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

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 multistory frames, the majority of the potential plastic hinge zones are located in the beams. The inelastic rotation, which may develop in these zones, arises predominately from the tensile yielding of the reinforcement. The associated compressive strains are small and, as a consequence, elongation occurs. Test results show that elongation on the order of two to four 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 in this paper. 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.

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.

Problems associated with design of beam-column joints for shear have been studied extensively in many countries. Work in New Zealand on the performance of joints in reinforced concrete moment resisting frames in seismic zones served to alert designers all over the world to consider these problems. Fundamental studies conducted by Paulay and his colleagues and students contributed immeasurably to our understanding of the behavior of joints. However, the approaches used in design codes have not always been the same as those used in New Zealand. The reasons for these differences have much to do with design philosophies, research objectives, and code development procedures. Shear problems at locations other than joints and in elements where rehabilitation (repair and strengthening) is needed to improve performance of structures under earthquake generated deformations still lack definition sufficient for developing code provisions.

A new conceptual code format has been developed for earthquake- resistant design (EQ-RD) of buildings. It consists of guidelines for conceptual overall design of entire building systems and a conceptual methodology for numerical EQ-RD of building systems in compliance with the worldwide accepted EQ-RD philosophy and based on energy concepts, fundamental principles of structural dynamics, mechanical behavior of entire building facilities, and comprehensive design. The numerical EQ-RD methodology considers the desired seismic performance of the entire building system explicitly from the beginning of the EQ-RD process and concludes by evaluating whether such performance would be achieved. A discussion of the main aspects and problems involved in the preliminary numerical EQ-RD procedure is presented in this paper. Main results from its application to at 30-story reinforced concrete space-frame building are discussed and compared to results from analysis of the performance of the same building designed according to 1991 UBC, showing the weakness of present UBC seismic regulations when applied to tall buildings, particularly regarding performance under service level EQ ground motions. The main advantage of the proposed conceptual methodology is that uncertain quantifications of its concepts can evolve without changing the format of the codified methodology as new and more reliable data are acquired.