Methodology
Generally structural engineers adopt one of two methods for applying prestress to a structure for structural analysis. The first involves calculating the equivalent loads resulting from the tendon prestress, and applying these as nodal loads to the structure. The second involves modelling the tendons as elements in the structural model.
The advantage of the first method is that the model size (and hence analysis time) will not be increased, and the loads correctly model the tendon forces according to the parameters used for initial tendon stress and losses. However the disadvantage arising as a consequence of this is that subsequent loading on the structure, (including potentially loading from subsequent construction stages for staged construction) cannot alter the modeled tendon force.
Autodesk Structural Bridge Design therefore adopts the second method and models the tendons as elements in the model. In the stage at which the prestress is applied, the elastic modulus of the tendon elements is factored down by a factor of 106. Doing this ensures that the distortions in the model do not reduce the prestress at all. For subsequent loading effects, the elastic modulus of the tendons is reset to its correct value, so that the elastic deformation effects are correctly allowed for with no further input required.
Tendons are modeled as beam elements, with their area obtained from the tendon data specified, and other section properties assuming a circular section with the specified area.
Tendon forces after losses are converted to strains, and are applied as initial strains in the beam elements of the tendon.
Where the beam elements representing tendons coincide with finite element edges, the program automatically generates appropriate Conforming sides data.
Two principal concepts used by Autodesk Structural Bridge Design need to be grasped. Firstly the concept of the tendon reference axis. Secondly, the concept of the tendon profile. Each of these is explained next.