Prestressed concrete with internal or external tendons: Behaviour and analysis.

摘要

Internal or external tendons with or without bond are widely used in practise to prestress concrete structures. A method of analysis and a computer program are developed to predict the behaviour of concrete structures at any load level up to ultimate. When the prestressing tendon is internal unbonded or external, the analysis is more involved because the strain compatibility of the prestressing tendon and the adjacent concrete at a section is no longer valid. Thus, the section properties and internal forces at a section are not enough to calculate the stress change in the tendon. The change in tendon stress due to applied loads should be determined from the displacements of the anchorages and the deviators. As the concrete member deflects, the external tendon remains straight resulting in change in tendon eccentricity and causes nonlinearity in the analysis.; In the proposed method of analysis, a structure is idealised as an assemblage of plane frame members connected at the nodes. The concrete is modeled as short, prismatic members; external tendons are treated as members connected to the structure nodes by rigid links. The analysis determines the nodal displacements accounting for the geometric and material nonlinearities, the time-dependent effects of creep and shrinkage of concrete and relaxation of prestressed reinforcements, and the temperature effects. The Newton-Raphson iterative scheme combined with displacement method is employed. The stiffness matrix to be used consists of two matrices: elastic and geometric stiffnesses. Both matrices depend on updated nodal co-ordinates and stresses. The nodal displacements, member end forces, section strains and stresses are calculated with respect to a selected constant reference axis because the centroidal axis changes due to cracking and the time dependent effects.; The computer program is verified by comparison with experimental data. These experiments include beams prestressed with fibre reinforced polymers (FRPs). The verified program is employed to study the difference in structural behavior with different prestressing techniques used in practise.