Showing papers on "Earthquake resistant structures published in 1991"

Behavior of rc frame strengthened using structural steel bracing

Abstract: A retrofit strengthening technique for improving the seismic behavior of a nonductile reinforced concrete (RC) frame was investigated experimentally. The original frame contained deep, stiff spandrel beams and short, flexible columns that were susceptible to shear failure under lateral loads. The strengthening technique used a steel X‐bracing system attached to the exterior of the frame using epoxy‐grouted dowels. A two‐thirds‐scale frame model consisting of two bays and three levels was subjected to statically applied cyclic lateral load. Test results indicated substantial increases in both lateral stiffness and strength. The lateral capacity of the strengthened frame was governed by brace buckling and eventual connection failures and column shear failures. Measured brace buckling loads agreed well with predicted capacities assuming fixed (K=0.5) connections, and dowel performance was excellent. Bracing‐system elements attached to the side faces of the concrete columns also substantially increased column...

Aseismic Hybrid Control Systems for Building Structures

Abstract: Two aseismic hybrid control systems are proposed for protecting building structures against strong earthquakes. The hybrid control system consists of a base-isolation system connected to either a passive or active mass damper. The base-isolation system, such as elastomeric bearings, is used to decouple horizontal ground motions from the building; whereas the mass damper, either active or passive, is used to protect the safety and integrity of the base-isolation system. The performance of the proposed hybrid control systems is investigated, evaluated, and compared with that of an active control system. It is shown from the theoretical/numerical results that the proposed hybrid control systems are very effective in reducing the response of tall buildings under strong earthquakes. Likewise, the practical implementation of such hybrid control systems is easier than that of an active control system alone.

Evaluation of Torsional Provisions in Seismic Codes

Abstract: The effects of plan asymmetry on the earthquake response of code-designed, one-story systems are identified with the objective of evaluating how well these effects are represented by torsional provisions in building codes. The earthquake-induced deformations and ductility demands on resisting elements of asymmetric-plan systems, designed according to several different codes, are compared with their values if the system plan were symmetric. The presented results demonstrate that the design eccentricity in building codes should be modified in order to achieve the desirable goal of similar ductility demands on asymmetric-plan and symmetric-plan systems. The design eccentricity should be defined differently for elastic and inelastic systems; in the latter case, it should vary with the design force level or anticipated degree of inelastic action. However, it does not appear possible to reduce the additional element deformations due to plan asymmetry by modifying the design eccentricity; these deformations should be provided for in building design.

Development of Design Criteria for RC Interior Beam-Column Joints

Abstract: Synopsis: A series of research efforts, at the University of Tokyo, leading to the development of earthquake resistant design criteria for reinforced concrete interior beam-column joints are briefly summarized. The design criteria emphasize the protection of the joint to an acceptable deformation level of a frame structure during an intense earthquake. For the design against shear, shear resisting mechanism by truss and concrete compression strut, the role of joint lateral reinforcement, and the effect of transverse beams and slabs were studied experimentally. The requirement for beam bar bond was discussed on the basis of nonlinear earthquake response analysis.

Earthquake resistant design of concentrically braced steel frames

Abstract: New provisions of the CSA standard for steel structures (CAN/CSA-S16.1-M89) dealing with detailing of concentrically braced frames for seismic design are described and related to requirements of the National Building Code of Canada. The basis of the new requirements is outlined, and an example eight-storey frame is used to outline a methodology for the design process for a ductile braced frame and to illustrate the impact of the provisions. Key words: design, structural engineering, steel, earthquakes, braced frame, standards.

Seismic response of friction jointed precast panel shear walls

Abstract: The effectiveness of using friction type mechanical connectors along vertical joints to improve the earthquake resistance of large panel precast systems is investigated. In particular, the paper examines the relative roles of these connectors and gap-friction action in the horizontal joints. Nonlinear time history dynamic analysis is employed to determine the design connector slip load at which response is optimized . The results show that reduced seismic response and enhanced structural integrity are achieved through use of such connectors. Design implications, as well as a tentative procedure for selecting these devices, are also discussed.

Shearwalls - an answer for seismic resistance?

Abstract: The highlights are presented of observations of the behavior of buildings containing shearwalls in the earthquakes of the last 30 years. The earthquakes discussed here include the following: Chile (1960); Skopje, Yugoslavia (1963); Caracas, Venezuela (1967); San Fernando, California (1971); Managua, Nicaragua (1972); Bucharest, Romania (1977); Mexico City (1985); Chile (1985); and Armenia (1988). During these earthquakes, buildings containing shearwalls exhibited extremely good earthquake performance. In most cases, the shearwalls were reinforced in the traditional manner for gravity and overturning, without consideration to special details for ductility, as required in recent U.S. codes. Investigatitons of the behavior of modern structures in a dozen earthquakes throughout the world since 1963, has not revealed a single concrete building containing shearwalls that has collapsed.

Seismic shear capacity of reinforced masonry piers

Abstract: Masonry buildings have been, and remain, a popular form for economically enclosing space. Whereas such buildings are safe under gravity loads, most are vulnerable to horizontal loads due to earthquakes. Observations following an earthquake and experimental programs have shown that piers between openings are the most vulnerable part of a masonry building, and that the failure of such piers is due in the majority of cases to shear (or diagonal tension). Accordingly, the study described concerns the seismic response of reinforced masonry piers that exhibit a shear mode of failure. The study consists of two parts. First the results of an experimental program on reinforced masonry piers under cyclic lateral loads simulating seismic excitation are presented. Then several code provisions governing the seismic design of masonry are evaluated in view of the experimental observations, and a new seismic shear design concept is proposed for reinforced masonry piers and walls where the design strength is defined as th...

Modeling earthquake response of concrete masonry building structures

Abstract: The paper discusses considerations involved with modeling concrete masonry building structures at one-qurater scale and reports on tests carried out to evaluate mechanical properties of reduced-scale masonry materials. Procedures used to design and test two different 3-story building structures to failure on a shaking table are also presented. Samples of dynamic response are presented to demonstrate the relevance of using a scale model of earthquake engineering research.

Fiber Reinforced Concrete Joints for Precast Construction in Seismic Areas

Abstract: Six beam type specimens, each consisting of 2 precast reinforced concrete parts joined together by a cast-in-place fiber reinforced concrete connector, were tested by under cyclic third-point loading. The tests were part of a comprehensive study aimed at developing a strong, ductile, and energy-dissipating connection for preacast concrete construction in seismic zones. It was observed that the cast-in-place joints, despite experiencing a localized failure due to the presence of a single major crack, exhibited higher levels of ductility and energy dissipation compared to cast-in-place conventional concrete joints.

Seismic response of concentrically braced steel frames

Abstract: Braced frame structures designed according to the 1990 edition of the National Building Code of Canada and the CSA standard for steel structures (CAN/CSA-S16.1-M89) are analyzed under a number of d...

Cable Anchoring of Deteriorated Arch Dam

Abstract: The Stewart Mountain Dam is a 212 ft (64.6 m) high multicurvature thin-arch dam. The structure was completed in 1930. It has experienced alkali-silica reactions within the concrete and exhibited no bond across horizontal construction lift surfaces. The dam could be subjected to upgraded maximum credible earthquake (MCE) or probable maximum flood (PMF) loadings. Alkali-silica reactions and expansions have caused visible surface cracking. This structure was analyzed for gravity-, reservoir-, temperature-, and earthquake-induced loads. Results indicated an unsafe structure for earthquake conditions. Several measures for prevention of further deterioration and strengthening were considered. Posttensioned cables were selected to provide seismic strengthening. Cable design uses the stiffness, acceleration, and response spectra methods. Vertical posttensioned cables are being installed during 1990–92 construction phase. This paper summarizes various field and laboratory investigations, structural analyses, and design parameters needed for posttensioning a deteriorated arch dam. Posttensioned cables are found to be a viable solution for the dynamic stability of a thin-arch dam. Methodology presented in this paper is applicable to other deteriorated dams.

Contact between Adjacent Structures

Abstract: In this paper, the static contact forces developed between adjacent structures are studied. Activation of a unilateral contact situation between adjacent buildings can be caused by external horizontal loading, e.g., the case of a design static loading representing an earthquake motion as in the pseudostatic method of analysis for earthquake‐resistant structures. By an appropriate finite element model and making use of the simplifying assumption of linear elastic behavior for the buildings, we are led to the formulation of a quadratic programming problem that can be tackled by any Q.P. code available. Some numerical experimentation performed for the horizontal static design loads reveals certain aspects of the interaction problem. In particular, the role of the aseismic gap between the structures is parametrically investigated.

Performance of Unreinforced Masonry Buildings During the Newcastle Earthquake, Australia

Abstract: On December 28, 1989, an earthquake of Richter magnitude 5.6 shook the city of Newcastle. Newcastle is situated on the east coast of Australia only 100km north of Sydney. Eleven deaths and $300-$400 million damage to buildings have been attributed to the earthquake, with the total economic cost to the community estimated to be in excess of $1 billion. This paper presents a summary of the performance of unreinforced masonry buildings in the Newcastle area during the 1989 earthquake. In particular, a 3-storey masonry apartment building which suffered a combined soft-storey and torsional mode of failure is analysed. This example is then used to illustrate several of the short-comings in the current Australian Earthquake Code.

Earthquake Considerations in Design of the Los Angeles Metro

Abstract: This paper discusses the earthquake design of the Los Angeles Metro. The design earthquake parameters are presented and discussed in light of the behavior of buried structures subjected to earthquake movements. The authors describe and illustrate a new design approach whereby the structure is subjected to earthquake deformations rather than pseudo loads. Finally, results of computer modeling showing that the approach produces conservative designs are presented.

Recent advances in seismic design and retrofit of bridges

Abstract: This paper describes the seismic retrofit program implemented by the California Department of Transportation after the 1971 San Fernando Earthquake and accelerated after the 1989 Loma Prieta earthquake. The goal of the program is to reinforce most of the bridges designed and constructed prior to acquiring the current levels of knowledge of seismic loading and structure response (about 1978) to improve their structural ductility and resistance to the major factors which contributed to damage and collapse in the San Fernando and Loma Prieta events. The program has been executed in two phases. The initial phase, completed in 1988, provided reinforcement to superstructures of 1260 bridges by connecting all narrow expansion joint seats with hinge restrainers and anchoring girders and other superstructure elements to the substructure. The single and multiple column retrofit phase, now under active implementation, is designed to reinforce substructure elements and create ductile members by increasing confinement. A Risk Analysis procedure was developed to prioritize the bridges for seismic upgrading so that highest risk bridges are retrofitted first. Legislation was recently enacted to require the Department of Transportation to also take the lead role in seismically retrofitting all locally owned bridges in the state. Research has been conducted, and is currently underway, at the University of California, San Diego to test and confirm the validity of several proposed design solutions for seismic retrofitting of existing bridge single column bent substructure elements. Since the Loma Prieta earthquake additional research is being conducted at the University of California at Berkeley, Irvine and Davis to develop and test retrofit techniques for multiple column bents and double level structures, including abutment and footing details. Some full scale testing was completed on a standing portion of the Cypress Street viaduct in Oakland to determine its fundamental response characteristics and to test some techniques which may be applied to the double deck structures in San Francisco and on complex structures with outrigger supports and other non standard support configurations throughout the state.

Seismic retrofit of bridge columns by steel jacketing

Abstract: Inadequate flexural strength and ductility of shear strength of concrete bridge columns has resulted in collapse or severe damage of a number of California bridges in recent moderate earthquakes. In general these bridges were designed prior to the new seismic design methods which were implemented in the mid-seventies. Bridges constructed in accordance with the new design methods have performed well in recent earthquakes. However, the large number of older bridges that are in service, particularly freeway overpasses designed and constructed in the 1950's to 1970's, are now recognized to have substandard design details and are presenting a cause for major concern. This paper reports the results of a theoretical and experimental program investigating retrofit techniques for circular columns by encasing the critical regions with a steel jacket. The jacket is bonded to the column using grout. Results from six large-scale column tests show that the casing acts efficiently as confinement reinforcement enabling displacement ductility factor of greater than 6 to be achieved. The casing also inhibits bond failures at the laps of longitudinal reinforcement in the critical regions of the column by restraining the dilation and spalling of the cover concrete which degenerates into bond failure. Comparisons of "as-built" and retrofitted columns are presented, and experimental strengths and ductilities are compared with analytical predictions.

Seismic Performance of Low-Rise Steel Perimeter Frames

Abstract: The seismic response arising from strong ground motions of steel frames constructed with moment‐resisting connections causes significant inelastic deformations in the members and connections. The inelastic behavior is dependent upon several design parameters, such as proportioning and selection of members, magnitude of design base shear, and configuration of lateral force‐resisting system. The influence of these parameters was investigated by performing inelastic time‐history analyses of six variations of a structural frame for a typical five‐story building. The direct design procedure adopted in the 1988 edition of the Uniform Building Code was used in the seismic design of the steel frames that were providing all of the lateral stability. The computed inelastic behavior for each frame design is compared with the expected behavior as envisioned by the code. The results reveal the calculated story drifts are larger than the acceptable limits for the structural frames investigated in this study when subjec...

Studies on Seismic Isolation of Buildings

Abstract: This paper has studied the applicability of base isolation to structures. The Navy is evaluating the potential use of base isolation for essential structures that must remain functional in a postearthquake environment. The paper reviews preliminary building‐selection criteria and structural design criteria. For low‐rise regular‐frame construction situated on a rock or stiff site and housing‐sensitive equipment, base isolation of the columns offers the potential for significant damage reduction. The paper presents a comparison of analysis techniques treating the isolator as a nonlinear hysteretic element and as linear elements with and without nodal damping. The results indicate that for the case studied, significant differences in displacement and drift result while moments vary by about 10%. Vertical acceleration effects were found to cause an increase in maximum moment by about 15%.

Device for Control of Building Settlement and for Seismic Protection

Abstract: The device allows controlled transfer of the building loads to the foundation piles, such that the building neither settles nor emerges with respect to the surrounding ground, in areas where the subsoil is highly compressible, as is that of Mexico City. Additionally, the device dissipates energy when the building experiences rocking motion under the action of earthquake forces. The device is fixed to the head of each of the foundation piles, which pass freely through the building base slab. Also, the device is fixed to the base slab, such that it provides, by means of deformable elements that undergo a rolling-bending motion, a variable length connection between foundation piles and building.