Showing papers on "Earthquake resistant structures published in 1995"

Performance of Hybrid Moment-Resisting Precast Beam-Column Concrete Connections Subjected to Cyclic Loading

Abstract: Test results of 10 hybrid precast concrete beam-to-column connections are presented. These tests constitute Phase IV of an experimental program on one-third-scale model precast moment-resisting connections conducted at the National Institute of Standards and Technology (NIST). The objective of the test program is to develop guidelines for the design of moment-resisting precast connections in regions of high seismicity. The hybrid connections consist of mild steel used to dissipate energy and post-tensioning (PT) steel used to provide the required shear resistance. Variables examined were the amount and type of mild steel (ASTM A 615). The amount of post-tensioning steel was varied to control the relative moment capacity contributed by the PT and mild steel. The specimens were subjected to reversed cyclic loading in accordance with a prescribed displacement history. Connection performances were compared to previous NIST tests based on energy dissipation capacity, connection strength, and drift capacity. Hybrid precast connection can be designed to match or exceed the performance of a monolithic connection in terms of energy dissipation, strength, and drift capacity

Performance of Buildings With Shear Walls in Earthquakes of the Last Thirty Years

Abstract: The author describes observations of shear wall performance in severe earthquakes in which modern reinforced concrete buildings stood the test of violent shaking, starting with the Chilean earthquake of May 1960 through most of the subsequent strong earthquakes, up until the Armenian earthquake of December 1988. Despile the excellent behavior of shear wall-type concrete structures as compared to concrete frame-type structures, building codes continued, up until the last decade, to give preference to concrete ductile frame structures (which are subject to higher distortions) while placing a substantial penally on the use of shear walls. This code approach was due to the lack of experimental and analytical background information on shear wall behavior. While a large body of information on shear walls accumulated during the 1980s, still more experimental and analytical studies are needed to create a solid basis for a rational seismic design approach. The availability of such information should encourage a wider use of shear walls for earthquake resistance

Seismic retrofit of reinforced concrete buildings designed for gravity loads: performance of structural model

Abstract: A partially prestressed concrete alternative and two masonry retrofit alternatives are analyzed for improving the local and global response performance of reinforced concrete frame structures designed only for gravity loads and constructed in low-to-moderate seismic zones. The objective of these retrofits is to reconfigure the structural failure mode by averting a catastrophic soft-story collapse and enforcing a more ductile beam sidesway mechanism. A prestressed concrete jacketing alternative to limited columns was selected to retrofit the one-third scale model reinforced concrete frame building that had been previously tested by the authors. The seismic response of the retrofit model structure was investigated experimentally using the "shaking table" at State University of New York at Buffalo. A comparison is made of the experimental frame response before and after retrofitting. It is concluded that the seismic performance of vulnerable (soft-story prone) gravity load design frames may be effectively enhanced by strengthening limited columns and thus, changing the failure mode to a beam sidesway mechanism with acceptable story deformation levels.

Gravity-Load-Designed Reinforced Concrete Buildings--Part I: Seismic Evaluation of Existing Construction

Abstract: The seismic performance of nonductile reinforced concrete frame buildings in regions of low to moderate seismicity is evaluated. Several significant aspects of nonductile detailing are modeled using rotional simplifications of expected member behavior at critical sections to facilitate a complete inelastic time history analysis of the system. The detailing configurations included in the analysis are: discontinuous positive flexural reinforcement, lack of joint shear reinforcement, and inadequate transverse reinforcement for column core confinement. Seismic evaluations of three-, six-, and nine-story buildings are carried out under low- to moderate earthquake excitations. The essential parameters of the response are presented with a view to identifying vulnerability of such buildings to a potential seismic design event

Design of connections of earthquake resisting precast reinforced concrete perimeter frames

Abstract: Moment resisting perimeter frames are an excellent means of providing the earthquake resistance in medium height multistory buildings. These frames can be constructed using precast reinforced concrete elements designed to emulate conventional cast-in-place construction. Construction of earthquake resistant buildings incorporating ductile precast reinforced concrete perimeter frames is widespread in New Zealand. The main problem in the design and construction of structures incorporating precast concrete elements to provide seismic resistance is in finding economical and practical methods of connecting the elements. This paper discusses the design and detailing of the connection region between members of earthquake resisting perimeter frames.

Development and testings of a ductile connector for assembling precast concrete beams and columns

Abstract: The precast concrete industry has attempted for many years to develop connectors that will perform well during an earthquake. This article describes the development of an energy absorbing ductile connector that can be used to construct a seismic moment resisting frame of precast concrete components that will outperform comparable cast-in-place and structural steel systems. The system was developed by the author working in conjunction with Dywidag-Systems International. The design philosophy, test program, and adaptability of the connection system to real structures are described in detail. This paper is a companion to the largely non-technical article that was recently published in the September-October 1994 PCI Journal.

Reinforced Concrete Eccentric Beam-Column Connections Subjected to Earthquake-Type Loading

Abstract: Test results for four reinforced concrete eccentric beam-column connections subjected to reversed cyclic loading are reported. The specimens were part of an experimental program designed to investigate the effect of torsion, induced by earthquake motions, in eccentric frame connections. The major design parameters varied in the specimens were the beam width, the beam depth, and the amounts of top and bottom longitudinal reinforcement in the beam. From the test results, it was concluded that the presence of eccentricity in the joint reduced the strength of the connection to the extent that none of the specimens were able to attain their predicted story shear strength. To account adequately for the effect of torsion, a new equation for the effective joint width to estimate the nominal joint shear strength is proposed

Elongation in ductile seismic-resistant reinforced concrete frames

Abstract: To survive a major earthquake, current practice requires seismic resistant frames to be designed to be ductile. To achieve the required level of ductility in multi-storey frames, the majority of the potential plastic hinge zones are located in the beams. The inelastic rotation, which may develop in these zones, arises predominantly from the tensile yielding of the reinforcement. The associated compressive strains are small and as a consequence elongation occurs. Test results show that elongations of the order of 2 to 4 percent of the member depth develop in plastic hinge zones of beams subjected to cyclic loading before strength degradation occurs. The factors influencing elongation are reviewed. The results of a time history analysis, in which elongation effects are modeled, shows that this action, which is neglected in current design practice, has important implications for the detailing of columns and the design of supports for precast components and external cladding.

Seismic retrofit of beam-to-column joints with grouted steel tubes

Abstract: This paper presents the findings of an experimental study to evaluate a method of retrofit which addresses a particular weakness that is often found in reinforced concrete structures, especially older structures, namely the lack of sufficient reinforcement in and around beam-to-column joints. Many of these structures lack the required confining reinforcement within the joints and in adjoining beams and columns. The result is a reinforced concrete frame that is weak in the joint areas and lacks sufficient ductility during a seismic event. The proposed retrofit method consists of encasing the reinforced concrete joint with a grouted steel jacket that provides confinement to the joint area, and imparts ductility to the frame. In this study, two styles of retrofit jacket were tested: a circular steel tube and a rectangular casing. The two jacket styles are evaluated for strength, stiffness and ductility, and their relative merits are discussed.

Seismic isolation in bridges

Abstract: In the U.S., the use of seismic isolation in new bridge construction and retrofit projects has been growing as awareness of seismic risk has spread across the country since the 1971 San Fernando earthquake through the 1989 Loma Prieta and 1994 Northridge earthquakes--not to mention the false, but nonetheless consciousness-raising, prediction of a major earthquake on the New Madrid Fault in 1990. To date, isolator bearings have been used to outfit about 75 structures, of which slightly more than half are retrofit projects. Recent projects in New York State, St. Louis, Indiana and New Hampshire that involve the use of several systems demonstrate the use of seismic isolation in retrofitting bridges to better withstand earthquake forces.

Dynamic ductility demand and capacity design of earthquake-resistant reinforced concrete walls

Abstract: Reinforced concrete structural walls may provide efficient earthquake-resistance in multi-story buildings. In Europe they are commonly combined with gravity load dominated slender columns whereby the entire horizontal action is taken by the walls. The major findings of this paper pertain to three important design aspects as follows: (1) the dynamic rotational ductility demand may have a different distribution over various height to length aspect ratios of the wall than previously anticipated by static analysis. (2) the dynamic bending moment demand over the height of the wall may differ from the static assumption depending on the aspect ratio of the wall. This necessitates a modified moment capacity distribution. (3) the dynamic shear force at the base of the wall may exceed the previous assumptions of the capacity design method.

Earthquake-Resistance Design of Shearwalls With One Opening

Abstract: Shearwalls with one opening have been designed according to the Architectural Institute of Japan (AIJ) standard for structural calculation of reinforced concrete structures using allowable stresses. However, effective reinforcement details have been developed by current research in the search for a new and rational design method. This paper reports the outline of earthquake-resistant design of shearwalls with openings according to a method based on ultimate capacity.

Ductility of columns, wall, and beams -- how much is enough?

Abstract: Two hypothetical reinforced concrete buildings (one with special moment resisting frames and the other with structural walls) were designed. Using a time-history inelastic behavior approach, both buildings were analyzed. Drifts were determined for these structures when subjected to severe earthquakes similar to those expected in North America. In addition, drifts associated with an analysis based on ground motions measured for the 1985 Mexico City earthquake were also determined. Measured drifts from components detailed under 1990s North American code requirements are compared with calculated building drifts. These comparisons indicate that 1990s code requirements provide significantly more capacity than calculated to be needed for the structures and components considered. Finally, minimum drift requirements for components to be used in ductile frame buildings and in shear wall buildings are suggested.

Controversial Issues in the Seismic Design of Connections in Reinforced Concrete Frames

Abstract: This paper discusses aspects of the design of connections in reinforced concrete frame structures which often get overlooked. Detailed analysis of beam-column joint regions using the modified compression field theory demonstrates behavioral features that have important design implications. The use of non-linear finite element modelling of joint regions to design efficient, yet practical, retrofit measures is discussed. An alternate form of construction using ductile steel link beams to connect reinforced concrete walls is presented. The important design features for the connection of these beams to the walls are highlighted.

Performance Evaluation of a Dual-Level Design Using 1994 Northridge Earthquake Records

Abstract: National Earthquake Hazards Education Program under National Science Foundation Grant CMS 94-15726

Japanese 'presss' design guidelines for reinforced concrete buildings

Abstract: Synopsis: The paper briefly introduces an ultimate strength design method for reinforced concrete buildings on the basis of the capacity design concept. A design guideline was developed in Japan as a part of the U.S.-Japan PRESSS (Precast Seismic Structural System) project. The design for earthquake loading is specified for the serviceability limit state and ultimate limit state. This paper introduces the concept of earthquake resistant design for the ultimate limit state using a nonlinear static analysis under monotonically increasing force.

Lessons from the kobe quake

Abstract: The port city of Kobe, Japan, violently shook for 20 seconds on January 17, 1995, when an earthquake registering 7.2 on the Richter Scale struck. This particular earthquake was significant because it shattered many notions about seismic zones, earthquake resistance, and the adequacy of recovery plans. The Federal Highway Administration (FHWA) has worked for over two decades to create new sets of earthquake design criteria, codes, guidelines, and specifications based on the findings of actual earthquake investigations. These investigations yield valuable information for developing new technologies to promote structural resistance to earthquakes. This article describes the findings of a team of structural engineers that investigated 15 bridge sites in Kobe, Japan, after the catastrophic January 1995 earthquake. Highway bridges built 30 years ago before modern seismic codes suffered the most damage; although, some new bridges suffered serious damage, leading investigators to re-evaluate current design loads and procedures in the design specifications. Damage to bridge columns was spectacular, however, the incidence of bearing failure was high, pointing to the need for better damping devices and isolation bearing designs to decouple energy and motion. A long list of lessons learned from the Kobe earthquake is generated and a discussion presented regarding the vulnerability of many unsuspecting cities in the United States. The authors conclude by stating that the most cost-effective, long-term program for mitigating earthquake damage to highway bridges is to ensure that new construction is designed beyond existing specifications in accordance with the latest technology derived from investigations of real-world disasters. Retrofitting is not always the answer.