JOURNAL OF STRUCTURAL ENGINEERING / SEPTEMBER 2001 / 1063 BASE-ISOLATED FCC BUILDING:IMPACT RESPONSE IN NORTHRIDGE EARTHQUAKE By Satish Nagarajaiah, 1 Member, ASCE, and Xiaohong Sun, 2 Associate Member, ASCE ABSTRACT: The base-isolated Fire Command and Control (FCC) building in Los Angeles experienced strong motion during the 1994 Northridge earthquake. The California Strong Motion Instrumentation Program has instrumented the building and recorded the data during the Northridge earthquake; these data are available for performance evaluation. Impact was observed in the base-isolated FCC building during the Northridge earth- quake. The objective of this study is to evaluate the seismic performance of the base-isolated FCC building during the 1994 Northridge earthquake and the effect of impact. New analytical modeling techniques are de- veloped to analyze the base-isolated FCC building with impact and are verified using system identification. The response computed, using the developed analytical modeling techniques, is verified using recorded data. The response with and without impact is presented. The effects of impact on the structural response are evaluated. The seismic performance evaluations, comparing the response of the base-isolated building with the response if the building were fixed base, are presented. It is shown that the seismic performance of the FCC building in the Northridge earthquake was satisfactory, except for increased shear and drift due to impact. Impact should be avoided in base-isolated structures as it can cause damage. INTRODUCTION The Los Angeles County Fire Command and Control (FCC) base-isolated building is a two-story steel frame structure with a high damping elastomeric bearing isolation system. The building is located in the City Terrace area of East Los An- geles, approximately 1 mi southwest of the I-10 and I-710 interchange. The site consists of stiff clay fill underlain by siltstone bedrock (Bachman et al. 1990). The FCC building has been extensively instrumented by the California Strong Motion Instrumentation Program (CSMIP) (Shakal et al. 1994). The elevation, floor plan, and sensor locations are shown in Fig. 1(a). The building experienced strong motion during the 1994 Northridge earthquake. The CSMIP records (Shakal et al. 1994) of the response of the building (CSMIP station No. 24580) during the earthquake provide the data nec- essary for the evaluation. An examination of the records in- dicates that impact occurred at the base of the structure, which partially prevented free motion of the base. The objective of this study is to evaluate the seismic per- formance of the base-isolated FCC building during the 1994 Northridge earthquake and the effect of impact. This paper presents the developed analytical modeling of the base-isolated FCC building with impact, system identification, response computation and evaluation, and interpretation of structural behavior during the Northridge earthquake. The complete de- tails of this study can be found in Nagarajaiah and Sun (1996, 2000a,b) and Sun (1996). BASE-ISOLATED FCC BUILDING The FCC building is a two-story base-isolated building, 57.3 m (188 ft) long and 25.6 m (84 ft) wide, with seven bays in the north-south (NS) direction and three bays in the east-west (EW) direction. Fig. 1(a) shows the plan and elevation of the building. The superstructure is a braced frame consisting of 32 steel wide flange columns, which are in turn supported by 32 high damping rubber bearings. The superstructure chevron 1 Assoc. Prof., Dept. of Civ. Engrg., Rice Univ., Houston, TX 77005. E-mail: nagaraja@rice.edu 2 Des. Engr., CBM Engineers, Houston, TX 77004. Note. Associate Editor: Brad Cross. Discussion open until February 1, 2002. To extend the closing date one month, a written request must be filed with the ASCE Manager of Journals. The manuscript for this paper was submitted for review and possible publication on August 8, 2000; revised March 16, 2001. This paper is part of the Journal of Structural Engineering, Vol. 127, No. 9, September, 2001. ASCE, ISSN 0733- 9445/01/0009-1063–1075/$8.00 + $.50 per page. Paper No. 22502. bracing is located at the building perimeter. The base, first floor, and roof are made up of concrete slabs on metal decking. The metal decking is supported on a grid of steel beams. The base level steel beam framing system is moment connected to the columns to resist moments transferred from the isolators. Elastomeric Isolation System The isolation system is made up of 32 high damping rubber bearings. The bearings are 40.64 cm (16 in.) square with a height of 36.07 cm (14.2 in.); bearings are made up of 27 layers of rubber with 0.95-cm (0.373-in.) thickness and 26 layers of steel shims of 0.19-cm (0.0747-in.) thickness. The steel end plates are 2.54 cm (1 in.) thick and bolted to the column and the foundation. The bearings at the building pe- rimeter have an 11.68-cm (4.6-in.) diameter hole, which houses the ultimate restraint device (Seible and Priestley 1989); the ultimate restrainers engage at 31.75 cm (12.5 in.). Recorded Response in Northridge Earthquake The building has been instrumented by CSMIP (Shakal et al. 1994); the sensor locations are shown in Fig. 1(a). The sensors (accelerometers) are located at the foundation, first floor, second floor, and roof in the NS and EW directions. The peak values of the recorded acceleration, in the EW and NS directions, during the Northridge earthquake are shown in Ta- ble 1. The acceleration time histories and the acceleration re- sponse spectra are presented in Fig. 2. The foundation/ground acceleration, CHN5–CHN7, presented in Fig. 2(a) and the corresponding spectra presented in Fig. 2(b) indicate that the spectral accelerations were larger in the EW direction than the NS direction. The acceleration time histories of CHN9–CHN11 at the base level and of CHN14–CHN16 at the roof level in the EW direction, shown in Figs. 2(c and e), exhibit sharply increased higher mode response, in each upper half-cycle, between the 12- and 16-s time interval, increasing the acceleration from 0.22g at the foundation level to 0.32g at the roof level. The increased higher mode response was caused by one-sided im- pact against the concrete entry bridge [Fig. 1(c)] in the north- east corner of the building—reported in the reconnaissance report by the Earthquake Engineering Research Institute (EERI), El Cerrito, Calif. (1996)—which is evident from the analysis results to be presented in the following sections. The impact caused the higher acceleration of 0.35g in CHN11, which is the sensor close to the northeast corner of the build-