International Concrete Abstracts Portal

The difficulties in assessing the probable deformation and force states of structural wall buildings under lateral earthquake forces were evaluated by means of laboratory test results from a five-story full-scale reinforced masonry structural wall research building tested to failure at the University of California, San Diego, under simulated seismic loads. The individual structural components and actions which contribute predominantly to the seismic response characteristics of a structural wall building, such as axial loads on walls, coupling between structural walls, lintel beam force, and deformation capacities, as well as floor and wall-flange participation were evaluated based on individual component tests, the full-scale prototype test, and parallel diagnostic analyses. The importance of a realistic assessment of these parameters in a capacity design approach for structural wall buildings was evaluated. The outline for rational design models which allow a prediction of the complex behavior characteristics of structural wall buildings for all design limit states is presented in this paper.

The M7.8 earthquake which hit the Philippines in July 1990 caused extensive and varied damage to a wide range of structures, most of which were of reinforced concrete. Because U. S. codes are adopted in the Philippines, the event provides a unique opportunity for earthquake engineers worldwide to review their approaches to seismic design. This paper results from the author's involvement in a visit immediately after the event and his subsequent role, in 1991 and 1992, advising the Philippine government on reconstruction of damaged public buildings and infrastructure. Valuable insights into the real issues were gained through contact local consultants, government engineers, and government agencies, such as the Departments of Health and Education. The government's Earthquake Reconstruction Project is outlined and the effects of the earthquake briefly described as an introduction to the main issues: structural concepts, ductile detailing, construction practice and supervision, influence of "nonstructural" elements, and the value of site investigations. Examples are given to illustrate these issues in the Philippine context. The author concludes that proper attention to the basics is sufficient to significantly reduce earthquake risk, not only in the Philippines, but in many developing and other countries. In this International Decade for Natural Disaster Reduction, this has special relevance.

Waffle-flat plate buildings are very popular in different countries. Their seismic performance has been very poor. Several research projects on the seismic response of these buildings have been performed at the Instituto de Ingenieria, UNAM; their results and findings are summarized in this paper. First, the main features of the structural system and of its resistance to lateral loads are presented. The most common mechanisms of collapse are described and the observed performance during the 1985 Mexico earthquake is discussed. A case study of building performance during the earthquake is briefly presented. Results of an experimental research on a two-story model of a waffle-slab building are described. The specimen was first tested in a haking table and later subjected to cycles of static lateral loads. The specimen showed a rather poor behavior with very small lateral stiffness and limited energy dissipation capacity. The failure mechanism was mainly governed by the shear and flexural strengths of the columns and by flexural cracking in waffle slabs. Recommendations for the proper use of the system are given, emphasizing the need to combine it with structural walls, bracings, or stiff frames to achieve the necessary lateral stiffness and strength in a building.

Earthquake-resistant design of reinforced concrete structures has special problems in moderate seismic zones if the possibility of a very large rare earthquake exists. This is the situation in central and eastern North America. The questions and difficulties associated with introducing a seismic design code for the first time are discussed. The seismic risk to a populated region is not reduced much for many years after the code takes effect; only the rehabilitation of existing structures will reduce the risk significantly in a meaningful time frame. The overall behavior of buildings, especially of existing older reinforced concrete buildings, is often nearly elasto-plastic in nature because a mechanism forms soon after the formation of the first hinge and there is little or no overstrength. This may not be an optimum design in most cases. The response of reinforced concrete buildings to moderate ground motions designed only for gravity loads is better than expected, with moderate drifts and no premature brittle failures in most building types. That is not the case for the rare catastrophic earthquake.

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 circular steel jacket provided direct confinement as well as a ductile force transfer mechanism through the jacket itself, but it was more difficult and expensive to fabricate than the rectangular casing. Although the rectangular jacket did not provide the same amount of concrete core confinement, it seemed to be sufficient to prevent damage in the joint area. The load transfer mechanism of the rectangular jacket was found to be adequate in withstanding the loads and deflections typical for seismic events. In the paper, the two jacket styles are evaluated for strength, stiffness, and ductility and their relative merits are discussed.

The method of transversely reinforcing columns and beam-to-column connections with bellows square steel tubes was devised to develop a construction method necessary to realize reinforced concrete frame highrise buildings which are easy to design and execute in zones where high earthquake resisting performance is required. To secure a ductile seismic behavior for columns subjected to heavy load, strong shear reinforcement and transverse reinforcement are necessary to prevent brittle failure, such as shear failure, bond split failure along the longitudinal bars, and failure of the compressed extreme fiber of concrete, or to change it into ductile failure. It was manifested by concentric compression tests of 1/4 scale columns, combined compression, bending and shear tests of 1/3 scale columns, seismic load tests of 1/3 scale and 1/4 scale beam-column subassemblages, and bond tests of main bars embedded in 1/4 scale columns that no dangerous collapse of the building is likely to occur even if shear forces of some of the columns and/or beam-to-column connections in the same story reach the loading capacity, because the mechanical behavior of the columns and beam-to-column connections is very ductile even when the webs of their tube yield in shear. Field execution tests of this structure have been conducted.

Describes the essential features of the "modified compression field theory." A group of behavioral models based on these assumptions is presented. The use of these models is illustrated and reference is made to experimental data and to existing design codes. A simple, unified design method for shear that is able to approach both routine and unusual design problems is presented. The method is applicable to both prestressed and nonprestressed concrete members. It treats members subjected to either axial tension or axial

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 1990's North American code requirements are compared with calculated building drifts. These comparisons indicate that the 1990 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 shearwall buildings are suggested.

Summarizes the developments and changes to the seismic design provisions of the National Building Code of Canada (NBCC 1990) since its 1977 edition and discusses the changes to the seismic design provisions of the Canadian Standards Association Standard, Design of Concrete Structures for Buildings (CAN3-A23.3). The paper outlines the philosophy of seismic-resistant design of the Canadian code and discusses the changes to the zoning maps, static design procedures, and the introduction of the force modification factors. The paper also deals with the changes to the Canadian reinforced concrete code and especially with the section on ductile walls, changes to load combination factors, and the explicit links between the concrete code sections containing the detailing requirements and the NBCC sections on determination of the design seismic forces.

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.