Showing papers in "Earthquake Spectra in 1996"

Seismic Evaluation of Existing Reinforced Concrete Building Columns

Abstract: Past earthquakes have emphasized the vulnerability of reinforced concrete columns having details typical of those built before the mid‐1970's. These columns are susceptible to axial‐flexural, shear, and bond failures, which subsequently may lead to severe damage or collapse of the building. Research was undertaken to investigate the lateral and vertical load‐resisting behavior of reinforced concrete columns typical of pre‐1970's construction. Eight full‐scale specimens were constructed and were loaded with constant axial load and increasing cyclic lateral displacement increments until failure. Test data are presented and compared with behavior estimated by using various evaluation methods.

Application of Pseudodynamic Test Method to Structural Research

Abstract: The pseudodynamic test method is intended as an efficient and powerful alternative to the shaking table test method. Since its inception, a variety of new testing techniques and numerical schemes have been developed to improve the efficiency and accuracy of the method. The main objective of this paper is to present the pseudodynamic test method from a user's perspective. With this objective in mind, an overview of the pseudodynamic test method is provided. Some of the most up‐to‐date developments, including substructure testing techniques, are illustrated with test examples. It is shown that while the method is definitely more complicated than conventional quasi‐static tests, it can be implemented and applied to structural research with a small incremental cost.

Damage Scenarios Simulation for Seismic Risk Assessment in Urban Zones

Abstract: A methodology for simulating seismic damage of unreinforced masonry buildings for seismic risk assessment of urban areas is presented in this paper. The methodology is based on the Italian vulnerab...

Out-of-Plane Strength of Unreinforced Masonry Infill Panels

Abstract: Simple procedures are presented for estimating out‐of‐plane strength of unreinforced masonry infill panels. The procedures are based on an experimental study and subsequent development of an analytical model, which are described as a preface to derivation of a simple and approximate expression for estimating out‐of‐plane strength of a general class of infill panels. An example is given illustrating application of the proposed guideline.

Testing of Masonry Structures for Seismic Assessment

Abstract: The experimental evaluation of strength, deformability, and energy dissipation capacity of unreinforced masonry buildings subjected to seismic loadings presents unique and complex problems, both for laboratory and field evaluations. The paper addresses these problems, focusing on the relative merits and roles of several experimental techniques, including quasistatic, dynamic, and pseudodynamic loadings at full and reduced scale.

Seismic Input Modelling for Zoning and Microzoning

Abstract: The strong influence of lateral heterogeneities and of source properties on the spatial distribution of ground motion indicates that the traditional methods require an alternative when earthquake records are not available. The computation of broadband synthetic seismograms makes it possible, as required by a realistic modelling, to take source and propagation effects into account, fully utilizing the large amount of geological, geophysical and geotechnical data, already available. For recent earthquakes, where strong motion observations are available, it is possible to validate the modelling by comparing the synthetic seismograms with the experimental records. The realistic modelling of the seismic input has been applied to a first‐order seismic zoning of the whole territory of several countries. Even though it falls in the domain of the deterministic approaches, the method is suitable to be used in new integrated procedures which combine probabilistic and deterministic approaches and allow us to...

Earthquake Performance of Gas Transmission Pipelines

Abstract: Over 61 years of earthquake performance of steel transmission and distribution supply pipelines operated by the Southern California Gas Company are reviewed. The seismic record includes 11 major earthquakes with ML ≥ 5.8 and epicenters within the transmission system. An evaluation is made of the most vulnerable types of piping, failure mechanisms, break statistics, threshold seismic intensity to cause failure, and damage induced by permanent ground displacement. The database assembled represents one of the most comprehensive and detailed records of seismic response in a large, complex gas transmission system.

Implications of Experiments on the Seismic Behavior of Gravity Load Designed RC Beam-to-Column Connections:

Abstract: This paper summarizes recent experimental research at Cornell University conducted on the behavior of gravity load designed reinforced concrete building frame components subjected to reversing cyclic loads (simulated seismic effects). Reinforced concrete framing systems, designed primarily for gravity loads, with little or no attention given to lateral load effects, are typically characterized by non‐ductile reinforcing details in the joint regions and in the members. The seismic response of connection regions for gravity load design (GLD) frames has received relatively little attention in earlier studies, thus making it difficult to reliably evaluate GLD frames and to properly plan repair or retrofit strategies. Thirty‐four full scale bare interior and exterior beam‐to‐column joints have been tested under reversed cyclic bending to identify the different damage mechanisms and to study the effect of critical details on strength and deformations. The discussion of test results focuses on the defin...

Retrofit of concrete columns with inadequate lap splices by the use of rectangular steel jackets

Abstract: This paper describes an experimental research program on the use of rectangular steel jackets for seismic retrofit of non‐ductile reinforced concrete frame columns. Eleven large scale columns were tested to examine the effectiveness of various types of steel jackets for improving the ductility and strength of columns with an inadequate lap splice in the longitudinal reinforcement. Response of the columns before and after being strengthened with steel jackets was examined. Several types of steel jackets were investigated, including rectangular solid steel jackets with and without adhesive anchor bolts. The test results indicate that a thin rectangular steel jacket combined with adhesive anchor bolts can be a highly effective retrofit measure for reinforced concrete columns with an inadequate lap splice. Design guidelines for the use of rectangular steel jackets as a seismic retrofit for non‐ductile reinforced concrete columns are presented.

Cyclic Loading Histories for Seismic Experimentation on Structural Components

Abstract: In order to utilize results obtained from quasi‐static cyclic load tests on structural components for a general performance evaluation, the need exists to establish loading histories that capture critical issues of component capacity as well as seismic demands. In inelastic seismic problems capacity and demands cannot be separated since one may strongly depend on the other. Because of cumulative damage issues the capacity depends on the number of inelastic excursions and the magnitude of each excursion (not just the largest one). These two parameters depend on the frequency content of the ground motion, the period(s) of the structure, and the strength and inelastic deformation characteristics of the structure. The paper presents procedures how these characteristics can be considered in the selection of suitable loading histories.

Site Response Studies for Purpose of Revising NEHRP Seismic Provisions

Abstract: A strong motion database was compiled for California earthquakes of surfacewave 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: rock (class A), soft rock or stiff soil (class B), medium stiff soil (class C), and soft soil (class D). The results of analyses 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 (designated as Fa and Fv, respectively) are generally within 20 percent of those recommended during the 1992 Site Response Workshop conducted by the National Center for Earthquake Engineering Research (NCEER). The Fa and Fv values recommended from our study and those from the NCEER workshop are intended for...

Retrofit of Non-Ductile Moment-Resisting Frames Using Precast Infill Wall Panels:

Abstract: Many existing reinforced concrete moment‐resisting frames located in seismic zones lack strength and ductility. One approach for correcting these deficiencies is the construction of infill walls to strengthen and stiffen the structure. Cast‐in‐place construction is often used; however, there are conditions where cost, time constraints, or limiting disruptions to building operations may dictate other solutions. One possible modification is the use of infill walls constructed of precast concrete panels. A precast infill wall system eliminates the need for large formwork during construction. Elimination or reduction of connection hardware between precast panels or between panels and the existing frame element can provide additional efficiency. Problems associated with casting large quantities of concrete in an existing building are eliminated. Construction time and inconvenience to occupants may be reduced along with the costs. The precast system has the potential of reducing the overall costs of re...

Seismic Upgrading of Old Brick‐Masonry Urban Houses: Tying of Walls with Steel Ties

Abstract: The influence of tying the walls with steel ties on the seismic behavior of existing brick‐masonry houses has been investigated. Three models of simple two‐story brick masonry houses with wooden floors, with or without wall ties, have been tested on a simple earthquake simulator. In addition, a model with identical structural configuration, but with r.c. slabs instead of wooden floors, has been tested for comparison. Whereas wooden floors with joists not anchored to the walls did not prevent separation and disintegration of the walls, rigid slabs and steel ties significantly improved seismic behavior. On the basis of test results, a simple method for designing the ties has been proposed.

Seismic Design and Testing of an RC Slab-Column Frame Strengthened by Steel Bracing:

Abstract: This study is concerned with developing a rational design procedure for use of ductile steel bracing for strengthening existing seismically “weak” RC slab‐column building structures. A one‐third scale, two‐bay, two‐story RC slab‐column frame model was selected to represent existing seismically inadequate structures of its type. The design procedure, construction and test results of the steel bracing system for strengthening the RC frame are presented in this paper. The strengthened frame was subjected to a combination of gravity and cyclic lateral loads up to 2% overall frame drifts. The behavior of the strengthened frame improved dramatically over that of the bare RC frame. A maximum 2.75% drift in the first story was reached which is highly probable during severe earthquake motions.

Pseudodynamic Capabilities of the ELSA Laboratory for Earthquake Testing of Large Structures

Abstract: The paper describes the distinctive features of the pseudo-dynamic test method as implemented at the largest European reaction-wall facility. Both hardware and software aspects are considered. Part...

Seismic Test Methods for Architectural Glazing Systems

Abstract: An ongoing effort is being made at the University of Missouri‐Rolla to develop standard laboratory test methods and codified design procedures for architectural glass under seismic loadings Recent laboratory work has yielded some promising results regarding the development of an appropriate seismic test method for architectural glass, as well as identifying ultimate limit states that quantify the seismic performance and damage thresholds of various glass types Specifically, a straightforward “crescendo‐like” in‐plane dynamic racking test, performed at a constant frequency, has been employed successfully Two ultimate limit states for architectural glass have been defined: (1) a lower ultimate limit state corresponding to major glass crack pattern formation; and (2) an upper limit state corresponding to significant glass fallout Early crescendo tests have yielded distinct and repeatable ultimate limit state data for various storefront glass types tested under dynamic racking motions Crescendo

Seismic Rehabilitation of Reinforced Concrete Beam‐Column Connections

Abstract: As building codes are updated, some of the existing important structures may fall short of complying with current standards even though they may have been properly designed and constructed according to earlier codes. Many existing structures may be inadequate and may pose severe risk during seismic events. Rehabilitation measures to upgrade the capacity of these structures can be performed at some point in their useful lives especially when located in seismically active zones. A new method for improving the seismic performance of existing reinforced concrete structures is by jacketing the deficient connections using corrugated steel jackets. An experimental program was conducted to evaluate this method of rehabilitation. Corrugated steel jacketing addresses the particular weakness that is often found in existing reinforced concrete structures, namely the lack of sufficient shear reinforcement and the required confining reinforcement within the joints and in adjoining beams and columns. The perfor...

Considerations for the use of quasi-static testing

Abstract: While advances in servo‐hydraulic technology and computer simulation are making real‐time pseudo‐dynamic and shake table testing more feasible for research in earthquake engineering, quasi‐static testing will remain the most prevalent form of testing for the foreseeable future. This paper describes the evolution of quasi‐static testing, discusses by means of an example the main issues involved in developing a quasi‐static testing program, and describes some of the advantages and limitations of this mode of testing.

Seismic Strengthening of Unreinforced Masonry Piers with Steel Elements

Abstract: The system of wall piers and spandrels, created by openings, largely controls the inplane lateral resistance of the wall. For the “rocking‐critical” masonry wall piers, the overall hysteretic behavior can be significantly improved by installing a steel framing system consisting of vertical and horizontal elements around the wall — without any braces. Vertical elements provide the necessary hold‐down forces to stabilize the rocking piers. The stabilized piers “rocked” through a number of cycles of large displacements (up to 2.5%) without crumbling or shattering, displaying a ductile response. The strengthened system has excellent strength, stiffness and ductility, despite the brittleness of the masonry because of considerable load sharing between the existing masonry and the added steel elements. FE analyses predicted the envelope response of the rocking piers accurately. A simple mechanics based model was developed to predict the load‐deflection behavior of a stabilized rocking pier which can be ...

Evaluation of UBC‐94 Provisions for Seismic Design of RC Structural Walls

Abstract: New provisions for seismic design of reinforced concrete structural walls were incorporated into the 1994 version of the Uniform Building Code. The new provisions are based on the use of a displacement‐based design methodology, which is a significant departure from prior codes. The format of the UBC‐94 provisions offer significant advantages over previous design formats, with both simplified and detailed design approaches. As is the case with any significant code change, proper application of the new provisions requires a thorough understanding of the design methodology used to develop the provisions as well as its limitations. Given these needs, an overview of displacement‐based and the UBC‐1994 requirements is provided. The methods employed in UBC‐94 to estimate the maximum displacement response are found to significantly underestimate maximum displacement; therefore, direct application of the provisions could lead to unconservative designs. Recommendations to address this shortcoming, as well ...

Repair and Retrofit of Unreinforced Masonry Structures

Abstract: A comprehensive study was conducted to evaluate the efficacy of different repair and retrofit techniques for unreinforced masonry (URM) structures. The study was to identify suitable grouting materials and procedures for the repair and retrofit of URM structures, quantify the efficacy of different repair and retrofit techniques, and develop analysis tools to evaluate the performance of masonry structures before and after retrofit. To this end, a number of grouting materials and procedures were evaluated with material tests, masonry component tests and shear wall tests. The experimental results indicate that the strength and stiffness of damaged walls can be fully restored with grout injection. Furthermore, the strength and ductility of a URM wall can be greatly enhanced by the introduction of steel reinforcement.

Issues on the seismic retrofit of a building near resonant response and structural pounding

Abstract: Paper presents an analytical study on the possible application of commonly used retrofit schemes for an existing nine‐story apartment building in Mexico City. The building, located in Mexico City's lake‐bed region, was originally conceived as a RC waffle‐flat slab structural system and was built in the late sixties. The building suffered important structural damage during the March 14, 1979 Petatla´n Earthquake, primarily caused by structural pounding with neighboring structures due to its flexibility and resonant response with the ground. The building was retrofitted shortly after the Petatla´n Earthquake with steel bracing in the direction of pounding. The retrofitted building survived the September 19, 1985 Michoaca´n Earthquake with light structural damage. Some evidence of pounding with one neighboring structure was again observed. Because of the chronic pounding problems with adjacent structures, which are separated eight to fifteen cm from the apartment building, it is felt that an additio...

A Refined Model for Base‐Isolated Bridges with Bi‐Linear Hysteretic Bearings

Abstract: Recognizing that the inelastic displacement spectra with constant ductility ratios of an earthquake ground motion can be approximated by the equivalent elastic displacement spectra with the consideration of appropriate effective period shifts (or effective stiffness) and equivalent damping ratios, a refined equivalent linear model for the seismic analysis of base‐isolated bridges with bi‐linear hysteretic bearings is established in this paper using a system identification method. The parameters necessary to completely define a bi‐linear hysteresis loop such as the elastic stiffness, yielding force, strain harden ratio and ductility ratio are all considered in the modeling. Eighteen California, one Washington and one Japan earthquake ground motions are used for the identification. The equivalent linear model established through the identification process is characterized as a modification of the current equivalent linear model provided by the AASHTO isolation guide specifications. Numerical compar...

Effects of Scale and Loading Rate with Tests of Concrete and Masonry Structures

Abstract: Static and dynamic response of large‐scale and reduced‐scale test structures are correlated to discern effects attributable to scale and loading rate. Three case studies are presented where reduced‐scale models were subjected to dynamic excitation using a shaking table. The test structures were: (a) multi‐story reinforced masonry building systems, (b) two‐story unreinforced masonry bearing and shear wall systems, and (c) ten‐story reinforced concrete frame‐wall systems. For each study, static testing of either critical components, or of the complete structural system, was done at a large scale to examine differences attributable to the modeling method, or to the loading procedure.

Some Retrofit Options for the Seismic Upgrading of Old Low-Rise School Buildings in Mexico

Abstract: An analytical study regarding the seismic upgrading of typical 60's and 70's designs for public school buildings in Mexico is presented. Some schools with these designs were moderately damaged during the 1985 Michoaca´n Earthquake in Mexico City. The damage was primarily observed in their longitudinal direction where existing slender RC columns have their weak axis. In addition, these columns are confined and shortened by masonry walls that do not run all the story height. These walls are supposed to be non‐structural components, however, they experienced shear cracking during the quake due to the distress of the confined columns. Some school buildings were retrofitted after the Michoaca´n Earthquake adding post‐tensioned bracing systems composed of prestressed high‐slenderness steel strands (tension‐only bracing systems), a retrofit option that is economical. In fact, there is an interest on assessing the effectiveness of this retrofit scheme in other regions with different soil conditions, as f...

Dynamic Properties of a Circuit Breaker with Friction‐Based Seismic Dampers

Abstract: The design parameter of 1.05 g peak seismic ground acceleration for a 300 kV SF6 circuit breaker necessitated the provision of supplemental friction‐based dampers. This paper describes the dampers and the dynamic properties of the circuit breaker as obtained from a series of pull‐release tests with increasing force amplitudes. These tests permitted a determination of a wide range of damping ratios and natural frequencies as a function of displacements. A comparison is also presented between the measured and the calculated damping ratios and frequencies, using common engineering approximation of the energy dissipated per cycle for damping, and a discrete parameter and linearized stiffness approach for the calculation of natural frequency. Reasonable comparisons were achieved between the measured and calculated values.