas the 0.5-second-long cluster of high-amplitude, high-frequency motion near second 11.0 is believed to denote the time at which the columns along the building's east face collapsed.
Peak accelerations at'he roof level are approximately 0.59 g and
0.48 g in the nor th-south and east-west directions, respectively. Both peaks occur between second 6.8 and second 11.0.
Conclusions. Both the 13-channel CRA-1 accelerograph in the structurally damaged Imperial County Services Building and the triaxial St%-1 accelerograph approximately 340 feet east. of the building at ground level triggered and functioned properly during the October 15 earthquake. A preliminary review of the accelerograms obtained from these instruments indicates that there were significant differences between the motion recorded at the base of the building and the motion recorded at the adjacent ground site. Peak accelerations in the reference north-south, vertical, and east-west directions at the base of the building were 0.35 g, 0.19 g, and 0.32 g, respectively, whereas those at the adjacent ground site were 0.24 q, 0.27 g, and 0.24 g, respectively. A major feature of the motion recorded at the base of the building was the 1-second-long acceleration pulse occurring in the east-west component approximately 7 seconds after the instrument was triggered.. This pulse contained the east-west peak ground acceleration of 0.34 g and was succeeded by abrupt changes in the frequency content of motion recorded at the second, fourth, and roof levels in the same direction. These changes (period elongations) as well as numerous clusters of high-frequency, low-amplitude components of motion that occur in all upper-story records are highly suggestive of structural yielding. Approxi-mately four seconds after the abrupt changes in frequency content occurred in the upper-story east-west components, that is, approximately 7 seconds after the instrument was triggered, a 0.5-second-long cluster of high-amplitude, high-frequency (approximately. 50 Hz) motion occurred in the north-south
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direction on the second floor directly above the columns that failed du) ing the earthquake. This cluster- is believed to denote the time at which the column collapses occurred.,
In general the records from the building and the adjacent ground site provide an important data set for studying the failure mechanism of a typical recently-engineered, 6-story reinforced concrete building that was damaged by a. nearby earthquake of moderate magnitude and shallow depth.
~Ak 1d .fh hti t fthf dt 1 1 dth ff t f y individuals. We are particularly grateful'o the CDMG Office of Strong-Motion Studies technicians who installed and maintained the equipment, to Randy Rister of the Imperial'ounty Department of Buildings and Grounds for his continued support and interest in the project, to the other members of the California Seismic Safety Commission's Subcommittee on Instrumentation for Buildings (WilTiam E. Gates, Gary C. Hart, Kenneth K. Honda, John F.
Meehan, Chris D. Poland, John 0. Robb, Roland L. Sharpe, and James L. Stratta) who advise and actively support the building instrumentation phase of the California Strong-Motion Instrumentation Program, to the .SEAOSC Instrumentation Subcommittee for their role in selecting and instrumenting the building, and t'o Richard P. Maley of the U.S. Geological Survey who initially recom-mended the building for instrumentation.
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References.
Porcella, R. L.. and Nielsen, J. D., 1977, Preliminary report on the Calipatria, California earthquake swarm, November, 1976: Seismic Engineering Program Report, October-December 1976', U.S. Geological Survey Circular 736-D, 23 p.
Rojahn, C. and Matthiesen,, R. B., 1977, Earthquake response and instrumen-tation of buidings: Journal of the Technical Councils, ASCE, Vol.
103, No., TC1, pp.. 1-12.,
Rojahn,, C., and Ragsdale, J. 0'., 1979, Building instrumentation phase of the California Strong-Motion Instrumentation Program: Proceedings, 1978 Annual Convention of the Structural Engineers Association of California, Lake- Tahoe, 19 p, (in press).
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't figure 3.- Imperial County Services Building. South elevation of east end of buildinq. t1en in lower right-hand corner of photograph are viewing row of columns that. failed during the October 15 earthquake.
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a%a Q Figure -4.- Imperial County Services Building. One of four damaged reinforced concrete columns .
located along the east end between the ground and second floor levels. Damage to the other three columns was similar.
J Figure 5.- Fiberglass shelter (foreground) housing SHA-1 accelerograph located approximately 103 meters east of the Imperial County Services Building (shown in background).
Note solar cells mounted on pole adjacent to shelter; these cells provide current for the accelerograph's battery charger.
4 WWVB.time-code 1 4 ~ 4kMt Roof, N-S (west) Sensitivity = 1,82 cm/g 1
Roof N-S center ensl vl y = , cm g 2
Roof, N-S (east) Sensitivity = 1.74 cm/g 3
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1 2 Figure 6.- CRA-1 strong-motion accelerogram recorded on October 15, 1979 in the Imperial County Services Building, El Centro, California. Accelerometer locations are shown on schematic in figure 2.
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Figure 7.- SHA-1 strong-motion accelerogram recorded on October 15, 1979 at a ground site approximately 103 meters east of the Imperial County Services Building, El Centro, California. Ground site location is shown on schematic in figure 2.
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