This paper concerns in-plane vibration analysis of coupled bending and longitudinal vibrations in multi-story planar frame structures based on the advanced Timoshenko bending theory. It takes into account the effects of both rotary inertia and shear distortion. A wave based vibration analysis approach is proposed. From a wave vibration standpoint, vibrations propagate along a uniform waveguide (or structural element), and are reflected and transmitted at discontinuities (such as joints and boundaries). Reflection matrices at various boundaries, as well as transmission and reflection matrices at joint discontinuities are derived. Natural frequencies of coupled bending and longitudinal in-plane vibrations are obtained by assembling these propagation, reflection, and transmission matrices. Numerical examples are presented along with comparisons to results available in literature. The examples show good agreement with the results presented in the available literature.