To model external tendons, it is necessary to do the following:
- Model the tendon as elements (rather than as loads). This assures that the tension is carried in the tendon where it is external. Modeling as loads is possible, but does require further considerations.
- Make sure that the tendon is not inside the bounding box of any object where you want it to be external. If the tendon falls outside of all objects, this is automatic. If the tendon is inside of a box girder, you may have a problem if you model the girder as a frame element. You can either model the box girder as areas (shells) so that the tendon falls inside the open space within the box, or if you do want to use frame elements, then you can easily define a group of objects that the tendon is allowed to load, and exclude from the group those elements where the tendon is external.
- Modify the losses to account for the external segments. It may be most accurate to do this by using separate tendon objects for the internal and external segments. The distributed losses could be set to zero for the external segments, and non-zero for the internal segments. If the cable is essentially all external except for limited connection points, then a single tendon with distributed losses set to zero would be adequate.