Showing papers on "Earthquake resistant structures published in 1972"

Mechanisms of energy absorption in special devices for use in earthquake resistant structures

Abstract: A structure designed to resist earthquake attack must have a capacity to dissipate kinetic energy induced by the ground motion. In most structures this energy absorption is developed in the vicinity of beam to column connections. Recent research has shown that connections are not reliable when subject to cyclic loading, such as results from earthquake attack. Connections in steel frames deteriorate due to local instabilities in adjacent flanges, and in reinforced concrete frames alternating shear loads produce diagonal tension and bond failures which progressively reduce the strength of the connection. Much work in building research and earthquake engineering in laboratories throughout the world is directed toward increasing the reliability and energy absorption capacity of structural connections. In this paper an alternative approach to this problem is described. This approach is to separate the load carrying function of the structure from the energy absorbing function and to ask if special devices could be incorporated into the structure with the sole purpose of absorbing the kinetic energy generated in the structure by earthquake attack. To determine whether such devices are feasible a study has been undertaken of three essentially different mechanisms of energy absorption. These mechanisms all utilized the plastic deformation of mild steel. They included the rolling of strips, torsion of square and rectangular bars, and the flexure of short thick beams. These mechanisms were selected for intensive study since they were basic to three different types of device each of which was designed for a separate mode of operation in a structural system. The characteristics of these mechanisms which were of primary importance in this study were the load displacement relations, the energy absorption capacity and the fatigue resistance. This information was obtained with a view to the development of devices for specific structural applications. This report describes the tests used to explore the basic mechanisms and the data obtained. It also include s a brief description of tests on scale models of a device which was designed to be located in the piers of a reinforced concrete railway bridge. It has been shown by the tests that the plastic torsion of mild steel is an extremely efficient mechanism for the absorption of energy. It was found that at plastic strains in the range 3% to 12% it was possible to develop energy dissipation of the order of 2000-7500 lb in/in3 per cycle (14-50 x 106 N/M2 per cycle) with lifetimes within the range of 1000 to 100 cycles. It was also shown that the mode of failure in torsion is an extremely favourable one for use in an energy absorbing device in that it took the form of a gradual decay. The other two mechanisms studied were both less efficient and less reliable than torsion and had capacities of 500-2000 lb in/in3 per cycle (3.5 - 14 x 106 N/M2 per cycle) and life times of around 200 to 20 cycles. Nevertheless they lend themselves to more compact devices than does the torsional mechanism and furthermore the devices may be located in regions in a structure where they are readily accessible for replacement after attack.

Spectrum-Compatible Motions for Design Purposes

Abstract: A practical method is presented to obtain synthetic motions that closely match a reasonable specified deisgn spectrum for a seismic design of important earthquake-resistant structures and equipment. The principle of this method is to modify existing motions. For this purpose, a spectrum-suppressing technique and a spectrum-raising technique are developed. Examples are then given to illustrate their applicability to actual structure design.

Soil Conditions and Building Damage in 1967 Caracas Earthquake

Abstract: Although the magnitude of the 1967 Caracas earthquake was only 6.4 and its epicenter was located about 35 miles from Caracas, the shaking caused the collapse of four 10-story to 12-story apartment buildings in Caracas, with a loss of over 200 lives, and the partial collapse (top 4 stories) of a 12-story building at Carabelleda. Detailed studies were made of the distribution of structural damage to buildings in different story height ranges, the depth and dynamic characteristics of soils throughout the area, and the response of typical soil deposits during the earthquake. Both empirical and analytical evidence indicate that the locations of zones of heavy damage may be attributed to unfavorable combinations of soil conditions and building characteristics which resulted in particularly strong response of the damaged structures. Of particular significance was the finding that recently developed analytical techniques can predict the general pattern of damage in the 1967 earthquake.

Seismic design of building structures

Abstract: THE PURPOSE OF THIS PROJECT WAS TO FORMULATE POSSIBLE IMPROVEMENTS IN THE DESIGN CODE CONCERNING BUILDING STRUCTURES WHICH ARE SUBJECTED TO EARTHQUAKE LOADS. IN ADDITION, SUGGESTIONS WERE MADE CONCERNING WAYS TO INCORPORATE THE CONCEPTS AND METHODS OF DISCRETE MECHANICS, STATISTICAL ANALYSIS, AS WELL AS EARTHQUAKE BEHAVIOR OF CONCRETE AND METAL STRUCTURES INTO THE SEISMIC DESIGN CODE. AVAILABLE LITERATURE ON EXISTING DESIGN CODES IS SUMMARIZED HEREIN. DESIGN PHILOSOPHIES AND METHODOLOGIES IN EARTHQUAKE ENGINEERING WERE ALSO STUDIED. A "DESIGN-TREE" TECHNIQUE WAS THEN ADAPTED TO PRESENT THE RECOMMENDED IMPROVEMENTS IN THE CONTEXT OF EXISTING SPECIFICATIONS. MOREOVER, APPROACHES TO CERTAIN EXPECTED PROBLEMS FOR THE IMPLEMENTATION OF THESE IMPROVEMENTS WERE OUTLINED ALONG WITH A SET OF RECOMMENDATIONS. FURTHERMORE, LITERATURE REVIEWS AND SOME ORIGINAL CONTRIBUTIONS IN DISCRETE MECHANICS, STATISTICAL METHODS, AND DYNAMIC BEHAVIOR OF STRUCTURES ARE GIVEN IN THREE APPENDICES AS BACKGROUND INFORMATION. A SIMPLE ILLUSTRATION OF THE DIRECT APPROACH TO SEISMIC DESIGN IS CONTAINED IN THE FOURTH APPENDIX.

Earthquake resistant design of unbraced frames

Abstract: A seismic design procedure which forces all significant inelastic behavior into the girders of moment resistant frames is presented. A prescribed response spectrum and modal analysis are used to determine the seismic design forces. The members of the frame are proportioned using ultimate strength concepts. Important design parameters are the expected girder ductility requirement and the rate of strain hardening. Ten story single bay frames are designed according to the proposed procedure and according to the standard allowable stress procedure. These frames are subjected to two strong motion earthquakes and the inelastic response is evaluated.

Expected fatigue damage of seismic structures.

Abstract: Whenever a structure is subjected to strong-motion earthquake excitation, certain parts of the structure may undergo plastic deformations which are indications of structural damage. It has also been known that the damage due to repeated applications of plastic deformation is cumulative and could cause low-cycle fatigue failures. Nevertheless, fatigue problem is not being considered in the present design codes in earthquake engineering. Because of the low number of repeated plastic deformations involved, as well as the rare occurrences of strong earthquakes, attention is given to only low-cycle fatigue in this study. The linear cumulative damage criterion is used along with the counting process derived by Middleton. A method of computing the statistics of the cumulative low-cycle-fatigue damage of structures to earthquake excitation is presented herein. As examples, the expected cumulative damage of a structural member to three types of earthquake excitation is computed numerically at various magnitudes.

Multiphase cross bracing in earthquake resistant structures

Abstract: The concept of multiphase cross bracing is discussed and tests undertaken on a simple cross braced frame are described. From these experiments it is concluded that selected multiphase response may be achieved reliably in cross braced frames if sufficient attention is given to the determination of the material characteristics and to the structural detailing.

Performance of bridges during san fernando earthquake

Abstract: DURING THE 1971 SAN FERNANDO EARTHQUAKE SEVEN BRIDGES WERE EITHER DESTROYED OR SO BADLY DAMAGED THAT THEY HAD TO BE DEMOLISHED, WHILE AN ADDITIONAL 60 SUFFERED MODERATE TO EXTENSIVE DAMAGE. THE TWO PRINCIPAL REASONS FOR THE SEVERITY OF DAMAGE WERE THAT THE HORIZONTAL AND VERTICAL ACCELERATIONS WERE MANY TIMES GREATER THAN ANY PREVIOUSLY RECORDED, AND THAT THE DAMAGED BRIDGES WERE IN THE AREA OF GREATEST INTENSITY. BRIDGE DESIGN IN THE CALIFORNIA DIVISION OF HIGHWAYS IS BASED ON ACCELERATION RECORDS OF THE EL CENTRO EARTHQUAKE OF 1940 AND INCORPORATES FACTORS NOT ACCOUNTED FOR IN THE AASHO SPECIFICATION. BRIDGES BUILT TO THE CALIFORNIA SPECIFICATION AND 3--4 MILES FROM THE CENTER OF THE EARTHQUAKE EXPERIENCED NO NOTICEABLE DAMAGE. FAILURE MECHANICS IN THE AREA OF HEAVIEST DAMAGE ARE EXPLAINED, AND DESIGN LESSONS LEARNED FROM THE EARTHQUAKE ARE RECITED.

American practice in seismic design

Abstract: SEISMIC DESIGN PRACTICE IS REVIEWED AGAINST THE UNDERLYING ASSUMPTIONS OF THE BASIC SEISMIC DESIGN EQUATIONS IN THE "UNIFORM BUILDING CODE." PRACTICE AND ASSUMPTIONS ARE TREATED UNDER THE TOPICAL HEADINGS DESIGN CRITERIA, DYNAMIC ANALYSIS, LATERAL DUCTILITY, AND ROTATIONAL DUCTILITY. A SECTION ON TEST RESULTS DEALS WITH THE BEHAVIOR OF PRESTRESSED CONCRETE AND WITH THE BEHAVIORAL DIFFERENCES BETWEEN PRESTRESSED AND REINFORCED CONCRETE. FROM TESTS AND STUDY OF THE ENGINEERING PROPERTIES OF PRESTRESSED CONCRETE, IT IS CONCLUDED THAT UNDERREINFORCED PRESTRESSED SECTIONS CAN SUPPLY THE DUCTILITY IMPLIED IN CODE PROVISIONS TO RESIST CATASTROPHIC EARTHQUAKES WITH AMPLE RESERVE CAPACITY. IN THE RANGE OF SEISMIC DISTURBANCES SLIGHTLY GREATER THAN MODERATE, PRESTRESSED CONCRETE CAN PROVIDE THE NECESSARY DEFORMABILITY AND STILL RESPOND ALMOST ELASTICALLY.

Hindsight and foresight on the performance of prestressed concrete bridges in the san fernando earthquake

Abstract: THE 1971 SAN FERNANDO EARTHQUAKE WAS UNUSUAL IN THE MAGNITUDE OF THE VERTICAL MOTION. ONE RESULT OF THIS WAS FAILURE OF BRIDGE COLUMNS IN THE MIDDLE, AS IF CRUSHED RATHER THAN SHEARED. IN GENERAL, BRIDGE FAILURE WAS THE RESULT OF DESTRUCTION OF SUPPORT STRUCTURES AND CONNECTIONS. HINGE AND EXPANSION JOINTS CAME APART, ABUTMENT FILL SETTLEMENT RESULTED IN APPROACHES DROPPING AWAY AND IN FORCES AGAINST THE STRUCTURE ITSELF, PULVERIZATION OF BLOCK FOOTINGS ALLOWED COLUMN BARS TO PULL FREE. SOLUTIONS TO THESE PROBLEMS INCLUDE: STRONGER COLUMN WRAPPING, TIEING OF EXPANSION JOINTS, TOUGHER FOOTINGS, AND BRIDGE DESIGN TO WITHSTAND DYNAMIC STRESSES. BRIDGES CAPABLE OF WITHSTANDING EARTHQUAKES WITHOUT DAMAGE ARE CONSIDERED UNFEASIBLE; RATHER, BRIDGES SHOULD BE DESIGNED TO BE EARTHQUAKE RESISTANT, ELIMINATING THE FATALITIES CAUSED BY COLLAPSE OF THE ENTIRE STRUCTURE.

Japanese practice in seismic design of prestressed bridges

Abstract: DAMAGE TO AND RESISTANCE OF PRESTRESSED BRIDGES ARE RELATED TO DESIGN PRACTICE, WHICH IN TURN IS COMPARED WITH THE JAPANESE CODE VARIOUS TYPES OF PRACTICAL EARTHQUAKE- RESISTANT MEASURES ARE DESCRIBED DYNAMIC ANALYSES ON CONTINUOUS BRIDGES EQUIPPED WITH DAMPERS DEMONSTRATED THEIR EFFECTIVENESS IN AVOIDING EXCESSIVE AMPLIFICATION OF DYNAMIC RESPONSE IN THE LONGITUDINAL DIRECTION VISCOUS DAMPERS ARE EFFECTIVE ONLY IN THE LONGITUDINAL; DYNAMIC RESPONSE IN THE TRANSVERSE DIRECTION COULD BE 2--3 TIMES GREATER THAN THE JAPANESE CODE RESPONSE