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Composite and Hybrid Structures (CHS) provide many advantages over those employing conventional systems. Since the available design codes in Japan do not cover these systems, the current Japanese Building Standard Law requires detailed experimental and/or analytical studies before the building permit for a structure with CHS can be issued. As part of the U.S.-Japan Cooperative Earthquake Research Program on Composite and Hybrid Structures, three major types of composite and hybrid systems are being studied. These include concrete-filled tubular columns (CFT), reinforced concrete columns with steel beams (RCS), and reinforced concrete cores with steel perimeter frames (HWS). Among other objectives, these studies are expected to lead to design guidelines which can be incorporated in the Building Standard Law. The availability of these guidelines are expected to encourage the Japanese designers to use CHS more frequently. This paper provides an overview of the ongoing studies in Japan. After a brief historical review of each system, the important issues related to each system are summarized. Planned and ongoing studies for each group of composite systems are also described.

Diagonal Mar Centro Comercial is a 350,000 square meter (3.75 million square foot) mixed use commercial development in Barcelona, Spain. It is located at the terminus of the premier boulevard in Barcelona, avenida Diagonal and will be known as Diagonal Mar - the avenue by the sea. The project is located only two hundred meters from the shores of the Mediterranean Sea and approximately ten kilometers east of the Olympic Village - home to the athletes in the 1992 Olympic Games. A commercial venture of Diagonal Mar S.A., the first phase consists of a 165,000 square meter (1.75 million square feet) retail mall and hypermarket (the largest in Spain) and six levels of underground parking for 5,100 automobiles in 185,000 square meters (2.0 million square feet) of space. Construction cost for Phase I is estimated to be $180,000,000 US dollars. Residential housing and office buildings are planned for later phases of the project. The site is a very large triangular plot bounded by the extension of avenida Diagonal on the northwest, avenida Josep Pla on the west and avenida Taulat on the south. The sides of the triangle are approximately 333 meters (1,072 feet) along avenida Diagonal, 285 meters (935 feet) along avenida Taulat and 236 meters (774 feet) along avenida Joseph Pla. The 24 meter (79 feet) deep excavation required for the underground parking, located 18 meters (59 feet) below the shallow water table, will create the largest basement substructure in the world and will remove more than 1 .O million cubic meters (1 .3 million cubic yards). The sheer size of the project and its location so close to the sea posed a whole host of enormous engineering challenges for the design and construction planning team as follows: (1) Excavation retention method; (2) Foundation system selection and design; (3) Excavation/substructure construction method and sequence, (4) Substructure system selection and design.

Design of composite structures for earthquake loading has to address different problems to static design, as the advantageous greater damping may be offset by the disadvantage of increased mass and stiffness, leading to higher seismic loads. However, since composite construction is used extensively, especially for high-rise construction, the seismic performance of this form of structure requires investigation and the development of specific design guidance. European work over the past ten years or so confirmed that, with minimum design and detailing alterations, composite structures offer a most economical and reliable design alternative to steel and reinforced concrete structures. This paper reviews some of the European work on composite members. Particular emphasis is placed on work at Imperial College, since this was mostly carried out by the writer and his co-researchers. The work on a novel type of composite member is described,with special emphasis on ductility-based design recommendations. This is followed by a discussion of the role of composite beam-column connections and beam members in providing lateral stiffness, resistance and energy dissipation. Hierarchical assessment limit states are defined and are used to arrive at earthquake yield and ultimate response accelerations. These are used to calculate analytical behaviour factors of typical composite frames, which are shown to be more economical than steel frames designed for the criteria. Finally, brief comments regarding current and future work on seismic resistance of composite structures in Europe are given.

Equivalent macromodel-based analytical tools, comprised of flexibility-based element models, are used to accurately represent the non-linear moment-curvature (force-deformation) response characteristics in structural systems using columns of reinforced concrete (RC) or composite steel sections encased in reinforced concrete (SRC), structural steel beams, and composite beam-column joints. To facilitate the modeling of inelastic deformations in joint regions, a panel element capable of representing joint shear distortions and joint bearing deformations was developed and incorporated into an existing computer program, IDARC. The inelastic shear-deformation characteristics of the joint panel were partly established from guidelines published by an ASCE Task Committee (1). Various hysteretic control parameters for members of the subassemblage, such as strength degradation, stiffness deterioration, and pinching (slip), were quantified based on observed experimental response. Potential failure modes in the steel beam, RC or SRC column, and composite joint of the frame subassemblage can be represented in the proposed formulation. Experimental subassemblage testing performed at Cornell University (2) was used to validate the analytical platform. It is shown that the revised IDARC program can be used for seismic evaluation of composite structures and for development of design guidelines to ensure desirable mechanisms in RCBRC structures.

A five-year research program on Composite and Hybrid Structures as Phase 5 of the U.S.-Japan cooperative Earthquake Research Program was recommended to be initiated in 1993 in both countries. Presented in this paper is a summary of the research program which is based on a number of technical meetings of the U.S. and Japan Planning Groups and a Joint Planning Workshop. Because of diverse and broad scope of the subject area, the research program is organized into the following four groups: New Materials, Elements and Systems; Concrete Filled Tube Column Systems; Reinforced Concrete (RC) and Steel Reinforced Concrete (SRC) Column Systems; and RC/SRC Wall Systems. A theme structure with well selected layout, geometry and design loads for the research is also presented, which provides a common focus for various systems to be studied, and also a common prototype structure from which the components and sub-assemblages are drawn.