International Concrete Abstracts Portal

Current practice in seismic analysis and design is examined in this paper, with particular reference to reinforced concrete structures. The attitude of the paper is deliberately iconoclastic, tilting at targets it is hoped will not be seen as windmills. It is suggested that current emphasis on strength-based design and ductility leads in directions that are not always rational. A pure displacement-based design approach is advanced as a viable alternative. Improvements resulting from increased sophistication of analyses are seen to be largely illusory. Energy absorption is shown to be a mixed blessing. Finally, accepted practices for flexural design, shear design, and development of reinforcement and the philosophic basis of capacity design are questioned.

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.

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.

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.

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.