Bending Stiffness of a Continuous Concrete Beam Post-tensioned with Unbonded Tendons

The ratio of bending stiffnesses at different critical points in the continuous prestressed concrete beam exerts an influence on the redistribution of internal forces, generally moment from support to field. The increasing load will result in the formation of a large number of cracks in the maximum moment regions. This, in turn, leads to a reduction of the flexural stiffness of the member and finally the formation of plastic hinges. Four continuous two-span concrete beams prestressed with unbonded tendons were constructed and loaded up to failure at the laboratory of Civil Engineering in Tampere University. This article focuses on the examination of the flexural stiffness of the test beams during the loading tests. The average bending stiffness of the test beams, as determined through the measured deflections, was clearly less than the elastic value E_cmI_g, even at the low loading levels. The calculated rotation values in the centre support exhibited a good correlation with experimental results. However, these values remained below the observed values because, in the plastic hinge regions, the stiffness underwent a more rapid decrease. The total plastic rotation obtained from test results was approximately three times the plastic rotation capacity calculated using the Eurocode.