SMIP 1994 Seminar

Some of the highest acceleration ever recorded at structural and ground response sites occurred in the Northridge earthquake. These accelerations are greater than most existing attenuation models would have predicted. The thrust mechanism of this event as well as its location under a metropolitan area may have contributed to the number of high acceleration recordings. Although the accelerations are high, the correspondence between measured acceleration and damage requires further study, since some sites with high acceleration experienced only moderate damage. Some vertical accelerations were larger than the horizontal, but in general this event fits the pattern observed in previous earthquakes. Strong-motion records processed to date show significant differences in acceleration and velocity waveforms and amplitudes across the San Fernando Valley.

Analysis of processed data from four buildings in the San Fernando Valley indicate that the stiff, short-period building experienced large forces and relatively low story drift during the Northridge earthquake. On the other hand, three moment frame buildings (periods between 1 and 3 seconds) experienced large drifts. The two non-ductile concrete moment frame buildings suffered column cracking and other damage. For this earthquake, accelerations did not always amplify from base to roof for flexible structures like these three buildings, but the displacements were always larger at the roof. The records from a base-isolated building indicate that high- frequency motion was reduced significantly by the isolators, which only deflected 3.5 cm. The records from a parking structure show important features of the seismic response of this type of structure.

A strong-motion database was compiled for California earthquakes of surface-wave magnitudes, Ms ≥ 6, occurring from 1933 through 1992. The database consisted of horizontal peak ground acceleration and 5 percent damped response spectra of accelerograms recorded on four different local geologies: bedrock (class A); soft rock or stiff soil (class B); medium stiff soil (class C); and, soft soil (class D). The results of regression analysis of the database within each of these site classes were used to derive a set of site-dependent spectral amplification factors for oscillator periods between 0.1 and 4.0 sec and ground acceleration levels between 0.1 and 0.4 g. The amplification factors at 0.3 and 1.0 sec periods are generally similar to those recommended during the 1992 NCEER Site Response Workshop.

The California Strong Motion Instrumentation Program has obtained significant records of earthquake motions of non-ductile concrete moment frames in Southern California. This research was performed to verify or develop methods for better understanding and prediction of the seismic performance of these buildings.Three dimensional models of non-ductile reinforced concrete buildings were developed to test a variety of analytic techniques and materials assumptions against the recorded data. Linear elastic models which explicitly account for the stiffness contribution of diaphragms, in addition to the building frames, provided fairly accurate prediction for the low to moderate levels of earthquake motions.

A multi-span, curved, concrete box-girder bridge has been extensively instrumented by the California Strong Motion Instrumentation Program (CSMIP) in cooperation with the California Department of Transportation (Caltrans). On June 28, 1992, the bridge was shaken by the magnitude 7.5 Landers and magnitude 6.6 Big Bear earthquakes in Southern California. The epicenters of these earthquakes were 50 and 29 miles (81 and 46 km) from the bridge, respectively. All thirty-four strong-motion sensors installed on the bridge recorded its response to these earthquakes and provided an insightful set of response data. A striking aspect of the response is the presence of intermittent sharp spikes in nearly all of the acceleration records from sensors on the deck of the bridge. Among these the highest spike was 0.80g for the Landers and 1.00g for the Big Bear earthquake. The peak ground acceleration at the bridge site was only about 0.10g for both these earthquakes. With the aid of visual examination and simple analysis it is deduced that: (i) the spikes were caused by forces generated at separation joints between adjacent bridge segments by impacts and stretching of the cable restrainers; and (ii) the forces of impacts and cable stretching are directly proportional to the size of the spikes and can be estimated by the use of a simple formula.

The strong motion records from the Northwest Connector (located in Colton, Calif.) in the 1992 Landers and Big Bear earthquakes provide valuable information about the seismic behavior of this common type of freeway bridge. Modeling of the bridge with reasonable estimates of the column stiffness and pile foundation properties and with gap elements for the intermediate hinges provides a good correlation between the computed and the recorded earthquake response of the bridge. Pounding of the hinges produces large acceleration spikes, causing sharp increases in the column shear forces. Since modeling the hinge opening-closing is relatively simple, this type of analysis is recommended for the design of multiple-frame bridges.

Abutment stiffnesses are determined directly from the earthquake motions recorded at the US 101/Painter Street Overpass using a simple equilibrium-based approach without finite-element modeling of the structure or the abutment-soil systems. The calculated abutment stiffnesses, which include the effects of soil-structure interaction and nonlinear behavior of the soil, are used to investigate variation of the abutment stiffness with its deformation during the earthquake and torsional motions of the road deck. Also evaluated are the CALTRANS, ASSHTO-83, and ATC- 6 procedures for estimating the abutment stiffness. It is demonstrated that stiffness of the abutment depends significantly on its deformation during the earthquake: larger is the deformation, smaller is the stiffness. The road deck of this structure experienced significant torsional motions in part because of eccentricity created by different transverse stiffnesses at the two abutments. It is also shown that the CALTRANS procedure leads to good estimate of the abutment stiffness provided the deformation assumed in computing the stiffness is close to actual deformation during the earthquake, and ASSHTO- 83lATC-6 procedure results in stiffer initial estimate of the abutment stiffness.

The California Strong Motion Instrumentation Program (CSMIP) of the Division of Mines and Geology has obtained and processed a number of significant building response records from its network of seismographs in California. This study was conducted using strong motion data to investigate the performance of nonstructural elements and building components under actual earthquake loadings and to improve the seismic provisions of the building codes. The study includes the analysis of building response records obtained primarily from the Loma Prieta earthquake of 1989.

The strong motion instrumentation program (SMIP) of the California Division of Mines & Geology (CDMG) has been designed to instrument specific building types in specific areas of California where strong ground motion records may be readily obtained from active seismic sources. The records are intended for use by structural engineers and researchers in developing analytical and design procedures that more accurately represent the building's behavior in earthquakes. Recent California earthquakes have provided significant data on a series of instrumented shear wall buildings along with observable data on their earthquake performance.

This paper presents a detailed investigation of three high-rise shear wall buildings in the nine- to ten- story range with three different shear wall configurations: perimeter walls, core walls and distributed walls. The dynamic earthquake response of these buildings is assessed to evaluate overturning forces in the shear walls under three recent northern California earthquakes: 1984 Morgan Hill, 1986 Mt. Lewis; and 1989 Loma Prieta. Two methods of data reduction and analysis are employed in the investigation to assess the significance of soil- structure interaction on building overturning forces. These include: simplified data analysis procedures using recorded motions, mode shapes and building weights to assess dynamic performance and three dimensional hear elastic dynamic analyses using soil-structure models for the shear walls and foundation systems.

Realistic three-dimensional models of the structures refined through system identification techniques are used to study the response to the three earthquakes. These analyses indicated that under the larger earthquakes structural softening occurred that was associated both with soil strain levels as well as shear wall cracking. The analytical results are compared with code procedures for predicting the periods of the structures as well as the distribution of overturning forces.