Traditional definition of the effective width of steel-concrete composite beam is based on a certain beam section.Section-based effective width changes from one section to another along the beam span. Therefore, effective width for deflection calculation of composite beam should be evaluated based on an element, rather than a specific section. This paper firstly presents the development of two theoretic models for composite beams. One is shear-lag slip beam model (SSM), which takes into consideration both slip effects and shear-lag effects. The other is slip beam model (SLM), which only considers the interface slip between steel beam and concrete slab. Validation of the theoretic models is performed through comparison of the theoretic predictions with the results obtained from more complex finite element simulations. Based on the theoretic models, an element-based definition of effective width for deflection analysis of composite beams is proposed. Parametric studies are performed to find out the most important parameters influencing effective width. It is demonstrated that the effective width is mostly related to the width of the concrete slab, the span of the beam and the thickness of the floor slab. Simplified design formulas for computing the effective width are proposed. Comparisons between the results of the simplified formulas and the test results indicate the accuracy of the proposed formulas. Key word composite beam, effective width, shear-lag, deflection, design  , a c s J J J

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