Vitelmo V. Bertero

Evaluation of Strength Reduction Factors for Earthquake‐Resistant Design

Strength reduction factors which permit estimation of inelastic strength demands from elastic strength demands are evaluated. Results from various investigations of strength reduction factors carried out over the last 30 years are reviewed, and their results are presented in a common format which facilitates their comparison. The main parameters that affect the magnitude of strength reductions are discussed. The evaluation of the results indicates that strength reductions are primarily influenced by the maximum tolerable displacement ductility demand, the period of the system and the soil conditions at the site. Simplified expressions of strength reduction factors to estimate inelastic design spectra as functions of these primary-influencing parameters are presented.

Earthquake Engineering : From Engineering Seismology to Performance-Based Engineering

This multi-contributor book provides comprehensive coverage of earthquake engineering problems, an overview of traditional methods, and the scientific background on recent developments. It discusses computer methods on structural analysis, provides access to the recent design methodologies, and serves as a reference for both professionals and researchers involved in earthquake engineering. With an entire chapter dedicated to seismic resistant design through supplemental damping and structural control, this volume includes important advances in the characteristics of earthquake ground motions, behavior and design of structures, seismic design of non-structural systems, and more.

Seismic Testing of Steel Plate Energy Dissipation Devices

In a research program, co-sponsored by Bechtel Power Corporation and CounterQuake Corporation, the seismic performance of steel plate added damping and stiffness (ADAS) elements was investigated through a series of sub-assemblage experiments and by the earthquake simulator testing of a three story ductile moment resisting space frame (DMRSF) upgraded with ADAS elements. The sub-assemblage testing of the ADAS elements demonstrated their stable hysteresis for a large number of yielding cycles. The addition of the ADAS system (ADAS elements and chevron braces) to the DMRSF limited the frames response to acceptable levels during severe earthquake simulation. The research program clearly demonstrated that ADAS elements can be used to effectively upgrade moment resisting frames (MRFs) and concentrically braced frames (CBFs) to achieve a moderately stiff building with extremely good energy dissipation characteristics. The implications of the research results for the design and upgrade of buildings using ADAS elements, and potential uses for ADAS elements, are presented in the latter part of the paper.

Lessons learned from recent earthquakes and research and implications for earthquake-resistant design of building structures in the United States

Following an overview of the special problems inherent in the design and construction of earthquake-resistant buildings in regions of high seismic risk, the techniques that will be required to solve these problems in the U.S. are discussed. Some lessons learned from recent earthquakes, particularly those in Chile and Mexico in 1985, are discussed as are some results of integrated analytical and experimental research at the University of California, Berkeley. The implications of the ground motions recorded during the 1985 Mexican and Chilean earthquakes, the performance of buildings during the Mexican earthquake, and the research results previously discussed are then assessed with respect to seismic-resistant design regulations presently in force (UBC) as well as those formulated by ATC 3-06 and the Tentative Lateral Force Requirements recently developed by the Seismology Committee of SEAOC. The rationale for and reliability of the values suggested by the ATC for the “Response Modification Factor R” and by the SEAOC Seismology Committee for the “Structural Quality Factor R w ” are reviewed in detail. In the conclusion to the paper, two solutions for improving the earthquake-resistant design of building structures are proposed: an ideal (rational) method to be implemented in the future, and a compromise solution that can be implemented immediately.

The Mexico Earthquake of September 19, 1985—Performance of Low‐Rise Buildings in Mexico City

This paper summarizes the results of analytical studies conducted to understand the observed performance of low-rise buildings located in the soft-soil zone of Mexico City during the 1985 Michoacan earthquake. Two low-rise reinforced concrete moment resistant space frames were designed in accordance with the 1976 Code for the Federal District of Mexico. They were subjected to a series of static and time history dynamic analyses. The results indicate that the designed buildings have significantly larger lateral strengths than required by the Code and that these overstrengths were the main reason for the excellent performance of most of the low-rise buildings in Mexico City during the 1985 Michoacan earthquake.

Seismic fragility of short period reinforced concrete structural walls under near-source ground motions

The Bayesian parameter estimation technique is used to develop probabilistic displacement and strength capacity and demand models for reinforced concrete structural walls. Experimental data are used to develop the capacity models, and nonlinear dynamic analysis is employed to develop the demand models. Both flexural and shear failures are accounted for. These models are used to assess the seismic fragility of an example RC structural wall. As a new measure of the ground motion intensity, the significant peak ground acceleration is defined and incorporated in the probabilistic demand models and fragility assessment. It is shown that, for short period structures, this measure better correlates with the inelastic response than the elastic response spectrum.

EVALUATION OF PRE-NORTHRIDGE STEEL MOMENT- RESISTING FRAME JOINTS

The 1994 Northridge earthquake caused widespread and unexpected damage to steel moment-resisting joints and connections. Shortly following the earthquake, the Federal Emergency Management Agency funded a series of full-scale tests of steel moment-resisting joints and connections to characterize the behavior of pre-earthquake connections and to evaluate the efficacy of a selected number of repair schemes. Twelve pre-earthquake connections were tested. Three of the twelve connections were tested by the authors to failure, and then repaired and re-tested. The response of the pre-earthquake connections was highly variable and uniformly poor. Premature fractures were observed in all twelve connections, and the types of fractures were similar to those observed in the field following the earthquake. The mean beam plastic rotation was 0.005 rad: one-sixth of the target value of 0.03 rad. The response of those moment-resisting connections that were repaired by replacing fractured weld and parent metal with toughness-rated weld filler metal was also poor. On the basis of the studies described in the paper, the rotation capacity of large-size moment-resisting connections built prior to the Northridge earthquake is smaller than the target values established following the earthquake; rehabilitation of earthquake-damaged moment-resisting connections by re-welding only will likely be ineffective; beam-column panel zones should be designed to remain elastic for the forces associated with plastic hinging in the beams; design equations for continuity plates should be revised; and design checks for flange compactness should be based on expected rather than nominal material properties. Copyright

Seismic Rehabilitation of Infilled Non‐Ductile Frame Buildings Using Post‐Tensioned Steel Braces

In recent years, it has been shown that the seismic performance of existing buildings can be enhanced considerably by bracing them with post-tensioned rods or cables. This upgrading technique has several advantages, which include architectural versatility, low cost and fast construction. Furthermore, it does not add a significant mass to the existing building. The objectives of the present paper are to: investigate the use of post-tensioned steel bracing for seismic retrofit of non-ductile reinforced concrete frames with unreinforced masonry infills, discuss some of the issues that need to be addressed in the design of the rehabilitated building, assess the advantages of this rehabilitation technique by studying the seismic performance of an infilled non-ductile frame building before and after it has been upgraded with post-tensioned braces, and propose research recommendations.

SEISMIC PERFORMANCE OF AN INSTRUMENTED TEN-STOREY REINFORCED CONCRETE BUILDING

Results from analytical studies conducted on an instrumented ten-storey reinforced concrete building which experienced ground accelerations in excess of 0.6g during the 1987 Whittier-Narrows California earthquake and suffered only minimal damage are presented. Using the dynamic characteristics inferred from accelerations recorded in the building during the earthquake, a mathematical model was calibrated to study the response of the building and to explain its good behaviour despite the apparent severity of the motions recorded in the basement of the building. Very good correlation was obtained between the computed and recorded response of the building. Non-linear analyses were conducted to evaluate the strength and deformation capacity of the building and to estimate its response in the event of more severe earthquake ground motions. Special emphasis is given to the evaluation of the overstrength of the building. Lateral overstrengths larger than 4.2 and larger than 5.7 were computed for the longitudinal and transverse directions of the building, respectively. It is concluded that these high levels overstrength in the building played an important role in limiting the damage during the Whittier-Narrow earthquake. Since the estimation of inelastic deformations during severe earthquake ground motions depends on the actual strength of the building, it is recommended to consider explicitly probable values of this overstrength in the strength reduction factors.

Seismic Testing of Eccentrically Braced Dual Steel Systems

Two six-story eccentrically braced dual steel systems (EBDSs) were tested as part of the U.S.-Japan Cooperative Earthquake Research Program. The first, a full-scale structure (prototype) was pseudo-dynamically tested in the Large Size Structures Laboratory of the Building Research Institute in Tsukuba, Japan. The second, a similitude scaled replica of the first, was tested on the earthquake simulator at the University of California at Berkeley. The prototype was designed for the minimum earthquake forces specified by the 1981 Japanese Aseismic Code and satisfied the current earthquake-resistant design regulations in the U.S.A. (1985 UBC, 1984 ATC 3-06 and 1986 SEAOC). The performance of the EBDS (both prototype and model) was outstanding in terms of its elastic strength and stiffnesses during minor earthquake shaking and its ability to absorb and dissipate energy, without strength and stiffness degradation, during severe earthquake shaking. Substantial overstrengths of both EBDSs with respect to their nominal yielding strengths were observed during severe earthquake shaking. However, the response modification factors currently adopted by the ATC and SEAOC significantly overestimated the experimental values in both instances.