Analysis and Design of Steel-Concrete Composite Frame Systems
The lack of clear guidance regarding the value of elastic flexural rigidity (EI) that should be used in the analysis of steel-concrete composite columns is a significant unresolved issue that hinders the design of frame systems that include these members. The common approach of utilizing the rigidity of either the steel or the concrete only tends to be overly conservative. Other approaches that combine the rigidities have not been fully validated for use in strength and stability design procedures or for the determination of lateral deflections. To address this issue, research has been conducted to develop comprehensive recommendations for the elastic flexural rigidity of composite columns. A complimentary goal has been to validate common strength design procedures, including interaction diagrams, axial compression and bending moment anchor points, and analysis procedures to determine required strengths. In this paper, an assessment of current strength design methodologies is presented along with recommendations for potential improvements. Both the AISC Direct Analysis method and the ACI elastic second-order analysis method are investigated. INTRODUCTION The wide availability of structural analysis software packages that perform accurate geometric nonlinear analyses has made second-order elastic analysis design approaches very common in practice. Such design provisions exist for steel-concrete composite members in both the AISC Specification (AISC 2010b), as the Direct Analysis method, and the ACI Code (2011), as the second-order analysis method. However, neither methodology has yet been comprehensively assessed for validity and accuracy, as has been done in the past for structural steel (Kanchanalai 1977; Surovek-Maleck and White 2004) and for reinforced concrete (Hage and MacGregor 1974). This paper presents the results of a large parametric study that has been conducted to assess the current design methodologies with a particular emphasis on the elastic flexural rigidity (EI) recommended for analysis. 2605 Structures Congress 2014 © ASCE 2014
Assessment of Seismic Behavior Factor of Code-Designed Steel–Concrete Composite Buildings
In this study, the seismic behavior factor of steel–concrete composite buildings was examined. For this purpose, 5-, 10-, 15- and 20-story composite moment-resisting frame (CMRF) buildings having concrete-filled steel tube columns and composite beams were designed at medium ductility (DCM) and high ductility (DCH) levels as per the regulations of Eurocode 8. Examined DCM structures were designed for a PGA of 0.2 g while DCH structures were designed for a PGA of 0.4 g. SeismoStruct software was utilized in the design and performance assessment of the case study structures. Nonlinear static pushover and incremental dynamic analyses were used. In the pushover analysis, the uniform and triangular load distributions were adopted while 12 earthquake ground motions were employed in the dynamic analysis. The effect of ductility levels and number of stories on the seismic behavior of CMRFs was studied using the results of the nonlinear analysis. To this, the variation in the lateral response, global yielding value, code-based behavior factor and component, and dynamic behavior factor were presented for CMRF structures with DCM and DCH. It was observed that the behavior factor of all CMRFs exhibited well above the design assumptions, especially DCM class CMRFs.
Stability Analysis and Design of Concrete Frames
In recent years tall slender buildings and slender building components have necessitated new methods of analyzing stability problems. During the past 5 yr second-order analyses or PΔ analyses have been extensively examined in the literature. This paper reviews the recent developments in this field and presents several methods in which design engineers can include PΔ moments in their analyses. The first part of the paper reviews methods currently available for carrying out second-order analyses and the second deals with the application of such analyses in the design of reinforced concrete buildings considering stability. It reviews procedures to determine when second-order analyses are required, recommends stiffness values for use in the analysis of concrete frames, and presents the steps necessary in design oriented second-order analyses.