This work presents a semi-passive concept to reduce structural vibrations over a wide frequency regime. Therefore, a purely resistive passive electrical network is designed and connected to a piezoelectric element. This concept allows an enhancement in structural damping without using sensitive sensor electronics and amplifiers. Furthermore, it yields constant vibration reduction over a wide temperature range; limitations arise for temperatures above the Curie temperature. Firstly, the damping capabilities of a resistively shunted piezoelectric element are discussed and the optimal resistance for the passive electrical network is outlined. Next, a design concept for optimal placement of the piezoelectric elements is presented. This concept is designed for placement on two dimensional structures, such as plates. It is based on an energy ratio, which is defined for each structural mode. In this context, the effective strain energy of a two dimensional piezoelectric element is introduced. It allows calculation of the electrical energy, generated by the piezoelectric element as it is mechanically loaded. Adaption of the shunt resistor, and thus, maximal reduction of structural vibrations, is obtained by a new concept, which uses digital potentiometers in combination with a feedforward control concept. Depending on the excitation signal, two different resistances can be realized by the adaptive passive electrical network. In this manner two structural modes can be optimally damped. Finally, experiments are conducted, in which fixed resistors as well as adaptive resistors are implemented. Results show the advantage of using adaptive resistors for the passive electrical network in terms of enhanced vibration reduction capabilities.