A modified model combining electromechanical impedance (EMI) techniqu e with reverberationray matrix method (RMM) was presented to quantitatively rel at e crack parameters with piezoelectric signatures for structural health monitorin g. The structural members of portal frames were modeled as Timoshenko beams for flexural motion and as the classical longitudinal rods for axial motion as well as the cracks were treated as massless rotational springs. For a structural segm ent with surfacebonded PZT wafer, a coupled system was considered. Then, an analytical expression of electro-mechanical impedance involving information of cra c ks was derived. By using the method, the EMI signatures extracted from the PZT wafers can be used to detect cracks in a portal frame.