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This paper presents a simplified beam design method for semi-rigid composite frames with vertical loading at the serviceability limit state. Equations were developed to determine the deflections of the composite beam allowing for both joint flexibility and beam sectional properties, along with a formula for the connection secant stiffness. The equations for the connection stiffness are more accurate than previous equations used because it considers the beam-to-column stiffness ratio and the beam-to-connection stiffness ratio. The equations were validated by the experimental results for two semi-rigid composite frames. The equations agree well with the experimental data because they take into account the actual beam-to-column connections and the composite action between the steel beam and the concrete slab.