Recent Findings from Phase V of the United States–Japan Cooperative Earthquake Research Program

This special issue is a compilation of papers that documents research results from the recently completed United States-Japan cooperative earthquake research program on steel-concrete composite and hybrid structures ~CHS!. The issue has a variety of papers that fall into the following three main categories: hybridwall systems ~HWS!, concrete-filled tube ~CFT! systems, and reinforced-concrete steel ~RCS! systems. A forum by Subhash Goel, the American coordinator of the program, leads off the issue. The forum briefly describes the history of the United States-Japan cooperative earthquake research program and highlights some of the main products of the CHS phase of the research. For example, the findings from the research have resulted in design guidelines, especially for CFT and RCS structures in Japan, where design and construction of those systems has become more common. In the United States, this has not yet happened, but it is expected that a similar trend will occur. The first technical paper by Spacone and El-Tawil is also general in nature and discusses the current state of the art of nonlinear analysis of steel-concrete composite structures. The paper addresses a variety of topics including section, connection, frameelement, and system-modeling techniques. Modeling applications to the analysis of composite steel-concrete frames are also presented. ‘‘Seismic behavior and design of high strength square concrete-filled steel tube beam-columns’’ by Varma et al. is the first of eight CFT papers. The writers conducted tests in which reversed cyclic moment was applied to axially loaded CFT columns of high-strength steel. The tests are designed to investigate the effect of a variety of geometric and material parameters and provided information that is used to critique the American Concrete Institute and modified Architectural Institute of Japan design methods. Sakino et al. in ‘‘Behavior of centrally-loaded concretefilled steel-tube short columns’’ conducted a battery of tests to investigate the synergistic interaction between the steel tube and filled concrete. The parametric tests provided information about the influence and relative importance of several design variables and are used to propose a design model for estimating the axial compressive strength of both square and circular CFT columns. The proposed model accounts for size effect, concrete strength increase due to confinement, and reduction in the tendency for the steel tube to buckle locally as a result of the concrete infill. In ‘‘Behavior of concrete-filled steel tube beam-columns’’ by Inai et al., test results are presented that shed light on the behavior of CFT beam columns constructed with different grades of steel and concrete and subjected to different loading regimes. Computer models are also presented and their results are shown to compare favorably with the experimental results. Fujimoto et al. present more test results for CFT columns under eccentric loading in ‘‘Behavior of eccentrically loaded concrete-filled steel tubular columns.’’ They used their parametric test results to calibrate an analysis model that describes the flexural behavior of CFT columns. In ‘‘Moment connections to circular concrete-filled steel tube columns’’ by Azizinamini and Schneider, the writers tested various types of CFT connections under quasi-static loading in a collaborative research effort at the University of Illinois and the University of Nebraska at Lincoln. Of the different details tested, the through beam configuration, in which the beam passes through the column, provides best behavior. The test results are used to develop an understanding of the force transfer mechanism and to develop design guidelines for the through-type connection. Ricles et al. discuss the behavior of 10 full-scale momentresisting subassemblages under simulated seismic loading in ‘‘Seismic behavior of composite CFT column WF beam moment connections.’’ The writers show that CFT moment connections can be economically designed to provide more than 0.045 rad of inelastic story drift under cyclic loading. Attention is drawn to various details that performed well in the tests. In ‘‘Brace-beamcolumn connections for concentrically braced frames with CFT Columns,’’ MacRae et al. describe analytical and experimental studies that were carried out to better understand the transfer and distribution of forces in the brace-beam-column joint. The test program considered a number of variables, all of which showed that the majority of force is transferred into the connection by bearing on concrete rather than by friction. The results also showed that slip deformations between concrete and steel are likely to be too small to mobilize the strength of shear studs in such connections. In the last CFT paper ‘‘Inelastic forcedeformation response of joint shear panels in beam-column moment connections to concrete-filled tubes,’’ Nishiyama et al. discuss the AIJ design models for CFT connections. Experiments are conducted to investigate the validity of the model for various material strengths and to calibrate a shear panel model. The first two of five HWS papers are companion papers titled ‘‘Outrigger beam-wall Connections: I and II’’ by Shahrooz et al. In Part I, the writers discuss the experimental behavior of a group of studs under the combined action of gravity shear and cyclic diaphragm forces. The test results show that a failure mode involving yielding and eventual fracture of the connection shear tab is more ductile and thus preferable to stud pull out. A new design technique based on this observation is proposed. In Part II, two one-quarter scale subassemblies involving a cantilever wall and outrigger beams with and without floor diaphragms are tested under cyclic loading. Based on the test results, the writers found that the outrigger beams transferred the majority of the diaphragm forces into the wall, and thus they recommend that floor slab-wall connections can be based on simple details that resist only gravity loads unless the connections are specifically designed to transfer