Abstract
The estimate of internal prestressing in concrete beams is essential for the assessment of their structural reliability. Many scholars have tackled multiple and diverse methods to estimate the measurable effects of prestressing. Among them, many experimented with dynamics-based techniques; however, these clash with the theoretical independence of the natural frequencies of the forces of internally prestressed beams. This paper examines the feasibility of a hybrid approach based on dynamic identification and the knowledge of the elastic modulus. Specifically, the author considered the effect of the axial deformation on the beam length and the weight per unit of volume. It is questioned whether the uncertainties related to the estimate of the elastic modulus and the first natural frequency yield reasonable estimates of the internal prestressing. The experimental testing of a set of full-scale concrete girders with known design prestressing supports a discussion about its practicability. The author found that the uncertainty in estimating the natural frequencies and elastic modulus significantly undermines a reliable estimate of the prestressing state.
Highlights
The question of a prestressing estimate may lie between two issues: (1) Is the resisting bending moment of a beam affected by the prestressing force?; (2) Why is the prestress value necessary for the assessment of the structural reliability? The prestressing force does not affect the bending resistance of a prestressed concrete beam: the resistance depends on the resistances of concrete and steel, as depicted in Figure 1, which represents a simplified rectangular cross-section with an eccentric prestressing cable
The author tested the effect of the axial deformation due to internal prestressing on the estimate of the prestressing force
The procedure is tested on a set of seven -supported prestressed concrete girders, using the first experimental natural frequency
Summary
The Role of Prestressing in Structural ReliabilityThe question of a prestressing estimate may lie between two issues: (1) Is the resisting bending moment of a beam affected by the prestressing force?; (2) Why is the prestress value necessary for the assessment of the structural reliability? The prestressing force does not affect the bending resistance of a prestressed concrete beam: the resistance depends on the resistances of concrete and steel, as depicted in Figure 1, which represents a simplified rectangular cross-section with an eccentric prestressing cable. The question of a prestressing estimate may lie between two issues: (1) Is the resisting bending moment of a beam affected by the prestressing force?; (2) Why is the prestress value necessary for the assessment of the structural reliability? The prestress force enhances the performance of the beam in operational conditions by reducing its deformability [1] and preventing the cross-section cracking [2,3,4]. Since the tensile stresses are not admitted or allowed in a limited range, a loss in the prestress force compromises the operational performance of the beam but not its ultimate resistance. The knowledge of the prestressing state is determined when assessing the maximum admitted loads of a bridge and the performance in operational conditions [5,6]
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