Study On Natural Frequency Of Externally Prestressed Concrete Beams

Externally prestressed technique has been widely used in civil engineering constructionswith the development of decades. It has been the research focus to estimate the use situtationand prestress of prestressed structure by detecting the change of natural frequencies, butresearchers have not reached an agreement on influence of frequency due to prestress.The research on natural frequencies of three externally prestressed concrete beams iscarried out. Three different supports are imposed respectively on two ends of beam L1, thenanalysis the first three frequencies. It shows that the supporting conditions on two ends of beamaffect natural frequencies a lot and have more influence on the higher order frequencies. Next,three beams are subjected to prestress in grades in the same form of constraint, then tested thefirst three frequencies. Experimental results indicate that the change of prestress has little effecton basic frequency of beams, however, higher order frequencies grow with the increase ofprestress, and the eccentricity and shape of external tendons have great influence on vibrationfrequency of beams. It is because that the first frequency is little sensitive to small damage inbeams and the prestress closes cracks inner beams which leads to growth of higher orderfrequencies, the simple-supported beam with eccentricity or fold line external tendons which issubjected to large presress will make the upward arch results in change of the stiffness.Surveying vibration frequency can be carried out after the text of vertical load on prestressedbeams,then take comparative analysis with frequencies of general beams. The conclusionindicates that the first frequencies of prestressed beams subjected to vertical load in early timeskeep invariant and the second and third frequencies decrease a little. In later period of load, allorder frequencies decrease obviously, but descent rates of all order frequencies of prestressedbeams are less than ones of general beams in ultimate bearing state.The theoretical relation model between prestress and vibration frequency is established onthe base of classical Timoshenko theoretical model in this thesis, then the stiffness of beam ismodified based on test data. The formula calculated in the paper manifests that vibrationfrequencies of beams increase with the increment of prestress, the first frequency of the straightexternal tendon beams calculated by the formula in the paper is little different from which istested from the experiment, but not the higher order frequencies. Otherwise, calculationformula between prestress and frequency of tendons is founded which makes possibility ofcalculating the effective prestress by basic frequency in tendons.