Showing papers in "Earthquake Spectra in 1989"

Slotted Bolted Connections in Aseismic Design for Concentrically Braced Connections

Abstract: The use of concentrically braced steel frames to satisfy lateral force requirements is a common design practice in regions of strong seismicity. They provide a very efficient means of fulfilling the dual objectives of earthquake resistant design, that is, damage control and collapse prevention. While tensile yielding and inelastic buckling of bracing elements provide the basic energy absorbing mechanism, there are inherent problems with member behavior under multiple cycles of inelastic deformations. Inelastic energy dissapation, however, may also be provided by friction resistance in slotted bolted connections, thus eliminating the need for inelastic member buckling. Limited laboratory tests indicate that this concept may be a viable alternative for use as a lateral force resisting system for both new construction and the seismic upgrading of existing structures.

Response Modification Factors for Earthquake Resistant Design of Short Period Buildings

Abstract: Most recent seismic codes include response modification factors in the definition of the equivalent lateral forces that are used for the design of earthquake resistant buildings. The response modification factors (R) are used to reduce the linear elastic design spectrum to account for the energy dissipation capacity of the structure. The evaluation of these response modification factors for various sets of earthquake records and ductility factors is presented herein. Special attention is given to the short period range where the reduction of linear elastic response spectra is smaller than the values for intermediate and long period structures. An idealized and simple variation of the response modification factor as a function of the period of vibration, suitable for seismic codes formulation, is also presented.

On Design of Eccentrically Braced Frames

Abstract: Seismic‐resistant Eccentrically Braced Frames (EBFs) are becoming a widely used lateral resisting system for steel buildings, with even wider application anticipated as design requirements are put into building code format. This paper addresses a number of EBF design issues, which in the opinion of the authors are inadequately considered either in current practice or in the emerging code provisions. The overall design philosophy for EBFs is reviewed, with specific reference to the concept of “Capacity Design”. Application of capacity design principles assures that yielding will be restricted primarily to the ductile link elements, an important goal of EBF design. Further, through careful choice of frame geometry and link length at the preliminary stages, many potential design difficulties can be avoided. The paper also presents some important observations from experimental work currently underway on EBFs with long, flexural yielding links.

The Mexico Earthquake of September 19, 1985—Behavior of Steel Buildings

Abstract: During the September 19, 1985 earthquake about 60 post‐1957 multistory steel buildings in Mexico City were subjected to a severe test. In most cases, the damage in the post‐1957 structures was minor to moderate. The notable exception is the Pino Suarez complex in which one 21‐story building collapsed onto a 14‐story building, and two other 21‐story buildings were severely damaged. The behavior of these buildings as well as two other case studies are analyzed. Conclusions are drawn on the ductility demands imposed on steel buildings in Mexico City, utilizing simplified structural models and considering the actual strength of buildings that may be much higher than the code design strength.

The Mexico Earthquake of September 19, 1985—Design Spectra for Mexico's Federal District

Abstract: We describe the methods used to obtain seismic design spectra adopted for different zones of Mexico's Federal District in 1987 Building Code. Paper exposes the two approaches followed in the study, deterministic and probabilistic. The assumptions adopted are presented and justified. Several aspects that require detailed scrutiny are pointed out.

The Mexico Earthquake of September 19, 1985—Statistics of Damage and of Retrofitting Techniques in Reinforced Concrete Buildings Affected by the 1985 Earthquake

Abstract: The 1985 Mexico City earthquake caused very serious damage, especially in reinforced concrete buildings, making it necessary to repair and strengthen many structures. The authors had the opportunity to study 114 of these retrofitted buildings. The data base created with the collected information contains the description of the building, its damages and the retrofitting techniques used. The statistical analysis of this information makes it possible to identify the correlation among the structural and foundation types, the dominant failure modes, the causes of damage and the repair and strengthening techniques used in this group of buildings.

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

Abstract: 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...

The Mexico Earthquake of September 19, 1985—Case Studies of Seismic Strengthening for Two Buildings in Mexico City

Abstract: The earthquake that shook Mexico City on 19 September 1985, destroyed several hundred buildings and took thousands of lives. Two buildings located in the most highly damaged part of the city experienced strong shaking, but suffered only slight damage. These reinforced concrete frame buildings had been retrofit with steel bracing systems and infill reinforced concrete walls prior to the earthquake. Forced vibration tests, analytical studies and geotechnical investigations for each building have been conducted. The results indicate that the steel bracing systems strengthened the buildings and stiffened them, moving their natural periods away from the 2.0‐second predominant ground period in that part of the city. Implications for the design of strengthening systems have been determined.

Magnitude and Distance Scaling of Response Spectral Shapes for Rock Sites with Applications to North American Tectonic Environment

Abstract: Response spectral shapes (Sa/a) computed from the BLWN (Band Limited White Noise) ground motion model combined with RVT (Random Vibration Theory) compare favorably with shapes computed from WNA (Western North American) earthquakes recorded at rock sites. The comparisons are made at close distances and for moment magnitudes 4 1/2, approximately 5, 6, and 6 1/2. Predicted spectral shapes for the ENA (Eastern North America) show reasonably good agreement with empirical results at frequencies of 1 and 10 Hz and with other random process models at frequencies of 0.5, 1, 2, 5, 10, and 20 Hz. Magnitude dependence of Sa/a is predicted to be significant for periods greater than 0.2 seconds in WNA and for periods greater than 0.1 seconds in ENA and increases with increasing period. Distance scaling appears to be significant only in WNA and at distances exceeding 50 km.

Past and Possible Future Earthquakes of Significance to the San Diego Region

Abstract: The potential for earthquakes that may significantly affect the San Diego, California, region is examined from the viewpoint of geology, seismic history, and strong ground motion. We have compiled the best available data on slip rates and recurrence times for all major faults in southern California and northern Baja California, identified possible fault segments that might rupture in single earthquakes and obtained repeat times for these events which are consistent with (or at least not contradicted by) trenching studies. The most important faults for San Diego's seismic hazard are the Rose Canyon fault, the Elsinore fault, and faults immediately offshore (Coronado Banks, San Diego trough). There have not been any major earthquakes on any of these nearby faults in historical time, but the geological evidence is clear that such events will eventually occur. San Diego is located on the western flank of the Peninsular Range batholith, and there is weak evidence that attenuation in this batholith mig...

Demonstration of Torsional Coupling Caused by Closely Spaced Periods—1984 Morgan Hill Earthquake Response of the Santa Clara County Building

Abstract: The parameters of the dominant modes of vibration of the steel‐framed Santa Clara County Office Building in San Jose, California, are determined using “the modal minimization method” for structural identification. The optimal estimates of the model parameters are determined by minimizing a selected measure‐of‐fit between the responses of the structure and the model. Two types of models are used: (1) A planar linear model with classical damping and (2) A three dimensional linear model consisting of rigid floor decks, where each floor is allowed three degrees of freedom ‐ two orthogonal translations plus a rotation. The Santa Clara County Office Building continued vibrating in a free vibration manner with very low damping, long after the intense part of ground motion had ended. The records of its torsional motion exhibit a strong beating effect which is explained by the strong coupling of torsional and translational modes of vibration. Such a strong coupling of modes of vibration is attributed to t...

The Mexico Earthquake of September 19, 1985—Analysis of Building Response

Abstract: Response of simple structural models to the peculiar ground motions recorded in the lake‐bed area of Mexico City was analyzed in order to explain the performance of typical buildings during the 1985 earthquake. Results for one‐degree of freedom systems with different kinds of inelastic behavior show that when hysteresis loops remain stable under several cycles of large inelastic deformations, exceptionally large reductions in the structural response take place. Nevertheless, if the hysteresis loops show some degradation of strength for large displacements, the response increases sharply for the same critical periods. Results for multi‐degree of freedom systems show large concentrations of ductility demands at lower stories. Static and dynamic analyses of linear and non‐linear models of several typical mid‐rise concrete buildings were also performed. The results agreed with reasonable accuracy with the observed behavior and with measurements of the response under low amplitude ambient vibrations.

Seismic Response of a Large‐Span Roof Diaphragm

Abstract: Records obtained from the West Valley College Gymnasium in Saratoga, California during the 1984 Morgan Hill earthquake are used to study the dynamic behavior of the overall gymnasium as well as its flexible diaphragm. The ground‐level motions recorded in the two orthogonal axes of the structure differ considerably in peak acceleration and amplify by approximately 1.5 times at the roof edges and by 4‐5 times at the center of the diaphragm. The diaphragm responds with a frequency of approximately 4 Hz in both orthogonal axes. A simple finite‐element model is used to match the fundamental frequency of the diaphragm with that from the records. Using this model and the ground‐level motions as input, the diaphragm center displacements are calculated by varying the structural damping. Best comparisons are obtained for 5% damping. These results are discussed in terms of the code provisions.

The Mexico Earthquake of September 19, 1985—The Seismic Performance of Buildings with Weak First Storey

Abstract: During the Michoacan earthquake of Se´ptember 19, 1985, 8 percent of the damaged buildings in the Mexico City area were characterized by a first storey much weaker than the upper ones. Although it is recognized that these failures may have resulted from the combination of several features, in this paper it is intended to study the possible influence of the lateral strength discontinuity in the ductility demand at the first storey, and to try to understand the behaviour of this type of structures under the actions of the East‐West component of the highest acceleration record obtained on soft soil in Mexico City during the mentioned earthquake. A parametric study for five and twelve‐storey buildings with weak first storey is presented in this paper. The infill walls in the upper storeys were brittle in some cases and ductile in others. For certain cases, the results show the existence of a range of values of the ratio of seismic lateral resistance of the upper storeys to that of the lowest one for ...

Seismic Velocities and Attenuation from Borehole Measurements near the Parkfield Prediction Zone, Central California

Abstract: Shear (S)‐ and compressional (P)‐ wave velocities were measured to a depth of 195 m in a borehole near the San Andreas fault where a recurrence of a moderate Parkfield earthquake is predicted. S‐wave velocities determined from orthogonal directions of the S‐wave source show velocity differences of approximately 20 percent. An average shear‐wave Q of 4 was determined in relatively unconsolidated sands and gravels of the Paso Robles Formation in the depth interval 57.5‐102.5 m.

Inelastic Response of a Spectrum‐Compatible Artificial Accelerogram

Abstract: The validity of evaluating the inelastic response of a structure subjected to an artificial accelerogram in lieu of a suite of eight recorded ground motions is determined by analyzing the inelastic response of single‐degree‐of‐freedom oscillators over a range of frequencies from 1.0 to 10.0 Hz. A normalization procedure to minimize the dispersion in the ductility response of the oscillators subjected to the recorded ground motions is investigated. The artificial accelerogram is derived by superimposing closely spaced sine waves in order to match a target response spectrum. The results show that the artificial accelerogram is expected to produce the same amount of damage as the average of the recorded strong ground motions for structures with an initial frequency of less than 5.0 Hz and close to the average for the entire suite of ground motions for structures with frequencies greater then 5.0 Hz.

Generating Seismic Design Power Spectral Density Functions

Abstract: The most widely used way to describe earthquake motions for purposes of design is the response spectrum, but it is often difficult to apply a response spectrum when dealing with multiple degrees of freedom or with complex representations of structural behavior. The power spectral density function, which is a more fundamental description of the frequency content of ground motion, has found increasing use and is essential in the most popular methods of developing artificial earthquake time histories. Although in theory the response spectrum and the power spectral density are closely related, in practice it has proven difficult to compute one from the other. Two integration schemes described in the literature have been implemented in an interactive micro‐computer program SPEED and are found to give substantially identical results. When they are used to find a power spectral density function that corresponds to a standard design response spectrum, the results do not converge at frequencies above 10 H...

The Mexico Earthquake of September 19, 1985—Typical Cases of Repair and Strengthening of Concrete Buildings:

Abstract: It is the aim of this paper to describe the most important techniques that are being used in Mexico City to retrofit buildings damaged during the 1985 earthquake. The information is presented by means of four case studies. These cases offer practical design examples of the typical solutions adopted and illustrate the various aspects of the repair and strengthening process such as damage assessment, emergency measures for temporary protection, the retrofitting technique employed in each case and considerations and decisions made in the analysis and design.

Emergency Earthquake Damage and Usability Assessment of Buildings

Abstract: An earthquake damage inspection form as well as criteria for assessing earthquake damage and usability of buildings under emergency conditions are presented. These criteria and the inspection form are key elements of the emergency inspection and have been based on recent experiences from damaging earthquakes in the Balkan region. They have been developed with a number of objectives in mind and notably the efficient use in the field and the reliable assessment of a builging's capacity. Since the emergency inspections of building damage will be a large scale operation, organizational aspects of this operation are discussed and specific recommendations are made.

A Simplified Procedure for Constructing Probabilistic Response Spectra

Abstract: This paper provides a simplified procedure for computing the probabilistic maximum response of a linear single‐degree‐of‐freedom system subjected to earthquake ground motion. The formulation uses the root‐mean‐square (rms) response of the system and a probabilistic peak factor which is modified to account for the wide‐band characteristics. The modification uses a set of damping‐dependent coefficients obtained from correlating the peak factor with the ratio of the maximum response to the rms response from accelerograms recorded on alluvium and rock. The procedure incorporates the influence of soil condition and the strong motion duration as parameters in computing response spectra.

The Mexico Earthquake of September 19, 1985—Behavior of Heavy Cladding Components

Abstract: A combined field study of building cladding performance during the 1985 Mexico earthquake and supporting analytical and experimental studies of cladding systems, typical of those used in Mexico City, are described. Results reported are from an on‐going research effort which is broken down into three phases: (I) Nonstructural damage survey and evaluation for selected buildings in Mexico City; (II) Laboratory testing of cladding connections representative of Mexican practice; and (III) Analytical evaluation of a case study building for cladding‐structure interaction effects. This study of the behavior of architectural cladding systems in the Mexico Earthquake is complementary to earlier work involving laboratory testing and analytical studies of cladding connection designs typical of U.S. practice. The data gathering, laboratory experimental, and analytical phases were designed to provide a balanced and coordinated attack on the problem of nonstructural performance in earthquakes and to extraction ...

Comparisons among Earthquake Codes

Abstract: This review paper compares ANSI, NEHRP, SEAOC, and UBC. A few essential differences among these documents are as follows: (a) The NEHRP document gives force levels corresponding to a strength‐based or limit states design, while the other three documents give force levels that correspond to working or service stress design; (b) the importance factor is used as a multiplier of base shear level in all documents except NEHRP, which treats building importance by a seismic hazard exposure group; (c) NEHRP and UBC‐1988 contain detailing requirements for all common construction materials and all seismic zones, while UBC‐1985 contains detailing requirements for zones of high seismicity but only limited requirements for zones of moderate seismicity; (d) P‐delta analysis is specified by NEHRP for all buildings that must be analyzed, by SEAOC for buildings that exceed drift limits, by UBC‐1988 for all buildings except those in Zones 3 and 4 meeting drift limits, and is not specified by ANSI.

Seismic Testing of Eccentrically Braced Dual Steel Systems

Abstract: 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 ...

Some Statistical Aspects of Earthquake Occurrence in Iraq

Abstract: Owing to Iraq's unique geographic location (29‐38.5 N;39‐49 E) the eastern and northeastern parts of the country are directly influenced by the seismic activity of the Tauros‐Zagros tectonic zones. An earthquake data file containing nearly 550 entries covering the period 1905‐1984 is compiled and an analysis of completeness is performed using Stepp's (1973) model. It is observed that the file is complete for 4.8 Ms and greater over the whole 80‐year sample of data while during the past 25 years this threshold drops to about 4.1 Ms. The “completed” data file is then employed in re‐evaluating the magnitude‐frequency formula for the country. To further investigate the problem of earthquake occurrence a simple statistical treatment is applied to a “constant‐b” seismicity model whereby an increase in the frequency of occurrence of approximately 35% is identified in the latter half of the seismic history of the country.

The Mexico Earthquake of September 19, 1985—Evaluation of the Seismic Capacity of Buildings in Mexico City

Abstract: As a consequence of the 1985 Mexico City earthquake, the authorities of the city have decided to evaluate the seismic capacity of the buildings without damage with the purpose of detecting those with the most vulnerability, in order to take the preventive actions necessary to improve the security of the citizens in future earthquakes. Based on the census of the structures of the city, the procedure of evaluation used considers three levels of increasing precision permitting a progressive selection of the buildings in bad condition. Work is now under process in the main districts of the city with good results.

Dynamic Site Periods in Charleston, SC

Abstract: The 1985 experience in Mexico City has again demonstrated that building damage from earthquakes is related to the ground conditions beneath the building When the natural period of the building matches the natural site period, resonance can occur, resulting in large building deformations and severe damage Mapping dynamic site periods for a city, and governing building construction accordingly can greatly reduce the threat of damage from earthquakes Using dynamic soil property data and a one‐dimensional linear elastic computer program, a dynamic site period map for Charleston, SC, is developed and presented Comparison of the building damage distribution for the 1886 earthquake with the distribution of dynamic site periods shows the important role of dynamic site period An approximate method of estimating the linear site period is given

Predicted Ductility Demands for Steel Moment Resisting Frames

Abstract: Recent changes to the United States seismic design provisions permit the use of weak column‐strong beam steel moment resisting frames. This design concept has not been used in the past, because it results in plastic hinges in the columns during moderate or extreme earthquakes. This paper shows the results of inelastic dynamic response calculations on a weak column frame and a comparable strong column system. The results show that the ductility demand is much greater for the weak column strong beam framing system with some acceleration records. The required ductility is then compared for the different structural systems and both are compared to the results of experiments. The comparison suggests that the weak column system may not be able to develop the required ductility. The results of this paper should help define the viability and limits in applicability of the weak column system.

Toward a Philosophy for Seismic Retrofit: The Seattle Experience

Abstract: By examining seismic retrofit experience with three buildings in Seattle, important practical issues regarding the underlying economic, political, and social conflicts are explored. The philosophy of seismic retrofit and its resulting consequences for life safety, building usefulness both before and after seismic events, and the economic consequences of retrofit are examined in light of the professional and regulatory atmosphere. While a great deal of information is generated nationally in the field of seismic retrofit, its assimilation by the practising professional and regulatory official is incomplete and uncoordinated. The work proposes, therefore, a seismic retrofit design philosophy for discussion and possible use as a premise for development of codes. The proposed philosophy coordinates the economic, social, and life safety aspects of seismic retrofit in a way that meets the needs of designers and regulators who are currently forced to make social policy in their everyday design interactions.