In the assessment of truss structure damage, the piezoelectric impedance-based technique showed good performance, especially by using the longitudinal wave. However, a satisfactory longitudinal impedance output is not easy to obtain in most practical applications. In our previous study, we found that longitudinal vibration modes can be generated by selecting suitable PZT patches, and used this method for damage assessment. The objective of this paper is to investigate how to design the PZT patch to generate an efficient impedance response in the expected longitudinal modes. First, the influence of PZT patch design parameters, such as the bonding position and length of the PZT patch, on the piezoelectric impedance response are investigated theoretically. Next, for the purpose of generating a large longitudinal impedance output at the expected frequency, a scheme for determination of the bonding position and length of the PZT patch is proposed. To validate the proposed method, several examples are demonstrated and discussed. Finally, as an application to use longitudinal modes for damage assessment, crack detection in beams is investigated experimentally and analytically.