# State-of-the-Art Review on Determining Prestress Losses in Prestressed Concrete Girders
## 2 Works Conducted by Researchers Worldwide
### Laboratory Investigations  
Hop [31] monitored the vibration responses of a number of PC beams. Investigation focused on the influence of prestress forces on frequency and damping. The author found that applying an increase in prestress force, acting unevenly on the member, would increase the vibration frequency. In many cases, it was measured that the application of further levels of prestressing increase would result in drop of vibration frequency.  
Similar experimental results were obtained by [PERSON] et al. [32] on a PC girder with concentric tendon. The research proved an increase of the first eigenfrequency from 11.41 Hz for the case of null prestressing, to 15.07 Hz for the maximum magnitude of prestressing. Besides, the authors observed that an increase in prestressing seems to influence microcrack closure and, consequently, increment stiffness and natural frequencies of PC beams.  
[PERSON] et al. [33] tested the dynamic response of PC girders, strengthened with external tendons. According to their results, prestress forces applied to external tendons influence the frequency vibrations of PC girders.  
[PERSON] and [PERSON] [34] tested a PC beam with a straight concentric tendon. Two conditions were studied, id est, with and without the prestress force of 66.7 kN. The authors noted that the prestressing induced an increment in the first three eigenfrequencies within a range of 0.4-2.1%.  
[PERSON] and [PERSON] [35] tested three simply supported PC girders with different configurations of external tendons. The authors observed that the natural frequency initially increased with the increase in prestress force. Vice versa, the natural frequency decreased after cracks induced by prestressing.  
[PERSON] et al. [36] tested a PC girder with many damage scenarios of prestress losses. Starting from a state of absence of prestress losses, the prestress force was then gradually reduced to zero. During this unloading process, vibration measurements allowed to estimate reductions of the first four eigenfrequencies up to values of 4.0-4.4% from the initial stage to the final one.  
[PERSON] et al. [37] tested a number of six PC beams with a bonded tendon. By applying continuously an increase of prestressing from 0 to 523 kN, the authors observed a progressive increase of the first eigenfrequency from 7.6 to 8.7 Hz.  
[PERSON] et al. [38] performed tests on three PC girders with different configurations of tendons. The researchers detected that the natural frequency generally increases as tension force in prestressing steel increases. Additionally, they observed that natural frequencies of PC girders are affected by other parameters, such as beam camber, geometric stiffness of tendon, and stiffness effect of beam-tendon system.  
The results of the aforementioned relevant works, id est, of [PERSON] [31], [PERSON] et al. [32], [PERSON] et al. [36], and [PERSON] et al. [37], declared that for lower values of prestress force, an increase in prestressing generates an increase in eigenfrequencies, especially for the fundamental frequency. Conversely, for higher levels of prestressing, the rate of increase of the eigenfrequencies tends to decrease. Moreover, changes in vibration frequencies were observed to be higher for smaller eccentricities of prestressing tendon. By considering such investigations, it can generally be claimed that prestress force slightly affects the dynamics of PC girders (Table 1). Nevertheless, relevance of these modifications depends on many characteristics (e.g., cracking and nonlinearity of concrete, bonding and eccentricity of tendon) which generate counterbalancing effects making it difficult to determine an evident relation between dynamic properties and prestress force.