For post-tensioned bridges there are several analysis problems to overcome:

• The definition of the cable profiles, especially in decks that are curved in plan.
• The calculation of the losses such as friction, wobble, elastic shortening, jacking releases etc.
• Calculation of the moments and forces induced into the structure by the cable.

SAM-LEAP5 now has a facility that addresses all of these issues.

To define a cable, an axis is defined in 3D by selecting the nodes to which the cable will be attached. Then a cable profile is defined by specifying offsets from the axis as may be necessary. A powerful feature is the ability to define a number of fixed offsets and then any other offset will always fit a quadratic or cubic curve. Once one cable is defined it is easy to copy it into other locations. It is important to specify the geometry of the cable accurately as this effects the loss calculations. The figure below shows cables that are parabolic in the vertical plane of the deck, projected onto the finite element webs, which are sloping and curved in plan. The tools in SAM-LEAP5 make this a simple process.

Short term losses are calculated automatically by the program from user defined parameters such as friction coefficient, cable profile, wobble factor, anchorage slip etc. Long term losses, such as shrinkage and creep, can be specified by entering an actual strain (as suggested in BS5400) or by a percentage of the initial pre-stress force. A full set of loss calculations can be printed for checking and the loss profile can be viewed graphically.

Finally, SAM-LEAP5 calculates the internal load at each point and from this derives the combined primary and secondary pre-stress moments and forces within the structure. Because the cable is represented by cable elements, with offsets, then the interaction between concrete and cable is fully modelled.

By using the composite member facility in SAM-LEAP5, (see Case Study #1), the overall effects on one box, or indeed the whole structure, can be output or displayed.

As always in SAM-LEAP5, it is simple, easy to use, and incredibly useful for bridge engineers.