# Effect of Environmental Parameters on Structural Health Status Assessment Using OMA Techniques
## 9 Conclusions
In the case of correction with ambient temperature, the results show a highly variable trend over time; on the contrary, when the correction is done using the axial forces in the rods, the results are significantly stable over time. Cleaning of the raw frequencies by means of a parameter not measured directly on the beam, such as ambient temperature, leads to variability in the results which can hide the presence of damage on the specimens. In the presented experimental campaign, the vibration of the rods is influenced by a single parameter affected by temperature: the axial force. This is a very simple case, as the specimens' geometry is extremely simple. Moreover, the presence of a load cell and strain gauges allows to directly measure this parameter on both rods, therefore cleaning the frequency response using a parameter that is not available for more complex real structures in the field. In real SHM cases [27; 28; 29; 30; 31], when bridges or historical buildings are under investigation, the frequency response of the structure is not related to a single mechanical parameter (axial force); therefore, it is usually not possible to measure physical parameters that are as good as the axial force in controlling the test results, as is the case in the present work. Therefore, the frequency correction is commonly done using environmental temperature, or in the best situations, using the temperature obtained by thermocouples fixed on the surfaces of the structure. The data coming from real-life SHM applications are then more similar to those obtained from the experimental dataset of this campaign corrected with ambient temperature. Therefore, it is extremely difficult to assess small damages causing frequency variations close to those caused by ambient temperature. The following key points emerge from the study reported in this paper:  
* Mechanical models (such as F.E.M) are very useful to check the results obtained from operational modal analysis. In the study reported in this paper, the knowledge of the theoretical frequencies was very important to be able to exclude non-physical modes and external forcing (the fatigue test). * The peak-picking method is only effective for identifying the frequency of the first mode of vibration, as the signal-to-noise ratio was very low, whereas Cov-SSI and PolyMAX provide better and more robust results. * The correction of frequency data using ambient temperature is not entirely effective and leads to results affected by a variability that can have an order of magnitude similar to the effect of the damage that need to be individuated. In SHM applications of structures like bridges or buildings, it is therefore very important to evaluate the data provided by OMA techniques with care, as they are highly affected by external environmental factors. In many cases the damages that are under investigation generate very little frequency variation in serviceability conditions (especially in prestressed concrete structures); thus, they can be easily hidden within the thermal noise. Damage such as reinforcement corrosion in prestressed structures can in fact generate sensible reduction of bearing capacity with very small reduction of stiffness; therefore, generating very small frequency variation. For these reasons, OMA techniques are undoubtedly useful for dynamic identification, but their limits in open field use should be assessed with care to avoid misleading results. Conceptualization, G.B. and A.C.; methodology, M.F. and G.B. ; software, M.F. ; validation, M.F., A.C. and F.L. ; formal analysis, M.F. ; investigation, M.F. ; resources, G.B. ; data curation, A.C. and F.L. ; writing--original draft preparation, M.F. and G.B. ; writing--review and editing A.C., G.B. and F.L. ; visualization, M.F. ; supervision G.B. ; project administration, G.B. and A.C.; funding acquisition, A.C. All authors have read and agreed to the published version of the manuscript.