Purpose: It is the intention of the present study to develope a new beam theory for the analysis of functionally graded compopsite beams to overcome the shortcomings present in the existing beam theories. Design/methodology/approach: Within the displacement field of a first-order shear deformation theory and by using the Hamilton principle the governing equations of motion are obtained for both the new and the existing beam theories. The beams are assumed to have isotropic, two-constituent material distribution through the thickness. Findings: It is found that the procedure used is simple and straightforward and similar to the one used in the development of shear deformation plate and shell theories. It is analytically showed that the new approach yields identical results as those obtained by using the existing first-order shear deformation theory. Research limitations/implications: The new approach can be adopted in developing higher-order shear deformation and layerwise theories. It is believed that the new approach has advantage with respect to the existing beam theories especially for developing beam layerwise theories. Practical implications: The new shear deformation beam theory can be used to develop a new beam element for analysis of practical composite beam structures. Originality/value: The paper introduces an approach to develop a new theory for modeling composite beams. The resulting equations of motion may be solved analytically or by using finite element method.

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