Energy harvesting from ambient vibrations in civil structures provides a potential permanent power source for sensor networks used in structural health monitoring. These structures exhibit a narrow, natural frequency-response range, which is generally one to two orders of magnitude lower than the operating frequency spectrum of most piezoelectric-scavenging devices. This considerably limits the levels of harvestable power and thus hinders the implementation of advanced sensing functionalities. In this paper, the improvement of the energy-harvesting characteristics of a bimorph cantilever lead zirconate titanate (PZT) piezoelectric beam is studied. The application of a variable prestress-loading condition is used as a solution to modify the system's properties. A generalized model that takes into account all the vibration-mode shapes of the beam and the back-coupling effect is derived using the Hamiltonian principle. The model describes the effect of the prestress parameters on the harvestable energy levels. An analysis of the variations of the frequency response, amplitude, damping ratio, and efficiency of the device with respect to the preload is presented. Experimental verification of the model is also performed. The time and frequency domain responses of the piezoelectric bimorph are measured and compared with theoretical results. Measured acceleration from a concrete bridge deck under ambient loading and recordings from extreme events (earthquakes) are used to show the variations of the harvested power with respect to different prestress conditions. DOI: 10.1061/(ASCE)EY.1943-7897.0000077. (C) 2012 American Society of Civil Engineers.