SMIP 2007 Seminar

For shaking periods less than about 1 second, the observed ground motions from the 2005 Anza earthquake are significantly higher (around +1 sigma) than predicted by three recent NGA attenuation relations, with little systematic dependence on distance, Vs30 or Z2.5. This same trend is found comparing these data to motions computed from a broadband simulation technique. For shaking periods greater than about 1 second, both the empirical models and the numerical simulations do well at reproducing the median level of the data. We obtain a significant improvement in the fit to the shorter period motions by increasing the corner frequency in our broadband simulation by a factor of 1.6. These results suggest that the strong short period motions resulted from a rupture process having a relatively high dynamic stress drop.

Procedures for the three-dimensional (3-D) dynamic analysis of earth dams have been available for over 25 years. However, additional case histories are needed to assess whether such procedures are capable of simulating the seismic response of dams in narrow canyons, and to further evaluate the effects of 3-D behavior on the response of such dams. This paper describes a study aimed at identifying the vibration characteristics of Seven Oaks Dam during the 2005 Yucaipa and 2001 Big Bear Lake earthquakes, and at evaluating the applicability of 3-D and two-dimensional numerical procedures to simulate the recorded response of the dam.

The U.S. Geological Survey (USGS) and the California Geological Survey (CGS) have established a cooperative U.S. National Center for Engineering Strong Motion Data (NCESMD), which will have mirrored operational centers in Sacramento and Menlo Park, CA. The National Center integrates earthquake strong-motion data from the CGS California Strong Motion Instrumentation Program, the USGS National Strong Motion Project, and the regional networks of the USGS Advanced National Seismic System (ANSS), thus serving as a provider of uniformly processed strong-motion data for seismic engineering applications. The NCESMD builds on the Engineering Data Center of the California Integrated Seismic Network, and so will continue to serve the California region while expanding to serve other ANSS regions. The National Center will assimilate the Virtual Data Center, which was developed at U.C. Santa Barbara with support from the Consortium of Strong Motion Observation System (COSMOS), NSF and SCEC. A Center Management Group with input from an external Advisory Committee manages the NCESMD. Products will be generated by both CGS and USGS facilities, thus ensuring robustness. Each ANSS region is responsible and credited for the data recorded by its regional network. The National Center is co-hosted by CGS and USGS at www.strongmotioncenter.org.

This paper evaluates current Nonlinear Static Procedures (NSPs) specified in the FEMA-356, ASCE-41, ATC-40, and FEMA-440 documents using strong-motion data from reinforced-concrete buildings. For this purpose, peak roof (or target node) displacements estimated from the NSPs are compared with the value derived from recorded motions. It is shown that: (1) the NSPs either overestimate or underestimate the peak roof displacement for several of the buildings considered in this investigation; (2) the ASCE-41 Coefficient Method (CM), which is based on recent improvements to the FEMA-356 CM suggested in FEMA-440 document, does not necessarily provide better estimate of roof displacement; and (3) the improved FEMA-440 Capacity Spectrum Method (CSM) generally provides better estimates of peak roof displacements compared to the ATC-40 CSM. However, there is no conclusive evidence of either the CM procedures (FEMA-356 or ASCE-41) or the CSM procedure (ATC-40 or FEMA-440) leading to better estimate of the peak roof displacement when compared with the value derived from recorded motions.

The recorded response of a number of reinforced concrete buildings to real earthquakes are used to test the predictive capability of nonlinear static procedures (NSP). Response parameters such as drifts and inter-story shears are obtained by blending measured acceleration signals and model based information, using an observer. The buildings are represented by 3D nonlinear Finite Element models with elastic stiffness updated from eigenproperties identified from small amplitude response. The fidelity of the models for behavior at large amplitudes is validated by contrasting time history predictions with measured strong motion acceleration response. As one anticipates from theoretical considerations, the prediction accuracy of NSP is found to be significantly higher in the lower levels of the buildings considered. The ratio of “measurements” to predictions over the full height for the cases analyzed has a mean and coefficient of variation of around {1.05 and 0.22} for shears and {1.2 and 0.45} for inter-story drifts. Differences in predictive capability between the various NSP are found statistically insignificant.

The strong motion observation in Japan was initiated in 1953, 20 years behind from California. In 2004, celebrating 50th anniversary of strong motion observation in Japan was held sponsored by the Strong-Motion Earthquake Observation Council in the National Institute for Earth Science and Disaster Prevention (NIED), Japan Association for Earthquake Engineering, and Earthquake Research Institute, University of Tokyo. The first part of my presentation is a summary or review of the symposium and an introduction of the strong motion observation networks by the individual organizations.

The NGA project has developed give new ground motion attenuation models for application to California. The models are applicable to all relevant shallow crustal earthquakes in California: M5-8.5 for strike-slip earthquakes and M5-8.0 for reverse earthquakes; distance of 0-200 km; strike-slip, reverse, and normal mechanisms. The models are also defined for spectral periods up to 10 seconds. As a result, the user of the models does not need to extrapolate them for most applications.

Key changes from the previous models include the use of VS30 for the site condition, inclusion of a depth of rupture factor, inclusion of hanging wall factors, and inclusion of depth of soil factors. All of the models include non-linear site response effects. Three of the models include the effects of the soil non-linearity on the standard deviation.

Comparisons of the differences between the five NGA models and between the NGA models the previous models that they are replacing are shown. The new models show a reduction in the median ground motion close to large earthquakes and an increase in the standard deviation for large earthquakes. There is an increase in the short period median ground motion for sites over the hanging wall of thrust earthquakes. Overall, the NGA models lead to reduced design ground motions based on IBC procedures. About half of the reduction is due to use of VS30, consistent with the building code, rather than using generic rock models.

The Design Ground Motion Library (DGML) is a software package for searching for ground motion time histories suitable for use by engineering practitioners for the time history dynamic analysis of various facility types in California and other parts of the western United States. The DGML was developed in a project funded jointly by the California Geological Survey-Strong Motion Instrumentation Program (CGS-SMIP) and the Pacific Earthquake Engineering Research Center-Lifelines Program (PEER-LL). The project was carried out by a multidisciplinary project team of practitioners and researchers in structural engineering, geotechnical engineering, and seismology. Currently, the DGML is limited to recorded time histories from shallow crustal earthquakes of the type that occur in California and other parts of the western United States. The software package includes a database of ground motion records and a software tool for selecting, scaling, and evaluating time histories for applications. The DGML is currently on a DVD, and consideration is being given to converting the DGML to internet web-based usage. The ground motion database used in the DGML consists of the PEER-NGA data base created for the Next Generation of Attenuation (NGA) relationships project. The database includes time histories from CGS-SMIP, U.S. Geological Survey (USGS), and other reliable sources including selected record sets from international sources. The DGML is documented and supported by a Users Manual and a report.

The DGML has the broad capability to search for ground motion time history records on the basis of (1) the response spectral shape of the records in comparison to design or target response spectra and (2) other characteristics of the records. Ground motion response spectral shape over a period range significant to structural response has been found to be closely correlated to inelastic structural response in a number of research studies. The period range of significance may include periods shorter than the fundamental structure period because of higher mode effects and periods longer than the fundamental structure period because of structure softening during inelastic response. Accordingly, a key capability of the DGML software tool is searching for and ranking time history records on the basis of the degree of match of the response spectral shapes of the time histories with design or target spectra over a user-specified period range. To support this capability, the software tool can construct design or target spectra using different approaches.

Other criteria are also specified by the user in constraining searches for ground motion time history records. These search criteria include: parameters for earthquakes that produced the ground motion records (ranges of earthquake magnitude, type of faulting); ranges of earthquake source-to-site distance for records; recording station site conditions (characterized by ranges of site shear wave velocity in the upper 30 meters, Vs30); ranges of significant duration for records; presence of pulses in near-fault records; direction of horizontal component of records (fault-strike-normal (FN) direction, fault-strike-parallel (FP) direction, either FN or FP direction, or two-component pairs in FN and FP directions); maximum number of records to search for; and ranges of acceptable scaling factors for scaling records to the level of the target spectrum.

The software tool includes a graphic interface for data input, processing, and plotting of target response spectra, spectra of individual or multiple time histories, and average spectra for selected time histories. In addition, acceleration, velocity, and displacement time histories can be plotted for the selected time histories.

We present correction factors that may be applied to the NGA models of Abrahamson and Silva, Boore and Atkinson, Campbell and Bozorgnia, and Chiou and Youngs to model the azimuthally varying distribution of the GMROTI50 component of ground motion (commonly called 'directivity') around earthquakes. Our correction factors are non-zero for M > 6.0 and rupture distance less than 70 km. They may be used for planar or nonplanar faults having any dip or slip rake. The correction factors are a product of an approximation, based on isochrone theory, of true directivity, a term simulating the earthquake's slip distribution, and a point-source approximation of the earthquake's radiation pattern.

Several west coast cities are seeing an upsurge in the construction of high-rise buildings. Many of these buildings feature framing systems, materials, heights, and dynamic properties not envisioned by our current building code prescriptive provisions. Rather than force these buildings to conform, many jurisdictions are allowing these new designs to proceed under the alternative procedures provision of the building code, which allows alternative lateral-force procedures using rational analyses based on well- established principles of mechanics in lieu of the prescriptive provisions. Most designs are opting for a performance-based approach in which a rational analysis demonstrates serviceability and safety equivalent to that intended by the code prescriptive provisions. Several questions arise in a performance-based design. What is equivalent performance? How should it be demonstrated? If dynamic analysis is conducted for a range of anticipated earthquake ground motions, how should the ground motions be selected and how should the design value determined? How should performance designs be reviewed?

The Tall Buildings Initiative is funding a range of short to intermediate-term projects in 2006-2009. The final product will be a set of written guidelines containing principles and specific criteria for tall building seismic design. The document is intended to support ongoing guidelines and code-writing activities of collaborating organizations, as well as being a stand-alone reference for designers of high-rise buildings. The main points relevant to the CSMIP program include: Selection of earthquake ground motions; Modification of earthquake ground motions to represent the design shaking level, including scaling and spectrum matching methods including considerations of epsilon and conditional mean spectra; Interactions between incoming ground motions and building foundations; and Interpretation of the results of multiple response history analyses for determination of design values.

This project is under way at the time of this presentation, including new funding from the CSMIP program. The presentation reviews the salient issues and current progress in addressing them.

As stated in the recent Guideline for ANSS Seismic Monitoring of Engineered Civil Systems, the mission of response monitoring within the ANSS program is to provide data and information products that will (1) contribute to earthquake safety through improved understanding and predictive modeling of the earthquake response of engineered civil systems and (2) aid in post-earthquake response and recovery. The second mission component intersects with the distinct field of Structural Health Monitoring (SHM). SHM is a broad field encompassing research and applications in Mechanical, Aerospace, and Civil Engineering. It is a very active area, with dedicated conferences such as the annual International Workshop on Structural Health Monitoring and several journals dedicated to SHM research.

The goal of SHM is the timely detection, location, and quantification of structural damage. At the present time there are successful SHM applications in other fields where the structures are well-defined and standardized (such as aircraft or rotating machinery). In these fields, the benefit is clear and the benefit/cost ratio is favorable. In civil engineering, research and development abounds and instrumentation technology exists for providing the data needed for SHM of buildings, bridges, and other large structures. However, uncertainty in the assessment of damage clouds the benefit, and costs are high. That said, there is now opportunity for overlap between earthquake monitoring of structures and SHM. Multi-disciplinary advances in the technologies of sensor networks, data acquisition, communication, real-time computation and system identification techniques have the potential to provide a useful and reliable post-earthquake damage assessment for instrumented structures. Testbeds such as the CSMIP-instrumented Vincent Thomas Bridge and the ANSS-instrumented UCLA Factor Building demonstrate the future potential of this overlap, combining earthquake monitoring with continuous monitoring and recording of data for SHM applications.

The recently published 2007 California Building Code (CBC) represents the most significant change in seismic design and construction in California in a decade. The 2007 CBC requirements are adopted from the 2006 International Building Code (IBC), which in turn adopts seismic provisions essentially by reference from ASCE 7-05 - Minimum Design Loads for Buildings and Other Structures. The ASCE 7-05 requirements are presented in an entirely new format, which is more logical and which places the more commonly used sections in earlier chapters and the more specialized or advanced topics (e.g., response history procedures, and requirements for isolated or damped systems) in later chapters.

Ground Motion is defined by a design spectrum based on spectral values maps adopted from the 2003 NEHRP Provisions. The updated maps reflect recent USGS research, resulting in refinements in spectral values in many locations, compared to the 2000 NEHRP, and more importantly for California, significant changes in ground motion compared to the UBC zone maps with accompanying near fault adjustments. The design spectrum also introduces a new long-period branch, following the constant velocity branch, which will affect very long period structures.

A Seismic Design Category (SDC) is assigned to each structure as a means of capturing both the seismic hazard, in terms of Mapped Acceleration Parameters (spectral values), Site Class (defining the soil profile), and the Occupancy Category, which is based on its importance or hazardous material contents. The SDC affects analysis, design and detailing requirements as well as the structural system that is allowed to be used and its height. The traditional UBC approach was to capture such requirements strictly based on zone.

Requirements for Nonstructural Components and for Nonbuilding Structures are substantially modified and expanded. Nonstructural Components are assigned the same SDC as the building to which they are attached; however, a given building, and therefore its components, may contain more than one occupancy category, and thus more than one SDC. Components are also assigned their own Importance Factor, based on either importance or hazardous materials content. There are also special Certification requirements for equipment that must remain operable after an earthquake or that contains hazardous materials. Nonbuilding Structures are addressed in a separate chapter, covering those that are similar to buildings and those that are not. The former are addressed in a manner similar to buildings, with factors for response modification, overstrength and displacement amplification. The later contains expanded requirements for tanks and vessels.